ML20138A256
| ML20138A256 | |
| Person / Time | |
|---|---|
| Issue date: | 04/21/1997 |
| From: | NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | |
| Shared Package | |
| ML20138A252 | List: |
| References | |
| REF-WM-69 NUDOCS 9704280026 | |
| Download: ML20138A256 (74) | |
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FINAL TECHNICAL EVALUATION REPORT FOR THE REMEDIAL ACTION PLAN AND SITE DESIGN AT THE MAYBELL URANIUM MILL TAILINGS SITE MAYBELL, COLORADO i
i 9704280026 970421 PDR WASTE WM-69 PDR
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TABLE OF CONTENTS Section Paae 1
1.0 INTRODUCTION
1-1 1.1 EPA Stand
.a 1-1 1.2 Site and Proposed Action...................
1-1 1.3 Review Process........................
1-2 1.4 TER Organization.......................
1-7 1.5 Summary of Open Issues 1-7 1
l 2.0 GEOLOGIC STABILITY.........................
2-1 l
2.1 Introduction.........................
2-1 2.2 Location...........................
2-1 l
2.3 Geology 2-1 2.3.1 Physiographic Setting 2-1 l
2.3.2 Stratigraphic Setting 2-4 2.3.3 Structural Setting..................
2-4 2.3.4 Geomorphic Setting..................
2-8 2.3.5 Seismicity.
2-9 2.3.6 Natural Resources 2-11 l
2.4 Geologic Stability.....................
2-11 l
2.4.1 Bedrock Suitability 2-13 2.4.2 Geomorphic Stability.................
2-13 l
2.4.3 Seismotectonic Stability...............
2-13 l
2.5 Con cl u s i on s........................
2-15 3.0 GE0 TECHNICAL STABILITY.......................
3-1 3.1 -Introduction................
3-1 l
3.2 Site and Material Characterization..............
3-1 3.2.1 Site Description...................
3-1 3.2.2 Geotechnical Investigations 3-1 3.2.3 Testing Program 3-2 3.2.4 Site Stratigraphy 3-3 i
l 3.3 Geotechnical Engineering Evaluation 3-4 3.3.1 Slope Stability 3-4 3.3.2 Settlement and Cover Cracking 3-5
.3.3.3 Liquefaction Potential................
3-5 3.3.4 Cover Design.....................
3-6 3.4 Geotechnical Construction Details 3-7 3.4.1 Construction Methods and Features 3-7 3.4.2 Testing and Inspection................
3-7 3.5 Conclusions 3-7 l
4.0 SURFACE WATER HYDROLOGY AND EROSION PROTECTION...........
4-1 4.1 Hydrologic Description and Site Conceptual Design 4-1 4.2 Flooding Determinations 4-1 4.2.1 Selection of Design Rainfall Event 4-2 4.2.2 Infiltration Losses 4-2 i
i MYBELL fiER l
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..s 4.2.3 Times of Concentration 4-3 4.2.4 Rainfall Distributions.............
4-3 4.2.5 Computation of PMF 4-4 4.2.5.1 Top and Side Slopes 4-4 4.2.5.2 Apron / Toe 4-4 4.2.5.3 Diversion Ditches 4-4 4.2.5.4 Natural Gullies 4-5 4.2.5.5 Rob Pit Overburden Pile 4-5 4.3 Water Surface Profiles and Channel Velocities 4-5 4.3.1 Top and Side Slopes 4-5 4.3.2 Apron / Toe 4-5 4.3.3 Diversion Ditches 4-6 4.3.4 Natural Gullies 4-6 4.4 Erosion Protection 4-7 4.4.1 Sizing of Erosion Protection.............
4-7 4.4.1.1 Top Slopes ar.d Side Slopes 4-7 4.4.1.2 Apron / Toe 4-7 4.4.1.2.1 Lower Side Slope 4-7 4.4.1.2.2 Toe 4-7 4.4.1.2.3 Collapsed Slope 4-8 4.4.1.2.4 Natural Ground 4-8 4.4.1.3 Diversion Ditches 4-8 4.4.1.3.1 Ditch Side Slopes 4-9 4.4.1.3.2 Ditch (Main Section) 4-9 4.4.1.3.3 Ditch Outlets 4-9 4.4.1.3.4 Sediment Considerations 4-9 4.4.1.4 Natural Gullies 4-10 4.4.1.5 Rob Pit Overburden Pile 4-11 4.4.2 Rock Durability 4-11 4.4.3 Testing and Inspection of Erosion Protection 4-12 4.5 Upstream Dam Failures 4-13 4.6 Conclusions 4-13 5.0 WATER RESOURCES PROTECTION.....................
5-1 5.1 Introduction.........................
5-1 5.2 Hydrogeologic Characterizatisn................
5-1 5.2.1 Identification of Hydrogeologic Units 5-2 5.2.2 Hydraulic and Transpo t Properties..........
5-2 5.2.3 Geochemical Conditions and Extent of Contamination..
5-3 5.2.4 Water Use 5-4 5.3 Conceptual Design Features to Protect Water Resources 5-6 5.4 Disposal and Control of Residual Radioactive Materials (RRM).
5-7 5.4.1 Water Resources Protection Standards For Disposal 5-7 5.4.1.1 Hazardous Constituents............
5-7 5.4.1.2 Concentration Limits.............
5-8 5.4.1.3 Point of Compliance 5-8 5.4.2 Performance Assessment..
5-9 5.4.3 Closure Performance Demonstration 5-9 5.4.4 Ground-Water Monitoring and Corrective Action Plans 5-10 5.5 Clean-Up and Control of Existing Contamination.......
5-11 5.6 Conclusions 5-11 ii MM BELL ffER
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6.0 RADON ATTENUATION AND SITE CLEANUP 6-1 6.1 Introduction.........
6-1 6.2 Radon Attenuation 6-1 6.2.1 Evaluation of Input Parameters............
6-1 6.2.2 Evaluation of Radon Attenuation Model 6-5 6.3 Site Cleanup....
6-5 6.3.1 Radiological Site Characterization..........
6-5 6.3.2 Cleanup Standards 6-6 6.3.3 Verification...................
6-6 6.4 Conclusions 6-7
7.0 REFERENCES
7-1 l
l iii MAYBELL fTER l
1
o x
l LIST OF FIGURES Fiqure Paae 1
FIGURE 1.1, LOCATION MAP 0F THE MAYBELL TAILINGS SITE..........
1-3 FIGURE 1.2.
LOCATION DETAILS OF THE MAYBELL TAILINGS SITE........
1-4 i
FIGURE 1.3.
APPR0XIMATE B0UNDARIES OF Ra-226 50Il CONCENTRATIONS EXCEEDING 5pCi/g AT THE TAILINGS SITE NEAR MAYBELL.
1-5 FIGURE 1.4.
M"'. LL DISPOS *'. cE!' *ND COVFR 1-6 FIGURE 2.1.
PHYSIOGRAPHIC PROVINCES OF THE MAYBELL SITE REGION.
2-2 FIGURE 2.2.
REGIONAL AND AREAL DRAINAGE SYSTEM.
2-3 FIGURE 2.3.
STRUCTURAL SETTING OF THE MAYBELL SITE REGION........
2-5 FIGURE 2.4.
SEISM 0 TECTONIC PROVINCES OF THE MAYBELL SITE REGION....
2-10 FIGURE 2.5.
ENERGY AND MINERAL RESOURCES MAYBELL SITE REGION.
2-12 FIGURE 2.6.
MAJOR TRIBUTARIES OF JOHNSON WASH MAYBELL SITE, CO.
2-14 l
LIST OF TABLES Table Paae le 1.'
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40-MI (65-KM) RADIUS OF THE MAYBELL, COLORADO SITE 2-6/2-7 l
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LIST OF ACRONYMS Acronym Definition CFR Code of Federal Regulations
[
t COE U.S. Army Corps of Engineers 1
CR Completion Report DOE U.S. Department of Energy DOT U.S. Department of Transportation EPA U.S. Environmental Protection Agency LTSP Long-Term Surveillance Plan MCL Maximum Concentration Limit, or Maximum Contaminant Level NEPA National Environmental Policy Act NRC U.S. Nuclear Regulatory Commission PMF Probable Maximum Flood 1
PHP Probable Maximum Precipitation
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POC Point of Compliance RAIP Remedial Action Inspection Plan l
RAP Remedial Action Plan RAS Remedial Action Selection SRP Standard Review Plan TER Technical Evaluation Report UMTRA Uranium Mill Tailings Remedial Action UMTRCA Uranium Mill Tailings Radiation Control f.:t v
IMYBELL fiER l
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1.0 INTRODUCTION
The Maybell site was designated as one of 24 abandoned uranium mill tailings piles to receive remedial action by the U.S. Lepartment of Energy (D0E) under the Uranium Mill Tailings Radiation Control Ac.; of 1978 (UMTRCA).
UMTRCA requires, in part, that the U.S. Nuclear Regulatory Commission (NRC) concur with DOE's selection of remedial action, such that the remedial action meets appropriate standards promulgated by the U.S. Environmental Protection Agency (EPA). This final Technical Evaluation Report (TER) documents the NRC staff's review of the DOE final Remedial Action Plan (RAP), Remedial Action Inspection Plan (RAIP) and all associated documentation pertinent to the proposed remedial action.
1.1 EPA Standards As required by UMTRCA, remedial action at the Maybell site must comply with regulations' established by the EPA in 40 CFR Part 192, Subparts A-C.
These regulations are summarized as follows:
1.
The disposal site shall be designed to control the tailings and other residual radioactive material for 1000 years to the extent reasonably achievable and, in any case, for at least 200 years [40 CFR 192.02(a)].
l 2.
The disposal site design shall provide reasonable assurance that releases of radon-222 from residual radioactive materials to the atmosphere will not exceed 20 picocuries/ square meter /second, or increase the annual average concentration of radon-222 in the air at any location outside of the disposal site by more than 0.5 picocuries/ liter [40 CFR 192.02(b)].
3.
The remedial action shall ensure that radium-226 concentrations, in land that is not part of the disposal site and is averaged over any area of 100 square meters, do not exceed the background level by more than 5 picocuries/ gram (pci/g) averaged over the first 15 I
centimeters of soil below the surface, and 15 pCi/g averaged over any 15-centimeter-thick layer of soil more than 15 centimeters below the land surface [40 CFR 192.12(a)].
On January 11, 1995, EPA published a final rule for groundwater standards for remedial actions at inactive uranium processing sites (40 CFR 192, Subparts A through C). The standards consist of two parts:
a first part, governing the control of any future groundwater contamination that may occur from tailings piles after remedial action; and a second part, governing the cleanup of contamination that occurred before the remedial action of the tailings.
In accordance with UMTRCA Section 108(a)(3), the remedial action shall comply with the EPA standards.
1.2 Site and Proposed Action The Maybell uranium mill site is approximately 25 miles west of the town of Craig, Colorado, in Moffat County, located in the northwest part of the state 4
1-1 mAvsett fira
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(Figure 1.1).
The site is five miles north of the Yampa River and is located in relatively flat terrain broken by low, flat-topped mesas.
It is situated between Johnson Wash to the east and the Rob Pit Mine to the west.
Numerous reclaimed and unreclaimed mines are located in the immediate vicinity.
The site covers approximately 110 acres and consists of a concave-shaped tailings pile and rubble from the demolition of the mill buildings, which are i
buried in the former mill area. The tailings area within the designated site l
boundary is shown in Figure 1.2.
Contaminated materials at the Maybell i
processing site include the tailings pile that has an average depth of 20 feet (ft) and contains 2.8 million cubic yards (cy) of tailings.
The former mill processing area is on the north side of the site and contains approximately 20,000 cy of contaminated demolition debtis.
The volume of off-pile 1
l contamination to be placed in the disposal cell is approximately 550,000 cy, and is composed of contamination adjacent to the tailings pile, as well as l
windblown and waterborne contamination.
Figure 1.3 presents the areal extent of contamination exceeding 5 pCi/g Ra-226 in the area surrounding the tailings l
pile and outside of the designated site.
The total volume of contaminated i
materials to be disposed of as part of the remedial action is estimated to be
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3.5 million cy.
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The remedial action will consist of stabilization of the existing tailings i
pile in place. The pile will include demolition debris from the former mill j
processing yard, windblown contaminated soils, waterborne contamination, and j
vicinity property contamination.
The completed disposal cell will contain the estimated 3.5 million cy of contaminated materials on approximately 66 acres.
1 The disposal cell embankment will rise an average of 40 ft above the surrounding topography with side slopes of 20 percent, and a top slope of i
three percent to the west (Figure'1.4).
1.3 Review Process The NRC staff review was performed in accordance with the Standard Review Plan (SRP) for UMTRCA Title I Mill Tailings Remedial Action Plans (NRC,1993) and i
consisted of comprehensive assessments of DOE's final RAP and site design.
1 Staff review of the final RAP and designs submitted by DOE indicate that there l
are no remaining open issues, as presented in Section 1.5 and discussed in further detail in Chapters 2 through 6 of this final TER.
The NRC has reviewed and concurs with all DOE revisions to the preliminary final RAP (DOE, l
1993 a-e).
These revisions were incorporated into the final RAP.
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o The remedial action information assessed by the NRC staff was provided primarily in the following documents:
1.
DOE, " Remedial Action Plan and Site Design for Stabilization of the Inactive Uranium Mill Tailings Site at Maybell, Colorado,"
(MAY RAP), "UMTRA-00E/AL/62350-24F," 1996a - Remedial Action Selection Report (RAS) (December 1996),
2.
MAY RAP, 1993b, Attachment 1: Subcontract Documents (May 1992),
Calculations (Volumes I-V, May 1992)), Information for Reviewers (May 1992), and Information for Bidders (Volumes I-IV, May 1992),
3.
MAY RAP, 1996c, Attachment 2: Geology Report (December 1996),
4.
MAY R?P, 1996d, Attachment 3: Ground-Water Hydrology Report (December 1996),
5.
MAY RAP, 1996e, Attachment 4: Water Resources Protection Strategy (December 1996),
6.
MK-Ferguson Company,1996, Remedial Action Inspection Plan, Rev 0, August 1996.
1.4 TER Organization The purpose of this final TER is to document the NRC staff review of DOE's RAP and RAIP for the Maybell site.
The following sections of this report have been organized by technical discipline relative to the EPA standards in 40 CFR Part 192, Subparts A-C.
Sections 2, 3, and 4 provide the technical basis for the NRC staff's conclusions with respect to the long-term stability standard in 192.02(a). Section 5, Water Resources Protection, summarizes the NRC staff's conclusions regarding the adequacy of DOE's compliance demonstration with respect to EPA's ground-water protection requirements in 40 CFR Part 192.
Section 6 provides the basis for the staff's conclusions with respect to the radon control standards in 192.02(b), and soil cleanup standards in 192.12.
1.5 Summary of Open Issues The NRC staff review of the preliminary final RAP identified 18 open issues.
These issues have been satisfactorily addressed by DOE in the final RAP, and are discussed in more detail in the following sections. A brief summary of these open issues and their status is provided in Table 1.1.
1-7 MAYBELL fTER
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TABLE 1.1.
SUMMARY
OF OPEN ISSUES OPEN ISSUE TER STATUS SUBSECTION 1.
DOE should-add a requirement to test 3.4.2 Closed bentonite -..;ded radon cover Atterterg limit values during production and placement.
2.
DOE should provide a revised design 4.4 Closed of the erosion protection for the natural gullies downstream of the i
disposal cell, particularly Gullies 1 and 2.
DOE should consider the effects of high flow velocities produced by large floods in Johnson Wash on the stability of the riprap to be placed in the gullies.
DOE should also evaluate the effects of scouring in Johnson Wash and the potential for undermining of the riprap in the gullies.
3.
Several discrepancies have been noted 5.2.2 Closed among the RAP text and calculations MAY-08-90-14-06-00, and MAY-08-90 07-00.
DOE should review the measured potentiometric data for the site and provide an accurate estimate of the horizontal hydraulic gradient and the average linear velocity for the site.
4.
DOE has not addressed the potential
. 2.3 Closed ground-water contamination resulting from the prior tailings fluid releases into Johnson Wash and Lay Creek.
DOE should provide and evaluation of the ground-water impact from these prior tailings fluid releases, considering that at least one domestic well is situated within the alluvial soils of Lay Creek j
downgradient of the site.
j 1-8 MAYBELL ffEt
e
- e OPEN ISSUE TER STATUS
]
SUBSECTION 5.
DOE has not evaluated the potential 5.3 Closed I
adverse impact on the ground-water quality caused by the proposed preloading of the existing tailings pile. DOE should quantify the amount of anticipated seepage into the underlying soils caused by the preloading and evaluate the impact to the ground-water quality beneath the l
site.
6.
DOE should revise the list of 5.4.1.1 Closed hazardous constituents in the RAP to include antimony, beryllium, and nickel, and should provide information on the organic hazardous constituent concentrations in the l
tailings.
7.
DOE should revise calculations MAY-5.4.2 Closed l
08-90-12-06-00 and MAY-09-90-14-03-00 l
to reflect the hydraulic conductivity i
specified in the design and account for the additional flux resulting I
from the preloading of the tailings l
pile.
8.
NRC staff does not agree with DOE's 5.4.3 Closed l
proposal to defer the development of l
a conceptual performance monitoring program to the Long-Term Surveillance Plan (LTSP). At a minimum for l
demonstrating compliance with l
fl92.02(b), DOE should provide a conceptual monitoring program or alternative measures which will be i
effective in demonstrating the disposal cell performance during the specified post-closure period.
l Details of the monitoring plan can be l
provided in the LTSP.
9.
DOE should demonstrate compliance 5.5 Closed with EPA's final ground-water cleanup standards in 40 CFR 192, Subparts B and C.
l-9 m4vsttt fire
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1 l
OPEN ISSUE TER STATUS i
SUBSECTION 10.
DOE should indicate the basis for 6.2.1 Closed chosing the density and specific 4
l gravity values for the off-pile and 1
windblown materials.
11.
DOE should provide diffusion 6.2.1 Closed coefficient test data at the i
appropriate densities for the i
estimated final upper 16 feet of a
contaminated material, or provide a I
conservative estimate for these i
values for the radon model.
12.
DOE should provide the radon 6.2.1 Closed i
emanation test data for the off-pile and windblown material and 4
demonstrate that the data is representative of these materials.
i 13.
DOE should provide the diffusion 6.2.1 Closed i
coefficient test data for the radon barrier material.
i
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14.
DOE should state in the RAS that the 6.2.1 Closed j
final cover design will be based, in i
~
part, on parameter values (e.g. Ra-i 226 concentration, density) measured i
during construction under standard procedures. The analysis should be i
provided for review as part of the i
Completion Report.
15.
DOE should provide a calculation in 6.2.1 Closed the final RAP that incorporates all the information needed to review the input parameters, or refer to specific pages in this RAP.
16.
DOE should determine if any 6.3.1 Closed designated contaminated areas are actually natural mineralized deposits.
17.
DOE should adequately characterize 6.3.1 Closed Th-230 at this site, considering site history and potential sources for Th-230 contamination, in the absence of Ra-226 that requires removal.
1-10 MAYBELL fiER
OPEN ISSUE TER STATUS SUBSECTION 18.
DOE should consider planning more 6.3.2 Closed extensive cleanup in Johnson Wash and Lay Creek, and provide the supp'
.ntal r+>.nd'-d
'rplicatin-for these areas in the final RAP.
1-11 MAYBELL ffER
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2.0 GE0 LOGIC STABILITY 2.1 Introduction l
This section of the TER documents the staff's review of regional and site geologic information for the proposed remedial action at the Maybell uranium mill tailings site in northwestern Colorado. The EPA standards (40 CFR 192) do not include generic nr site-specific requirements for characterizatinn of geologic conditions at Uranium Mill Tailings Remedial Action (UMTRA) Project sites.
Rather, 40 CFR 192.02(a) requires that control mechanisms be designed to be effective for up to 1,000 years, to the extent achievable, and in any case for at least 200 years.
NRC staff has interpreted this standard to mean that certain geologic conditions must be met in order to have reasonable l
assurance that the long-term performance objectives will be achieved.
l Guidance with regard to these conditions is specified in the SRP.
2.2 Location l
For location description, see Section 1.2.
2.3 Geology l
DOE characterized regional and site-specific geology by referring to l
published and unpublished geologic literature and maps; reviewing subsurface geologic data, including logs of exploratory boreholes drilled on the site; l
and conducting field investigations as recommended in Sections 1.3.2 through l.3.5 of the SRP. A summary of DOE's geologic characterization is presented l
below.
2.3.1 Physiographic Setting The Maybell processing site is located in the Sand Wash Basin subprovince of l
the Wyoming Basin physiographic province (Figure 2.1).
The Wyoming Basin is characterized by.subbasins containing thick deposits of Tertiary-age sediments with low to moderate topographic relief. The basin is an elevated depression separating the Middle and Southern Rocky Mountains, and is continuous with the l
Great Plains to the northeast. The Colorado Plateau lies to the south. Minor folding and faulting have separated the region into subbasins.
l The Sand Wash basin contains some Tertiary volcanic mountains and ridges, and is rimmed by faulted Tertiary deposits.
Topography of the site area is typical of the basin region; however, the area is underlain by eroded, folded and faulted structures that are more typical of the Vinta Basin which lies to the west.
The Green and Yampa Rivers have been superimposed upon the uplifted structures of the bordering provinces.
The Yampa River and its tributary, Lay Creek, drains the site region (Figure 2.2).
Elevations vary from 6280 to 6340 feet above mean sea level.
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PHYSIOGRAPHIC PROVINCES OF THE MAYBELL SITE REGION i
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2-2 MAYBELL ffER 2
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REGIONAL AND AREAL DRAINAGE SYSTEM 2-3 MAYBELL fiER i
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l 2.3.2 Stratigraphic Setting DOE characterized the regional and site stratigraphy by referring to published and unpublished geologic literature and maps: reviewing site-specific subsurface geologic data, including logs of exploratory boreholes drilled on the site; and conducting field investigations as recommended in Section 1.3.2 of the SRP. A summary of DOE's characterization of the stratigranhy is provided below.
Bedrock underlying the site area and a large portion of the site region consists of the Browns Park Formation of Oligocene and Miocene age. A few miles north of the site in the Sand Wash Basin, bedrock consists of the lower l
Tertiary Green River, Wasatch, and Fort Union Formations, the upper Cretaceous Lance and Lewis Shale Formations, and the Mesaverde Group.
These rocks in turn overlie the Mancos Shale. At the site, the Browns Park Formation immediately overlies the Mancos Shale.
The younger rocks previously mentioned have been removed by erosion prior to deposition of the Browns Park Formation at the site.
The Browns Park formation is estimated to'be between 800 and 900 feet in thickness beneath the site.
It contains widespread tuff beds (Luft 1985),
which are believed to be the primary source of the uranium mined at the site.
The NRC staff has reviewed the details of the regional and site stratigraphy as provided in the final RAP by DOE, and concludes that the characterization of the Maybell site establishes the regional and site stratigraphy sufficiently to support DOE's assessment of geologic stability.
2.3.3 Structural Setting DOE characterized the region's structural setting by referring to published regional geologic maps, aerial reconnaissance, field observations, and mapping of features critical to assuring the long-term stability of the remedial action.
These studics were recommended in Section 1.3.3 of the SRP.
A summary of DOE's structural characterization is presented below.
The site is on the Axial Basin Arch, a collap
' anticlinal structure (Figure 2.3).
This is a narrow, northwestward-trending structural province that lies along the trace of the Vinta Mountains uplift.
It occupies the region of transition between the Colorado Plateau to the south and the Wyoming Basin to the north. The Axial Basin Arch is bordered on the north by subbasins of the Wyoming Basin and on the south by the Piceance Basin, a subprovince of the Colorado Plateau.
The northern and southern boundaries are formed by nort:.sest trending thrust faults of Laramide age. The Axial Basin section of the Arch collapsed in Mid-Tertiary time (Hansen 1984).
Subsequent erosion along the axis produced the trough (Browns Park syncline) in which the Browns Park Formation was deposited.
Table 2.1 gives a summary of m:pped faults and lineaments within a 40 mile radius of the Maybell site. All faults are considered noncapable.
2-4 MAYBELL fiER
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STRUCTURAL SETTING 0F THE MAYBELL SITE REGION i
2-5 mArsett fren l
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6 Table 2.I.
SbMMARYANALYSISOFMAPPEDFAULTSANDLINEAMENTSWITHINA40-MI(65-KM)RADIUSOFTHEMAYBELL, COLORADO, SITE Capehes ICI er 8'
Longik Distance trem she
-p no.
F" Source Indi gung Intel bl peCllandt 1
tesegetJohneen Ftt This study 1.2 2.0 0.1 0.1 NC 2
uinta-Spasks flow 6es et al. (1979) 40 mas 66 max 0.6 0.9 NC Scene (19861 3
Wolf CreeW flowies et al. (1979) 60 man 80 mas 6.0 S6 NC Junger Raountaan Stone (19848 2a CedarKnehe This study 1.4 2.3 0.6 0.9 NC 2h Lay Peak Twete (19761 3.6 6.8 4.4 7.1 NC Katham and flogers (1981) 4 Latte Junger Twees (1976) 1.6 2.6 4.0 6.4 NC I
amounson Hancock (1826) i 6
Junger taounemen Rowles et al (1986) 2.2 3.6 6.6 S.O NC l
2c East of Lay Twees (19761 6.6 8.8 8.6 13.7 NC i
Katham and flogers (1981) 6 Temple Canyon Rowles et at (19861 4.6 7.2 10.6 16.9 NC least et at (19864 i
2d Csang Soushwest Twees (19788 S.0 14.6 14.0 22.6 NC Kapidessa and flagers (1981) 7 Cedar heeunsaan Twees (1978) 9.6 16.3 16.6 24.9 NC t
Katham and flagers (1981)
This oeudy 4
hannument ausse Hancock (19268 7.0 11.3 21.0 33.8 NC Twees (19781 l
9 Cdadet Plateau laett et al. (19861 8.0 S.6 21.0 33.8 NC newees et al. (1986) j 2-6 MAYBELL fTER
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SUMMARY
ANALYSIS OF MAPPED FAULTS AND LINEAMENTS WITHIN A 40-MI (65-KM) RADIUS OF THE MAYBELL SITE (Concluded)
CapaMe ICI er
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l Fedt Lamelk Distance essen este mancapable gre e me.
Few Sewee toiti Gunt lau8 Dauel peCl feidt 10 Cross Mowitaan Rowles et d. (19868 12.0 19.3 22.0 36.4 NC Dyni (19681 l
11 Sie Hate Gdch Rowles et at (19861 6.0 8.0 23.0 37.0 NC Dyd (19886 11a Scanenevien Gdch Reedes et d. (19861 S.0 9.8 24.0 38.8 NC Dyal (ISSSI t
12 Fasetscation Fault Rowles et d. (IM61 3.8 6.8 27.0 43.4 NC Dyni (19681 13 Webesas Fosk Rowles et W- (IM61 6.0 8.0 28.0 46.0 NC l
Dyni (19848 14 Meeker Routes et W. (1S861 6.0 8.0 29.6 47.5 NC i
Dyni (19841 16 whsee River Anstes et d. (19861 10.0 18.1 33.0 63.1 NC Dyse (19888 i
- 18 Dougies temuntaan Rowles et al. (19861 9.6 16.3 30.0 48.3 NC i
Dyni (19888 17 Cherokee Rdge Rowles et al. (19861 9.3 14.9 34.0 64.7 NC (Wyesseing Gesup4 Dyd (19845 18 Yasupe Faidt noustes et at (19861 9.4 noen 16.2 nian 32.0 61.6 NC Dyni (19881 taE = 8.02 + 0.729 Los I sende et at,19841
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The NRC staff has reviewed the details of the regional and site structure as provided in the RAP by DOE and concludes that the characterization of the Maybell site establishes the regional and site structural setting sufficiently to support DOE's assessment of geologic stability.
2.3.4 Geomorphic Setting DOE characteriW the site oeomorphology by referring to published literature and topographic maps, as recommenaea in Section 1.3.4 of the SRP.
Site geomorphic conditions were characterized by aerial photographic interpretation and field observations. A summary of DOE's oeomorphic characterization is provided below.
Development of the present landforms in the Naybell area has been influenced most strongly by tectonic activity prior to and during deposition of the Browns Park Formation, and by changes in the locations of the Green River and Yampa River drainage systems; By late Miocene age, the Browns Park Formation had filled the depentional valley 'and covered the adjacent uplands, except for the highest peaks (Hansen, 1986).
The present distribution of the Browns Park Formation reflects the greater thickness of sediment accumulated in the Maybell area because of the development of a depositional syncline within the Axial Arch basin.
This formation has subsequently been removed by erosion from most areas outside the northwest trending syncline.
The basin is characterized by superimposed meandering rivers and streams and tributary streams.
The lack of relationship between the drainage pattern in the region and any pre-Miocene structures suggests that the stream system was established on top of a formerly continuous cover of Browns Park Formation.
As the Browns Park formation was steadily eroded, the drainage system was superimposed onto the underlying rocks and structures.
Landscape development is controlled by the dominant regional geomorphic cycles of stream aggradation and degradation. Major rivers and their tributaries have dissected the basin by headward erosion and nickpoint migration.
Capture of the ancestral upper Yampa River (tributary to the Green River) flowing on the Browns Park Formation has greatly affected erosion and incision rates in the Maybell area since Pliocene time Headward extension of the Yampa River tributaries across the Axial Basin Arch allowed the ancestral Lay Creek to grow eastward.
Capture of the headward reaches of the northern tributaries of the Yampa River by Lay Creek resulted in the deeply entrenched tributaries on the northern -ide of Lay Creek such as Johnson Wash.
Landscape equilibrium in the Maybell area may be affected by the fairly recent drainage adjustments due to the extension of the lay Creek drainage system.
Processes of slope modification, such as landslides, slumps, and debris flows, are generally absent in the low elevation site area.
Small localized talus deposits are present along some vertical bedrock outcrops. Minor, shallow soil failures occur along oversteepened slopes next to stream channels.
No landslide or debris flows are known to exist in the site area.
Fluvial processes constitute the main geomorphic controls on the landscape.
Both stream aggradation and erosion are occurring in major drainage systems.
2-8 MAVBELL ffER
t The alluvial fill within stream channels reflects cycles of deposition and incision during the Quaternary period.
The NRC staff has reviewed the details of the regional and site geomorphology, as provided in the final RAP by DOE, and concludes that the characterization of the Maybell site establishes the regional and site geomorphology sufficiently to support DOE's assessment of geclogic stability.
2.3.5 Seismicity DOE characterized the regional seismicity by obtaining earthquake data bases provided by the National Oceanographic and Atmospheric Administration (NOAA),
by applying accepted techniques to determine earthquake magnitudes, and by i
employing methods suggested in Section 1.3.5 of the SRP for calculating peak horizontal ground acceleration generated by a design basis event.
A summary of DOE's seismic characterization is provided below.
l The Maybell site lies within the Vinta-Elkhead seismotectonic province (Kirkham and Rogers 1981).
This province is bounded by the Colorado Plateau on the south, by the Western Mountains, northern Rio Grande Rift, and Eastern Hountains on the southeast and east and by the Wyoming Basin on the north (Figure 2.4).
The area of significance for seismic investigation was determined to have a radius of 40 miles around the Maybell site based on attenuation-distance relationships (Campbell, 1981). Although the site region is an area of complex structural elements, under the present seismotectonic regime it is relatively stable. Most of the structures are Laramide uplifts and basins.
Some late Tertiary activity resulted in volcanism and movement along old fault systems.
No areas of recurring seismicity exist within the 40-mile site radius. There are several locations with epicenters between 40 and 125 miles of the site, with the nearest location being at Steamboat Springs.
The maximum instrumentally recorded earthquake within a 125-mile radius of the site was a 1973 g - 5.4 event, located 51 miles from the site in the Piceance Basin of the Colorado Plateau.
The largest historical earthq cke in the 125-mile radius was an 1882 event of intensity VII (DGE, 1993c).
No known faults are associated with scismic activity within a *0-mile radius of the site.
The nearest confirmed Quaternary faults are located in the Vinta Basin of the Colorado Plateau (Nakata et al.,1982).
The NRC staff has reviewed the details of the regional and site seismicity as provided in the final RAP by DOE, and concludes that the characterization of the Maybell site establishes the regional and site seismicity sufficiently to support DOE's assessment of geologic stability.
2-9 MAYBELL fiER
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SEISM 0 TECTONIC PROVINCES OF THE MAYBELL SITE REGION 2-10 MAYBELL ffER
o 2.3.6 Natural Resources DOE characterized the regional and site-specific natural resources by an analysis of regional and local publications, regional geologic maps, topographic maps, and field observations. A summary of DOE's character-ization of the natural resources is as follows.
The economic mineral resources in the Maybell site region include uranium and vanadium ores, coal, oil and gas, oil shale, gold, silver, copper, gypsum, and potash (Steele et al., 1979).
Uranium is the only mineral resource that has been exploited in the immediate site area.
Figure 2.5 shows the location of mineral resources for the region.
Uranium, the only mineral resource to be developed in the site area, is found within the underlying Browns Park formation.
The uranium is concentrated on numerous steeply dipping normal faults as irterstial fillings on sand grains.
Between 1957 and 1964, 355,000 tons of locally mined uranium ore were processed at the Maybell mill site.
Heap leaching of the mill tailings was conducted during the 1970's with no new mining.
The uranium ore milled averaged 0.098 percent uranium, which was one of the lowest in the industry.
Although hydrocarbons have been found and exploited in the area surrounding the Haybell site, there has been no production in the immediate vicinity of the mill site.
There are no coal or oil shale deposits at the site because of past erosion of the host beds.
There is a potential for oil and gas deposits.
The closest gold was found 8 miles away in a placer deposit.
The Browns Park Formation is not considered to be a good prospect for gold or other metallic minerals.
There has been some geothermal activity 7.5 miles southeast of the site near Juniper Mountain.
Heat flow maps have shown no anomalies in the vicinity of the site.
Host rocks for potash are not present at the site.
There are no mineral resources which will be adversely affected by the Haybell disposal site.
2.4 Geologic Stability Geologic conditions and processes are characterized to determine the site's ability to meet standards in 40 CFR 192.02(a).
In general, site lithologic, stratigraphic, and structural conditions are considered for their suitability i
as a disposal foundation and their potential interaction with tailings leachate and ground-water. Geomorphic proce:.ses are considered for their potent.ici impact upon long-term tailings stabilization and isolation.
Potential geologic hazards, including seismic shaking, liquefaction, on-site fault rupture, ground collapse, and volcanism are identified for the purpose of assuring the long-term stability of the disposal cell and success of the remedial action design.
I 2-11 MAYBELL ffER
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ENERGY AND MINERAL RESOURCES MAYBELL SITE REGION 2-12 m ysttt n ot j
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i
i 2.4.1 Bedrock Suitability DOE's evaluation of the site region, as described in TER Section 2.3 and the t
final RAP, indicated no evidence of bedrock instability, nor any capable 4
tectonic faulting within 40 miles of the disposal site, nor other structural i
conditions affecting the stability of the site.
Therefore, the NRC staff concurs with DOE's assessment that bedrock stratigraphic and structural conditions at the sP
.:hould have no effect on the design's ability t meet standards for long-term stability of the remedial action.
i 2.4.2 Geomorphic Stability i
The fluvial processes of most concern to long-term stability of the pile are gully formation and migration, stream channel erosion, sheetwash erosion, and the effects of base-level changes.
The large scale processes controlling the geomorphology of the Lay Creek basin, of which Johnson Wash is a tributary, are temporary sediment storage and flushing, and continued incision due to fall of base-level.
Several j
cycles of recent (Holocene) erosion and periods of stability have been i
experienced by the site area.
Episodes of gully filling are recorded by exposures in mine pit walls where V-shaped cuts 10 to 20-feet deep are filled with surficial deposits.
Current erosion is expressed primarily by headward extension and deepening of gullies and by slumping along gully sides. The existence of the filled gullies indicates that future erosion may result in headward migration of gullies.
Several incised gullies extend westward from the upper reach of Johnson Wash to the edge of the tailings pile.
Actively eroding nickpoints occur in each of the western gullies near its junction with the main wash.
However, except where gullies drain significant surface areas, the Browns Park Formation offers a relatively erosion-resistant surface to gully formation.
Figure 2.6 shows the major gully tributaries to Johnson Wash.
The tributaries labeled T5 through T8 are growing headward towards, cr in, the tailings area.
Long-term stability of the disposal site requires that this gullying situation be controlled. Section 4.4 discusses the erosion procection measures which will be provided for long-term stability of the site.
Ba:ed on the information provided by 60E in the final RAP, The NRC staff has reasonable assurance that geomorphic conditions of the site have been adequately characterized, and that stability of the site can be achieved.
2.4.3 Seismotectonic Stability Studies by DOE to analyze seismic hazards included search for a design-basis fault, selection of a design earthquake, calculation of the estimated peak horizontal ground acceleration, evaluation of potential on-site fault rupture, and recognition of potential earthquake-induced geologic failures at the site.
2-13 MAYBELL fiER
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MAJOR TRIBUTARIES OF JOHNSON WASH MAYBELL SITE, COLORADO j
2-l4 MAYBELL ffER
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The seismic characterization for the site region, as described by DOE and presented in Section 2.3.5 of this final TER, identifies the significant structures and delineates the tectonic provinces.
That section provides i
analysis of the regional characteristics to determine sources that could generate ground motion that would most affect the site.
The maximum earthquakes (ME) for the adjacent seismotectonic provinces are based on recommendations by Kirkham and Rogers (1981).
This study had estimated a range of 5.5 to 6.5 for the Colorado Plateau Province and a range of 5.5 to 6.5 for the Vinta-Elkhead Province.
The larger magnitudes are consistent with the fault lengths of the significant faults of these provinces and are selected as the MEs for these adjoining provinces.
Based on the d
closest approach of these province boundaries to the site, the distance i
attenuation relationship of Campbell (1981) indicates a resultant peak horizontal acceleration at the site of 0.16 g for the Colorado Plateau i
Province and 0.14 g for the Rio Grand Rift Frovince.
The floating earthquake (FE) is the largest event not associated with a specific structure and is determined on the basis of seismic history and i
tectonic character.
Since the largest earthquake considered possible without ground rupture is a magnitude 6.2 event, this magnitude is considered as the FE, representing the seismic potential of unknown structures in the region.
j This event is considered to occur at a radial distance of 15 km (9.3 miles) from the site and would result in an acceleration of 0.21 g at the disposal i
site.
None of f'.e identified faults are considered to be capable.
The NRC staff has reviewed the data presented by DOE in the Maybell final RAP and agrees that the peak horizontal acceleration from a 6.2 magnitude earthquake at a distance of 15 km, using Campbell's (1981) 84th percentile 4
value of 0.21 g.
The staff finds the data inputs and the cited results to be reasonable and conservative for DOE's calculation of the seismic coefficient for the site.
Section 3.0 of this final TER provides the review of the application of this seismic coefficient to the geotechnical stability of the disposal cell.
2.5 Conclusions Based upon review of the geologic aspects of the final RAP, the staff has reasonable assurance that regional and site geologic conditions have been characterized adequately to meet 40 CFR Part 192.
Conditions hindering long-term stability of the site have been identified and mitigated by features in the remedial design.
2-15 MAYBELL fiER 4
i I
l
i i
3.0 GE0 TECHNICAL STABILITY l
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3.1 Introduction This section presents the results of the NRC staff review of the geotechnical engineering aspects of the remedial action proposed at the Maybell, Colorado, UMTRA Project site.
The remedial action consists of the consolidation and 2
movement of all mtaminated materials from the processing site to the l
tailings pile, wnich is about rive miles east-northeast of the town of l
Maybe11, Colorado.
The final disposal cell will be an above-grade engineered embankment extending an average of 20 feet above the existing pile.
Contaminated material will be added to the cell, and will be covered with a i
six-foot, eight-inch-thick multiple layer cover on top, and a seven-foot-thick i
multiple layer cover on the sides.
The geotechnical engineering aspects reviewed include:
(1) information related to the disposal and borrow sites; (2) materials associated with the l
remedial action, including the foundation and excavation materials, tailings, and other contaminated materials; and (3) design and construction details j=
related to the disposal site, disposal cell, and its cover.
Staff evaluation of related topics such as geology, geomorphology, an.d seismic characterization are presented in Section 2.0 of this TER.
Surface water and erosion control evaluations are presented in Section 4.0, and ground-water condition evaluations are presented in Section 5.0 of this report.
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3.2 Site and Material Characterization 3.2.1 Site Description l
For site description, see Section 1.2.
lL 3.2.2 Geotechnical Investigations 1
A.
Disposal Cell and Processing Site Areas i
Several subsurface investigations have been performed at the Maybell processing site in order to characterize the ' ' lings and contaminated 4
materials for geotechnical engineering and radiological aspects of the temedial action.
Drawings in the final RAP, Attachment 1, Subcontract i
Documents, identified as MAY-PS-10-0224 and -0225 illustrate the test i
locations.
Logs of the boreholes and test pits were also provided in.
l l
j A study by Sergeant, Hauskins & Beckwith (SHB) in 1981, to determine the extent of the tailings, consisted of approximately 130 boreholes. Additional i
investigations by Jacobs Engineering Group (1985,.1986), Western Technologies (1989), and Western Engineers (1990), yielded samples for laboratory analysis, j
Geotechnical engineering characteristics and strength parameters for the j
tailings, contaminated soil, and natural soils have been determined through laboratory analyses of samples from these investigations.
3 j
3-1 MAYBELL f7ER l
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.~ -.. - _-.-. -
s l
In addition to the conventional test drillina discussed above, Rogers &
Associates conducted radiological laboratory testing (1985); and Jacobs Engineering Group performed piezocone soundings (1986), and monitoring well 1
installation (1989).
Finally, test pits were excavated in 1989 and 1990.
B.
Borrow Areas Proposed radon barrie Tnd frost protection soils from the Rob Pit overburden i
pile were evaluated by Jacobs Engineering Group (1986, 1989), Western j
Technologies (1989), and Western Engineers (1990).
DOE determined that Rob Pit overburden soils would be amended with the addition of ten percent bentonite, 4
j The Maybell Gravel and River Gravel sources were identified as potential sources of riprap and bedding.
Based on investigations by Western 4
1 Technologies (1989) and Morrison Knudson (1989), Maybell was identified as the l
better of the two sources, and was selected for use.
i Of several potential sources identified for 1arger riprap, the Juniper 6
Mountain Limestone Quarry and Craig Basalt Quarry were determined to be optimum sources.
Rock from these two sources was more resistant to weathering effects than alternate-source material, and greater in quantity.
j j
Based on its ;eview, the staff concludes that from a geotechnical engineering j
perspective, the site investigations at the processing / disposal site and
{
borrow materials sites are in general conformance with the applicable
)
provisions of Chapter 2 of the SRP (NRC,1993), and they are adequate to 1
support the assessment of the materials occurring at the sites.
i 3.2.3 Testing Program The staff has reviewed the geotechnical engineering testing program for the Maybell site.
The testing included gradation, Atterberg Limits, specific gravity, moisture-density (Proctor) determinations, consolidation tests, saturated hydraulic conductivity, capillary moisture, and triaxial shear tests.
The tests identified above were conducted on representative tailings materials.
Proposed cover materials were evaluated for durability, including Los Angeles Abrasion, sulfate soundness, absorpti:n, specific gravity, Schmidt Hammer, and Brazilian disk tensile tests.
Petrographic analyses were also conducted.
Tests on the proposed radon / infiltration barrier materials from the Rob Pit overburden pile, which will be amended with bentonite, inciuded gradation, Atterberg. Limits, saturated hydraulic conductivity, capillary moisture, triaxial ~ shear strength, and specific gravity.
Based on its review, NRC staff finds that the number and type of tests conducted in the testing program were appropriate to support the engineering analyses performed, and that the scope of the testing program and the utilization of the test results to define the material properties are in general agreement with the applicable provisions of the SRP (NRC,1993).
3-2 W BELL f m
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3.2.4 Site Stratigraphy The elevation of the Maybell site is near 6300 feet above mean sea level.
Borings included standard geotechnical and piezocone techniques.
Geotechnical methods included drilling with hollow-stem augers and sampling at regular intervals with the Standard Penetration Test (SPT).
On occasion, a 2.5-inch 2
inside-diameter ring-lined split-barrel tube was used to sample the materials.
f The SPTs were conducted in accordance with ASTM D-1586 procedures.
Drawings MAY-PS-10-0224, -0225, and -0226 of the final RAP show locations of the borings, monitoring wells, and piezocone soundings.
Section 2 of this report presents an evaluation of the geologic, geomorphic, and seismic characteristics of the site.
The most prominent site feature is the tailings pile, which consists of interlayered sands and slimes overlying alluvial soils.
The thickest slime layers (up to 24 feet) are in the south-central portion of the pile, with thinner zones along the western edge of the pile.
The slimes generally grade into sand-slime mirtures, then sands, in the northern portion of the pile.
Starter dikes were used in the construction of the pile on the south and east sides, and these consist of silty sands.
The pile surface was covered with approximately six inches of rooting medium.
Materials in the piles range from coarse to fine-grained tailings.
The coarse j
materials are medium to fine-grained sands and silty sands. The fine-grained materials are slimes of low-plasticity.
The soil cover is classified as clayey sand.
Subgrade soils are light-brown to reddish-brown alluvial silty sands.
Bedrock in the area is the Browns Park Formation, a light gray and white, silty, fine-grained sandstone.
The Browns Park is 800 to 900 feet thick in the site area.
The bedrock surface slopes to the south and is broken only by drainage channels incised into the soil cover.
The soil cover is thickest in the center of the basin of Johnson Wash, and thins at the edges of the basin.
Section 2 of this report presents a detailed evaluation of the bedrock conditions at the site.
)
Unconfined ground-water occurs within the Browns Park formation at depths ranging from 35 to 300 feet below the ground surface.
Ground-water recharge is principally from the infiltration of rain or snow.
Ground-water generally flows away from the tailings site in a southwesterly direction.
No perched water zones are known to exist in the immediate vicinity of the tailings site.
NRC staff has reviewed the sulsurface exploration information present in the RAP and discussed above. The staff concludes that the geotechnical investigaticns conducted at the processing / disposal and borrow sites establish the stratigraphy, that the explorations are in general conformance with applicable provisions of Chapter 2 of the SRP, and that they are adequate to support the assessment of the geotechnical stability of the stabilized tailings and contaminated material in the disposal cell.
5-3 MAYBELL ffER
i 3.3 Geotechnical Engineering Evaluation 3.3.1 Slope Stability The evaluation of the geotechnical stability of the slopes of the disposal cell containing stabilized tailings and other contaminated materials is presented in this section.
The staff has reviewed the exploration data, test results, slope characteristics, and methods of analyses pertinent to the slope stability aspects of the RAP.
The analyzed cross-sections with 5 horizontal (H) to I vertical (V) side slopes have been compared with the exploratory records and design details.
The staff finds that the characteristics of the slope have been satisfactorily represente<i and that the most critical slope sections have been considered for stability analyses.
Soil parameters for the various materials in the disposal cell slope have been adequately established by appropriate testing of representative materials.
Soil parameter values have been assigned to other layers (riprap, gravel bedding, bedrock, etc.) on the basis of data obtained from geotechnical explorations at the site and data published in the literature.
The staff finds that the determinations of these parweters for slope stability evaluation follow conventional geotechnical engineering practice, and are also in compliance with the applicable provisions of Chapter 2 of the SRP (NRC, 1993).
The staff also finds that appropriate methods of stability analysis (Spencer and infinite slope methods) have been employed to address the extreme adverse conditions to which the slope might be subjected.
Factors of safety against failure of the slope for seismic loading conditions have been determined for both short-term (end of construction) and long-term states.
Factors of safety for the static loading conditions were 2.2 (short-term) and 2.5 (long-term), which are in excess of minimum allowable values of 1.3 and 1.5, respectively.
The seismic stability of the slope was invostigated by the pseudo-static method of analysis using horizontal seismic coefficients of 0.14 for the end-of-construction case and 0.18 for the long-term case.
The values of the seismic coefficients were calculated as per guidance in the SRP, and are acceptable to the staff.
The staff finds the pseudo-static method of analysis to be acceptable considering the degree of conservatism in the soil parameters and the flatness of the slopes (5H:lV).
The minimum factors of safety against failure of the slopes were 1.1 for the end-of-construction seismic and 1.3 for the long-term seismic conditions, compared to an allowable minimum of 1.0 for both conditions.
Based on review of these analyses and the results, NRC staff concludes that the slopes of the disposal cell are designed to endure the effects of the geotechnical natural forces to which they may reasonably be subjected during the design life, and that the analyses have been made in a manner consistent with Chapter 2 of the SRP.
3-4 MAYBELL ffER
6 3.3.2 Settlement and Cover Cracking Long-term settlement of materials in the disposal cell, which could result in either local depressions or cracks on top of the cover, has been adequately addressed in the final RAP.
If depressions in the cover were to form, they could initiate erosion gully pathways followed by a potential exposure of tailings material. A crack in the cover might open up a pathway for surface water to infilt % into or throuah the tailings material, or would allow radon to escape.
For tailings and contaminatr. aatorials of a granular nature, much of the settlement will be instantaneous or will occur during the construction phase.
Long-term (consolidation) settlement is expected to occur principally in areas which have significant slimes layers. Affected areas are identified in Section 3.3.3.
Primary consolidation settlement was estimated to require up to nine years to occur.
Despite the presence and nature of the in-situ sliuies, settlements were predicted to be within tolerable limits for satisfactory cell behavior.
The staff concludes that the long-term settlement of in-situ and newly-placed materials in the disposal cell will not have adverse impact on the performance of the disposal cell cover.
From a long-term settlement perspective, there is reasonable assurance that there will be ro adverse effects on the ability of the disposal cell to meet the EPA Standards.
3.3.3 Liquefaction Potential Based on a review of'results of the geotechnical investigations, including boring logs, test data, soil profiles, and disposal cell design, the NRC staff concludes that the DOE has adequately addressed the potential for liquefaction at the Maybell site.
The compacted dry density of relocated materials within the cell will be 90 percent of the maximum dry density by the ASTM D-698 test.
By design these materials are in an unsaturated condition, and therefore, not susceptible to liquefaction.
The southeastern edge of the pile has potentially liquefiable strata.
The following critical areas were identified in liquefaction calculations:
Area 1:
Borings D-10 ana D-12 Area 2:
Borings B-5 and 526 Area 3:
Borings B-6, C-71, and 202 Area 4:
Borings G-12 and 209 The slimes zones are reportedly not interconnected and are limited in lateral extent, thus broad areal failure, or excessive settlement due to liquefaction, is unlikely.
The disposal cell is. founded on granular soils not susceptible to liquefaction.
The ground-water table is reported to be approximately 50 feet below prevailing grades at the base of the existing cell.
Considering the placement density of the relocated tailings, liquefaction is judged not to be a potential problem. Whereas localized liquefaction for the in-situ tailings is possible under extreme geological conditions, such an occurrence should not jeopardize the integrity of the composite cell.
The applicable provisions of 3-S MYBELL ffER
s Chapter 2 of the SRP (NRC, 1993) have been..ct with respect to liquefaction potential.
3.3.4 Cover Design The final cover over the tailings will consist of a 1.5-foot-thick radon / infiltration barrier (sandy clay / sandy silt amended with seven percent bentonite), covered L, 4 4-foot-thick frost protection layer, a 6-inch. nick sand and gravel bedding / drainage layer, and a riprap / erosion protection layer of 8-inch-thickness on the top and 12-inch-thickness on the side slopes (Figure 1.4).
The disposal cell embankment will rise an average of 40 ft above the surreunding topography with side slopes of 20 percent and top slope of three percent to the west. A rock apron 20 ft wide.will be placed along the south and east sides of the pile.
Permanent ditches will be constructed on the north and west sides of the stabilized pile.
The cover system described above will provide 6 feet-8 inches and 7 feet of protection on the top and sides of the cell, respectively.
The system has been designed to limit the infiltration of precipitation, protect the pile from erosion, and to control the release of radon from the tailings below.
Details of the staff's review of the cover's performance related to limiting infiltration are addressed in Section 5.0 of this report; the review of the cover's erosion protection features is presented in Section 4.0, and the review of the radon attenuation aspects of the cover is presented in Section 6.0.
Certain other design aspects of the proposed cover are discussed below.
DOE has determined the frost depth using the BERGGREN. BAS computer code developed at the U.S. Army Corps of Engineers (C0E, 1968), which has been used for other-UMTPA Project Sites.
The total frost penetration depth at the disposal site is calculated to be 63.8 inches on top, and 65.4 inches on the sides.
The cover design provides for the appropriate cover by the total thickness of riprap (8 to 12 inches), bedding (6 inches), and frost protection layer (48 inches) above the radon barrier. Although up to 2 inches of frost penetration may occur on the top slope, the cover integrity should not be substantially affected. The staff has reviewed the input data used in determining the total frost penetration depth and these values are a reasonable representation of the extreme site conditions to be expected.
Therefore, DOE's evaluation of the frost penetration depth is acceptable to the staff. However, it should be noted that DOE's calculation was based on the 200-year, and not on the 1000-year frost depth.
NRC staff accepts this approach for the Maybe11 site because DOE has been conservative in using soil parameter values in the model.
The radon barrier layer material is the sand from the Rob Pit overburden pile.
The specifications require that the material shall consist of soils with 100 percent of material finer than one inch and with 5 to 100 percent passing the No. 200 sieve. Testing has indicated that the borrow soil shc'ild generally meet the requirements, and that inspection procedures will verity gradation.
Since the ability of the radon barrier to withstand cracking and to retard radon emanation is unrelated to the Plasticity Index (PI) of the soil cover at this site, the radon barrier need not be tested for PI during construction.
3-6 MAYBELL fiER c--r
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The Remedial Action Selection Report (MS) (D0E,1996a) states that the radon barrier material will be amended with seven percent (by weight) bentonite.
The design hydraulic conductivity value is 1x10' cm/sec.
Based on laboratory testing, it should be possible to construct the barrier to the required hydraulic conductivity.
I The cover design has been evaluated by NRC staff for geotechnical long-term j
stability and the design is acceptable. The radon attenuation ability of the cover is discussed in Section 6, and the hydraulic cond6-tivity aspects of the cover in Section 5 of this TER.
3.4 Geotechnical Construction Details i
f 3.4.1 Construction Methods and Features J
)
The staff has reviewed and evaluated the geotechnical construction criteria d
provided in Attachment 1 to the final RAP.
Based on this review, the staff
[
concludes that the plans and drawings clearly convey the proposed remedial action design features.
In addition, the excavation and placement methods and specifications represent accepted standard practice.
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3.4.2 Testing and Inspection k
The staff has reviewed and evaluated the testing and inspection quality control requirements provided in the Remedial Action Inspection Plan (RAIP).
In general, the RAIP is found to provide a program for testing and inspection that is consistent with the Staff Technical Position on Testing and Inspection (NRC, 1989a).
The RAIP is in general agreement with the final RAP.
3.5 Conclusions The NRC staff has reviewed the geotechnical engineering aspects of the design of the Maybell remedial action as presented in the RAP and the RAIP.
The NRC staff concurs with the plan with respect to long-term stability aspects of the EPA standards (40 CFR Part 192.02(a)). All outstanding open geotechnical engineering issues have been satisfactorily addressed.
3-7 MAYBELL ffER I
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I
s 4.0 SURFACE WATER HYDROLOGY AND EROSION PROTECTION 4.1 Hydrologic Description and Site Conceptual Desigr.
The Maybell site is located in northwest Colorado, approximately 25 milcs west of Craig, Colorado. The site is situated between Johnson Wash to the east and the Rob Pit Mine to the west.
The site is drained by Johnson Wash, which is deeply incised in the immediate site area, with branching gullies up to 40 i
feet deep.
The gullies have eroded downward into the r. oft sandstone bedrock layer on which the pile is located.
In order to comply with EPA standards, which require stability of the tailings for 1,000 years to the extent reasonably achievable and, in any case, for at least 200 years, DOE proposes to stabilize the contaminated materials in an engineered embankment to protect them f rom flooding and erosion.
The design basis events for design of the ero; ion protection included the Probable Maximum Precipitation (PMP) and the Probable Maximum Flood (PMF), both of which are considered to have low probabilities of occurrence during the 1000-year stabilization period.
As proposed by DOE, the tailings will be cansolidated into a single pile, which will be protected by a rock cover.
The rock cover will have a maximum slope of three percent on the top slopes and 20 percent on the side slopes. The disposal cell will be surrounded by aprons which will safely convey flood runoff away from the cell and prevent gully intrusion into the contaminated materials.
In addition, two diversion ditches will be constructed to divert flood flows from the upland drainage area around the disposal cell. Also, four existing gullies downstream of the pile will be stabilized using large riprap.
These four gullies pose a serious threat to the stability of the pile because active erosion is occurring and the gullies are advancing toward the tailings pile area.
The gullies are advancing at a rate which requires stabilization to prevent erosion of the tailings during the required 1,000-year design period.
A large segment of the Rob Pit overburden pile, which is located immediately upstream of the reclaimed Maybe11 tailings pile, will be stabilized with rock riprap. The purpose of this erosion protection is to prevent excessive amounts of sediment from entering the diversion channel located between the overburden pile and the reclaimed tailings.
4.2 Flooding Determinations The computation of peak flood discharges for various design features at the site was performed by DOE in several steps.
These steps included:
(1) selection of a design rainfall event; (2) determination of infiltration losses; (3) determination of times of concentration, and (4) determination of appropriate rainfall distributions, corresponding to the computed times of concentration.
Input parameters were derived from each of these steps and were then used to determine the peak flood discharges to be used in water surface profile modelling, and in the final determination cf rock sizes for erosion protection.
4-1 MAYBELL ffER
i 4.2.1 Selection of Design Rainfall Event One of the most disruptive phenomena affecting long-term stability is surface water erosion.
DOE has recognized that it is very.important to select an appropriately conservative rainfall event on which to base the l
flood protection designs. DOE has concludeo and the NRC staff concurs i
(NRC,1990) that the selection of a design flood event should not be
)
based on the erMolation of limited historical flood data, due to the unknown level of accuracy associated with such extrapolations.
- Rather, i
DOE utilized the PMP, which is computed by deterministic methods (rather than statistical methods), and is based on site-specific i
hydrometeorological characteristics.
The PHP has been defined as the most severe, reasonably possible rainfall event that could occur as a l
result of a combination of the most severe meteorological conditions occurring over a watershed.
No recurrence interval is normally assigned to the PMP; however, DOE and the NRC staff have concluded that the 1
probability of such an event being equalled or exceeded during the 1000-year stability period is small.
Therefore, the PHP is considered by the NRC staff to provide an acceptable design basis.
l Prior to determining the runoff from the drainage basin, the flooding analysis requires the determination of PMP amounts for the specific site location. Techniques for determining the PMP have been developed for the entire United States primarily by the National Oceanographic and Atmospheric Administration (NOAA) in the form of hydrometeorological reports for specific regions.
These techniques are widely used and l
provide straightforward procedures with minimal variability.
The staff, j
therefore, concludes that use of these reports to derive PHP estimates is acceptable.
A PHP rainfall depth of approximately 7.4 inches in one hour was used by DOE to compute the PMF for the small drainage areas at the disposal site.
l This rainfall estimate was developed by DOE using Hydrometeorological Report i
(HMR) 49 (Department of Commerce, 1977). The staff performed an independent l
check of the PHP value, based on the procedures given in HMR 49.
Based on this check of the rainfall computations, the staff concludes that the PMP was acceptably derived for this site.
4.2.2 Infiltration Losses Determination of the peak runoff rate is dependent on the amount of i
precipitation that infiltratas into the ground during the occurrence of the rainfall.
If the ground is saturated from previous rains, very littie of the rainfall will infiltrate and most of it will become surface runoff. The loss rate is highly variable, depending on the vegetation and soil characteristics of the watershed.
Typically, all runoff models incorporate a variable runoff coefficient or variable runoff rates.
Commonly-used models such as the Rational Formula (USBR,1977) incorporate a runoff coefficient (C); a C value of 1 represents 100 percent l
l runoff and no infiltration. Other models, such as the U.S. Army Corps of i
4 -2 MAYBELL fTER i
i
Engineers (COE) Flood Hydrograph Package HEC-1, separately compute infiltration losses within a certain period of time to arrive at a runoff amount during that time period.
In computing the peak flow rate for the aesign of the rock riprap erosion protection at the proposed disposal site, DOE used the Rational Formula.
In this formula, the runoff coefficient was assumed by DOE to be unity; that is, DOE assumed +"t no infiltration would occur.
Based on a rev W of the computations, the staff concludes that this assumption is acceptable.
4.2.3 Times of Concentration The time of concentration (tc) is the amount of time required for runoff to reach the outlet of a drainage basin from the most remote point in that basin.
The peak runoff for a given drainage basin is inversely proportional to the time of concentration.
If the time of concentration is computed to be small, the peak discharge will be conservatively large.
Times of concentration and/or lag tima are typically computed using empirical relationships such as those developed by Federal agencies (USBR, 1977).
Velocity-based approaches are also used when accurate estimates are needed.
Such approaches rely on estimates of actual flow velocities to determine the time of concentration of a drainage basin.
Various times of concentration for the riprap design were estimated by DOE using several methods, such as the Kirpich Method (USBR, 1977) and the Manning's Equation (Chow, 1959).
Such velocity-based methods are considered by the staff to be appropriate for estimating times of concentration.
Based on the precision and conservatism associated with such methods, the staff concludes that the tc's have been acceptably derived. The staff further concludes that the procedures used for computing tc are representative of the small steep drainage areas present at the site.
4.2.4 Rainfall Distributions After the PMP is determined, it is necessary to deter'ine the rainfall intensities corresponding to shorter rainfall durations and times of concentration.
A typical PMP value is derived for periods of about one hour.
If the time of concentration is less than one hour, it is necessary to extrapolate the data presented in the various hydrometeorological reports to shorter time periods.
DOE utilued a procedure recommended in HMR 49 and by the NRC staff (NRC, 1990). This procedure involves the determination of rainfall amounts as a percentage of the one-hour PHP, and computes rainfall amounts and intensities for very short periods of time.
DOE and the NRC staff have concluded that this procedure is acceptable.
4-3 MAYBELL fiER
s In the determination of peak flood flows, approximate PMP rainfall intensities were derived by DOE as follows:
Rainfall Duration Rainfall Intensity (minutes)
(inches / hour) 2.5 48.0 5.0 40.0 10.0 28.0 60.0 7.4 The staff checked the rainfall intensities for the short durations associated with small drainage basins.
Based on a review of this aspect of the flooding determination, the staff concludes that the computed peak rainfall intensities are acceptable.
4.2.5 Computation of PMF 4.2.5.1 Top and Side Slopes The PMF was estimated for the top and side slopes using the Rational Formula, which provides a standard method for estimating flood discharges for small drainage areas.
For the top slope and the side slope (west side), DOE estimated the peak flow rates to be about 1.4 cubic feet per second per foot of width (cfs/ft.) and 1.0 cfs/ft., respectively.
Based on staff review of the calculations, these estimates are considered to be acceptable.
4.2.5.2 Apron / Toe A PMF flow rate of 0.5 cfs/ft. for the apron on the east and south sides was computed similarly to the design flow rate for the top and side slopes. As discussed above, the flow rate is considered to be acceptable.
4.2.5.3 Diversion Ditches The diversion ditches are trapezoidal channels designed to intercept and divert runoff from the upland area around the site.
Ditch No. I will be cos.structed along the edge of the embankment toe on the north, west, and southwest sides of the cell to drain runoff from the embankment and from the upland drainage area. Ditch No. 2 will drain a portion of the upland drainage area and will discharge into Ditch No. 1.
Flows from Ditch I will discharge into an existing gully, where outlet protection will be provided.
In the PMF analyses, the Rational Formula was used to compute peak flow rates at different locations along the lengths of the ditches. Maximum flow rates of about 4,240 cfs and 2,580 cfs were estimated as the peak PMF discharges at the downstream ends of Ditch I and Ditch 2, respectively. These flow rates are based on drainage areas of 285 acres and 173 acres.
Based on a check of the calculations of drainage area, time of concentration, and rainfall intensity, the staff concludes that the PMF estimates have been acceptably derived.
4-4 MAYBELL fiER
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4.2.5.4 Natural Gullies Surface runoff from the side slopes of the pile will discharge into four separate natural gullies downstream of the cell.
These gullies will be heavily protected with rock riprap.
Using the Rational Formula, as discussed above, DOE computed peak PMF flow rates of 186, 387, 175, and 290 cfs fo'r Gullies 1, 2, 3, and 4, respectively.
Based on staff review of the calculations, the estimates are considered to be acceptable.
1 4.2.5.3 Rob Pit Overburden Pile The maximum flow rate for the side slope of the Rob Pit overburden pile was
)
computed similarly to the flow rates for the tailings pile cover.
Using the Rational Formula, DOE calculated the maximum flow rate to be 0.26 cfs/ft.
Based on a check of the calculations, this estimate is considered to be acceptable.
4.3 Water Surface Profiles and Channel Velocities Following the determination of the peak ficod discharge, it is necessary i
to determine the resulting water levels, velocities, and shear stresses 1
associated with that discharge. These parameters then provide the basis I
for the determination of the required riprap size and layer thickness i
needed to assure stability during the occurrence of the design event.
l 1
4.3.1 Top and Side Slopes In determining riprap requirements for the top and side slopes, DOE utilized the Safety Factors Method (Stevens, et al., 1976) and the Stephenson Method i
(Stephenson, 1979), respectively.
The Safety Factors Method is used for i
relatively flat slopes of less than 10 percent; the Stephenson Method is used j
for slopes greater than 10 percent.
The validity of these design approaches has been verified by the NRC staff through the use of flume tests at Colorado i
State University.
It was determined that the selection of an appropriate design proceduse depends on the magnitude of the slope (Abt, et al.,1987).
j The staff therefore concludes that the procedures and design approaches t
used by DOE are acceptable and reflect state-of-the-art methods for i
designing riprap erosion protection.
l 4.3.2 Apron / Toe 1
The design of the apron at the toe of the disposal cell is based on the i
j_
following considerations:
{
1.
Provide riprap of adequate size to be stable against the design storm j
(PHP);
i 2.
Provide uniform and/or gentle grades along the apron and the adjacent i
ground surface such that runoff from the cell is distributed uniformly at a relatively low velocity, minimizing the potential for flow concentration and erosion; and 4-5 MAYBELL fiER i
j i
3.
Provide an adequate apron thickness to prevent undercutting of the disposal cell by (a) local scour that could result from the PMP, or (b) potential gully encroachment that could occur due to gradual headcutting l
over a long period of time.
j The key elements which DOE evaluated in the design of riprap protection for
]
the apron / toe are:
k 1.
The lowet part of the w pe,unt side slope immediately upstream of the grade break formed when the side slope meets the apron; 2.
The toe. which is the relatively flat lower slope (five percent) immediately downstr'am of the grade break; 3.
The downstream portion of the apron, which is assumed to have j
collapsed due to scour or long-term erosion; and i
4.
The ground surface adjacent to the apron.
1 i
DOE used several analytical methods for designing the riprap for the apron / toe. Additional detailed discussion of the riprap design of various j
components of the apron / toe can be found in Section 4.4.1.2, below.
4.3.3 Diversion Ditches C0E Water Surface Profiles Package HEC-2 was used to estimate depths and velocities under the estimated discharge conditions in the channels.
The maximum flow depth and velocity in Ditch 1 are 6 feet and 12.4 feet per second, respectively. The invert slope of the channel is about one percent throughout most of its length.
The outlet of the channel, where it discharges i
into a natural gully, will be protected with oversized large riprap; thus, DOE considers that the maximum outlet velocity will not cause headcutting or further erosion of the natural gully. The design of erosion protection for the outlets of the ditch is further discussed in Section 4.4.1.3.3.
The Safety Factors Method was used to determine riprap sizes for the ditch.
Based on staff review of the calculations, the nalysis is considered to be acceptable. Additional detailed information related to the design of erosion protection for the ditches may be found in Section 4.4.
4.3.4 Natural Gullies DOE designed the riprap for the downstream guilies using the Safety Factors Meth:d, assuming that the flows in the gullies would be uniformly spread across the gullies. Additionally, DOE considered flows occurring directly down the side slopes of the gullies. The outlets of the gullies will be designed to prevent scour and undercutting of the erosion.
Since the layer cannot be keyed into competent rock, a large toe will be provided.
Based on a review of the toe design, the staff concludes that the design is acceptable.
Additional discussion of the design may be found in Sections 4.4.1.2 and 4.4.1.3.2 of this TER.
4-6 MAYBELL ffER i
4.4 Erosion Protection 4.4.1 Sizing of Erosion Protection Riprap layers of various sizes and thicknesses are proposed for use at the site.
The design of each layer is dependent on its location and purpose.
4.4.1.1 Top Slopes W Side Slopes The riprap on the top slope has been sized to withstand the erosive velocities l
resulting from an on-cell PHP, as discussed above.
DOE proposes to use an 8-inch-thick layer of rock with a minimum D of 1.5 inches (Type A).
The w
l riprap will be placed on a 0.5-foot-thick bedding layer. The Safety Factors Method was used to determine the rock size.
l The rock layer on the side slopes is also designed for an occurrence of the local PHP. DOE proposes to use a 1-foot-thick layer of rock with a minimum Dw of approximately 4.5 inches (Type C) on the west side slope and 3.0 inches (Type B) on the east slope. The rock layer will be placed on a 0.5-foot-thick bedding layer.
Stephenson's Method was used to determine the required rock size.
Conservative values were used for the specific gravity of the rock, the l
rock angle of internal friction, and porosity.
Based on staff review of the DOE analyses and the acceptability of using design methods recommended by the NRC staff, as discussed in Section 4.3 of this report, the staff concludes that the proposed rock sizes are adequate.
4.4.1.2 Apron / Toe L
l DOE evaluated the design of the apron / toe in four separate segments, as previously discussed in Section 4.3.2.
Following is the staff evaluation of each of the segments.
4.4.1.2.1 Lower Side Slope For the lower portion (last 10 feet) of the side slopes (upstream portion of the apron), DOE proposes to use a layer of rock with c. minimue, Om size of about 11 inches (Type D), gradually increasing in thickness from2 to 5 feet.
Several methods were used to check the rock size required for the toe.
DOE determined the shear forces associated with PMP flows down the side slope and assumed that turbulence would be created on the lower portion of the slope where it meets the toe. To account for this turbulence and energy dissipation, DOE increased the shear stress by 50 percent, in accordance with l
COE recommendations. The maximum required rock size of about eight inches was computed using the Safety Factors Method. Based on staff analysis of DOE's I
methods and assumptions, the 11-inch Type D rock proposed for this portion of
(
the slope is acceptable.
4.4.1.2.2 Toe I
For the actual toe area, which will have a 20-foot length and a five percent slope, DOE used the Safety Factors Method to determine the required rock size.
l 4-7 MYSELL ffER I
1
e The flow rate was increased by a factor of three to account for flow concentrations near the downstream end of the apron. The D rock size 3a calculated using this method was found to be about five inches, which is smaller that the proposed D30, size of 11 inches.
Based on our review of DOE's calculations, the rock size is acceptable.
4.4.1.2.3 Collapsed Slope As part of the analysis of the toe area, DOE conservatively assumed that the natural ground downstream of the toe would be eroded due to cumulative
]
local scour and/or erosion at its base, resulting in the collapse of he rock into the eroded area.
DOE assumed that the collapsed slope of the rock would be IV:3H. Using the Stephenson Method, the required rock size is calculated to be about 7-8 inches.
Since this computed size is less than the proposed size of 11 inches, the rock is acceptable.
4.4.1.2.4 Natural Ground In order to determine the depth to which the toe must be placed, it is necessary to estimate the depth of scour which will occur to the graded natural ground slope just downstream of the tae.
DOE assumed that the ground slope would be about five percent and assumed that a flow concentration factor of three would occur for gully flows. Using the Lacey Regime Equation (Davis and Sorenson, 1969), the scour depth was estimated to be about three feet. However, DOE proposes to place the toe to a depth of five feet, based on an analysis of existing gullies in the area. DOE assessed the gullies in the area using an analysis of drainage area versus gully depth to arrive at the depth estimate.
Staff review of the calculations indicates that the methods are appropriate.
4.4.1.3 Diversion Ditches The riprap design of the interceptor ditches was analyzed by DOE in the following areas:
1.
Design of the ditch side slopes for runoff directly down the side slopes from the embankment and from the upland drainage area; i
2.
Design for runoff directly through the ditch; 3.
Design of ditch outlet; and 4.
Sediment considerations.
4.4.1.3.1 Ditch Side Slopes A Type D riprap layer is proposed for a substantial length of the ditch.
For the upland side slope of the channel, conditions exist where the flows from natural gullies could impinge. The proposed Type D rock size of 11 inches is adequate to prevent erosion of the slopes under PMF conditions, even if a natu-al gully forms and discharges directly onto the ditch side slope.
DOE checked the rock size and determined that rock with an average size of 6-8 4-8 MAYBELL fTER
inches would be necessary. Therefore, the 11-inch rock is considered acceptable.
4.4.1.3.2 Ditch (Main Section)
For flows directly through the ditch, the Safety Factors Method was used to determine the rock size.
Based on a review of the calculations, the proposed Type D rock size of 11 inches for the channel bottom and side slopes is considered to be adequate.
4.4.1.3.3 Ditch Outlets Maximum potential scour depths due to the PMF flows were computed using several calculational methods, including the U.S. Department of Transportation (DOT,1975) formula, Lacey's formula (Davis and Sorensen,1979), and several other methods.
The potential scour depth was estimated by DOE to be about 17 feet, using a range of estimates from several of these formulas.
DOE proposes that the riprap at the ditch outlets will be extended down to 17 feet below grade.
The outlet section of the ditch is assumed to collapse due to either gully headward erosion over a long period of time, or the PMF flow in the ditch.
DOE assumed that the outlet slope of IV on 6H will be formed following erosion and collapse.
This slope requires a stable rock size of j
about 24 inches, calculated using the Stephenson Method. Type E riprap with a D
of about 24 inches will be used in the imediate area of the outlet and g
wul also be placed for a length of about 80 feet past the outlet, to prevent headward gully development. This outlet section is located several hundred feet from the reclaimed tailings, which will also contribute to the stability of the tailings over the design life of 1,000 years.
Based on a review of the calculations by the staff, the design of the outlet area is considered to be acceptable.
4.4.1.3.4 Sediment Considerations In general, sediment deposition can be a problem in diversion ditches when the slope of the diversion ditch is less than the slope of the natural ground where flows enter the ditch.
It is usually necessary to provide sufficient slope and capacity in the diversion ditch to flush or store any sediments which will enter the ditch.
In particular, enhanced design features may be necessary in areas where natural gullies are intercepted by the diversion ditch. Concentrated flows and high velocities could transport large quantities of sediment, and the size of the particles transported by the natural gully may be larger than the man-made diversion ditch can effectively flush out.
For this site, a considerable amount of sediment from the upland drainage area can be expected to enter the diversion ditch for the following reasons:
1.
The upland drainage area has an average slope which is greater than the relatively flat-sloped ditch, which has a slope of about one percent in 4-9 MAYBELL ffER
t the reaches adjacent to the tailings embankment.
Flow velocities in the ditches may not be as high as those occurring on the natural ground.
Therefore, sediment, cobbles, and boulders may be transported to the ditch and may not be easily flushed out by the lower velocities in the ditch.
2.
The potential for gully development (and resulting high flow velocities) in the un' W drainaga area, and subsequent transport of bed-load material into the diversion ditch is nigh.
Gullies and areas of flow concentration are evident upstream of the diversion ditch, based on review of topographic maps of the area and a staff site visit to the area.
Flows moving towards the diversion ditch will tend to concentrate in these gullies, increasing the potential for gully incision and transport of sediment.
]
In order to document the acceptability of the ditch design, DOE provided j
analyses which indicated that the diversion ditch, with a slope of about one percent, will be able to flush out much of the sediment other than the larger gravels and cobbles.
Using storm events ranging in magnitude from the annual flood to the PMF, DOE calculated the critical shear stresses and velocities needed to transport materials of various sizes.
DOE concluded that the slope of the ditch was sufficient to transport 'much of the smaller-sized materials during most flood events.
Based on an evaluation of the sediment calculations, the staff considers that the analyses are acceptable.
4.4.1.4 Natural Gullies To prevent gully erosion and headcutting into the tailings area, DOE proposes to stabilize four natural gullies which are currently actively eroding headward toward the pile. The erosion protection for Gullies 1, 2, 3, and 4 was designed in accordance with standard channel design procedures, and the staff concludes that DOE has appropriately designed the erosion protection for flows directly in the gullies.
Further, DOE evaluated the effects of Johnson Wash, which is located very close to Gullies 1 and 2, and several hundred feet from Gullies 3 and 4.
DOE evaluated the adequacy of the gully erosion protection as a result of flooding on Johnson Wash, and evaluated the potential for significant erosion and scour t,.ccur in the channel of Johnson Wash.
Johnson Wash was identified (in the DOE geomorphic studies and Section 2.0 of this TER) as a potential hazard to the stability of the tailings pile. Active erosion and base level lowering in the immediate area are occurring.
Johnson Wash appears to be relatively unstable, and lateral erosion and meandering are definite possibilities.
Such erosion, meandering, and other channel changes could subject the riprap to greater forces than those which would occur under present conditions, j
1 DOE's estimate of flooding and velocities indicates that very high velocities (up to 16.5 feet per second) could occur during major flood events in Johnson Wash.
The riprap toes and riprap side slopes for Gullies 1 and 2 are designed j
to accommodate the maximum expected velocities and scour depths in Johnson Wash.
The proposed toe depth is adequate to accommodate the calculated scour 4-10 MMBELL f TER
s l
depth of about 10 feet in Johnson Wash.
Gullies 3 and 4 are located a l
sufficient distance from Johnson Wash to re...uin unaffected by scour and l
erosion in Johnson Wash.
Based on staff evaluation of DOE calculations and the design of the gullies, the staff concludes that flooding and erosion in Johnson Wash will not affect the stability of the tailings.
4.4.1.5 Rob Pit Overburden Pile The erosion protectic,, ror the Rob Pit overburden pile was designed us...g both the Safety Factors Method and the Stephensen Method.
DOE proposes to place a 12-inch thick layer of Type C riprap with a 0 of 4.7 inche on a large 3o portion of the pile side slopes.
Based on staff review of t'
.alculations associated with the design, the proposed rock layer is considered to be adequate.
4.4.2 Rock Durability The EPA standards require that control of residual radioactive materials be effective for up to 1,000 years, to the extent reasonably achievable, and in any case, for at least 200 years. The previous sections of this report examined the ability of the erosion protection to withstand flooding events reasonably expected to occur in 1,000 years.
In this section, rock durability is considered to determine if there is reasonable assurance that the rock itself will survive and remain effective for 1,000 years.
Rock durability is defined as the ability of a material to withstand the forces of weathering.
Factors that affect rock durability are (1) chemical reactions with water, (2) saturation time, (3) temperature of the. water, (4) scour by sediments, (5) windblown scour, (6) wetting and drying, and (7) freezing and thawing.
DOE identified three acceptable sources of rock in the immediate site vicinity.
The suitability of the rocks as protective covers was then assessed by laboratory tests to determine their physical characteristics.
DOE conducted the tests and used the results of these tests to classify the rock's quality, and to assess the expected long-term performance of the rock.
In accordance with past DOE rock-testing practice, the 'asts included 1.
Petrographic Examination (ASTM C295).
Petrographic examination of rock is used to determine its physical and chemical propertiet.
The examination establishes if the rock contains chemically unstable minerals or volumetrically unstable materials.
2.
Bulk Specific Gravity (ASTM C127).
The specific gravity of a rock is an indicator of its strength or durability; in general, the higher the specific gravity, the better the quality of the rock.
3.
Absorption (ASTM C127). A low absorption is a desirable property and indicates slow disintegration of the rock by salt action and mineral hydration.
l 4-11 MAYBELL fTER t
i 1
l 4.
Sulfate Soundness (ASTM C88).
In locations subject to freezing or exposure to salt water, a low percentage is desirable.
5.
Schmidt Rebound Hammer. This test measures the hardness of a rock and can be used in either the field or the laboratory.
6.
Los Angeles Abrasion (ASTM Cl31 or C535). This test is a measure of rock's resistance to abrasion.
7.
Tensile Strength (ASTM D3967 or ISRM Method).
This test is an indirect test of a rock's tensile strength.
DOE then used a step-by-step procedure for evaluating durability of the rock, in accordance with procedures recommended by the NRC staff (NRC, 1990), as follows:
Step 1.
Test results from representative samples are scored on a scale of 0 to 10.
Results of 8 to 10 are considered " good"; results of 5 to 8 are considered " fair"; and results of 0 to 5 are considered
" poor".
Step 2.
The score is multiplied by a weighting factor.
The effect of the weighting factor is to focus the scoring on those tests that are the most applicable for the particular rock type being tested.
Step 3.
The weighted scores are totaled, divided by the maximum possible score, and multiplied by 100 to determine the rating.
Step 4.
The rock quality scores are then compared to the criteria which determine its acceptability, as defined in the NRC scoring procedures.
J In accordance with the procedures suggested in the NRC Staff Technical Position (NRC, 1990), DOE conducted durability testing of three rock sources.
Based on these tests, the rock quality score was determined. Type A and Type B rock, which will be produced from the Maybell gravel source, scored 4
less than 80, and DOE proposes to oversize the rock, as r.ecessary, using NRC criteria (NRC, 1990).
Types C, D, and E riprap, which will be produced from j
the two other sources, scored above 80 and will not require oversizing, even if placed in frequently-saturated areas. The staff concludes that the rock sources proposed meet the criteria suggested by the staff (NRC, 1990) and are of sufficient quality to meet EPA standards.
4.4.3 Testing and Inspection of Erosion Protection The staff has reviewed and evaluated the testing and inspection quality control requirements for the erosion protection materials.
Based on a review of the information provided by DOE, the staff concludes that the proposed testing program is acceptable.
4-12 mAvstLL fita
4.5 Upstream Dam Failures There are no impoundments near the site whose failure could potentially affect the site.
4.6 Conclusions Based on staff review of the information submitted by DOE, the NRC staff concludes that the site design meets EPA requirements as stated in 40 CFR 192 with regard to flood design measures and erosion protection.
The staff concludes that an adaquate hydraulic design has been provided to reasonably assure stability of the contaminated material at the disposal site for a period of 1,000 years, or in any case, at least 200 years.
4-13 MAYBELL fTER
i l
5.0 WATER RESOURCES PROTECTION 5.1 Introduction The staff reviewed the Remedial Action Plan (DOE,1996) and supporting documents for the Maybell, Colorado UMTRA Project site for compliance with EPA's groundwater protection standards in 40 CFR Part 192, Subparts A through C.
All field 4 stigatione. analvses, and conclusions presented in the RAP were performed by DOE, or its contractors. The staff reviewed the RAP in accordance with EPA's groundwater protection standards (EPA, 1995) and relevant portions of the staff's Standard Review Plan for UMTRCA Title ! Mill Tailings Remedial Action Plans (NRC,1993). All site descriptions and characterizations presented in this chapter are based on information and data provided by DOE.
Specific analyses and conclusions made by staff, as a part of this review, are designated as such.
The Maybell processing mill was operated by Union Carbide (UMETC0) from 1957 to 1964. The Maybell milling process extracted uranium oxide from the ore by a sulfuric acid leaching method in combination with a continuous, counter-current, resin-in-pulp process. An ammonium nitrate solution was used to elute the uranium from the loaded resin and the uranium was precipitated with anhydrous ammonia.
In 1976, low-grade ores left from the milling operation were heap leached at the site.
Initially, an ion exchange unit was installed to concentrate the uranium, but was later replaced by a solvent extraction process.
Leaching continued through 1981, when those operations ceased.
Tailings fluids had been released into Johnson Wash and Lay Creek on several occasions during operations at the site.
DOE proposed to incorporate tailings and contaminated off-site materials in an engineered disposal cell at the present location of the tailings pile. 00E concluded that the proposed remedial action complies with the proposed EPA groundwater protection standards under supplemental standards in 40 CFR 192.22, because the uppermost aquifer beneath the tailings materials meets the limited-use designation as defined in 40 CFR 192.11(e)(2), and that hazardous constituents released from the site do not represent a substantial hazard tv human health and the environment.
DOE proposes no groundwater cleanup at the Maybell site. This proposal is based on DOE's characterization of the uppermost aquifer as limited use, containing wide-spread ambient contamination not related to uranium milling activities; no current or projected future water use of the aquifer within a 4.8 km (3 mile) radius of the site; no apparent discharge of tailings contaminated groundwater to surface-water bodies or deeper aquifers in the vicinity; and continued groundwater monitoring of the existing contamination to assure conditions remain unchanged.
5.2 Hydrogeologic. Characterization The hydrogeologic characterization provides the basis for determining which strategies will be appropriate for groundwater resources protection in the vicinity of the processing and disposal sites.
The characterization sets the groundwork for complying with the standards of 40 CFR 192 Subparts A through 5-1 MAYBELL ffER
)
s
, cc C.
Critical elements of the hydrogeologic characterization are:
(1) identi-fication of the hydrogeologic units, (2) hydraulic and transport properties, (3) geochemical conditions and contamination extent, and (4) present and potential water use.
Each of these elements are presented below for the processing and disposal sites, as provided in DOE's final RAP.
5.2.1 Identification of Hydrogeologic Units The uppermost hydrogeologic unit at the Maybell site is the Miocene-Age Browns Park formation (Browns Park aquifer), which overlies the truncated rocks of the Cretaceous Mancos Shale.
The Browns Park Formation consists of fluviolacustrine and eolian sandstones overlying a fluvial basal conglomerate.
The lower conglomerate unit is from about 0 to 45 m (0 to 150 ft) thick, while the upper sandstone unit is approximately 305 m (1000 ft) thick and consists of fine-to medium-grained quartz sandstone with minor interbeds and lenses of conglomerate, chert, siltstone, inoristone, and volcanic ash and pumice. The Browns Park, in this area, generally dips less than 10 degrees to the north toward the east-west trending Lay Syncline.
Groundwater in the Browns Park aquifer is encountered from about 10.7 m (35 ft) to over 91 m (300 ft) below the ground surface and is under unconfined conditions. Groundwater surface fluctuations are generally less than 0.6 m (2 ft) and likely represent responses to temporal and spacial variations in precipitation and aquifer recharge.
Shallow groundwater also occurs in the Lay Creek and Yampa River Valley alluvium. The Yampa River is located approximately 4.8 km (3 miles) southeast of the site, and Lay Creek is located about 2.4 km (1.5 miles) to the south.
Groundwater _in the Yampa River alluvium is encountered at depths ranging from 3 to 6 m (10 to 20 ft) and is unconfined.
Projections of the Browns Park potentiometric surface indicates that the Yampa River alluvium is likely recharged by groundwater from the Browns Park Formation.
Staff reviewed the hydrogeologic characterization of the Maybell site and agrees with the designation of the Browns Park format %n as the uppermost aquifer.
5.2.2 Hydraulic and Transport Properties DOE evaluated the hydraulic and advective transport properties of the uppermost aquifer with three pumping tests, single-well slug tests, and water-level measurements at the site and the surrounding area.
The pumping tests confirmed the unconfined hydraulic condition of the aquifer with delayed yield during the later stages of the tests.
The average horizpntal hydraulic 2
conductivities measured from the pumping tests is 0.16 m / day (1.7 ft / day).
The late time-drawdown data from tpe tests provided a calculated average specific yield of about 8.12 x 10'.
By comparison, the hydraulic conductivities from the slug tests are less reliable than those derived from the pumping tests, and were used by DOE only to provide a relative indication of the variability in the hydraulic conductivity in the formation, 5-2 MAYBELL fiER 1
,a Groundwater flow is generally southwesterly toward the Yampa River, and generally follows the gentle slope of surface topography.
DOE calculated the average horizontal hydraulic gradient in the vicinity of the site as approximately 0.02.
The average linear groundwater velocity, as calculated by DOE in Calculation MAY-ll-93-14-07-00, is 5x10'2 m/ day (0.17 ft/ day).
5.2.3 Geochemical Conditions and Extent of Contamination Ambient groundwater quality in the uppermost aquifer is influenced by tht.
presence of naturally occurring uranium mineralization.
Several unreclaimed open pit mines are located near the tailings pile.
A characteristic feature of the Maybell ore deposits is the large amount of low-grade to sub-economic mineralized material that is still present in the Browns Park Formation.
The background water quality at the site is described as a calcium bicarbonate /
sulfate type with a pH that ranges from 4.94 to 7.56.
The average ambient Total Dissolved Solids (TDS) concentration in the uppermost aquifer is approximately 1680 mg/L.
Background water quality is also characterized by low concentrations of chloride and nitrate, both of which are contaminants present in the tailings leachate.
DOE's assessment indicates that groundwater contamination from the processing site is readily distinguished from ambient groundwater conditions, due to the elevated nitrate levels in the contaminated waters.
Ambient groundwater quality measurements within the uppermost aquifer indicate that arsenic, selenium, and uranium exceed the proposed EPA maximum contaminant levels (MCLs) upgradient of the site. Additionally, iron, manganese, and sulfate are elevated above the Secondary Drinking Water Standards in wells upgradient of the pile and in wells downgradient, but beyond the hydraulic influence of the pile.
These constituents likely result from the ore bodies and mineralized zones in the Browns Park. Mining activities may also have contributed to elevated levels of these constituents in the ambient groundwater.
DOE's assessment of the site's geochemical characteristics indicates that conditions beneath the tailings pile are favorable for neutralizing the acidic tailings seepage; however, the relatively low ion exchange capacities in the Browns Park Formation do not attenuate all of the hazardous constituents.
Neutralization of the tailings seepage and precipitation of acid-soluble iron in the unsaturated zone appears to enhance the sorption of some hazardous metals such as arsenic, molybdenum, selenium, radium, and uranium.
The sorption of these contaminants by the sub-pile soils may partially explain the relatively low concentration of these constituents in groundwater samples I
collected downgradient of the tailings pile.
Past seepage from the tailings materials created a measurable mound in the
{
potentiometric surface beneath the pile. The mound distorts local flow patterns in the immediate vicinity of tne pile and caused some contamination to travel 'upgradient' of the tailings pile.
The contaminated groundwater is 4
characterized by elevated nitrate concentrations and appears limited to a radius of about 610 m (2000 ft) or less from the pile.
5-3 MAvstto fica I
I
l
.s DOE documented the past release of tailings fluids into Johnson Wash and Lay Creek on several occasions during the operation of the site. These releases caused va*ying degrees of soil contamination, which DOE characterized.
Soils l
with elevated radium-226 levels in these areas will be remediated and incorporated into the disposal cell.
DOE evaluated the potential for groundwater contamination in Johnson Wash by reviewing the groundwater quality data from monitoring wells located along Johnson Wash. Data from wells 673, 675, 609, 667, and 694 were evaluated.
Well 673 is the nearest to the tailings pile along the Wash and has been impacted by tailings seepage.
The water quality from well 673 shows elevated concentrations of nitrate, selenium, and uranium which exceed the MCLs by one to two orders of magnitude.
Water quality data from the remaining wells along the Wash (975, 609, 667, and 694), which are further downgradient from the tailings pile, show no indication of tailings contamination.
DOE indicated that Well 609 had single-event MCL exceedances for lead and selenium in 1987 and 1989, respectively, which were attributed to analytical or recording errors from the laboratory.
l Surface water bodies occur at three locations near the site. Two ephemeral seeps (719 and 701) are situated in Johnson Wash, south of the site; and permanent surface water in the Rob Pit, next to the tailings pile.
Seep 719 i
is nearest the tailings pile and may result from surface runoff infiltrating i
i the alluvium, since the seep is perched above the more impermeable Browns Park l
Formation.
Seep 701 is situated near the confluence of Johnson Wash and Lay Creek, and is likely an expression of groundwater.
The distance of these i
seeps from the tailings and the direction of groundwater flow across the site indicate that tailings contamination could not have reached the surface through these seeps.
The water surface in the Rob Pit represents the local groundwater surface near the site. DOE indicates that it is possible for tailings contamination to eventually reach Rob Pit; however, constituent l
concentrations measured in Rob Pit are likely associated with naturally-occurring uranium mineralization adjacent to and beneath the pit.
DOE concludes that there is no geochemical evidence of tailings contamination in the waters of Rob Pit, since constituent concentrations are within the variance measured at background locations and other mine pits. The lower concentrations of uranium and elevated radium-226 in the Rob Pit waters likely 1
indicate the presence of ore mineralization.
Staff reviewed the geochemical characterization of the site and agrees that l
measurements indicate elevated concentrations of arsenic, selenium, and uranium in the ambient, upgradient groundwater. Staff also agrees with the l
conclusion that tailings contamination has not likely impacted nearby surface l
water bodies.
l 5.2.4 Water Use l
DOE performed an inventory of domestic, municipal, agricultural, and industrial wells surrounding the Maybell site.
The inventory consisted of reviewing well registration records at the Colorado Division of Water Resources.
The well records indicate that there are no domestic wells within a 4.8 km (3 mile) radius of the tailings pile.
The nearest domestic well is 5-4 MAYBELL fiER l
i
located more than 4.8 km (3 miles) southwest of the pile in the alluvium of Lay Creek.
Several domestic wells are in or near the town of Maybell, about 8 km (5 miles) southwest of the site.
These wells are typically less than 39 m (130 ft) deep and are situated in the Yampa River Alluvial Valley, with the exception of one well located in the Browns Park aquifer.
DOE indicates that the domestic and municipal wells near the town of Maybe11 will not be impacted by the tailings leachate because of favorable geochemical conditions which prevent the downaradient movement of contaminants.
The Browns Park aquifer provided water for the uranium milling operations during the active life of the facility.
No industrial groundwater users are currently extracting water from the Browns Park aquifer within a 4.8 km (3 mile) radius of the site.
Two windmill-powered wells provide water for livestock within the 4.8 km (3 mile) radius of the site.
One windmill is located about 3.7 km (2.3 miles) northeast of the site, and the second windmill is about. 6 km (1 mile) south of the site within Johnson Wash.
DOE performed an engineering evaluation of the treatability of the ambient groundwater in the vicinity of the site.
This evaluation was based on the concentration of naturally-occurring hazardous and non-hazardous constituents that would require treatment to make the groundwater potable.
The particular constituents of concern are selenium and uranium, which occur at levels above the EPA Primary Drinking Water Standards (40 CFR 141), or the 40 CFR 192 Subpart A MCLs; also iron, manganese, and sulfate, which are present above the EPA Secondary Drinking Water Standards (40 CFR 142).
The treatment-train needed to treat the Maybell groundwater to meet EPA Drinking Water Standards would require several conventional treatment processes performed in series or the use of reverse osmosis.
DOE also confirmed that groundwater is not normally treated, beyond chlorination, for drinking water purposes in Colorado; except where alternative drinking water sources are not readily avail able.
Historically, there has been little groundwater development in the vicinity of the Maybell tailings pile.
DOE's assessment indicates that future groundwater use in the affected hydrogeological environment will be minimal because of the remotewss of the tailings site to the existing and potential future groundwater users, and the generally poor groeWater quality in the site vicinity.
Additionally, the availability of alternative water supplies, such as the Yampa Mver Valley Alluvium near the town of Maybell, will also likely curtail the development of groundwater in the area of the tailings pile. Given these factors, the expected future value of the Browns Park aquifer for water supply appe" s to be low in the area of the tailings pile.
Staff reviewed the water-use information and treatability evaluation provided by DOE and agrees that future development of the groundwater resource in the vicinity of the tailings pile will likely be minimal.
5-5 MAYBELL fTER
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5.3 Conceptual Design Features to Protect Water Resources DOE included several factors, both engineering design and siting considerations, which should provide protection of the water resources at the Maybell site.
DOE proposes to incorporate the tailings and contaminated materials from the Maybell processing site into an engineered disposal cell at the present location of the tailings materials.
The existing tailings will be pre-loaded with conta
- ated soil materials to induce tailings consolidation, and the relocated contaminated materials will ultimately be placed in um lifts and compacted at near optimum moisture levels.
The disposal cell's cover design consists of the following five om*nh, in ascending order:
(1) 46 cm-thick (1.5 ft-thick) radon barrier constructed of j
clayey sands amended with 10 percent bentonite and compacted.
The constructed radonbprrierwillhaveasaturatedhydraulicconductivityofapproximately 1 x 10' cm/sec; (2) 1.2 m-thick (4 ft-thick) frost protection layer composed of sandy soils; (3) 15 cm-thick (6 in-thick) drainage / bedding layer which will serve as a capillary break, and hydraulically separate the radon barrier and frost protection layer from the overlying cover component; and (4) 20 cm-thick (8 in-thick) riprap layer for erosion protection.
The top slope of the cover will have a three percent slope to the west and runoff will be diverted to a ditch adjacent to the disposal cell.
All cover components will be constructed of natural, stable materials to ensure the long-term stability of the disposal cell.
The disposal cell components are designed to comply with the EPA groundwater protection standards and ensure that long-term performance does not rely on active j
maintenance.
The overall low permeability of the cover is designed to mitigate the long-term seepage from entering the tailings.
The main natural components that will serve to protect the water resources at i
the disposal site are the chemical attenuation capacity of the unsaturated portion of the Browns Park Formation, and the favorable climate conditions.
Measurements and modeling performed by DOE indicate that the majority of the hazardous constituents are attenuated within the upper few feet of the Browns Park Formation.
A summary of the attenuation capacity is presented in Section 5.2.3.
The climate at the Maybell site is considered semi-arid with a mean annual precipitation of about 33.8 cm (13.3 in), with an annual pan evaporation rate of 122 cm (48 in). DOE also stated that the available capacity within the Browns Park should be sufficient to neutralize all of the tailings porewater that is expected to seep from the disposal cell.
DOE also evaluated the potential amount of water that will be released from the tailings during preloading of the existing pile (Calcuhtion MAY-03-94 05-00).
DOE estimates that the flux caused by the preloading and construction activities will be transient and dissipate to a near steady-state condition in approximately 1.5 years after initial loading.
DOE also evaluated the impact to groundwater, once the flux leaves the pile (Calculation MAY-03-94-12-06-00).
DOE's evaluation examined the impact of nitrate as a conservative contaminant, since nitrate is not readily attenuated or retarded in groundwater flow. DOE's calculation estimates that the nitrate 5-6 MAYBELL ffER
1 s
concentrations would begin to increase after about five years at a distance of 152 m (500 ft) downgradient of the pile. Nitrate concentrations would begin to dissipate after 25 to 30 years.
Staff reviewed the conceptual design features of the disposal cell and agrees that the combination of the engineered features, the natural attenuation component, and the favorable climate conditions should protoct the uppermost aquifer in accordance with 40 CFR 192.20(a)(3)(iii).
Staff agrees with DCE's analysis that temporary transient drainage from loading and construction of the disposal cell will likely impact the groundwater quality in the vicinity of the disposal cell.
5.4 Disposal and Control of Residual Radioactive Materials EPA standards in Subparts A and C of 40 CFR Part 192 require DOE to demonstrate that the disposal of residual radioactive material complies with site-specific groundwater protection standards and closure performance standards in four areas: Water Resources Protection Standards for Disposal (Section 5.4.1), Performance Assessment (Section 5.4.2), Closure Performance Standards (Section 5.4.3), and Groundwater Monitoring and Corrective Action Program (Section 5.4.4).
5.4.1 Water Resources Protection Standards for Disposal The site specific groundwater protection standards for disposal contained in 40 CFR 190.02 consist of three elements for groundwater monitoring: (a) a list of hazardous constituents, (b) a corresponding list of concentration limits, and (c) a point of compliance (P0C); or the supplemental standards established under 40 CFR 192.22.
DOE proposes the use of supplemental standards for groundwater protection at the disposal site in accordance with the limited use provision of 40 CFR 192.21(g). DOE states that the groundwater in the uppermost aquifer contains widespread, ambient contamination unrelated to activities from the former processing site which cannot be cleaned up using treatment technologies reasonably employed in public water systems. DOE conducted chemical analyses of groundwater both upgradient and downgradient (beyond the influence of the processing site); engineering evaluations of treatability; and investigations of past and current groundwater use in the area. The results of these analyses and evaluation are presented in Sections 5.2.3 and 5.2.4.
Supplemental standards shall, "come as close to meeting the otherwise applicable standards as is reasonable under the circumstances," as stated in 40 CFR 192.22(a). DOE's demonstration for meeting these standards is presented in Sections 5.4.1.1 through 5.4.1.3.
5.4.1.1 Hazardous Constituents Hazardous constituents were identified from the tailings pore fluid characterization, and from knowledge of the uranium recovery process at the processing site.
Hazardous constituents are those compounds listed in Table 1 or App?ndix 1 of 40 CFR 192. DOE identified seven inorganic hazardous 5-7 MAYBELL f1ER
constituents in the tailings pore fluids that exceed the maximum contaminant limit (MCL) from Table 1 of 40 CFR 192.
These constituents are:
- arsenic, cadmium, molybdenum, nitrate, combined radium-226 and radium-228, selenium, and uranium.
Additionally, the following seven hazardous constituents or elements of hazardous constituents listed in Appendix I or 40 CFR 192 were also detected in the tailings fluids:
aluminum, antimony, beryllium, copper, nickel, vanadium, and zinc. Aluminum and zinc are components of aluminum phosphide and zinc phosphide. Copper and zinc are components of copper cyanide and zinc cyanide.
Vanadium is a component of vanadium pentoxide.
The phosphide and cyanide anion components were not detected in the tailings fluids, when analyzed, and are not expected to occur under the geochemical conditions within the tailings.
Vanadium pentoxide is also not expected to occur under the geochemical conditions in the tailings. D0E's analyses did not detect the presence of organic hazardous constituents in the groundwater directly beneath the tailings pile.
Staff reviewed DOE's hazardous constituent characterization according to the following three criteria:
(1) whether the constituents are reasonably expected to be in or derived from the tailings; (2) whether they are listed in Table 1 or Appendix I of 40 CFR 192; and (3) whether they were detected in the tailings or groundwater at the site (NRC, 1988).
Based on this review, staff agrees with DOE's determination of arsenic, cadmium, lead, molybdenum, nitrate, combined radium-226 and -228, selenium, uranium, antimony, beryllium, manganese, and nickel as hazardous constituents for the Maybell disposal site.
5.4.1.2 Concentration Limits DOE proposes a narrative supplemental groundwater compliance standard at the Maybell site, rather than establishing numerical concentration limits at a specified P0C.
DOE indicates that a narrative supplemental standard is appropriate at the site because: (1) the background groundwater quality in the uppermost aquifer is variable with concentrations of several hazardous constituents ranging from below the method detection limit to well above the MCL; (2) the groundwater in the vicinity of the tailings pile has no historic or current beneficial use downgradient of the affected area; and (3) the proposed remedial action at Maybell comes as close to meeting the otherwise applicable standards as is reasonable under the circumstances.
Staff reviewed the ambient groundwater quality data, water use, and conceptual disposal design from the disposal site and agrees with using a narrative compliance standard at the Maybell site.
5.4.1.3 Point of Compliance The P0C is defined as a vertical surface that extends downward into the uppermost aquifer along the hydraulically downgradient limit of the disposal area. Monitoring wells should be located as close to the disposal cell as practicable, without disturbing the engineered components intended for the long-term tailings isolation.
5-8 MAYBELL fiER
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DOE does not propose to conduct monitoring along the P0C at the Maybe11 site i
because monitoring for the purpose of numerical compliance would not provide 1
additional protection of human health and the environment beyond the narrative compliance standards for a limited-use aquifer.
Based on its analyses, the 4
staff agrees that P0C monitoring at the Maybell site is not appropriate for i
protecting human health and the environment.
l 5.4.2 Performt...e Assessmer.;
DOE must demonstrate that the disposal cell performance will comply with EPA groundwater protection standards in 40 CFR 192 Subparts A and C.
DOE evaluated the potential flux through the bottom of the disposal cell caused by i
transient drainage during pile preloading and construction (Calculation MAY-1 03-94-12-05-00), and the impact to groundwater (Calculation MAY-03-94-12 !
00).
DOE's calculations estimate that contaminant concentrations, using J
nitrate as a conservative constituent, would begin to increase after about five years at a distance of 152 m (500 ft) downgradient of the pile.
j Contaminant concentrations would begin to dissipate after 25 to 30 years.
DOE i
also recognizes that downgradient monitoring of cell performance may be 1
]
obscured by the transient effects of tailings consolidation for several years.
The staff reviewed the information and calculations used to evaluate the flux from the disposal cell and the resulting groundwater impact.
In turn, the staff agrees with DOE's evaluation and conclusion that performance monitoring of the groundwater near the disposal cell may be obscured by transient drainage effects for several years.
5.4.3 Closure Performance Demonstration
)
DOE must demonstrate that the proposed disposal design will (1) minimize and control groundwater contamination, (2) minimize the need for further maintenance, and (3) meet initial performance standards of the design, in accordance with the closure performance standards of 40 CFR 192.02.
DOE's remedial design for combining the off-pile contaminated soils with the existing tailings materials in an engineered (Heoosal cell should minimize contaminant releases to groundwater and surface water.
In particular, limiting the infiltration flux with a compacted, bentonite-am oarrierwithananticipatedhydraulicconductivityof1x10'pndedradon cm/sec, will minimize future contaminant releases to the groundwater.
Future maintenance of the disposal cell will be minimized through the use of stable, natural earth materials.
DOE proposes to apply supplemental standards for groundwater protection, which means that compliance will be based on a narrative standard of coming "as close to meeting the otherwise applicable standard as is reasonable under the circumstances." As a primary component of the remedial design, DOE performed various engineering assessments to evaluate whether the design would meet the disposal compliance standards in Subpart A.
Designating an uppermost aquifer as " limited use" assures that the same level of protection for human health and the environment will be provided by conform ng to the narrative standards 5-9 MAYBELL fiER
}
.s as would complying with the numerical stand'rds at a site with an unrestricted i
water-use designation.
DOE plans to demonstrate disposal cell performance through annual site inspections of the disposal cell components, and through measurements of 1
groundwater quality in cell performance monitoring wells during the specified post-closure period.. DOE anticipates that comparisons of actual. settlement measures with predict ' post-closure settlement projections will provi' an assessment of cell performance for groundwater compliance because the anticipated transient drainage is primarily related to the pore water within the tailings slimes.
DOE also proposes to confirm cell performance by monitoring groundwater l
contaminant levels in the Browns Park aquifer.at selected locations.
DOE i
proposes to establish baseline constituent levels in two cell performance l
monitoring wells downgradient of the disposal cell.
The designated l
performance monitoring wells may require the installation of new wells in order to locate the wells near the disposal cell, but beyond the construction zone influence of the surface remediation. DOE indicates that the travel time of transient drainage from the disposal cell will be considered before any I
elevated constituent levels in the performance monitoring wells are construed l
as indicators of cell failure.
The staff reviewed DOE's conceptual approach for demonstrating post-closure l
cell performance through visual inspections, settlement monitoring, and selected groundwater monitoring, and agrees with DOE's approach.
Details of post-closure monitoring will be provided.in the Long Term Surveillance Plan j
(LTSP).
5.4.4 Groundwater Monitoring and Corrective Action Plans 40 CFR 192.03 requires DOE to implement groundwater monitoring during the post-closure period for the purpose of demonstrating that the disposal cell l
will perform in accordance with the design.
40 CFR 192.04 requires the implementation of a corrective action program if the.=nitoring shows an exceedance of concentration limits. The monitoring plan required under 40 CFR 192.03 should be designed to include verification of the site-specific assumptions used to project the disposal cell's performance.
Prevention of groundwater contamination may be assessed by indirect methods, such as measuring the moisture migration within various components of the cover (within or beneath the tailings), as well as direct groundwater monitoring.
DOE does not plan to monitor groundwater quality in the uppermost aquifer through the P0C wells,along the downgradient edges of the disposal cell. DOE proposes to monitor groundwater quality in selected performance monitoring wells downgradient of the disposal cell. Details of the monitoring program
)
will be provided in the LTSP for concurrence by NRC.
l DOE provided reasonable failure scenarios of the disposal unit and also i
provided a conceptual corrective action to address each failure scenario. DOE agrees to provide and implement a corrective action plan no later than 18 months after detecting an excursion.
j 5-10 MAYBELL fTER
,..~4 m,
)
.c The staff reviewed the ambient groundwater quality data for the Maybell site and agrees that P0C groundwater monitoring of the uppermost aquifer would not be necessary for protection of human health and the environment, given the limited use designation of the Browns Park aquifer.
Staff also reviewed the f
credible failure scenarios provided in the final RAP and agrees with the conceptual corrective actions proposed by DOE.
j 5.5 Cleanup and Control of Existing Contamination DOE must demonstrate compliance with the EPA standards listed in 40 CFR Part 192, Subparts B and C, for the cleanup and control of existing groundwater contamination.
DOE has demonstrated that the uppermost aquifer at the Maybell site is limited-use, due to widespread ambient contamination not associated with uranium mill tailings.
DOE proposes supplemental standards for groundwater cleanup in accordance with 40 CFR 192.22.
i DOE proposes no groundwater cleanup at the Maybell site.
This proposal is based on the following considerations:
characterization of the uppermost aquifer as " limited use," containing wide-spread ambient contamination not related to uranium milling activities; no current or projected future water use of the aquifer within a 4.8 km (3 mile) radius of the site; no apparent discharge of tailings-contaminated groundwater to surface-water bodies or deeper aquifers in the vicinity; and continued groundwater monitoring of the existing contamination to assure conditions remain unchenged.
DOE conducted a baseline risk assessment of the groundwater contamination associated with the Maybell site.
Staff reviewed the Maybell baseline risk assessment solely as supporting documentation for DOE's groundwater cleanup j
approach.
The baseline risk assessment concluded that no complete exposure pathways have been identified for current uses of contaminated groundwater because contaminants have not been trsnsported beyond the land area proposed for transfer to DOE under the long-term care provisions of 10 CFR 40.27. DOE recognizes that transient drainage associated with disposal cell closure may impact local groundwater and potentially cause contaminated groundwater to migrate beyond the long-term care boundary.
If this occurs, an exposure pathway may exist, witn the mcst likely receptor for exposure being livestock.
Staff concurs with DOE's proposal of no groundwater cleanup at the Maybell site, based on DOE's conclusions that exposure pathways outside the long-term care boundary will likely not exist.
DOE commits to continued groundwater monitoring to assure that post-closure site conditions do not change.
In accordance with EPA's requirements, the proposed supplemental standards would require only such management of contamination from tailings as necessary to prevent additional adverse impacts on human health and the environment. DOE's proposed monitoring of the tailings contamination comes as close to meeting the otherwise applicable standards as is reasonably achievable.
5.6 Conclusions DOE proposes no groundwater cleanup at the Maybe11 site. This proposal is based en DOE's characterization of the uppermost aquifer as limited use, containing wide-spread ambient contamination not related to uranium milling E-11 MAYBELL ffER
.s activities; no current or projected future water use of the aquifer within a 4.8 km (3 mile) radius of the site; no app 6 rent discharge of tailings contaminated groundwater to surface-water bodies or deeper aquifers in the vicinity; and continued groundwater monitoring of the existing contamination l
to assure conditions remain unchanged.
Based on its review of DOE's prcposal, the staff agrees with DOE's findings and concludes that DOE has demonstrated compliance with all groundwater protection provisions of 40 CFR 192, Subparts A through C.
1 5-12 MYBELL ffER
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l
,.5 6.0 RADON ATTENUATION AND SITE CLEANUP 6.1 Introduction This section of the TER documents the staff evaluation of the radon attenuation design and processing site cleanup for the planned remedial action at the Maybell, Colorado, UMTRA Project site.
The material characterization, radon barrier P
.;n, pronow mmMial action, and site verification plan were evaluated to assure compliance with the applicable EPA standards.
The review followed the NRC Standard Review Plan for UMTRCA Title I sites (NRC, 1993).
6.2 Radon Attenuation To meet the EPA standards for limiting release of radon (Rn-222) from residual radioactive materials to the atmosphere, contaminated material will be stabilized in place.
Windblown and waterborne contamination, along with mill yard and ore storage contamination, will be excavated and placed on the existing tailings pile.
DOE is proposing a multi-layer cover for the disposal cell consisting of the following layers:
radon barrier, frost protection, bedding, and riprap.
The 1.5-foot radon barrier layer will consist of clayey-sand material from the Rob Pit Overburden Pile amended with 10 percent bentonite by weight, and has been designed to limit the average release of 2
radon to meet the EPA standard of 20 pCi/m /s.
NRC staff review of the cover design for radon attenuation included evaluation of the pertinent design criteria for the contaminated materials and radon barrier soil, and a review of the specifications for materials placement.
The staff considered that the barrier layer is designed to satisfy criteria for construction, settlement, cracking, and infiltration of surface water, as well as reduction of radon gas release at the surface of the completed cell. Also, the parameters of the other layers of the cover were evaluated for their ability to protect the radon barrier layer from drying and disruption and the stability of the cell as a whole was assessed because of the potential for cracking of the barrier layer due to settlement or heaving.
TER Sections 3 and 4 provide discussion of the cell materials and cell design from the aspect of stability (subsidence, freeze-thaw damage, m asion, etc.).
as discussed below, NRC staff evaluated DOE's RAECOM computer code parameter input values that were used to calculate the radon barrier thickness required to meet the radon flux limit. The staff then performed an independent analysis of the design usir.g the RAD 0N code (NRC, 1989b), which is a modification of the PAECOM code.
6.2.1 Evaluation of Input Parameters The required thickness of the radon barrier depends on the characteristics (parameters) of the radon barrier soil (s) and the underlying contaminated materials. NRC staff evaluated the physical and radiological data for the contaminated materials and the radon barrier soils used for input into the RAECOM and RADON computer codes.
In some cases, conservative estimates instead of measured values were used for input, and in other cases 6-1 mAractt frER
,s measurements were made, although not always "nder design conditions.
NRC staff evaluated the justification and assumptions made to confirm that each input value was representative of the material, consistent with anticipated construction specifications, and that each input value was conservative or based on long-term conditions.
The sampling and testing methods for the a
materials were also reviewed to determine their appropriateness, and to ensure that the data was adequate.
The design parameters of the contaminated and radon barrier materials that were evaluated include:
long-term moisture content, bulk density, specific 4
l gravity, porosity, material layer thickness, and the radon diffusion coefficient, in addition, the radium (Ra-226) content and radon (Rn-222) emanation coefficients of the contaminated materials were evaluated.
A. Contaminated Materials The excavated contaminated material is to be compacted to 90 percent of maximum dry density at, or to 6 percent dry of, optimum moisture. The more contaminated material is to be placed on the pile first to minimize radon flux from the completed cell.
1 Physical characteristics of the existing pile tailings were determined from 4
i laboratory testing of representative samples.
The results of in-situ density l
tests averaged 1.20 gm/cc (75 pcf) and specific gravity tests averaged 2.76.
A porosity of 0.57 was then calculated for the tailings materials. These values are conservative for the radon model because the expected long-term condition is further compaction of this material (higher density and lower porosity), due to the added weight of the relocated material and the cover.
The corresponding properties for the off-pile materials (mill yard / ore storage), based on seven compacted samples, were measured to be:
density of 1.60 gm/cc (100 pcf), specific gravity of 2.64, and a calculated porosity of 0.39.
DOE assumed windblown (including waterborne) contamination had the same values.
This will be substantiated during construction.
DOE selected a conservative long-term moisture conten., value of 10 percent (by weight) for the existing pile tailings.
The average in-situ moisture content for 18 samples was 43.7 percent.
DOE also selected a conservative value of 6.0 percent for the off-pile and windblown contamination; the placement moisture will be about '.3.7 percent.
i al.
DOE calculated radon diffusion coefficients for the contaminated matep/s and ting diffusion coefficient for the pile tailings was 0.05 cm The resu]/s for windblown materials.
0.031 cm DOE assumed that off-pile material would have the same moisture saturation level as windblown material, and assigned the same diffusion coefficient value. DOE should confirm this assumption for the final radon flux analysis in the Completion Report (CR).
Testing indicated that the radon emanation property of the material at the Maybell processing site is independent of its moisture content and Ra-226 concentration.
Measurements on samples from four areas of the tailings pile resulted in an average emanation fraction of 0.30.
DOE reported nieasurements of radon emanation performed on the off-pile and windblown materials averaged 6-2 MAYBELL ffER
'O 4
o O 30 and 0.24, respectively.
However, DOE chose to use the conservative default value of 0.35 in the flux modt1 for both of these materials.
The radium content of the contaminated materials was determined primarily by gamma spectroscopy. Average Ra-226 concentrations were calculated by volume-weighting individual measured increments.
Average Ra-226 levels for the tailings pile, off-site, and windblown materials were 156, 168, and 36 pCi/g, i
respectively.
It is not clear if the samples from the pile were from tha upper 8 feet, which is the area of concern for this radon model. According to one summary (Table 8.5 of calculation 334-01), over a thousand samples were taken on the pile, 284 in the off-pile areas, and 76 in the windblown areas.
Also, Ra-226 concentrations were assumed (no measurements) for five areas on the site that total approximately 301,000 cubic yards (cy).
This lack of data is not considered an open issue because it is standard practice for DOE to measure Ra-226 levels after contaminated material is placed in the cell, in order to perform a final radon attenuation analysis based on as-built conditions.
The thicknesses of the various contaminated layers used in the radon flux analysis are:
original pile 65 ft (19.8 m), off-site material 5.25 ft (16 m),
j and windblown tailings 3 f t (9.2 m).
These contaminated material thicknesses are acceptable, based on estimated volumes in the final RAP of the various contaminated materials derived from analysis of borings and soil samples.
i B.
Radon Barrier The physical properties of the radon barrier materials were selected by DOE based primarily on the results of laboratory testing of samples from the borrow site, which is the Rob Pit Overburden Pile.
The specifications require the radon barrier soil to be compacted to 95 percent Standard Proctor density at, or to three percent wet of, optimum moisture.
For use in the radon barrier model, DOE used design parameters based on bentonite-amended (10 percent by weight) radon barrier material.
The measured specific gravity for the radon barrier soil was 2.66.
The 95 percent Proctor density (five tests) was 1.70 gm/cc (111.8 pcf), resulting in a calculated porosity of 0.32.
However, DOE's model input (calculation 326-01-00) and Remedial Action Selection (RAS) Report, Section 6 used the less conservative density and porosity values corresponding to 100 percent compaction.
The NRC staff does not consider this error significant because of the recently proposed radon barrier design change (reduction of bentonite content, Project Interface Document 14-S-08) that required additional barrier material testing and radon flux modeling.
Four capillary moisture saturation tests on radon barrier material yielded an average moisture content (long-term) of 15.8 percent by weight.
DOE also considered in-situ moisture values and chose the more conservative value of 13.7 percent for the long-term moisture content input for the flux model.
The radon diffusion coefficient for the bentonite-amended radon barrier erial, based on tests of five samples compacted to 100 percent, was 0.0006 ma}/s.The test data was obtained at the wrong compaction, resulting in an cm 6-3 MAYBELL ffER
,a i
i underestimated of the diffusion coefficient.
However, as stated above, new l
test data obtained at placement compaction was provided to support the proposed barrier design change.
The radon barrier soil (overburden material) Ra-226 concentration averaged 1.7 pCl/g, based on 7 samples.
The highest reported value was 7.9 pCi/g, and was excluded from the average without explanation (calculation 326-01-00).
In the draft RAP, 72 samples were said to average 2.1 pCi/g (excluded deep sample of 26 pCi/g). Only two samples were analyzed for Th-230, but both indicated 3.0 pCi/g (background is 1.3 pCi/g).
An emanation fraction of 0.3 was assumed for modeling purposes.
The EPA standard for radon flux only applies to residual radioactive material, but the radon emissions from the covering materials should be estimated as part of developing a remedial action plan.
Because the radon barrier / frost protection borrow site (Rob Pit Overburden Pile) is known to contain areas of elevated Ra-226, the NRC staff has requested that DOE provide Ra-226 test data
)
in the CR for both the radon barrier and frost protection layers.
The staff also notes that procedure RAC-0P-006 " Rob Pit Overburden Pile Excavation at the Maybell Site", Section 5.4 states that ao to 15 pCi/g Ra-226 material may be used for radon barrier except the top 13 cm of the frost protection material, which shall contain less than 5 pC1/g.
The procedure does not indicate if these values include background.
In any case, these limiting values are high relative to the stated background Ra-226 value (1.4 pCi/g) and the value used in the flux model.
It is expected that DOE will use the average measured Ra-226 value for the radon barrier and frost protection layers in the final flux analysis, and will be verified by NRC staff in the CR.
The thickness of the radon barrier layer was set at 1.5 feet due to construction considerations.
This thickness is acceptable as it appears to provide sufficient radon attenuation as discussed below in the analysis of the radon attenuation model.
C.
Ambient Radon The ambient air radon concentration is another required parameter value for the RAECOM model, and has been measured in the Maybell area. The average value is 0.7 pCi/1. A value of 2.0 pCi/l was used in the DOE model, but this difference has an insignificant effect on the estimated radon flux.
D.
Conclusions on Model Parameters
]
NRC staff has reviewed the physical and radiological parameter values assigned to the contaminated and radon barrier materials by DOE, and concludes that most are a reasonable representation of expected conditions. Although some parameter values are based on a limited amount of testing or estimates, they can be considered acceptable for the radon barrier design.
DOE indicated in the RAS Report that the final cover design will be based, in part, on parameter values measured under standard procedures during construction.
The final flux analysis will be provided for review as part of the CR.
6-4 maysttt fita i
3 1
6.2.2 Evaluation of Radon Attenuation Model 1
l DOE used the RAECOM computer code to evaluate the radon attenuatip/s and n of the i
cover for compliance with the EPA radon flux standard of 20 pCi/m presented various RAECOM analyses in several calculations (358-01, 344-02).
Use of the mean parameter values for input (cciculation 326-02-00),
demonstrates that a radon barrier 2 inches (5 cm) thick would just meet the i
radon flux criterion.
Since DOE has stated that the barrier layer will be i
18 inches (46 on thick, inese o an ample aargin of safety in the design thickness.
1 j
NRC staff determined the radon flux for the 18-inch barrier layer using DOE's input values, except where the staff applied DOE's calculated standard error 4
i of the mean to the Ra-226 concentration and the emang/s for pile and 0.032 for tion fraction, and used the code-calculated diffusion coefficients (0.043 cm the off-pile and windblown contamination).
The resulting flux for this
^
2 reasonably conservative model was 9.7 pCi/m /s.
DOE did not model the radon barrier for frost damage. According to i
Calculation MAY-398-03, frost could penetrate two inches of the radon barrier on top of the cell.
Frost penetration would not be a problem on the sides of i
the cell where the riprap is four inches thicker.
NRC staff modeled four I
4 inches of frost (freeze-thaw) damaged barrier, with associated parameter changes, and used the calculated, cp/s.
nservative diffusion coefficients. The i
estimated radon flux was 11.9 pCi/m Therefore, the increased permeability 4
}
due to frost penetration would not cause the radon flux to exceed the j
standard.
i i
Based on review of the design and analyses presented in the final RAP and i
associated documents, NRC staff concludes that the radon attenuation model 4
supports the radon barrier design, but that model input should be substantiated by complete documentation and by further testing and radon flux analysis during material placement, as DOE has indicated will be done. This will be verified by the NRC staff in its review of DOE's CR.
6.3 Site Cleanup 6.3.1 Radiological Site Characterization a
i Field sampling and radiological surveys at the Maybell site identified l
approximately 3.5 million cy of contaminated materials (includes 2.8 million j
cy in the existing pile). Calculation 377-01 estimates the volumes of i
excavated materials to be:
244,148 cy around the pile, 240,246 cy windblown, and C,338 cy waterborne. Debris will acccunt for about another 20,000 cy and i
vicinity property material was not mentioned as a separate category.
Background levels of Ra-226 were measured in the Maybell area and the average value is 1.3 pCi/g.
This value appears unreasonably low for the immediate surrounding area because Maybell was formerly an open pit mine, and several other uranium mine pits are near-by.
This would indicate that areas i
containing elevated levels of naturally occuring uranium chain radionuclides
]
(eg., Ra-226 and Th-230) may exist at or near tne surface.
Since the UMTPA
)
6-5 wanL fmt i
4 s
4 l
1
,A I
Project is tasked to remediate only residual radioactive material, DOE committed to investigate the situation and aistinguish naturally occurring radioactive material from tailings contamination at the site.
DOE has characterized Th-230 contamination on the processing site and found elevated levels in tributaries of Johnson Wash.
However, an Oak Ridge National Laboratory Radiological Survey (ORNL,1980) mentions a settling pond i
that had been covered This pond was not identified on the DOE site characterization maps.
Since Th-230 could have leached below the pond (assuming it contained raffinate solution), samples for Th-230 analysis should be taken in this area and the data should be presented in the CR.
6.3.2 Cleanup Standards DOE has committed to excavate contaminated areas to meet the 5 pCi/g (surface), and 15 pCi/g (subsurface) plus background EPA standard for Ra-226 in soil, and to place the contaminated materials in an engineered disposal cell.
DOE states that for Th-230 contamination, supplemental standards will be based on the NRC-approved generic thorium cleanup / verification policy.
The cleanup standard for Th-230 will insure that 15-cm-thick layer:, averaged over an area of 100 square meters, will meet the EPA Ra-226 criteria for 1000 years.
Excavation will be monitored to ensure that cleanup efforts are complete.
j There are no buildings or equipment remaining to be decontaminated at the processing site. Demolition debris is stored on the site and will be buried in the disposal cell.
Johnson Wash and a section of Lay Creek are contaminated from past accidental and routine releases of liquid effluents.
DOE proposes to apply supplemental standards to these areas (mostly on vicinity properties), based on environmental harm to riparian wetlands, cost, detrimental effects on geomorphic stability of Johnson Wash, low radon flux, and low predicted effects from human consumption of area livestock. DOE has provided data on this contamination and committed to provide the appli ation for supplemental standards for these vicinity properties as soon as possible.
6.3.3 Verification The final radiological verification survey for land cleanup will be based on 100-square-meter areas.
DOE may use a variety of measurement techniques, depending on particular circumstances.
The standard method for Ra-226 verification is analysis of composite soil samples, by gamma spectrometry.
Verification for Th-230 will follow the generic thorium policy.
Areas known i
or suspected of.containing elevated levels of Th-230 below the depth of Ra-226 remediation will have 100 percent of the grids analyzed. Windblown areas will not be analyzed for Th-230, and other areas will have 10 percent of the grids verified for Th-230.
6-6 MYBELL ffEK
.,s No on-site structures at the processing site will require radiological verification since all structures have been demolished and the debris will be buried in the disposal cell.
6.4 Conclusions Based on review of the radon attenuation design of the Maybell remedial action plan, as presented in the final RAP and supporting documents, NRC staff finds that there is adequate, information to support the conclusion that the raaon barrier, as designed, will meet the long-term radon flux limit as required by 40 CFR 192.02.
Also, the staff finds the soil verification plan acceptable, and the proposed processing site cleanup plan should result in the site meeting the EPA standards contained in 40 CFR 192.12.
As a result of discussions and correspondence with DOE, the NRC staff expects DOE to provide Ra-226 test data in the CR for both the radon barrier and frost protection layers, and to use the average measured Ra-226 value for these layers, along with other parameter data acquired during construction, in the final (as-built) radon flux analysis.
Also, additional Th-230 data is expected in the CR (see TER Issue 17, Section 1.5), particularly from the settling pond, if the area was located.
In conclusion, DOE is expected to provide the application for supplemental standards for the vicinity properties associated with Johnson Wash and Lay Creek as soon as possible.
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