Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology (1995)

The Low-Level Radioactive Waste Policy Act (1980, amended 1985) legislates state responsibility for non-government low-level radioactive waste generated within states. California, Arizona, North Dakota, and South Dakota formed the Southwest Compact to share a disposal facility for these wastes. The Ward Valley site west of Needles, California, was investigated and proposed for the first facility to serve the compact.

As the siting and licensing process was ending, the U.S. Department of the Interior (DOI) was asked by the State of California to transfer the site lands, presently held by the Bureau of Land Management, an agency of DOI, to California for site development. While DOI was considering the land transfer, three geologists from the U.S. Geological Survey (USGS) expressed seven concerns about the site and its evaluation in a memorandum to the Secretary of the Interior, Bruce Babbitt. Although Howard Wilshire, Keith Howard, and David Miller (referred to as the Wilshire group in this report) acted as individuals rather than in official USGS capacities, the DOI asked the National Research Council (NRC) to convene a committee to evaluate their seven technical concerns prior to the DOI decision on the land transfer.

The seven issues, as originally stated in the Wilshire group's memorandum, are:

1. Potential infiltration of the repository trenches by shallow subsurface water flow.¹

2. Transfer of contaminants through the unsaturated zone and potential for contamination of ground water.

3. Potential for hydrologic connection between the site and the Colorado River.

4. No plans are revealed for .monitoring ground water or the unsaturated zone downgradient from the site.

5. Engineered flood control devices like those proposed have failed in past decades at numerous locations across the Mojave Desert.

6. Alluvium and colluvium derived from Cretaceous granite appears to make a very high quality tortoise habitat. Sacrifice of such habitat cannot be physically compensated.

7. Misconceptions about revegetation enhancement may interfere with successful reestablishment of the native community.

The committee's charge (see Appendix A of this report for details) was to evaluate the validity of these seven issues.

It should be noted that the committee was not asked to and did not take any position on the overall suitability of the Ward Valley site for a LLRW disposal facility. Although the seven concerns raised by the Wilshire group relate to site suitability, this evaluation of the technical validity of these concerns does not constitute approval or disapproval of the site.

The committee offers the following summary of the major conclusions to the seven technical issues related to the Ward Valley site that it was asked to review, with a cautionary note: first, as noted in the footnote on page 1 of Chapter 3, Issues 1 and 2 of the Wilshire group have been reversed in order, so that the committee's Issue 1, the potential for transfer of contaminants through the unsaturated zone, is the Wilshire group's Issue 2 and vice-versa; secondly, these conclusions should not be read without, nor taken out of context of, the discussions that describe the bases for the conclusions, the limitations of the data, and the levels of uncertainty which may accompany some of these conclusions. These are summarized briefly in this Executive Summary and extensively discussed in the body of the report.

ISSUE 1 (Issue 2 of the Wilshire group): GENERAL CONCLUSION: The committee concludes from multiple lines of evidence that the unsaturated zone at the Ward Valley site is very dry, and that recharge or potential transfer of contaminants through the unsaturated zone to the water table, as proposed by the Wilshire group, is highly unlikely. However, because of the limitations of the data, the committee recommends specific initial baseline and subsequent monitoring measurements, summarized on page 10 of this Executive Summary, to enhance the data base for monitoring the complex unsaturated zone.²

The Wilshire group divided the evaluation in the license application of the nature of water movement in the unsaturated zone into five subissues that dealt with (1) the adequacy of the treatment of the unsaturated zone variability and complexity; (2) the possibility of rapid water migration down preferential pathways; (3) the tritium measurements at 30 meter depth

suggesting rapid vertical water transport; (4) possible recharge to the ground water below the major drainage, Homer Wash; and (5) a possible interpretation of stable and radioactive isotopes in the ground water suggesting recent recharge to the ground water.

• With respect to the subissues, two unresolved data sets remain: the observed vertical hydraulic gradient between two monitoring wells, relating to subissue (2) and the presence of tritium in the unsaturated zone (subissue (3)).³The majority of the committee considers that subissue (1), unsaturated zone variability and complexity, has been adequately addressed in the modeling and analyses of the unsaturated zone, with the exception of the modeling of a complete cover failure; subissue (2), the presence of preferential pathways, is not supported by any consistent evidence for rapid downward water migration or ground water recharge below the site, despite arguments to the contrary³, as discussed later in this Executive Summary; subissue (4), recharge below Homer Wash, is likely but will have no consequences for the containment of contaminants because of the distance of the wash from, and its elevation below, the site and the waste trenches as presently designed; and subissue (5), recent recharge to the ground water below the site, is not supported by the solute concentrations in the ground-water chemistry.

  With respect to subissue (2), monitoring wells WV-MW-01 and WV-MW-02 show an apparent downward hydraulic gradient that could be caused by local recharge, an explanation that is inconsistent with most other data. Any deviations from the vertical in the boreholes could lead to erroneous depth measurements because the measurement of depth would be the distance down the borehole to the water table rather than the actual depth.

• The committee finds that the cause(s) of the observed vertical gradient in the saturated zone cannot be conclusively determined with the available data. The most probable sources of the apparent gradient are measurement and drilling errors. Detailed discussion of this issue and conclusion can be found in Chapter 3 in the section on ground water gradients.

• Regarding subissue (3), the committee finds that the conclusion in the license application that gas diffusion is responsible for the tritium reported in the unsaturated zone is conceptually incorrect. The committee concludes that inappropriate sampling procedures most probably introduced atmospheric tritium into the samples. Except for three data points at depths of 5.1 m and 5.4 m, the tritium data from deeper levels (11-30 m) are not distinguishable from zero owing to inadequate evaluation of the uncertainty of the tritium values resulting from the sample-collection procedure. The three results from the uppermost sampling depths may represent atmospheric contamination, or they may indicate small amounts of shallow infiltration. Due to these uncertainties, the tritium data are not adequate for the evaluation of infiltration. The

committee recommends that further analysis and sampling be conducted to resolve this subissue. Detailed discussion of this issue is in Chapter 3 in the section on environmental tracers and in the conclusions.

The committee reviewed multiple lines of evidence to evaluate water flux in the unsaturated zone at the Ward Valley site. We based our conclusion that the unsaturated zone is very dry in part on the following information: (1) In 82 samples from near the surface to 27 m depth, water contents were generally very low (94 percent of the samples had water contents less than 10 percent, and 6 percent of the samples had water contents between 10 and 15 percent); (2) Water content monitored in a neutron probe access tube installed to 6-m depth showed that the maximum depth of penetration of water after rainfall was about one meter; (3) Water potentials monitored to 30-m depth were very low (-3 to -6 MPa); (4) Chloride concentrations measured in three boreholes to 30-m depth were very high (up to 15g/1), suggesting little infiltration and downward percolation of water since accumulation of the chloride. The time required to accumulate these large quantities of chloride to 30-m depth was calculated to be approximately 50,000 yr; (5) Estimated water fluxes based on chloride data were very low (0.03 to 0.05 mm/yr below 10-m depth).

The above discussion is based on natural conditions. Of course, the facility itself must be designed so as not to alter this naturally low recharge. Additional discussion and recommendations on this point can be found in Chapter 7 dealing with the Wilshire group's Issue #5.

The committee notes that monitoring hydraulic parameters in dry soils like those at the Ward Valley site is very difficult and may be one of the causes of the limitations in collecting field data. Limitations of field data during site characterization are grouped into three classes: (1) restrictions imposed by the extremely low water fluxes, which can cause difficulties, for example, in resolving rate and direction of water movement and in collecting adequate water samples for tritium analysis; (2) limitations of the monitoring equipment in arid unsaturated zones, because of the lack of methods, procedures, and reliable instruments to measure precisely the hydraulic and hydrochemical parameters used to estimate water flux in dry desert soils, and because some of the instruments used for the site studies have a high failure rate; and (3) limitation in the quantity and quality of the data whereby the number and distribution of observations and quantity of data collected were restricted. Specifically these include errors in installation and operation of the unsaturated zone monitoring equipment, inconsistencies and errors in the methodologies, analysis, and presentation of data in the license application, and/or project decisions on where, how often, and how deep to test. Detailed discussion of the limitations and inconsistencies of some of the data can be found in

Chapter 3, especially in the sections on the nature, direction and magnitude of water flux, environmental tracers, and evaluation of recharge at Ward Valley.

• In the opinion of the committee, thick unsaturated alluvial sediments in arid environments such as that found at the Ward Valley site generally represent a favorable hydrologic environment for the isolation of low-level radioactive waste, because of the very small amount of water and very slow rate of water movement throughout most of these unsaturated zones. However, the committee attributes some of the incomplete and/or unreliable data sets that it reviewed to the fact that hydrologic processes in arid regions are characterized by extreme events which do not follow a one-year calendar. For this reason, regulatory and/or budgetary guidelines that permit one-year characterization periods, or other short-duration time frames not suitable for arid-soil characterization, can easily lead to incomplete or ambiguous results. In the committee's opinion, characterization activities should receive priority over arbitrary regulatory timetables, or short time-frame budgetary constraints, particularly in arid regions.

• To guard against deficiencies in characterization and monitoring efforts, and as more emphasis is placed on arid regions for waste disposal, the committee recommends that an independent scientific peer review committee be established to provide oversight early in the permitting process, to assess and suggest improvements in the site characterization plans and monitoring investigations, and to guide the interpretation of the long-term monitoring data. In this way, conflicts in, and other concerns with, characterization data and observations from the unsaturated zone can be resolved as they arise. This recommendation is also discussed later with reference to monitoring.

• As water content and water potential monitoring, tritium analyses, and ground-water levels are proposed for operational and post-closure monitoring, the committee recommends several actions to establish base levels for monitoring, including additional testing for tritium, sampling for chlorine-36 (³⁶CI) to help resolve the reported tritium found in the unsaturated zone, drilling and sampling of the unsaturated zone from well below the current characterization depth of 30 meters, and further investigation of the apparent vertical hydraulic gradient found between monitoring wells WV-MW-01 and WV-MW-02. Details of these and other recommendations can be found at the end of Chapter 3 and in Chapter 6 on monitoring during operations and post-closure.

ISSUE 2 (Issue 1 of the Wilshire group): The committee concludes that shallow subsurface (lateral) flow, as proposed by the Wilshire group, is not a significant issue at the Ward Valley site, because under low-water fluxes (1) the soil carbonate, thought to be a low-permeability horizon causing local ponding, or perching of surface water, is sufficiently permeable to allow water to move predominantly downward, and (2) calculations show that, with a two-percent slope of the layering and soil horizons, lateral flow into the trenches would be insignificant even under a worst-case scenario.

Lateral flow under natural conditions in arid soils depends on several factors. Among them are (1) lateral continuity of a perching (or low-permeability) horizon, (2) the relative permeabilities of the soil horizons, and (3) the slope of the less permeable layer.

Although the limited information available for Ward Valley suggests that some of the shallow subsurface carbonate or ''calcrete'' horizons are laterally continuous, and less permeable than the surrounding soil, studies from other arid regions and incomplete experimental data from Ward Valley indicate that the permeability of the calcrete is high enough to allow downward movement of water under conditions of low-water content and potentials, and extremely low-water fluxes.

Moreover, at the Ward Valley site, both ancient buried surfaces and the modem surface of the alluvial fan have a slope of only about 2 percent, which Darcy flow calculations indicate is too low to allow significant lateral flow in the unsaturated zone. Under low water-content conditions, the very small downslope gravity component of subsurface flow is negligible compared to the downward diffusion component.

• The committee strongly recommends that conditions that could cause local lateral flow, such as ponding and enhanced percolation through runoff-control structures, be avoided in and immediately surrounding the trenches.

ISSUE 3: While there are conceivable, but unlikely, flowpaths for some ground water within Ward Valley to reach the Colorado River, the committee concludes from conservative bounding calculations that, even if all 10 curies (Ci) of plutonium-239 expected in the facility were to reach the river, the potential impacts on the river water

quality would be insignificant relative to present natural levels of radionuclides in the river and to accepted regulatory health standards.

• Based on limited hydrologic and geologic data and the topographic conditions, in the committee's view, the major part of the ground-water flow beneath the proposed site in Ward Valley appears to discharge at Danby Dry Lake. It cannot be ruled out, however, that some portion of the ground water passing beneath the proposed site may leave the Ward Valley basin.

• Although, in the committee's evaluation of the pathways, four of the five postulated routes appeared to be possible, the committee judged that it would not be possible under any reasonable expectation for site characterization to either confirm, or eliminate with absolute certainty, any of the regional bedrock pathways postulated by the Wilshire group.

Assuming that a ground-water pathway is possible, the committee assessed the potential impact of some concentration of long-lived radionuclides reaching and entering the Colorado River. For the bounding calculation, the committee used a total inventory of about 10 Ci of plutonium-239 (²³⁹Pu) emplaced in the disposal site over the 30-year life of the facility, equivalent to 0.33 Ci per year. This order of magnitude was agreed upon by the Nuclear Regulatory Commission and the Congressional Research Service in separate analyses and is less than that proposed by opponents of the site by two orders of magnitude. Radionuclide composition of the waste is certified by the waste generators and will be monitored by the California Department of Health Services and the site operator. DHS has enforcement authority over radionuclide quantity restrictions.

With some overly conservative, non-credible assumptions, and an assumed plutonium release of 0.33 Ci per year, (which would be a release of all the estimated plutonium to be received over the life of the site) all of which is assumed to reach the river through the ground water, the calculations showed that the total annual concentration of alpha-emitting radionuclides from Ward Valley ground water that could be added to the Colorado River would be equivalent to a concentration of 0.07 picocuries (pCi) per liter (1). The reported concentration of alpha-emitting radionuclides being transported annually by the river is 44 Ci, equivalent to an average concentration of 4.4 pCi/1, based on recent river flow rates, which are much higher than the future flow rate assumed for the above plutonium calculation. The committee concludes that the addition of 0.33 Ci per year would be insignificant compared to the natural alpha load of 44 Ci per year carried by the Colorado River. The addition of 0.07 pCi/l from waste plutonium to the existing load in the river of 4.4 pCi/l would result in a total load of 4.47 pCi/1, well below the health-based regulatory standard of 15 pCi/l for gross alpha-emitting radionuclides. Detailed discussion of the

pathways, and of the calculations and assumptions used, can be found in Chapter 5 of this report.

ISSUE 4: With respect to the performance monitoring of the unsaturated zone and compliance monitoring of the ground water, the committee concludes that the Wilshire group's concerns for the absence of such plans are not borne out, as the administrative record provides definite plans for post-closure monitoring downgradient in the unsaturated zone beneath the trenches and at the water table at the site boundary. However, although remediation plans are described for ground water contamination, none are described in the revised plan for the unsaturated zone.

Two basic types of monitoring are proposed for Ward Valley: (1) regulatory compliance monitoring of the ground water to assure that contaminant releases do not exceed regulatory levels at the disposal system boundaries and (2) performance monitoring of the unsaturated zone to provide an early warning of releases that may exceed regulatory levels. The compliance boundaries are the air, vegetation, and water table at the edge of the buffer zone. Monitoring in the unsaturated zone, which constitutes performance monitoring because the unsaturated zone is not a regulatory compliance boundary, is critical because this is the primary natural barrier to radionuclide migration. The data provided will be essential in evaluating the performance of the Ward Valley facility and will be compared with the results of performance assessment models of the site. Monitoring beneath the trenches for changes in water content and presence of radionuclides in the gas and liquid phases constitutes downgradient monitoring in terms of the unsaturated zone, thus responding to the Wilshire group's concern.

Although traditionally, site characterization, monitoring, and performance assessment have each been conducted independently, various agencies involved in waste disposal, such as the Nuclear Regulatory Commission (US NRC) and the Department of Energy (DOE), increasingly recognize that these three activities should be integrated. More emphasis should be placed on continued site characterization in the proposed operational monitoring programs because reliance is ultimately placed on the natural system as the primary barrier to contain the waste. In the opinion of the committee, monitoring and performance assessment should

be integrated with continued site characterization during operation of the Ward Valley site.

• The committee considers that the proposed spacing of monitoring points along the perimeter of the radiological control area may not be adequate and recommends the installation of additional monitoring wells, including wells to examine changes in fluid potential and water quality with depth. In the opinion of the committee, the addition of these wells will provide sufficient monitoring points for ground water, and that no downgradient, offsite wells are required.

• Although fundamental site characterization data are collected prior to the license application, it is the committee's opinion that site characterization should be continued through the operational phase.

• As federal regulations provide investigation and action levels for compliance monitoring, but not for performance monitoring in the unsaturated zone, the committee strongly recommends developing and documenting investigation and action levels for monitoring in the unsaturated zone. In the unlikely event of contaminant movement through the unsaturated zone, it is not prudent to wait for contaminants to reach the compliance boundary before investigating the contaminant movement and developing an action plan.

• Unsaturated-zone hydrology is a relatively young science, and new technologies are continually being developed. In view of the complexities in monitoring unsaturated zones in arid systems and the poor quality of the monitoring data collected for the license application, the committee recommends that future monitoring be directed and overseen by a peer-review advisory panel. It should include experts especially knowledgeable in the state-of-the-art of unsaturated-zone hydrology, soil physics, arid zone water-balance modeling, performance assessments, and ground-water hydrology in arid regions. These experts would also assist in reviewing the monitoring data and in recommending ways of rectifying any problems that may arise. In addition, the peer-review advisory panel should be involved in evaluating iterative processes of site characterization, monitoring, and performance assessment. Continuing scientific peer review would build credibility and public confidence in the monitoring program.

• The committee recommends that each of the southern and eastern perimeters of the radiological control area have no fewer than four monitoring wells, inclusive of corner monitoring locations (i.e. a total of eight monitoring wells). In addition, to establish better background databases, the western and northern perimeters should be equipped with no fewer than three monitoring points (i.e. a total of three background wells).

Additional concerns and recommendations with respect to the monitoring plan can be found in Chapter 6 in the sections on performance monitoring and compliance monitoring.

ISSUE 5: The committee concludes that the proposed flood protection barrier (berm), which is designed to surround and shield the waste site from flooding and erosion, appears to be effectively engineered with thick stone (rip rap) and gravel (filter) layers to protect the trenches and cover from a rare, desert surface runoff flood event, such as the probable maximum flood (PMF) which is often associated with return periods ranging from 1,000 to 1 million years. Furthermore, any postulated formation of channels by storm water runoff toward the site and resultant scouting around the upstream comers of the flood protection berm appear to be adequately addressed by the tip-rap design above and below ground surface level.

• The proposed system of shallow ridges (or flow break-up hems) to be built of natural site material and placed upslope from the waste facility site in a chevron pattern, to provide roughness to reduce the velocity of water coming off the fan toward the site, will likely be eroded and breached over a period of decades but will probably continue to function to provide the desired flow resistance for several additional decades, and will have no impact on the stability of the site.

• In the committee's opinion, concerns over possible floodwater ponding along the upstream edge of the flood protection berm and possible water seepage through the berm and into the trench area can be effectively ameliorated through easily-engineered defensive measures.

Offsite storm water will be prevented from entering the trench area during operation and after closure by a permanent flood-protection berm surrounding the disposal site. The berm, to be built when site construction begins and incorporated into the final site cover at

closure, is designed to withstand the probable maximum flood (PMF) and also to divert flow around the north and south sides of the facility during operations and after closure. Embankment armoring, consisting of a layer of stone rip rap (0.9 m thick) and filter base of gravel (46 cm thick), is proposed to stabilize the surface against wind and water erosion. The outer embankment armoring system is to be extended to a depth of 1.5 m into the subsurface to provide some scouting protection from adjacent surface water flow.

A series of shallow, flow breakup berms will be placed in a staggered, offset chevron pattern upslope and west of the disposal facility primarily (1) to create sheet flow roughness to decrease sheet flow velocity near the permanent primary flood control berm and thereby reduce scour potential and (2) to divert storm runoff to the north and south of the facility. These breakup berms, however, are meant to be temporary and will be constructed with materials removed from the trenches and maintained during the operations and institutional control periods. Although these berms are temporary, and are likely to erode and breach over a few decades, they will remain in some form to offer resistance to sheet flow for many decades after closure.

• To eliminate the possibility of ponding along the upstream edge of the flood protection berm and to reduce the possibility of infiltration and leakage into the adjacent trench, the committee recommends an engineered sloped and lined channel for conveying storm water around the west, north, and south sides and corners of the flood protection berm. A lined channel/berm would also reduce non-flood event rainfall infiltration and seepage into the trench area.

• The committee recommends developing a long-term monitoring plan for detecting significant differential settlement of the trench-cover area and a response program for mitigating its potential negative effect(s) on surface drainage and floods. This plan also should include a comprehensive, operational and long-term flood and erosion-facility monitoring and response program for identifying, repairing, or mitigating any stability problems which develop.

Additional Detailed Discussion and Recommendations Can Be Found in Chapter 7.

ISSUE 6: The committee has two primary concerns about potential effects of the proposed facility on desert tortoise habitat: (1) limited habitat degradation and fragmentation associated with development of the facility, and (2) the unknown

consequences of the relocation plan. The committee concludes that the parts of the mitigation plan dealing with potential growth of predator populations and increased tortoise/human interactions are likely to minimize these adverse effects, but the plan to relocate tortoises may be detrimental to the tortoise population in the vicinity of the site.

The site of the proposed facility is in a section of the Mojave Desert that contains one of the largest and most robust desert tortoise populations and is considered to be a vital area for recovery of the desert tortoise. The proposal for the Ward Valley site comprises several approaches for mitigating adverse effects of site construction and operation on the local desert tortoise population. These were initially based on the Bureau of Land Management (BLM) management plan for desert tortoise habitat, and include compensation for lost habitat, reduction of negative impacts on tortoises during facility construction and operation, and research to improve the understanding of desert tortoise ecology. These were also included as Reasonable and Prudent Measures in the USFWS November 1990 Biological Opinion.

First, the proposed facility in Ward Valley will result in the direct loss of 36 hectares (ha), disturbance of additional area by road widening and establishment of monitoring equipment, and fragmentation of desert tortoise habitat within the Desert Wildlife Management Area (DWMA) considered by the Recovery Plan to contain the largest and most robust of the remaining desert tortoise populations. While the number of tortoises affected and the habitat area lost are small compared to the whole Ward Valley area, loss of habitat through fencing and road improvement must still be considered habitat fragmentation, a condition that should be avoided according to the US Fish and Wildlife Service (USFWS) 1990 Biological Opinion.

The licensee intends to compensate for lost tortoise habitat with a two-part plan: (1) fencing Interstate Highway I-40 and upgrading freeway underpasses to improve habitat currently supporting a low density of tortoises and to facilitate movement throughout Ward Valley; (2) relocating tortoises displaced during site construction into the protected habitat north of I-40 created through fencing along the highway. Approximately 23-30 tortoises will be moved as part of the relocation effort.

• In the committee's opinion, although the relocation site will have geology and soils similar to the waste facility site, the committee sees several problems associated with the relocation plan, some of which are: (1) previous desert tortoise relocation studies have shown only limited success, (2) according to the guidelines for tortoise translocation in the USFWS Desert Tortoise (Mojave Population) Recovery Plan, displaced tortoises should not be released in DWMAs until relocation is much better understood, and (3) the relocation plan could facilitate the transmission of disease from tortoises in the Fenner DWMA to individuals in the Chemehuevi DWMA.

• The committee recommends that the relocation plan be reevaluated in the light of the Desert Tortoise (Mojave Population) Recovery Plan and the paucity of data on successful tortoise relocations.

• The committee supports the recommendation of the U.S. Fish and Wildlife Service to establish a research program designed to study the effects of tortoise relocation and further recommends that relocation be made only outside DWMAs.

• As a possible alternative for the relocation plan, the committee suggests that consideration be again given to (1) evaluating impacts on the adjacent tortoise population of a plan that would exclude, but not relocate, resident individuals from all locations of facility construction and operation activities, or (2) consultation with the U.S. Fish and Wildlife Service about designating all individuals lost during construction as "incidental take".

• The committee recommends that formal consultation with the U. S. Fish and Wildlife Service on the low-level radioactive waste disposal site at Ward Valley be reinitiated, which is required by the Endangered Species Act if critical habitat is designated which may affect a prior biological opinion.

Detailed discussion of the plans and additional problems identified with the relocation plan, and further recommendations, can be found in Chapter 8 of this report in the sections on the assessment of the plan, conclusions, and recommendations.

ISSUE 7: In the opinion of the committee, the guidelines presented as part of the revegetation plan have been developed with an understanding of desert plant ecology, and do not reveal any "misconceptions about revegetation enhancements", as charged by the Wilshire group.⁴

Although no active revegetation program is presently in place, the revegetation plan proposal calls for establishing a comprehensive revegetation program. This program will have three phases: (1) transplanting cacti and yuccas during construction, (2) revegetation of caps of completed trenches during operations, and (3) restoration of the entire site after closure. Qualified biologists will be invited to participate on an ad hoc committee to help develop revegetation procedures and criteria for evaluating success of revegetation efforts.

Because the trench cover will be elevated above the surrounding terrain, a moisture gradient will result from the upper end of the cover (caps) receiving only incident precipitation while the lower end receives some runon from the upper end. This in turn is likely to produce a vegetation cover gradient with the upper end of the trench caps having sparser plant cover and possibly less robust plants than the lower end.

If properly planned and fully restored according to established guidelines and expert input, the vegetation cover gradient should not cause a problem relative to soil erosion because the upper end of the trench cap will receive only rainfall and thus will not be impacted by surface flow erosion. At the lower end of the cover, and in the troughs between, the increased runoff is expected to be compensated by an increased vegetation cover.

• The committee recommends that, although moisture and vegetational gradients of the raised trench caps are expected, the revegetation program include from the start plantings of native plants designed to produce densities and cover equivalent to that expected in the high density areas, that is, equivalent to the natural desert plant distribution.

The committee emphasizes the need for continued monitoring of the revegetated areas as part of the long-term monitoring program.