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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology 1 INTRODUCTION In June, 1993, the Secretary of the Interior, Bruce Babbitt, received a memorandum from three United States Geological Survey (USGS) geologists. The geologists, Howard Wilshire, Keith Howard, and David Miller, expressed concern that, in their professional judgment, the site evaluation studies for the Ward Valley, California, proposed low-level radioactive waste (LLRW) facility in the eastern Mojave Desert were inadequate to determine the suitability of the site to isolate the waste and left several critical issues unresolved (Wilshire et al, 1993a). The site studies were done by a contractor to the California Department of Health Services (DHS), the state agency responsible for the licensing and regulation of the LLRW facility, and were accepted by the DHS. A license was issued to U.S. Ecology in September 1993, that has been set aside by the court pending the resolution of several legal challenges. The USGS disclaimed any role related to the issues raised by the geologists (who will be referred to throughout this report as the Wilshire group) and, moreover, had not participated officially in the site evaluation. In the view of the USGS, the Wilshire group was therefore speaking as individuals and not in any official capacity as USGS employees (USGS, 1993). Nevertheless, opponents of the Ward Valley site urged that the Wilshire group's views be given consideration because they were qualified earth scientists. The Wilshire group subsequently elaborated in a more detailed report on the seven issues that they had briefly summarized in the June memorandum (Wilshire et al, 1993b). The DHS and their contractor for development of the site, U.S. Ecology (USE), replied that the Wilshire group's arguments lacked scientific merit and that all of the issues they raised had been adequately addressed during the entire 2-year siting process (Romano, 1993). The controversy led the Department of the Interior (DOI), the department under which the USGS operates, to request that the National Academy of Sciences' National Research Council (NAS/NRC) convene a committee of experts to evaluate the seven issues raised by the USGS geologists (see Appendix A). This report is the product of that committee's review. Department of Interior Involvement Although the responsibility for storage, disposal, and management of LLKW has been assigned to the states by federal law, the Department of Interior became involved because the Bureau of Land Management (BLM), an agency of the DOI, owns the land on which the site is located. Title of the land must be transferred or sold to the state of California before the site can be developed as a LLRW disposal facility. The DOI wanted the results of the NAS/NRC review, along with other considerations, before making a decision on the transfer.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology FEDERAL LOW-LEVEL RADIOACTIVE WASTE POLICY ACT The effort of the state of California to site a LLRW disposal facility, as with other states across the country, has its origins in the Low-Level Radioactive Waste Policy Act of 1980. This bill gave to the states the responsibility of disposing and managing their commercial LLRW. The 1985 amendment to that act set milestones and incentives for developing such facilities, with penalties if progress and goals were not achieved. The need for LLRW legislation arose when the last three LLRW disposal facilities operating for commercial wastes decided in the late 1970's that they would no longer continue to accept radioactive waste from the entire country. Hanford, Washington and Barnwell, South Carolina experienced difficulties with corrosion and leaks of waste packages before the 1980 federal act and subsequent regulations governed such activity. As the number of on-line nuclear power plants was increasing, and the use of radioisotopes in medical research and treatment kept growing, the need for disposal capacity for the wastes resulting from these activities, and the need for regulations to protect the health and safety of the public, became more pressing. Nevada closed the Beatty site to low-level waste at the end of 1992. As of July 1, 1994, the two remaining disposal sites were closed to states outside their regional compacts. At present such states, including California, are maintaining their wastes in temporary storage facilities, usually at the locality where the waste is generated, such as university research centers, hospitals, and nuclear power plants. Regional Compacts The Low-Level Radioactive Waste Policy Act allowed each state to decide if it would proceed alone or join a group of states in its region to share a facility in fulfilling their responsibilities for providing disposal capacity for non-government LLRW generated within their borders. The regional groups of site-sharing states are called compacts and, under acceptable conditions, are approved by the United States Nuclear Regulatory Commission (US NRC)1 upon application. There are ten such compacts, two of which have some member states not within their geographic region, and several unaffiliated states that chose independent paths. California, a major generator of radioactive wastes by virtue of its nuclear power plants, of which two are now in operation, and its many university research and medical facilities, formed the Southwest Compact with Arizona, and North and South Dakota. California was designated the host state, responsible for building the first LLRW facility for the compact. 1 Note that the National Research Council and the Nuclear Regulatory Commission have the same initial letters. Throughout this report they will be distinguished by referring to the Council as NAS/NRC and to the Commission as US NRC.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Box 1.1 (From the Code of Federal Regulations) Title 10 -Energy Chapter I-Nuclear Regulatory Commission Part 61 -Licensing Requirements for Land Disposal of Radioactive Waste Subpart A-General Provisions § 61.2 …Waste means those low-level radioactive wastes containing source, special nuclear, or byproduct material that are acceptable for disposal in a land disposal facility. For the purposes of this definition, low-level waste has the same meaning as in the Low-Level Waste Policy Act, that is, radioactive waste not classified as high-level radioactive waste, transuranic waste, spent nuclear fuel, or byproduct material as defined in section 11e.(2) of the Atomic Energy Act (uranium or thorium tailings and waste).... Regulations Governing Low-Level Radioactive Waste The law also designated the US NRC to provide regulations and guidelines for site selection and safe disposal of civilian radioactive wastes. Among the guidelines in the US NRC's regulations, found in Title 10, Part 61, of the Code of Federal Regulations (10 CFR Part 61), is a definition of LLRW (see Box 1.1) and a description of the types of wastes that are allowable for a LLRW site and classification of radioactive wastes based on the concentrations, half lives of the isotopes, and the types and intensities of activity (See Appendix B). Low-level radioactive waste is broadly defined as any radioactive waste that is not spent nuclear fuel, transuranic waste, or uranium mill tailings. Classification of Wastes A classification of radioactive waste was developed by the US NRC. Class A, B, and C wastes, and Greater Than Class C refer to the relative hazard of the radionuclides in the waste. The particular class into which a waste falls is determined by the concentration in the waste of (1) specific short-lived radionuclides (half-lives of weeks to 100 yrs) and/or (2) relatively long-lived radionuclides (half lives of about 500 years or longer) that are below the activities required for classification as high level waste. Class A, B, and C wastes can be disposed of in shallow-land burial trenches or other near-surface facilities. Greater than class C waste, however, must be disposed of in a high-level radioactive waste facility or some facility licensed by the US NRC (US NRC, 10 CFR Part 61) (see Appendix B for details of waste classification).
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Composition of Low-Level Radioactive Wastes Low-level radioactive waste may be anything from test tubes, hypodermic needles and animal carcasses to contaminated rags, rubber gloves, tools, decontamination resins and solutions from nuclear power plants, and parts of nuclear power plants other than the core, fuel rods, or other highly active, long-lived radionuclide-contaminated parts. However, the wastes cannot be accepted for disposal as a fluid. In contrast to the way that some low-level radioactive waste has been disposed of in the past, only containers originally packed with dry waste are permitted in a LLRW disposal facility. Dry is defined in 10 CFR Part 61 as containing less than one percent free-standing, non-corrosive liquid. This interdiction results from the knowledge among earth scientists and regulators that ground-water pathways are the most likely way in which radionuclides can reach beyond the disposal site boundaries, and liquid wastes are more likely to find their way to the ground water if the containers fail. Although gaseous releases are also possible, dispersion in air is usually rapid enough that it is considered unlikely to reach the public in harmful concentrations. Potential leaks from containers with fluids are therefore avoided by the requirement for dry material only. Thus the kind of leaks that may have occurred in early disposal sites, whether plumes of contamination within the soil, or contaminated ground water, are considered less likely by federal and state regulators if current federal and state regulations are followed and the sites are managed responsibly. Siting Considerations Given the concern about ground water, it has long been recognized that the safest places to store hazardous wastes, especially radioactive waste, would be in the unsaturated zone in desert climates (Winograd, 1974; National Research Council, 1976). In these environments, rainfall is minimal, surface and soil water evaporate rapidly, and plants transpire water vapor to the atmosphere to remove most water within the upper part of the soil or surface layer of earth material. Evaporation and transpiration are referred to jointly as evapotranspiration. Such a condition, it is argued by proponents of areas with desert climates for disposal of hazardous wastes, would prevent accumulation of water in a subsurface trench or other near-surface facility above the water table, and thus minimize the possibility of water and contaminants passing through the unsaturated zone to the water table. However, there are circumstances for which assumptions of dryness in the unsaturated zone in arid climates may be challenged. Multiple Barriers Moreover, as the regulations require reasonable assurance that the public will be protected against exposure to releases of radioactivity beyond the established regulatory limits, the siting guidelines encourage multiple barriers to isolate the waste. The natural
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology barrier is regarded as the major line of defense. In the case of Ward Valley, it would be the 180-213 m thick unsaturated zone, that part of the surface material, including the soil, that lies above the water table. The water table, as defined here, is the uppermost surface of the saturated-zone ground water. Recent advances in the understanding of unsaturated-zone processes, and newly developing techniques in analysis, especially in desert climates, have led to considerable confidence in the ability of such a thick unsaturated zone in a desert environment to isolate the radionuclides in a radioactive waste facility (Bedinger et al., 1989; Reith and Thomson, 1992). In addition, the design of the facility is required to provide redundant protection, and to enhance the ability of the natural barrier to isolate the waste. The cover of the facility, for example, must be designed to minimize infiltration of water which can be accomplished by diverting flow from the waste and by revegetation of native plants on the cover. Any device or construct designed to protect the waste from contact with water is referred to as an engineered barrier. The type of container or waste package is an example of an engineered barrier. Berms or rip rap (barriers of blocks of rock) built up to prevent the effects of energetic surface runoff after a rain, such as during a flash flood, is another example of engineering to protect the facility from erosion that could weaken its first line of defense. Engineered barriers are intended to enhance the natural barrier. Ground-Water Travel Time As an Barrier Although it may appear to be a contradiction to what was stated previously concerning the ground water as a pathway to public water supplies, the ground water can be thought of as still another possible barrier to the transport of radionuclides in sufficient concentrations to pose a risk to the public. That is because under some circumstances ground water can move so slowly through earth materials that it may take hundreds or even thousands of years to travel a few kilometers. Moreover, precipitation or adsorption of contaminants may take place along the way. The slow rate of transport would allow decay of shorter-lived radionuclides to extremely low concentration levels below the natural levels found in ground water, referred to as background levels. Depending upon its rate of movement toward the disposal site boundaries and beyond, the ground water could have little or no effect on the concentrations of longer-lived radionuclides. However, the concentrations of a large number of radionuclides, especially shorter-lived species, would be much reduced in their groundwater path beyond the site boundaries because the slow travel time would allow time for radionuclides to decay to stable elements, or to adsorb to minerals in the aquifer through which the ground water flows and thus remove some contaminants. THE WARD VALLEY CONTROVERSY The preceding general remarks provide the context within which the debate arose concerning the Ward Valley site in California. The two-year siting and licensing process,
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology which included public involvement from the early stages, was coming to a close after several court challenges by opponents to halt the project. As the decision to transfer the land to the state of California was being considered, DOI received the memorandum from the Wilshire group. Issues Raised By the Wilshire Group The memorandum from the Wilshire group described briefly seven concerns that arose from their reading of the draft Environmental Impact Report/Statement (EIR/S). The seven issues, as stated by the authors in the memorandum (Wilshire et al., 1993a), are: Potential infiltration of the repository trenches by shallow subsurface water flow.2 Transfer of contaminants through the unsaturated zone and potential for contamination of ground water. Potential for hydrologic connection between the site and the Colorado River. No plans are revealed for monitoring ground water or the unsaturated zone downgradient from the site. Engineered flood control devices like those proposed have failed in past decades at numerous locations across the Mojave Desert. Alluvium and colluvium derived from Cretaceous granite appears to make a very high quality tortoise habitat. Sacrifice of such habitat cannot be physically compensated. Misconceptions about revegetation enhancement may interfere with successful reestablishment of the native community. The Wilshire group provided a brief paragraph of explanation for each of the seven issues (Wilshire et al., 1993a). The DHS/U.S. Ecology Response U.S. Ecology (USE), the contractor to DHS, replied to Secretary Babbitt in a letter dated June 25, 1993. In it, USE pointed out that the Wilshire group cited the draft EIR/S to identify the seven issues, a version that had been superseded by the final EIR/S after a comment period in which the public and others provided their reviews of the draft and offered suggestions for improvements. The final EIR/S therefore had incorporated many suggestions from the public comment period. During that time, there was ample opportunity for comment by interested agencies and parties, but the Wilshire group had not participated. The letter asserted that had the Wilishire group become familiar with the licensing record, they would have learned that all the issues belatedly enumerated by them had already been addressed. 2 This refers to subsurface lateral flow as confirmed by the Wilshire group.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology The Second Report The Wilshire group replied with a second more detailed report dated December 8, 1993. They elaborated on the first five issues, modifying the issues after reading more up-to-date sections of the administrative record than the draft EIR/S, including the final EIR/S. They did not comment further on the ecological issues of the desert tortoise habitat and revegetation. In the second report they added some information and concerns about the geochemistry, including tritium occurrences in the unsaturated zone and dating of the ground water by the carbon-14 (14C) method. The Wilshire group also postulated five specific pathways by which ground water might flow from Ward Valley to the Colorado River (Wilshire et al., 1993b). The NAS/NRC Committee's Review In response to a letter of request from Secretary Babbitt, dated March 14, 1994, the NAS/NRC convened a volunteer committee of experts with training, research, and field investigation experience in the specialized disciplines necessary to evaluate the seven scientific issues. The committee included two unsaturated-zone hydrologists, one soil physicist, four ground-water hydrologists, three geologists, one geophysicist, one flood control/civil engineer, three geochemists, and two ecologists. Limits of NAS/NRC Review The NAS/NRC, at the request of the DOI, agreed to review only the specific issues identified by the Wilshire group. Based on the agreement reached with DOI, the Committee to Review Specific Scientific and Technical Safety Issues Related to the Ward Valley, California, Low-Level Radioactive Waste Site, the official name of the NAS/NRC committee, reviewed the literature and other relevant material related to the seven issues to (1) assess the adequacy of the site studies relative to the enumerated issues and the validity of the conclusions concerning site performance that are the subject of the debate, and (2) determine if the enumerated concerns have merit and, if so, to assess the impacts on site performance. Source of Documents and Data To accomplish this, the committee reviewed many pans of the administrative record (AR) and other reports and documents relevant to the issues. The AR includes the license application (LA), the DHS interrogatories and USE responses relative to that application, reports related to siting, and data from the monitoring of the site. The Wilshire group also submitted other information to support their position on the issues.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology In addition, because of the great public interest in the Ward Valley issue, the committee invited all interested parties to submit written information that they thought would help the committee in its review and deliberations. The committee made it clear that its review was restricted to the seven issues raised by the geologists and that, despite urging from many individuals and organizations, the committee could not consider wider issues or issues other than those scientific and technical issues that NAS/NRC had contracted to review. Time Constraints The request for a limited and short-term review of the issues put a great time constraint on the committee which very quickly became inundated with information: 5000 pages selected from the AR, dozens of documents of characterization and monitoring data and reports from USE, new documents generated by the Wilshire group or others who supported their efforts, several reports from proponent and opponent organizations, and data from regulatory bodies. The effort to get through all of the information in as short a time as possible with no important piece of information omitted in the review was to be followed by an intense deliberation and writing period, in order to meet the requested time frame for the issuance of the report. However, concerned with the possibility that they may be sacrificing thoroughness and careful deliberation for an arbitrary or unrealistic deadline, the committee required more time to write its report than was originally allotted. Three executive sessions were held for deliberation and writing after the open information-gathering meetings. The Open Meetings The committee met twice for three days in open session in the city of Needles, California, the closest population center to the Ward Valley site. The Wilshire group and the license applicant and licensor, USE and DHS, were given the time they requested to present arguments and evidence in support of their positions. The Wilshire group presented their views and analyses of the site studies, with supporting information from the surrounding area and local experts. USE and DHS presented data, plans, and explanations to support their conclusions that the issues raised had already been adequately addressed. In addition, independent experts with relevant experience and information on the issues were invited, some by the committee, and some by one side or the other. Those who were not direct parties to the dispute or their invitees, both individuals and organizations, were invited to speak at an open microphone at the end of each day's technical session. The Beatty, Nevada, Site as Analog One team of experts invited by the committee were USGS hydrogeologists David Prudic and David Nichols, who have spent several years investigating the Beatty, Nevada,
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology LLRW disposal site that closed in 1992. It had been in operation for at least 30 years, mostly through the period prior to the promulgation of federal regulations for LLRW siting, operations, and waste form. The studies by Prudic and Nichols took place outside the site boundary to evaluate the behavior of the unsaturated zone in an arid climate, and to test monitoring methods. Prudic and Nichols regard the Beatty site as, in some respects, an analog for the Ward Valley site, as it is located in a similar type of terrain, with similar climate and hydrologic characteristics. There are, however, some uncertainties because of unexplained anomalies in the 30-year monitoring records of well data. Although the natural settings are similar, there are major differences between the Beatty site and the proposed Ward Valley site regarding the types of waste disposed and disposal methods. For example, the Beatty facility includes both a toxic chemical waste site and a low-level radioactive waste site adjacent to each other. There are also serious questions or uncertainties regarding the types, compositions, and physical forms of wastes that were accepted at the Beatty site, such as how much toxic chemical and/or radioactive waste were disposed of in liquid form. Another characteristic of the Beatty site is that huge disposal trenches were excavated and kept open for many years until filled, allowing accumulation and infiltration of rainfall. The committee therefore decided that the Beatty site may be useful in understanding some natural processes, but it is limited in evaluating the behavior of the Ward Valley site because of the historical uncertainties. The Plutonium-239 Issue In the committee's effort to understand the timespan of concern for the facility, it requested information on the likely inventory of radionuclides for the facility. The committee was at that time unaware that the potential amount of plutonium-239 (239Pu) was a controversial issue, not between the Wilshire group and the DHS, but between another organization opposed to the Ward Valley site and DHS (Committee to Bridge the Gap, 1994). The controversy came about because the U.S. Ecology performance assessment calculations assumed 3500 curies of 239Pu, based on guidelines from the US NRC. Subsequently, experience at nuclear power plants suggested that the amount of 239Pu that would result from the process that produces the 239Pu was two orders of magnitude less than the original assumption. As a result DHS has since maintained that at most ''a fraction of a curie to two curies of 239Pu from decontamination waste, and a fraction of a cure to several curies from other sources'' would be emplaced over the life of facility operation (Brandt, 1994). Opponents insisted that the larger number is correct. As the committee needed a reliable estimate in order to assess the potential impact of long-lived radionuclides reaching the Colorado River, it requested an evaluation of the positions of the DHS and Committee to Bridge the Gap, from an independent source, the US NRC. In addition, the committee learned that the Congressional Research Service (CRS) had done a similar study (Holt, 1994). The results of the two organizations' studies converged independently (Holt, 1994; US NRC, 1994a and 1994b), both agreeing that the appropriate estimate is two orders of magnitude lower than that used for the performance assessment.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology (The CRS is presently doing a second study because of newer available data). These studies were especially helpful in providing the guidance the committee needed to select an order of magnitude to do some bounding calculations. The committee, therefore, based the order of magnitude for 239Pu expected for such a facility on these two studies in doing the bounding calculations found in Chapter 5 of this report. Organization of the Report After this introduction to the study, the report begins with an overview of the geology, geophysics, geomorphology, hydrology, and ecology of the site and surrounding region in Chapter 2. The following chapters cover the seven issues of the USGS geologists. The format of each chapter follows a pattern. First is a statement of the issue, followed by a summary of the Wilshire group's position, the USE and DHS response or position, and the committee's discussion, analysis, observations, conclusions, recommendations and references cited. Chapter 2, then, is the site overview; Chapter 3 addresses the potential for water and contaminants to be transferred through the unsaturated zone to the ground water; Chapter 4 deals with lateral subsurface flow in the unsaturated zone; Chapter 5 analyzes the five postulated pathways that ground water could take to flow from the site to the Colorado River; Chapter 6 discusses the monitoring plans for the unsaturated zone and the ground water; Chapter 7 evaluates the flood control devices and other engineered structures to prevent flooding and erosion at the site; Chapter 8 considers the Desert Tortoise and its habitat; and Chapter 9 evaluates the revegetation plan to reestablish the native vegetation at the site after construction. Note that the sequence of treatment of issues 1 and 2 of the original Wilshire group memorandum has been reversed in this report, with Chapter 3 addressing the second of the enumerated issues, and Chapter 4 addressing the first. Thus, the Wilshire group's issue 2, the potential downward transport of contaminants through the unsaturated zone, is the committee's issue 1 in Chapter 3, and the Wilshire group's issue 1, the potential infiltration of the trenches by shallow subsurface lateral flow, appears as the committee's issue 2 in Chapter 4. The reason for the reversal was the need to introduce many of the concepts and characteristics of a thick add-region unsaturated zone in addressing potential downward flow of water through the unsaturated zone to the water table. The committee, therefore, agreed that the discussion of this issue was best presented first. The discussion of shallow subsurface flow of water that follows in Chapter 4 requires far less information about the unsaturated zone and draws upon information already provided in Chapter 3 as background. COMMITTEE DECLARATION The committee was not asked to, and did not, evaluate the suitability of the Ward Valley site for a low-level radioactive waste facility. The charge of the committee
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology (see Appendix A) was to evaluate the validity of the issues raised by the Wilshire group and their implications for the behavior of the unsaturated zone and for the other features of concern. The committee depended upon the information provided by the opposing groups, DHS/USE and the Wilshire group, any relevant information provided by outside interests, the scientific literature, and their own professional judgment. None of the conclusions should be read as either an endorsement of, nor condemnation of, the Ward Valley site. Approval of the Ward Valley site is the responsibility of those government agencies and officials entrusted to make such decisions. Recommendations offered herein by the committee, using its professional judgment, should not be construed as an endorsement of the site. The recommendations are made in the spirit of enhancing the scientific program and of decreasing the uncertainties in the earth science information, if the site is developed for a low-level radioactive waste disposal facility. REFERENCES Bedinger, M. S., Sargent, K. A., Langer, W. H. 1989. Studies of the geology and hydrology of the Basin and Range province, southwestern United States, for isolation of high-level radioactive waste-characterization of the Sonoran region, California. U.S. Geological Survey Professional Paper 1370-E, 30 p. Brandt, E. 1994. Letter to I. Alterman, dated September 22. Committee to Bridge the Gap. 1994. Informal presentation to the National Academy of Sciences on the proposed Ward Valley radioactive waste disposal facility. September 1. Holt, M. 1994. Plutonium Disposal Estimates for the Southwestern Low-Level Radioactive Waste Disposal Compact. Memorandum dated June 3. Congressional Research Service. Library of Congress. Washington, D.C. National Research Council. 1976. The Shallow Land Burial of Low-Level Radioactive Contaminated Wastes. Washington, D.C.: National Academy of Sciences. 150 pp. Reith, C.C. and B.M. Thomson. 1992. (eds.) The Disposal of Hazardous Materials in Arid Ecosystems. University of New Mexico Press. Albuquerque. Romano, S.A. 1993. Letter to Secretary Bruce Babbitt, dated June 25. U.S. Geological Survey. 1993. Letter from R. M. Hirsch, Acting Director, to Senator Barbara Boxer, October 8. U.S. Nuclear Regulatory Commission. 1994a. Letter from M. R. Knapp to Ina Alterman , National Research Council. Dated November 23. ——— 1994b. Letter from M.R. Knapp to Ina Alterman, National Research Council. Dated December 14. Wilshire, H. G., K. A. Howard, and D. M. Miller. 1993a. Memorandum to Secretary Bruce Babbitt, dated June 2. ——— 1993b. Description of earth science concerns regarding the Ward Valley low-level radioactive waste site plan and evaluation. Released December 8.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Wilshire, H. G., K. A. Howard, and D. M. Miller. 1994. Ward Valley proposed low-level radioactive waste site. A report to the National Academy of Sciences. Winograd, I. J. 1974. Radioactive waste storage in the add zone EOS. Transactions of the American Geophysical Union. 55(10):884-894.
Representative terms from entire chapter: