Disposition of the Nevada Test Site
Allen G. Croff
The committee visited a number of U.S. Department of Energy (DOE) sites in the course of this study. The purpose of these visits was twofold: first, to understand better the issues and interrelationships that affect the disposition of various types of DOE sites in differing locations, and second, to acquire information that would permit development by the committee of an integrated approach to site-specific disposition decisions. The Nevada Test Site (NTS) was chosen as one of the sites to be considered by the committee at the request of DOE and because it is representative of a large DOE site where substantial amounts of hazardous materials exist and are likely to remain.
INITIAL STATUS OF THE NTS
The NTS encompasses 3,496 km² of land area in southern Nevada reserved for the jurisdiction of the DOE. It features desert and mountainous terrain, and is larger than Rhode Island, making it one of the largest secured areas in the United States. The NTS is in a remote and arid region, mostly surrounded by federal lands, and has strictly controlled access. Some lands are open to public entry. Most of the NTS is located in Nye County, Nevada, with its southernmost point being just 105 km northwest of Las Vegas. The NTS is surrounded by the Nellis Air Force Range (NAFR) Complex on the west, north, and east, and land managed by the U.S. Bureau of Land Management (BLM) on the south and southwest. The NAFR Complex is used for military training. The BLM lands are used for grazing, mining, and recreation. Near the eastern boundary of the NTS, the NAFR Complex shares the use of land with the U.S. Fish and Wildlife Service's Desert National Wildlife Refuge.
The historic activities at the NTS are: atmospheric weapons testing, underground nuclear testing, safety testing of nuclear weapons, nuclear rocket development, near-surface disposal of radioactive wastes, crater disposal of contaminated soils and equipment, greater confinement disposal of radioactive wastes (a term denoting disposal more isolating that near-surface, but not a deep geologic repository (e.g., deep borehole disposal of radioactive wastes such as is used at NTS), and site support activities. From 1951 to 1992 over 820 underground nuclear tests and 100 atmospheric tests were conducted at the NTS (U.S. Department of Energy, 1994). For many of the underground tests, more than one weapon was tested. Ongoing and planned future activities at the NTS include helping to ensure the safety and reliability of the nation's nuclear weapons stockpile, disposal of low-level radioactive wastes, storage of wastes for disposal off site, non-defense research and development (e.g., alternative energy projects, Spill Test Facility, alternative fuels demonstration projects, environmental technology), and use of the site by other Federal agencies for military exercises and R&projects.
NTS END STATE
DOE/NTS defines “complete clean-up” as bringing a site to the point “that land, facilities, and materials are adequately safe to be available for alternative use, based on future land use policy decisions, with a minimum cost for long-term surveillance and monitoring” (M. Sanchez, 10 September 1997, presentation to the committee). Cleanup priorities and cleanup levels are subject to negotiation with regulators and involved stakeholders.
Stated in broad terms, the presently accepted future use of the NTS is for it to become:
. . . a diversified national test and demonstration site that can continue to support the reduced nuclear weapons defense program, while also attracting and supporting other high tech programs and industry that can make significant and long term contributions to local and national energy, environmental, defense, and economic needs (Nevada Test Site Economic Adjustment Task Force, 1994).
The DOE and state of Nevada have agreed on a disposition approach that requires residual contamination resulting from nuclear weapon testing be cleaned up to varying degrees consistent with the proposed land use (U.S. Department of Energy, 1996c). Present plans for assessment and remediation of the Nevada Test Site are summarized as follows:
Surficial soils that are typically contaminated with uranium and plutonium will be excavated or contained. Currently there is no single national standard establishing cleanup levels for surficial contamination of plutonium; rather, these levels are negotiated on a site-by-site basis between DOE, the state, and the U.S. Environmental Protection Agency (EPA) regional office. However, the final targeted remedial action levels are in the 200 pCi/g range.
Underground nuclear weapons testing site remediation plans (IT Corporation, 1998) will involve sequential development of:
regional and test-specific groundwater flow models for underground test sites that intersect the saturated zone,
a corrective action investigation plan that could include further characterization of the test sites,
modeling of test sites for five radionuclides to predict the contaminant boundary and leading to a documented decision concerning the disposition of each test site, and
a corrective action and closure plan.
Other support facilities and the debris from near-surface safety tests will be cleaned up or remediated to a degree yet to be determined based on their potential for future use.
The DOE states that because cost-effective technologies have not yet demonstrated an ability to effectively remove or stabilize radioactive contaminants from the groundwater at the various test sites, subsurface contaminants in and around the underground test cavities will be left as is, and subject to continuing monitoring and surveillance. This approach may be revised if advanced, cost-effective technology is developed. The committee could not find evidence that any such technology development is currently planned by DOE (U.S. Department of Energy, 1997b).
Institutional control of the NTS is assumed in perpetuity at the existing boundaries, and for the foreseeable future, the landlord is assumed to be DOE Defense Programs (U.S. Department of Energy, 1997b, p. 24). However, the defined end state is establishment of a monitoring network, program, and schedule acceptable to DOE, the state, and interested and affected parties, including long-term surveillance and monitoring of the UGTA for a period of 50 years is required (U.S. Department of Energy, 1996c). Extending this period to 100 years is under consideration DOE (U.S. Department of Energy, 1997b, Appendix A).
CHARACTERIZATION AND TECHNICAL ASSESSMENT OF THE NTS
There are two major aspects of the NTS that require characterization: the contaminant source term and the naturally occurring features of the NTS and surrounding area that are relevant to potential release of or access to the contaminants.
Essentially all of the contamination at the NTS results from the radiologically and/or chemically hazardous substances associated with nuclear explosions. The atmospheric radiological source term is composed of volatile species that are released by leakage from historic nuclear weapons test sites and by evaporation. The estimated release rates are 700 Ci/y of tritium and 160 Ci/y of krypton-85 (U.S. Department of Energy, 1996b, Volume 1, p. 4-150).
Surficial radiological contamination is estimated to be 36 Ci. The dominant source of surficial contamination is contaminated soils from nuclear safety tests, but there is also fallout from atmospheric testing. The primary radioelements that were released are plutonium, uranium, and americium, with lesser amounts of cesium, strontium, and europium (U.S. Department of Energy, 1996a, p. 65).
The total underground radiological contamination is about 310 MCi, essentially all from underground nuclear testing. However, the 112 MCi underground radiological source term considered in the NTS environmental impact statement as being available for potential migration is just the total activity from all underground tests that were conducted beneath the water table or within 101 m of the top of the water table, of which about 90 percent is due to tritium (U.S. Department of Energy, 1996a, p. 65). This assumption is apparently based on the belief that the arid nature of the NTS would preclude substantial amounts of radionuclides above this level from mobilizing.1 In addition, there are substantial uncertainties in the total radiological source term because calculation of the radionuclide composition used estimation, adjustment, and extrapolation techniques to account for (a) significant amounts of radionuclides from testing by Lawrence Livermore National Laboratory, (b) the amount of inventory actually beneath or within 101 m of the water table, and (c) the initial amounts of fissile materials and tritium, and the amount of fission products, actinides, and activation products generated (Borg et al., 1976, p. 79; U.S. Department of Energy, 1996a, p. 74). The Committee has not been able to find any unclassified quantification of these uncertainties, and classified information was not examined in this study.
The toxic materials present after a nuclear weapon detonation occur in three locations: incorporated in the melt glass that pools in the bottom of the cavity, deposited on the rubble and along fractured surfaces within and outside the cavity, and in gases that escape to the atmosphere within a short time after detonation. The distribution of radionuclides is complex, and their behavior during the explosion as well as the chemistry by which they are incorporated or deposited are not fully understood, especially for those species that partition between the melt glass, rubble, and fractures (Borg et al., 1976, p. 177, 187; Kersting, 1996, p. 23; Smith, 1993, pp. 5, 21).
Non-radioactive hazardous materials used in nuclear weapons testing have been surveyed (Bryant and Fabryka-Martin, 1991). Such materials could be introduced into the subsurface from pre- or post-detonation drilling activities, or during sealing of the shot hole before detonation; and as materials used to seal the borehole before detonation. In practice, the non-radioactive hazardous materials typically amount to several tons of lead, a “few kilograms” of other hazardous metals (e.g., arsenic, gallium) and unidentified hazardous organic compounds. It should be noted that nonhazardous organic compounds are also of interest because they may lead to species that complex with hazardous constituents and promote their transport. No unclassified estimates are available concerning the identity and quantity of hazardous and potentially important non-hazardous, non-radioactive materials that
DOE officials recently stated that in the future the entire radionuclide inventory would be assumed to be part of the source term (R.M. Bangerter, 1998, personal communication).
may still remain in the subsurface at the NTS. Information regarding the distribution and chemistry of non-radioactive residues that do not have radiological analogues is not evident.
Atmospheric characterization (e.g., wind direction and frequency, rainfall) related to the transport of gaseous and particulate contamination has been well characterized. However, the mobility of contaminated surficial deposits is less certain. The DOE believes the contaminated soil to be largely gathered around the base of vegetation in immobile positions unless the surface is disturbed (U.S. Department of Energy, 1996a, p. 82), but the basis for this conclusion and its dependence on assumptions concerning future vegetation patterns and surface disturbances are unknown.
In general, the subsurface characteristics (geology, hydrology, geochemistry) of the NTS are not understood at a sufficient level of detail to provide a basis for modeling contaminant transport for the purpose of predicting risks with an acceptable degree of accuracy. This is especially true at Pahute Mesa, which constitutes one of the largest and most difficult hydrogeologic regimes at the NTS (IT Corporation, 1998). This lack of understanding is due, in part, to a combination of the extremely complex and heterogenous geology of the site, and in part to a lack of historical interest in achieving more complete understanding. Recently, attempts to perform two- and three-dimensional hydrologic modeling have been pursued (R.K. Waddell, HIS GeoTrans, Inc., September 10, 1997, presentation to the committee). The data base available to validate these models is meager, but NTS has recently initiated a drilling program for the purposes of subsurface exploration between Pahute Mesa and Oasis Valley, a study of groundwater discharge in Oasis Valley, and a study of water infiltration through test craters (IT Corporation, 1998).
The extent of information and investigation concerning NTS geochemistry is even less than for hydrologic aspects, with the exception of areas having water chemistry and geology similar to that of the Yucca Mountain, which is being extensively investigated as a potential site for a high-level waste repository. While water composition per se is known adequately, the chemistry of its interactions with naturally occurring subsurface materials and characterization of naturally occurring chemicals that might affect radionuclide transport (e.g., colloid formers) is not (Kersting, 1996, p. 25). The DOE has recently initiated geochemical studies between Pahute Mesa and Oasis Valley to determine groundwater age and travel time, and to study colloid transport (IT Corporation, 1998).
A risk assessment builds on the foundation provided by the characteristics of the site and source term, and superimposes considerations related to the mobilization, transport, uptake, and impact of contaminants. The important uncertainties and unknowns in these characteristics have been described immediately above, and their implications will not be repeated here. The impact of the other considerations will be discussed below for surficial and subsurface contaminants.
Within the bounds of uncertainty noted above in relation to activities that disturb the soil, the risks from surficial contamination appear to be relatively well understood. The DOE has calculated the maximum effective dose at the site boundary from airborne contaminants to be 0.0048 mrem and the collective effective dose equivalent within 80 km of the NTS to be 0.012 person-rem (U.S. Department of Energy, 1996b, Volume 1, p. 4-152). The risks from various types of habitation of some of the plutonium-contaminated sites are estimated in Daniels (1993, p. 56). Most lifetime risks are low (cancer risk well below 10−6), but for a few sites the risk exceeds 10−4. Within the reports cited, there is no mention of scenarios that involve intrusion or other disturbance of the surface or subsurface.
There is considerable uncertainty concerning the actual quantity of radioactivity that can be mobilized by leaching of contaminated subsurface debris by groundwater. Smith et al. (1998) have summarized the uncertainties associated with leaching for the NTS and concluded that the radionuclides most likely to become mobile and migrate via the groundwater regime are: (1) tritium; (2) a number of anions and neutral species such as technetium-99, ruthenium-106, chlorine-36, and iodine-129, all assumed to migrate at the same rate as groundwater; and (3) cationic species, including strontium-90, cesium-137, antimony-125, cobalt-60, zirconium-95, plutonium-239, and others, that are believed to move more slowly than groundwater to varying degrees. It should be noted that zirconium-95, ruthenium-106, and antimony-125 all have half-lives less than three years and are not likely to pose a groundwater hazard; the same is probably true for cobalt-60 with a half-life of 5.2 years. However, quantitative estimates are highly uncertain to the point of being almost non-existent. There has been essentially no study of whether the substantial fraction of the radiological source term that was deposited above the water table is moving downward into the saturated zone (Borg, et al., 1976, p. 187; Kersting, 1996, p. 26).
The situation related to retardation of radionuclide transport by sorption onto rocks is somewhat better than for leaching, with several studies having been conducted. Tritium is appropriately assumed to move at the same rate as the groundwater. However, documentation for most other radionuclides indicates that retardation factors vary significantly with respect to water composition, experimental conditions, and rock type. The causes of the variations are speculative (Smith, 1993, p. 18; Kersting, 1996, pp. 23, 25). In fact, a recent study (Daniels, 1993, p. 76) assumed no sorption of any radionuclides because of the limited database.
Otherwise insoluble or highly retarded radionuclides can be transported by forming or attaching to colloidal particles, which then move essentially at the same rate as the groundwater in which they reside. A recent review (Kersting, 1996, p. 24) concluded that a substantial fraction of radionuclides can be associated with colloids, but the effects on transport are not known. Contaminant transport by non-radioactive organic chemicals or degradation products thereof has not been studied or taken into account.
A review of the literature concerning leaching and sorption of radionuclides from nuclear weapons testing melt glasses is given in Smith (1993). The reader should note one important observation from this report: “Most of these investigations were published over ten years ago; the number of tests and access to device debris has diminished during the subsequent decade” (Smith, 1993, p. 24). The committee's investigations support this observation and the continuation of this trend to the present.
Tritium, which is not sorbed and moves at the same rate as groundwater, is the radionuclide considered almost exclusively by DOE in risk analyses. Other radionuclides were assumed by DOE to move very slowly as compared with tritium and, therefore, were not generally considered in the assessments. However, before 1997 about a dozen instances of migration of radionuclides other than tritium have been documented (Nimz and Thompson, 1992). The largest distance of migration of radionuclides other than tritium was not then known to have exceeded 500 m (1,640 ft). Migration of tritium is more difficult to interpret, but is thought to have migrated no more than several kilometers, although tritium, with a half-life of 12.3 years, is not likely to pose a long-term threat to the groundwater resources at NTS.
Pahute Mesa, which is the location of most of the U.S. large nuclear explosions, contains approximately 70 percent of the tritium at the NTS (IT Corporation, 1998). Modeling results also indicate that groundwater flow paths from Pahute Mesa are the shortest of all those at the NTS site and constitute the highest potential for contamination migration to off-site public receptors (IT Corporation, 1998). Recent analysis of water from a well near the TYBO nuclear weapon test site on Pahute Mesa (Thompson, 1998) showed that plutonium as well as cesium, cobalt, and europium were unexpectedly present in the water about 1300 m from the source site associated with the BENHAM test. All of these were shown to be associated with colloidal particles. The plutonium was present at concentrations below drinking water limits (Kersting et al., 1999).
The uptake points for radionuclides are generally assumed to be springs in off-site locations such as Oasis Valley to the southwest of the NTS. This assumption has implications for institutional management of the NTS.
For underground tests conducted within the NTS boundaries, groundwater modeling studies have been performed by Daniels (1993) and GeoTrans (1995). Both of these studies evaluated the migration of tritium from test
locations on Pahute Mesa to Oasis Valley. In addition, the GeoTrans study examined migration flow paths from Pahute Mesa to Amargosa Valley and from Yucca Flat to the boundary of the NTS south of Mercury, Nevada. In general, the GeoTrans results for tritium were far below 20,000 pCi/L, which is EPA's allowable tritium concentration in drinking water. The study reported by Daniels (1993) predicted much higher values, some a factor of five less than the drinking water standard. However, these calculations were for screening purposes and used a number of conservative simplifying assumptions. Based on the combined results from these two studies, the estimated range of peak tritium concentrations at the closest uncontrolled use area varies from 5 × 10−4 pCi/L (arriving 150 years after the beginning of migration) to 3,800 pCi/L (arriving in 25 to 94 years). The hypothetical maximally exposed individual at this location is estimated to have a lifetime probability of contracting a fatal cancer between 8 × 10−13 (about one in one trillion) and 1 × 10−5 (about one in 100,000), depending on which model is used.
Very little work has been done on estimating the potential risks from radionuclides other than tritium. Such an attempt was made in Daniels (1993). These estimates are self-characterized as being conservative. The results indicate that at the Area 20 (Pahute Mesa) boundary of the NTS and at Oasis Valley the lifetime committed effective dose for other radionuclides is about 10 percent of that from tritium. Important radionuclides other than tritium were strontium-90, iodine-129, cesium-137, radium-226, plutonium-239, and americium-241. The risks from toxic chemicals used in nuclear weapons tests have not been estimated.
The DOE has prepared a report (U.S. Department of Energy, 1997a) evaluating the feasibility and cost of selected options for addressing the contamination. An initial list of options was taken from the U.S. Environmental Protection Agency 1994 Remediation Technologies Screening Matrix and Reference Guide (EPA/542/B-94/013). The options in the EPA guide were screened to yield the following list of options:
Intrinsic remediation (reliance on natural subsurface processes).
Pump and in situ treatment.
Excavation and on-site treatment and disposal.
All options were determined to be technically feasible, although the “no action” alternative was noted as not meeting EPA requirements for “no action” on a risk basis. All other alternatives were deemed feasible.
The NTS is relying on contamination reduction measures for a specific set of contaminated sites such as those having surficial contamination from safety and atmospheric testing and the industrial sites. The goals of most such activities are to reduce contamination levels sufficiently so that the sites do not pose unacceptable risks to inadvertent intruders or during proposed industrial development, but the levels are not sufficiently low to allow site control to cease. The measures generally involve physical removal of contaminated soil and removal of contaminated materials from facilities, followed by burial of the resulting waste. In contrast to this active approach, contamination reduction measures other than natural attenuation for medium-lived species such as tritium are not underway or contemplated for the contamination resulting from underground tests, including the contaminated rock and groundwater.
There is very little reliance by DOE on engineered measures to isolate the contamination at the NTS, especially as it relates to contamination resulting from underground tests. The underground test cavities provide a natural form of isolation that should be well characterized over time regarding migration of radionuclides. These local sites provide information that could be relevant in other add locations. One exception to this is that DOE has left open the possibility to pump and recycle groundwater if it were to be contaminated with unacceptable levels of
tritium at locations accessible to the public. This would presumably continue until radioactive decay made further recycle unnecessary. Other engineering measures for site-wide or high-risk locations have only been studied cursorily (U.S. Department of Energy, 1997a).
To compensate for the relatively small use of contamination reduction and isolation measures, the DOE is placing very heavy reliance on controlling access to the site. The most important of these is to prevent public access to the NTS for the indefinite future, which includes retaining government responsibility for the site and active patrolling to prevent unauthorized site entry. Efforts are also underway to ensure that the activities conducted at “brownfields” within the NTS are consistent with the degree of contamination in particular locations and facilities.
The DOE rationale for assuming indefinite institutional management of the NTS is stated as follows (U.S. Department of Energy, 1997a):
Institutional controls have been in place at the NTS for over 50 years, and these controls have taken the form of both active and passive; the public knows of the related risks and is aware that the U.S. Government strictly controls access to the NTS. Therefore, because of 50 years of ‘Institutional Memory,' it seems reasonable to believe that such controls could continue indefinitely as they complement ongoing clean-up and monitoring efforts.
That active institutional management efforts may prove necessary to maintain such controls is a view reinforced by an earlier report (Daniels 1993, p. 72) that explicitly acknowledges (a) the growth in population in the Las Vegas area and the associated demand for water in an otherwise add area, and (b) the potential for loss of buffer areas provided by the NAFR lands surrounding much of the NTS that could result from extended cessation of nuclear testing. Both are seen as factors that could increase exposure to hazardous materials presently on or beneath the NTS.
Implementation of DOE's currently operative NTS disposition decision is composed of ongoing remedial actions and institutional management measures.
The DOE is presently remediating contaminated soils that are near the NTS boundary and have high contaminant concentrations, and also selected facilities for the purpose of reindustrialization. Limited characterization activity (e.g., concerning plutonium migration from the BENHAM test) is underway.
The DOE has a comprehensive program for monitoring water and air at locations within and outside the NTS, and the state of Nevada performs independent monitoring. The DOE maintains an extensive guard force to prevent public access to the NTS to prevent exposure to legacy contamination and actively hazardous situations, as well as to protect classified activities.
Future Reconsideration of the Disposition Decision
The committee was unable to identify any specific commitment or process that would result in future reexamination of the major features of site remediation decisions being made today, although decisions will be made on specific details (e.g., cleanup levels for specific locations) on a continuing basis. There appears to be little driving force for such reconsideration at present. Thus, the destiny of the site appears to be a limited number of remedial actions consistent with re-industrialization in selected portions of the site, followed by an indefinite period of institutional control.
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