Assessment of Hydrologic Transport of Radionuclides from the Gasbuggy Underground Nuclear Test Site, New Mexico (Uc-703)

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ASSESSMENT OF HYDROLOGIC TRANSPORT OF RADIONUCLIDES FROM THE GASBUGGY UNDERGROUND NUCLEAR TEST SITE, NEW MEXICO Sam Earman, Jenny Chapman and Roko Andricevic Nevada Operations Office U.S. Department of Energy DISTRIBUTION OF THIS DOCTOEPTT IS tMlMroo

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ASSESSMENT OF HYDROLOGIC TRANSPORT OF RADIONUCLIDES FROM THE GASBUGGY UNDERGROUND NUCLEAR TEST SITE, NEW MEXICO Sam Earman, Jenny Chapman and Roko Andricevic Water Resources Center Desert Research Institute University and Community College. System of Nevada Nevada Operations Office U.S. Department of Energy Las Vegas, Nevada The work upon which this report is based was supported by the U.S. Department of Energy under

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The U.S. Department of Energy (DOE) is operating an environmental restoration program to characterize, remediate, and close non-Nevada Test Site locations that were used for nuclear testing. Evaluation of radionuclide transport by groundwater from these sites is an important part of the preliminary risk analysis. These evaluations are undertaken to allow prioritization of the test areas in terms of risk, provide a quantitative basis for discussions with regulators and the public about future work at the sites, and provide a framework for assessing data needs to be filled by site characterization. The Gasbuggy site in northwestern New Mexico was the location of an underground detonation of a 29-kiloton nuclear device in 1967. The test took place in the Lewis Shale, approximately 182 m below the Ojo Alamo Sandstone, which is the aquifer closest to the detonation horizon. The conservative assumption was made that tritium was injected from the blast-created cavity into the Ojo Alamo Sandstone by the force of the explosion, viafractures created by the shot. Transport calculations were performed using the solute flux method, with input based on the limited data available for the site. Model results suggest that if radionuclides produced by the shot entered the Ojo Alamo, they are most likely contained within the area currently administered by DOE. The transport calculations are most sensitive to changes in the mean groundwater velocity, followed by the variance in hydraulic conductivity, the correlation scale of hydraulic conductivity, the transverse hydrodynamic dispersion coefficient, and uncertainty in the source size. This modeling was performed to investigate how the uncertainty in various physical parameters affects calculations of radionuclide transport at the Gasbuggy site, and to serve as a starting point for discussion regarding further investigation at the site; it was not intended to be a definitive simulation of migration pathways or radionuclide concentration values. Given the sparse data, the modeling results may differ significantly from reality. If greater confidence in the transport predictions is needed, more site data would need to be obtained, including ascertaining whether the hypothesized release of tritium from the cavity to the Ojo Alamo has occurred, and determining accurate values for the mean groundwater velocity, the variance in hydraulic conductivity, and the correlation scale for hydraulic conductivity at the site.

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LIST OF FIGURES........................................................................................................................... iii LIST OF TABLES............................................................................................................................. iii Discharge Mixing Area.............................................................................................................. 10 Distance to Control Plane .......................................................................................................... 11 Mean Groundwater Velocity .......................................................................... 12 Spatial Variability in Hydraulic Conductivity......................................................................... 14

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1. Gasbuggy site location map (from DOE, 1986).................................................................•... 2 2. Generalized geologic cross section at the Gasbuggy emplacement hole.............................. 5 3. Locations of the boreholes drilled for the Gasbuggy Project and EPNG 10-36.................. 8 4. Vertical distribution of permeability measured in cores from well GB-1............................. 13 5. Modeled tritium concentrations at the drilling exclusion boundary resulting from the Gasbuggy detonation.................................................................................................... 15 6. Sensitivity of mean tritium concentration 183 m west of surface ground zero to changes in correlation scale......................................................................................................... 17 7. Sensitivity of mean tritium concentration 183 m west of surface ground zero to changes in mean groundwater velocity..................................................................................... 17 8. Sensitivity of mean tritium concentration 183 m west of surface ground zero to changes in variance...................................................................................................................... 19 9. Sensitivity of mean tritium concentration 183 m west of surface ground zero to changes in transverse hydrodynamic dispersion coefficient................................................... 19 10. Modeled tritium concentrations at the drilling exclusion boundary for the model incorporating source size in transport calculations, and the model assuming a 11. Tritium concentration for well EPNG 10-36............................................................................. 23 1. Sorption Coefficients Published for Radionuclides in Contact with the Ojo Alamo Sandstone ................................................................................................................ 10 2. Results of Groundwater Transport Calculations for Gasbuggy

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The U.S. Department of Energy (DOE) and its predecessor agencies are responsible for nuclear weapons research and development as part of the national defense program. These activities include underground nuclear testing, and a small number of such tests have been conducted at sites distant from the Nevada Test Site (NTS). The Gasbuggy site is located in northern New Mexico, approximately 90 km northwest of Los Alamos (Figure 1). The Gasbuggy event was part of the Plowshare Program, and was designed to increase the natural gas production of a low permeability reservoir formation. The device had a yield of 29 kilotons, and was detonated 1,292 m below ground surface on December 10,1967. DOE has implemented an environmental restoration program with the goal of characterizing, remediating, and closing the offsite nuclear test areas. An early step in this process is performing a preliminary risk analysis of the hazard posed by each site. These analyses will allow prioritization of the sites in terms of risk, provide a quantitative basis for discussions with regulators and the public about future work at the sites, and provide a framework for assessing data needs to be filled by site characterization. Desert Research Instimte (DRI) is tasked with performing hydrologic risk evaluations for the groundwater transport pathway. This report details the results of the groundwater-transport evaluation for the Gasbuggy site in terms of radionuclide concentrations that could cross the site boundary. There are also predictions of distances past which radionuclide concentrations are expected to be below concentrations of regulatory concern. These results will be included with evaluations of risk due to surface sources at Gasbuggy to present a comprehensive site risk analysis in a separate report. The basic scenario evaluated for this assessment is the groundwater transport of radionuclides introduced into the subsurface by the Gasbuggy nuclear detonation. This assessment strives to be as accurate as possible, but the lack of data requires that significant assumptions be made about some critical parameters. As a consequence of these limitations, the results of this modeling are meant to serve merely as a tool to guide further discussion and investigation, not as a definitive assessment of radionuclide migration at the Gasbuggy site. The analysis relies solely on unclassified data available to the general public. Although this may increase the uncertainty of the source term data, and result in the lack of transport calculations for certain radionuclides introduced by the Gasbuggy detonation, these issues can be investigated more thoroughly, and with much greater accuracy, after the acquisition of further data regarding contaminant transport at the site. Measured values were used wherever possible, but given the lack of data, calculations were performed for ranges of certain parameters. The assessment can be made more realistic with the acquisition of additional site data. A screening tool approach outlined in Daniels et al. (1993), Andricevic et al. (1994), and Andricevic and Cvetkovie (1996) was used to model radionuclide transport from the Gasbuggy site. The employed modeling approach incorporates real physical phenomena, such as instantaneous and/or slow release from the source, advection, dispersion, sorption, mass transfer, and possible uncertainty in the model parameters. The output is the expected concentration profile as a function

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of time (e.g., concentration breakthrough curves) at the compliance point downgradient from the source, as well as the uncertainty around the expected concentration resulting from the natural hydrogeologic heterogeneity in general, and from the spatially variable groundwater velocity in The solute flux method is described in detail by Andricevic and Cvetkovic (1996), while important elements of the approach can also be found in Daniels et al. (1993) and Andricevic et al. (1994). The following summary is derived from these sources, but the reader is directed to these references for a detailed treatment of the method. The contaminant migration process is described in the solute flux method through the Lagrangian concept of motion following a particle on the Darcy scale. In the absence of direct information on groundwater velocities near Gasbuggy, the mean velocity, U, is calculated using where K is the mean hydraulic conductivity, 7 is the mean hydraulic gradient, and ne is the mean effective porosity. Hydrogeologic parameters such as K and ne can be highly variable as a result of geologic heterogeneity. Numerous studies of the variability of hydraulic conductivity have concluded that conductivity is generally log-normally distributed (Freeze and Cherry, 1979; Hoeksema and Kitanidis, 1985). Thus, the natural logarithms of hydraulic conductivity data can be described by a normal distribution with a mean pintf and variance a2^. The variance represents the variability of K about its mean, and may range from near zero for homogeneous deposits to five, or higher, for extremely variable porous media (Hoeksema and Kitanidis, 1985). Because it is distributed in space, K usually has some degree of spatial correlation. The negative exponential function is often used to describe the K correlation structure because it is found to correspond to log K data and is easy to use. The correlation length of K, X, represents the distance beyond which data points show weak correlation. The higher the value of X, the greater the spatial continuity of K. When the log-normal distribution and the negative exponential covariance function are assumed, the heterogeneous, isotropic hydraulic conductivity field can be statistically characterized by three If the parameters on the right-hand side of the Darcy equation are log-normally distributed, estimated mean U, o2inu, can be calculated as the sum of the variances of the other parameters, if sufficient data are available. The estimation error in U, o2\nu, represents the magnitude of uncertainty in the estimate of U contributed by the estimation errors of K, J, and ne. The magnitude of the uncertainty in the mean velocity, a2\nu, depends on the number of measurements used to estimate the parameters in the Darcy equation. In the case of independent measurements, a2\nu = a2\nu/N, where o2\nu is the variance in the velocity field and N is the number of measurements. For spatially correlated measurements, 02int/ is scaled by N~1[1+q(N-1)], where p is an averaged spatial correlation between data points.

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The solute flux method evaluates movement of a solute from the source to a plane perpendicular to the direction of flow. Aquifer heterogeneity is included and represented by the variance of log-hydraulic conductivity, a2^, and the hydraulic conductivity integral scale, X. The combination of the spatial variability of aquifer properties and the uncertainty in the estimates of these properties causes the solute flux to be a random function described by a probability density function (PDF). The mean and variance of the solute flux are converted to the flux-averaged concentration needed for risk calculations by dividing by the groundwater flux, Q. The first two moments of the flux-averaged concentration are important in determining the total risk level. The larger the magnitude of variance in the flux-averaged concentrations, the larger the maximum The geology and hydrogeology of the Gasbuggy site are described by many authors, including Mercer (1967), Rawson and Korver (1967), Fassett (1968), Koopman and Ballance (1968a), Koopman and Ballance (1968b), Mercer (1968), Mercer (1969), Sokol (1970), Weir (1971), and The Gasbuggy event was conducted at a depth of 1,292 m below ground surface, within the Lewis Shale Formation (Figure 2). The Lewis Shale is directly overlain by the Pictured Cliffs Sandstone, the gas reservoir which the shot was designed to fracture. The Ojo Alamo Sandstone, the aquifer of concern at the site, is separated from the Pictured Cliffs by the Fruitland Sandstone and the Kirtland Shale. The Ojo Alamo is a fine- to medium-grained, clayey Tertiary sandstone containing minor shale beds (Mercer, 1967). The bottom of the Ojo Alamo is 182 m above the shot The Gasbuggy site is located in the San Juan Basin, which has a sedimentary thickness that ranges between 3,048 and 4,572 m. Although the San Jose Formation, Nacimiento Formation, and Ojo Alamo Sandstone all produce water at the Gasbuggy site, the Ojo Alamo was thought to be the only aquifer which could be affected by the nuclear shot. Based on this assumption, hydrologic testing was restricted to the Ojo Alamo, and the occurrence and movement of water in the other formations is poorly understood (Chapman and Hokett, 1991). Water in the San Jose-Nacimiento-surface alluvium system is present within 100 m of ground surface. Both the San Jose and Nacimiento Formations are composed of sandstones and mudstones, but no information is provided as to whether the mudstones are of sufficient extent to act as confining layers (Stone et al., 1983). The top of the Ojo Alamo Formation is 1,056 m below ground surface, but confining pressure causes the potentiometric surface to be approximately 300 m below ground surface. Although randomly oriented joints present throughout the San Juan Basin may conduct some groundwater flow, pore flow is believed to dominate. It is thought that no major natural connections between any of the water-bearing strata exist at the Gasbuggy site (Sokol, 1970). The Ojo Alamo probably has its primary recharge area in the southeastern portion of the San Juan Basin (DOE. 1986). The regional direction of groundwater flow is poorly defined, but is

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thought to be predominantly westward (Sokol, 1970) or northwestward (Mercer, 1967), with the San Juan River (Figure 1) being the major discharge area (Sokol, 1970). There are two potential sources for groundwater contamination by radionuclides at the Gasbuggy site: surface contamination from site activities (e.g., drillback operations and production testing), and subsurface contamination resulting from the radionuclides produced by the shot. Contamination of soil water appears to have occurred in the vicinity of surface ground zero (SGZ) (Eberline Instrument Corporation, 1979). This contamination resulted from the release of approximately 2.5 x 1015 pCi of 3H during gas flow testing operations (Holzer, 1970). The released tritium was dispersed over an unknown, but presumably large, area by prevailing winds. The maximum concentration found during an air monitoring program at the time of the flow testing was 483 m from SGZ (DOE, 1986). Assuming the released mass was spread twice the distance at which the maximum concentration was measured, it would cover a circle centered on SGZ with a radius of 966 m. Further assuming that the tritium was evenly distributed in this area and instantaneously mixed throughout the root zone (assumed to be 1 m thick, with a porosity of 30 percent, of which Figure 2. Generalized geologic cross section at the Gasbuggy emplacement hole. After DOE (1986) and Stone et al. (1983).

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15 percent is saturated) once deposited on the land surface, this would produce a soil water The highest soil-water tritium concentration measured during site clean-up operations was 1.3 x 106 pCi/L, near the gas flaring stack (Eberline Instrument Corporation, 1979) (a pre-cleanup survey conducted by the El Paso Natural Gas Company (EPNG) recorded a peak value of 1.12 x 107 …