Assessment of Hydrologic Transport of Radionuclides from the Rio Blanco Underground Nuclear Test Site, Colorado (WC-703)

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University and Community College System of Nevada Water Resources Center ASSESSMENT OF HYDROLOGIC TRANSPORT OF RADIONUCLIDES FROM THE RIO BLANCO UNDERGROUND NUCLEAR TEST SITE, Jenny Chapman, Sam Earman and Roko Andricevic Nevada Operations Office U.S. Department of Energy

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This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, mark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. This report has been reproduced directly from the best available copy. Available to DOE and DOE contractors from the Office of Scientific and Technical Information, P.O. Box 62, Oak Ridge, TN 37831; prices available from (423) 576-8401. Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 22161.

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Portions of this document may be illegible in electronic image products. Images are produced from the best available original

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ASSESSMENT OF HYDROLOGIC TRANSPORT OF RADIONUCLIDES FROM THE RIO BLANCO UNDERGROUND NUCLEAR TEST Jenny Chapman, Sam Earman 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 Rio Blanco site in west-central Colorado was the location of the simultaneous underground detonation of three 30-kiloton nuclear devices in 1973. The devices were located 1780, 1899, and 2039 m below ground surface in the Fort Union and Mesaverde formations. Although all the bedrock formations at the site are thought to contain water, those below the Green River Formation (below 1000 m depth) are also gas-bearing, and have very low permeabilities. The transport scenario evaluated was the migration of radionuclides from the blast-created cavity through the Fort Union Formation. Transport calculations were performed using the solute flux method, with input based on the limited data available for the site. Model results suggest that radionuclides from the test are contained entirely within the area currently administered by DOE. This modeling was performed to investigate how the uncertainty in various physical parameters affect radionuclide transport at the Rio Blanco 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 needed, confidence in transport predictions can be increased by obtaining more site data, including ascertaining the amount of radionuclides which would have been available for transport (i.e., not trapped in melt glass or vented during gas flow testing), and the hydraulic properties of the Fort Union Formation.

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LIST OF FIGURES.................................................................................................................. Hi LIST OF TABLES............................................................................................. Hi Discharge Mixing Area.............................................................................................. 12 Distance to Control Plane ......................................................................................... 13 Mean Groundwater Velocity ..................................................................................... 14 Spatial Variability in Hydraulic Conductivity........................................................ 16

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1. Rio Blanco site location map................................................................................... 2 2. Stratigraphic cross section of the Project Rio Blanco site showing the locations and depths of pertinent wells............................................................ 5 3. Schematic diagram of the emplacement well (RB-E-01) after plugging.......... 8 4. Postdetonation formation pressures at the Rio Blanco site.................................. 10 5. Tritium monitoring data from the Long-Term Hydrologic Monitoring Program conducted by the EPA............................................................................... 21 6. Modeled tritium concentrations at the drilling exclusion boundary that would result in the B aquifer subsystem of the Green River Formation from gaseous transport via the RB-E-01 annulus from the upper Rio Blanco chimney compared to tritium concentrations from monitoring wells in the Green River Formation downgradient of the test point........................................ 21 1. Results of groundwater transport calculations for radionuclides produced by the Rio Blanco test................................................................................................. 17

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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 Rio Blanco site is located in west-central Colorado, approximately 83 km northeast of Grand Junction (Figure 1). Project Rio Blanco was part of the Plowshare Program, and was designed to increase the natural'gas production of a low permeability reservoir formation. The project consisted of the simultaneous detonation of three 30-kiloton nuclear explosives in a 2,134-m-deep well on May 17,1973. The explosives were located at depths of 1,780,1,899, and 2,039 m below land surface (bis). 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. The Desert Research Institute (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 Rio Blanco 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 Rio Blanco 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 Rio Blanco nuclear detonations. This assessment strives to be as accurate as possible, but the lack of data requires that significant assumptions be made about release scenarios and several critical transport 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 investigations, not as a definitve assessment of radionuclide migration at the Rio Blanco site. The analysis relies solely on unclassified data availible 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 present at the Rio Blanco site, 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 Cvetkovic etal. (1992), Daniels etal. (1993), Andricevic etal. (1994), and Andricevic and Cvetkovic (1996) was used to model radionuclide transport from the Rio Blanco site. The employed modeling approach incorporates real physical phenomena, such

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Figure 1. Rio Blanco site location map (from Coffin et al., 1968).

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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 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 particular. 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 Rio Blanco, the mean velocity, U, is calculated using where K is the mean hydraulic conductivity, 1 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 pinK and variance a2^. The variance represents the variability of \nK 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, 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, a2\n(j, can be calculated as the sum of the variances of the other parameters, if sufficient data are available. The estimation error in U, o2in[/j 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, a2iny, depends on the number of measurements used to estimate the parameters in the Darcy equation. In the case of independent measurements, o2\nu - °2ln«>W, where o2\m is the variance in the velocity field and N is the number of measurements. For

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spatially correlated measurements, o2u is scaled by Na[1+q(N-1)], where ^ is an averaged spatial correlation between data points. 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, o2^, 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 Rio Blanco site are described by many authors, including Ege etal. (1968), Cordes (1969), CER Geonuclear Corporation (1971), Coffin etal. (1971), Weir (1972), Knutson (1973a), Knutson (1973b), and Knutson (1975). The Rio Blanco site is located in the Piceance Creek Basin, a large structural basin over 8,500 m thick, containing sedimentary rocks from Cambrian to recent age. Mineral resources in the Piceance Creek Basin include coal, oil, gas, and oil shale. Groundwater occurs in surficial alluvium found along major drainages, and in the Green River Formation (Figure 2). The Green River Formation is divided into three hydrologic units: the “A” subsystem, consisting of the Evacuation Creek Member and upper part of the Parachute Creek Member; the “B” subsystem, including the middle and lower parts of the Parachute Creek Member; and the Douglas Creek Member at the base of the formation, which is generally discontinuous and frequently contains gas (CER Geonuclear Corporation, 1971). The A and B subsystems are separated by the Mahogany Zone aquitard. The transmissivity of the aquifers generally decreases with depth, with the alluvium being the most permeable and the Douglas Creek Member the least. Below the Green River Formation (Wasatch, Fort Union, and Mesaverde formations), rocks are frequently gas-bearing, have low permeability, and consist of discontinuous sandstone lenses within clay and shale. The units below the Green River Formation are not known to yield water to wells (Coffin et al., 1975), and are not considered to constitute aquifers nor form viable groundwater systems (U.S. Atomic Energy Commission, 1972). In general, recharge to the bedrock aquifers occurs along the southern edge of the basin with groundwater flow northward through the center, eventually discharging into Piceance Creek, Yellow Creek, or the White River (Figure 1). There are two potential sources for groundwater contamination by radionuclides at the Rio Blanco site: near-surface contamination from site activities (e.g., flaring during production testing), and subsurface contamination resulting from the radionuclides produced by the detonation.

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Figure 2. Stratigraphic cross section of the Project Rio Blanco site showing the locations and depths of pertinent wells (from CER Geonuclear Corporation, 1973).

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Flaring of gas released 7.53 x 1013 pCi of tritium and 8.9 x 1011 pCi of 14C to the atmosphere (Colorado Department of Health, 1980), as well as isotopes of krypton, argon, and xenon that are not considered here because health effects are regulated for immersion of the human body in the gas, rather than ingestion (National Bureau of Standards, 1963). The amount of tritium released by the test was far less at Rio Blanco than at previous gas-stimulation tests, due to changes in the nuclear device design. During site cleanup operations in 1976,482 soil samples were collected from areas of known or possible fallout or spills (Eberline Instrument Corporation, 1978). The criterion for release of the area from DOE control was that residual tritium concentration in soil water be less than 3.0 x 107 pCi/L, and all samples collected had concentrations far below that level. The highest tritium concentration measured in soil water was 762,000 pCi/L, measured in a sample collected near the flare line separator. Nine samples had concentrations above 300,000 pCi/L and 78 had concentrations greater than …