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Transport of sorbing solutes in randomly heterogeneous formations: Spatial moments, macrodispersion, and parameter uncertainty

Description: Expressions for the spatial moments and macrodispersion tensor for sorbing solutes in heterogeneous formations were presented using a probabilistic model of a fluid residence time coupled with the particle position analysis. The fluid residence time was defined as a fraction of the actual time during which the particle stayed in the mobile fluid phase of the aquifer. The fluid residence time is a random variable whose variability comes as a result of the non-equilibrium sorption properties. The sorbing solute was assumed to be governed with first-order linear kinetics. The closed-form expressions were based on the stationarity in the kinetic process and on the first-order approximation in the hydraulic conductivity field and in the fluid residence time. The non-equilibrium effects were presented as a function of the spatial variability in hydraulic conductivity and temporal variability in the fluid residence time. The importance of the non-equilibrium processes in the field scale was found to be dependent on reaction rates, retardation factor, mean velocity, and on variance and correlation scale of the hydraulic conductivity. The time needed to reach the asymptotic macrodispersivity is dependent on the degree of non-equilibrium processes and distribution coefficient. The impact from the uncertainty in parameters upon the spatial moments was examined and compared with the organic tracer used in the Borden field experiment.
Date: June 1, 1993
Creator: Andricevic, R.
Partner: UNT Libraries Government Documents Department

Scoping calculations for groundwater transport of tritium from the Gnome Site, New Mexico

Description: Analytic solutions are employed to investigate potential groundwater transport of tritium from a radioactive tracer site near the Project Gnome site in southeastern New Mexico. The tracer test was conducted in 1963 and introduced significant quantities of radionuclides to the transmissive and laterally continuous Culebra dolomite. Groundwater in the Culebra near Gnome travels toward a regional discharge point at the Pecos River, a distance of about 10 to 15 km, depending on flow path. Groundwater transport of radionuclides from the Gnome site is therefore of interest due to the proximity of the accessible environment and the 31-year time period during which migration is likely to have occurred. The analytical stochastic solutions used incorporate the heterogeneity observed in the Culebra by treating transmissivity as a spatially correlated random field. The results indicate that significant spreading of tritium will occur in the Culebra dolomite as a result of the combination of relatively high transmissivity, high spatial variability, and high spatial correlation of transmissivity. Longitudinal spreading may cause a very small fraction of tritium mass to arrive at the Pecos River within the 31 years since the tracer test. However, dilution and transverse dispersion will act to distribute this mass over a very large volume, thereby reducing groundwater concentrations. Despite the high degree of spreading, the calculations indicate that most of the tritium remains near the source. At present, the center of mass is estimated to have moved approximately 260 m downgradient of the test location and about 95 percent of the mass is estimated to have remained within about 1 km downgradient.
Date: August 1, 1994
Creator: Pohlmann, K. & Andricevic, R.
Partner: UNT Libraries Government Documents Department

Three-dimensional mapping of equiprobable hydrostratigraphic units at the Frenchman Flat Corrective Action Unit, Nevada Test Site

Description: Geological and geophysical data are used with the sequential indicator simulation algorithm of Gomez-Hernandez and Srivastava to produce multiple, equiprobable, three-dimensional maps of informal hydrostratigraphic units at the Frenchman Flat Corrective Action Unit, Nevada Test Site. The upper 50 percent of the Tertiary volcanic lithostratigraphic column comprises the study volume. Semivariograms are modeled from indicator-transformed geophysical tool signals. Each equiprobable study volume is subdivided into discrete classes using the ISIM3D implementation of the sequential indicator simulation algorithm. Hydraulic conductivity is assigned within each class using the sequential Gaussian simulation method of Deutsch and Journel. The resulting maps show the contiguity of high and low hydraulic conductivity regions.
Date: September 1, 1996
Creator: Shirley, C.; Pohlmann, K. & Andricevic, R.
Partner: UNT Libraries Government Documents Department

Identification of potential groundwater flow paths using geological and geophysical data

Description: This project represents the first phase in the development of a methodology for generating three-dimensional equiprobable maps of hydraulic conductivity for the Nevada Test Site (NTS). In this study, potential groundwater flow paths were investigated for subsurface tuffs at Yucca Flat by studying how these units are connected. The virtual absence of site-specific hydraulic conductivity data dictates that as a first step a surrogate attribute (geophysical logs) be utilized. In this first phase, the connectivity patterns of densely welded ash-flow tuffs were studied because these tuffs are the most likely to form zones of high hydraulic conductivity. Densely welded tuffs were identified based on the response shown on resistivity logs and this information was transformed into binary indicator values. The spatial correlation of the indicator data was estimated through geostatistical methods. Equiprobable three-dimensional maps of the distribution of the densely-welded and nonwelded tuffs (i.e., subsurface heterogeneity) were then produced using a multiple indicator simulation formalism. The simulations demonstrate that resistivity logs are effective as soft data for indicating densely welded tuffs. The simulated welded tuffs reproduce the stratigraphic relationships of the welded tuffs observed in hydrogeologic cross sections, while incorporating the heterogeneity and anisotropy that is expected in this subsurface setting. Three-dimensional connectivity of the densely welded tuffs suggests potential groundwater flow paths with lengths easily over 1 km. The next phase of this investigation should incorporate other geophysical logs (e.g., gamma-gamma logs) and then calibrate the resulting soft data maps with available hard hydraulic conductivity data. The soft data maps can then augment the hard data to produce the final maps of the spatial distribution of hydraulic conductivity that can be used as input for numerical solution of groundwater flow and transport.
Date: September 1, 1994
Creator: Pohlmann, K. & Andricevic, R.
Partner: UNT Libraries Government Documents Department

Exposure assessment of groundwater transport of tritium from the Shoal Site

Description: The US 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). An NTS site-wide Environmental Impact Statement (EIS) is being prepared in 1995 and includes the two offsite test areas in Nevada: the Shoal site and the Central Nevada Test Area. At the time of these tests, evaluations of project safety and predictions of groundwater transport of contaminants were made, and the tests were deemed safe to the public. These early evaluations were not considered sufficient for the EIS, so DOE decided to perform a new exposure assessment for the Shoal site. The basic scenario evaluated for this exposure assessment is transport of tritium from the Shoal underground nuclear test by groundwater to a receptor well where an individual drinks the contaminated water for 70 years, centered around the time of peak tritium concentration. This scenario is entirely hypothetical because, as of 1995, there are no known occurrences of humans drinking water downgradient from the test. Four specific scenarios are analyzed because of uncertainty in flowpath direction. Two of these presume that wells are drilled at the boundary of the current DOE land withdrawal and are then used for drinking water supply. Wells do not currently exist at these locations and thus the resultant risks do not apply to any current populations; however, there are no controls to prevent such wells from being drilled in the future. The two other scenarios consider transport to the first existing wells along possible flowpaths. These wells are currently used only seasonally to water cattle, and as such, these risks also do not apply to ...
Date: April 1, 1995
Creator: Chapman, J.; Pohlmann, K. & Andricevic, R.
Partner: UNT Libraries Government Documents Department

Wellhead protection area delineation under uncertainty

Description: A program to protect groundwater resources used for water supply from all potential threats due to contamination was established in the Amendments to the Safe Drinking Water Act (SDWA). Goal of the State Wellhead Protection (WHP) Program is to ``protect wellhead areas within their jurisdiction from contaminants which may have any adverse effect on the health of persons.`` A major component of WHP is the determination of zones around water-supply wells called Wellhead Protection Areas (WHPAs) within which contaminant source assessment and management should be addressed. WHPAs are defined in the SDWA as ``the surface and subsurface area surrounding a water well or wellfield, supplying a public water system, through which contaminants are reasonably likely to move toward and reach such water well or wellfield.`` A total of 14 water-supply wells are currently being used at the Nevada Test Site (NTS). Eleven of the wells are used for potable water supplies and the remaining three wells are used for construction purposes only. Purpose of this study is to estimate WHPAs for each water-supply well at the NTS. Due to the limited information about the hydraulic properties needed for estimating the WHPAS, an approach that considered the uncertainty in the estimates of the hydraulic properties was developed and implemented.
Date: September 1, 1994
Creator: Jacobson, E.; Andricevic, R. & Hultin, T.
Partner: UNT Libraries Government Documents Department

Exposure assessment of groundwater transport of tritium from the Central Nevada Test Area

Description: This exposure assessment provides a range of possible human health risk at two locations due to groundwater transport from the Faultless underground nuclear test. These locations correspond to the boundary of the land under DOE control (where no wells currently exist) and the closest existing well (Six Mile Well). The range in excess risk is within the EPA goal for excess risk due to environmental contaminants (10{sup {minus}6}) at Six Mile Well. Calculations considering high spatial variability in hydraulic properties and/or high uncertainty in the mean groundwater velocity are also within the EPA goal. At the DOE boundary, the range in excess risk exceeds the EPA goal, regardless of the values of spatial variability and uncertainty. The range in values of excess risk can be reduced with additional field data from the site; however, incorporation of additional data, which would likely be obtained at great expense, is unlikely to result in significant refinement of the results.
Date: April 1, 1995
Creator: Pohlmann, K.; Chapman, J. & Andricevic, R.
Partner: UNT Libraries Government Documents Department

Assessment of hydrologic transport of radionuclides from the Rio Blanco underground nuclear test site, Colorado

Description: DOE is operating an environmental restoration program to characterize, remediate, and close non-Nevada Test Site locations used for nuclear testing. Evaluation of radionuclide transport by groundwater is part of preliminary risk analysis. These evaluations allow prioritization of test areas in terms of risk, provide a basis for discussions with regulators and the public about future work, and provide a framework for assessing site characterization data needs. The Rio Blanco site in Colorado was the location of the simultaneous detonation of three 30-kiloton nuclear devices. The devices were located 1780, 1899, and 2039 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 in 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 site, and to serve as a starting point for discussion regarding further investigation; 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. Confidence in transport predictions can be increased by obtaining more site data, including 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 formation. 38 refs., 6 figs., 1 tab.
Date: October 1996
Creator: Chapman, J.; Earman, S. & Andricevic, R.
Partner: UNT Libraries Government Documents Department

Assessment of hydrologic transport of radionuclides from the Gasbuggy underground nuclear test site, New Mexico

Description: 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, via fractures created by the shot. 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.
Date: September 1, 1996
Creator: Earman, S.; Chapman, J. & Andricevic, R.
Partner: UNT Libraries Government Documents Department

Near-field modeling in Frenchman Flat, Nevada Test Site

Description: The US Department of Energy (DOE) is investigating the effects of nuclear testing in underground test areas (the UGTA program) at the Nevada Test Site. The principal focus of the UGTA program is to better understand and define subsurface radionuclide migration. The study described in this report focuses on the development of tools for generating maps of hydrogeologic characteristics of subsurface Tertiary volcanic units at the Frenchman Flat corrective Action Unit (CAU). The process includes three steps. The first step involves generation of three-dimensional maps of the geologic structure of subsurface volcanic units using geophysical logs to distinguish between two classes: densely welded tuff and nonwelded tuff. The second step generates three-dimensional maps of hydraulic conductivity utilizing the spatial distribution of the two geologic classes obtained in the first step. Each class is described by a correlation structure based on existing data on hydraulic conductivity, and conditioned on the generated spatial location of each class. The final step demonstrates the use of the maps of hydraulic conductivity for modeling groundwater flow and radionuclide transport in volcanic tuffs from an underground nuclear test at the Frenchman Flat CAU. The results indicate that the majority of groundwater flow through the volcanic section occurs through zones of densely welded tuff where connected fractures provide the transport pathway. Migration rates range between near zero to approximately four m/yr, with a mean rate of 0.68 m/yr. This report presents the results of work under the FY96 Near-Field Modeling task of the UGTA program.
Date: December 1, 1996
Creator: Pohlmann, K.; Shirley, C. & Andricevic, R.
Partner: UNT Libraries Government Documents Department

Assessment of hydrologic transport of radionuclides from the Rulison Underground Nuclear Test Site, Colorado

Description: 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 Rulison site in west-central Colorado was the location of an underground detonation of a 40-kiloton nuclear device in 1969. The test took place 2,568 m below ground surface in the Mesaverde Formation. Though located below the regional water table, none of the bedrock formations at the site yielded water during hydraulic tests, indicating extremely low permeability conditions. The scenario evaluated was the migration of radionuclides from the blast-created cavity through the Mesaverde 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. The transport calculations are most sensitive to changes in the mean groundwater velocity and the correlation scale of hydraulic conductivity, with transport of strontium and cesium also sensitive to the sorption coefficient.
Date: September 1, 1996
Creator: Earman, S.; Chapman, J. & Andricevic, R.
Partner: UNT Libraries Government Documents Department

Risk-based screening analysis of ground water contaminated by radionuclides introduced at the Nevada Test Site (NTS)

Description: The Nevada Test Site (NTS) is located in the southwestern part of Nevada, about 105 km (65 mi) northwest of the city of Las Vegas. Underground tests of nuclear weapons devices have been conducted at the NTS since late 1962 and ground water beneath the NTS has been contaminated with radionuclides produced by these tests. This concern prompted this examination of the potential health risk to these individuals from drinking the contaminated ground water either at a location on the NTS (assuming loss of institutional control after 100 y) or at one offsite (considering groundwater migration). For the purpose of this assessment, a representative mix of the radionuclides of importance and their concentrations in ground water beneath the NTS were identified from measurements of radionuclide concentrations in groundwater samples-of-opportunity collected at the NTS. Transport of radionuclide-contaminated ground water offsite was evaluated using a travel-time-transport approach. At both locations of interest, potential human-health risk was calculated for an individual ingesting radionuclide-contaminated ground water over the course of a 70-y lifetime. Uncertainties about human physiological attributes, as well as about estimates of physical detriment per unit of radioactive material, were quantified and incorporated into the estimates of risk. The maximum potential excess lifetime risk of cancer mortality estimated for an individual at the offsite location ranges from 7 {times} 10{sup {minus}7} to 1 {times} 10{sup {minus}5}, and at the onsite location ranges from 3 {times} 10{sup {minus}3} to 2 {times} 10{sup {minus}2}. Both the offsite and the onsite estimates of risk are dominated by the lifetime doses from tritium. For the assessment of radionuclides in ground water, the critical uncertainty is their concentration today under the entire NTS.
Date: June 1, 1993
Creator: Daniels, J.I.; Anspaugh, L.R.; Andricevic, R. & Jacobson, R.L.
Partner: UNT Libraries Government Documents Department

Assessment of hydrologic transport of radionuclides from the Gnome underground nuclear test site, New Mexico

Description: 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 site 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 Gnome site in southeastern New Mexico was the location of an underground detonation of a 3.5-kiloton nuclear device in 1961, and a hydrologic tracer test using radionuclides in 1963. The tracer test involved the injection of tritium, {sup 90}Sr, and {sup 137}Cs directly into the Culebra Dolomite, a nine to ten-meter-thick aquifer located approximately 150 in below land surface. The Gnome nuclear test was carried out in the Salado Formation, a thick salt deposit located 200 in below the Culebra. Because salt behaves plastically, the cavity created by the explosion is expected to close, and although there is no evidence that migration has actually occurred, it is assumed that radionuclides from the cavity are released into the overlying Culebra Dolomite during this closure process. 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 may be present in concentrations exceeding drinking water regulations outside the drilling exclusion boundary established by DOE. Calculated mean tritium concentrations peak at values exceeding the U.S. Environmental Protection Agency drinking water standard of 20,000 pCi/L at distances of up to almost eight kilometers west of the nuclear test.
Date: September 1, 1996
Creator: Earman, S.; Chapman, J.; Pohlmann, K. & Andricevic, R.
Partner: UNT Libraries Government Documents Department

Evaluation of groundwater flow and transport at the Shoal underground nuclear test: An interim report

Description: Since 1962, all United States nuclear tests have been conducted underground. A consequence of this testing has been the deposition of large amounts of radioactive materials in the subsurface, sometimes in direct contact with groundwater. The majority of this testing occurred on the Nevada Test Site, but a limited number of experiments were conducted in other locations. One of these is the subject of this report, the Project Shoal Area (PSA), located about 50 km southeast of Fallon, Nevada. The Shoal test consisted of a 12-kiloton-yield nuclear detonation which occurred on October 26, 1963. Project Shoal was part of studies to enhance seismic detection of underground nuclear tests, in particular, in active earthquake areas. Characterization of groundwater contamination at the Project Shoal Area is being conducted by the US Department of Energy (DOE) under the Federal Facility Agreement and Consent Order (FFACO) with the State of Nevada Department of Environmental Protection and the US Department of Defense (DOD). This order prescribes a Corrective Action Strategy (Appendix VI), which, as applied to underground nuclear tests, involves preparing a Corrective Action Investigation Plan (CAIP), Corrective Action Decision Document (CADD), Corrective Action Plan, and Closure Report. The scope of the CAIP is flow and transport modeling to establish contaminant boundaries that are protective of human health and the environment. This interim report describes the current status of the flow and transport modeling for the PSA.
Date: July 1, 1998
Creator: Pohll, G.; Chapman, J.; Hassan, A.; Papelis, C.; Andricevic, R. & Shirley, C.
Partner: UNT Libraries Government Documents Department