Research poster: Climate change impacts to groundwater, springs hydrology and aquatic communities Amargosa Desert and Death Valley National Park, Nevada and California

Research poste

Letter Report: Contaminant Boundary at the Shoal Underground Nuclear Test

As part of the corrective action strategy reached between the U.S. Department of Energy and the State of Nevada, the extent and potential impact of radionuclide contamination of groundwater at underground nuclear test locations must be addressed. This report provides the contaminant boundary for the Project Shoal Site, based on the groundwater flow and transport model for the site, by Pohlmann et al

Groundwater Flow and Thermal Modeling to Support a Preferred Conceptual Model for the Large Hydraulic Gradient North of Yucca Mountain

This task will create a two-dimensional, saturated zone, vertical cross-section model of groundwater flow and thermal transport through the large hydraulic gradient (LHG). This model is referenced herein as the thermal model. The scope of this study is limited to presenting a postulated hydrogeologic configuration of the LHG. The conceptualization will include the use of postulated hydrogeologic structures and material properties. The thermal model will be spatially limited to the area immediately upgradient and downgradient of the LHG and will not reproduce the many hydrogeologic features of the existing regional and site-scale models. The thermal model will be orientated north to south, approximately along a saturated zone streamline. The results of the thermal modeling will be compared to temperature data reported for site wells by the U.S. Geological Survey (USGS) and in peer-reviewed journals. Most, if not all, of this reported data is non- qualified. This task will not qualify the reported data and the reported data will be used only as a basis of comparison for the model simulations

Value of Information Analysis Project Gnome Site, New Mexico

The Project Gnome site in southeastern New Mexico was the location of an underground nuclear detonation in 1961 and a hydrologic tracer test using radionuclides in 1963. The tracer test is recognized as having greater radionuclide migration potential than the nuclear test because the tracer test radionuclides (tritium, 90Sr, 131I, and 137Cs) are in direct contact with the Culebra Dolomite aquifer, whereas the nuclear test is within a bedded salt formation. The tracer test is the topic here. Recognizing previous analyses of the fate of the Gnome tracer test contaminants (Pohll and Pohlmann, 1996; Pohlmann and Andricevic, 1994), and the existence of a large body of relevant investigations and analyses associated with the nearby Waste Isolation Pilot Plant (WIPP) site (summarized in US DOE, 2009), the Gnome Site Characterization Work Plan (U.S. DOE, 2002) called for a Data Decision Analysis to determine whether or not additional characterization data are needed prior to evaluating existing subsurface intrusion restrictions and determining long-term monitoring for the tracer test. Specifically, the Work Plan called for the analysis to weigh the potential reduction in uncertainty from additional data collection against the cost of such field efforts

Value of Information Analysis Project Gnome Site, New Mexico

The Project Gnome site in southeastern New Mexico was the location of an underground nuclear detonation in 1961 and a hydrologic tracer test using radionuclides in 1963. The tracer test is recognized as having greater radionuclide migration potential than the nuclear test because the tracer test radionuclides (tritium, 90Sr, 131I, and 137Cs) are in direct contact with the Culebra Dolomite aquifer, whereas the nuclear test is within a bedded salt formation. The tracer test is the topic here. Recognizing previous analyses of the fate of the Gnome tracer test contaminants (Pohll and Pohlmann, 1996; Pohlmann and Andricevic, 1994), and the existence of a large body of relevant investigations and analyses associated with the nearby Waste Isolation Pilot Plant (WIPP) site (summarized in US DOE, 2009), the Gnome Site Characterization Work Plan (U.S. DOE, 2002) called for a Data Decision Analysis to determine whether or not additional characterization data are needed prior to evaluating existing subsurface intrusion restrictions and determining long-term monitoring for the tracer test. Specifically, the Work Plan called for the analysis to weigh the potential reduction in uncertainty from additional data collection against the cost of such field efforts

Data Decision Analysis: Project Shoal

The purpose of this study was to determine the most appropriate field activities in terms of reducing the uncertainty in the groundwater flow and transport model at the Project Shoal area. The data decision analysis relied on well-known tools of statistics and uncertainty analysis. This procedure identified nine parameters that were deemed uncertain. These included effective porosity, hydraulic head, surface recharge, hydraulic conductivity, fracture correlation scale, fracture orientation, dip angle, dissolution rate of radionuclides from the puddle glass, and the retardation coefficient, which describes the sorption characteristics. The parameter uncertainty was described by assigning prior distributions for each of these parameters. Next, the various field activities were identified that would provide additional information on these parameters. Each of the field activities was evaluated by an expert panel to estimate posterior distribution of the parameters assuming a field activity was performed. The posterior distributions describe the ability of the field activity to estimate the true value of the nine parameters. Monte Carlo techniques were used to determine the current uncertainty, the reduction of uncertainty if a single parameter was known with certainty, and the reduction of uncertainty expected from each field activity on the model predictions. The mean breakthrough time to the downgradient land withdrawal boundary and the peak concentration at the control boundary were used to evaluate the uncertainty reduction. The radionuclide 137Cs was used as the reference solute, as its migration is dependent on all of the parameters. The results indicate that the current uncertainty of the model yields a 95 percent confidence interval between 42 and 1,412 years for the mean breakthrough time and an 18 order-of-magnitude range in peak concentration. The uncertainty in effective porosity and recharge dominates the uncertainty in the model predictions, while the other parameters are less important. A two-stage process was used to evaluate the optimal field activities. For all of the field activities combined there were five activities that were found to be "optimal" in terms of uncertainty reduction per unit cost: two-well, natural-gradient, energy budget, and single-well tracer tests, and the vadose zone modeling. A subset of the field activities was chosen such that there would be no duplication in parameter characterization. Of this subset, the vadose zone model, barometric test, energy budget, and the two-well tracer test were found to be optimal for the peak breakthrough time metric, while the single-well tracer test and the hydraulic head measurements are also considered optimal for the peak concentration metric. The environmental tracer activity was not found to be optimal, yet this activity may provide additional information on the transport system. Care must be taken in using this analysis to design a field characterization plan, as many assumptions were required in the analysis. First, many subjective assumptions were required to assess the reliability of the field activities in terms of their ability to reduce the uncertainty in the mean parameters. Actual field characterization may not result in the same reduction in model output uncertainty as estimated by this analysis. Second, this analysis focused on the reduction in model uncertainty due to the reduction in the uncertainty in the mean parameters. If the uncertainty in the mean parameters is reduced to zero, there still exists uncertainty in the natural heterogeneity that can never be reduced to zero. Therefore, this analysis should be used in combination with expert judgement when designing a field characterization strategy

Use of Numerical Groundwater Modeling to Evaluate Uncertainty in Conceptual Models of Recharge and Hydrostratigraphy

Numerical groundwater models are based on conceptualizations of hydrogeologic systems that are by necessity developed from limited information and therefore are simplifications of real conditions. Each aspect (e.g. recharge, hydrostratigraphy, boundary conditions) of the groundwater model is often based on a single conceptual model that is considered to be the best representation given the available data. However, the very nature of their construction means that each conceptual model is inherently uncertain and the available information may be insufficient to refute plausible alternatives, thereby raising the possibility that the flow model is underestimating overall uncertainty. In this study we use the Death Valley Regional Flow System model developed by the U.S. Geological Survey as a framework to predict regional groundwater flow southward into Yucca Flat on the Nevada Test Site. An important aspect of our work is to evaluate the uncertainty associated with multiple conceptual models of groundwater recharge and subsurface hydrostratigraphy and quantify the impacts of this uncertainty on model predictions. In our study, conceptual model uncertainty arises from two sources: (1) alternative interpretations of the hydrostratigraphy in the northern portion of Yucca Flat where, owing to sparse data, the hydrogeologic system can be conceptualized in different ways, and (2) uncertainty in groundwater recharge in the region as evidenced by the existence of several independent approaches for estimating this aspect of the hydrologic system. The composite prediction of groundwater flow is derived from the regional model that formally incorporates the uncertainty in these alternative input models using the maximum likelihood Bayesian model averaging method. An assessment of the joint predictive uncertainty of the input conceptual models is also produced. During this process, predictions of the alternative models are weighted by model probability, which is the degree of belief that a model is more plausible given available prior information (expert opinion) and site measurements (hydraulic head and groundwater flux). The results indicate that flow simulations in Yucca Flat are more sensitive to hydrostratigraphic model than recharge model. Furthermore, posterior model uncertainty is dominated by inter-model variance as opposed to intra-model variance, indicating that conceptual model uncertainty has greater impact on the results than parametric uncertainty. Without consideration of conceptual model uncertainty, uncertainty in the flow predictions would be significantly underestimated. Incorporation of the uncertainty in multiple conceptual models renders the groundwater flow model predictions more scientifically defensible

Assessing Groundwater Model Uncertainty for the Central Nevada Test Area

The purpose of this study is to quantify the flow and transport model uncertainty for the Central Nevada Test Area (CNTA). Six parameters were identified as uncertain, including the specified head boundary conditions used in the flow model, the spatial distribution of the underlying welded tuff unit, effective porosity, sorption coefficients, matrix diffusion coefficient, and the geochemical release function which describes nuclear glass dissolution. The parameter uncertainty was described by assigning prior statistical distributions for each of these parameters. Standard Monte Carlo techniques were used to sample from the parameter distributions to determine the full prediction uncertainty. Additional analysis is performed to determine the most cost-beneficial characterization activities. The maximum radius of the tritium and strontium-90 contaminant boundary was used as the output metric for evaluation of prediction uncertainty. The results indicate that combining all of the uncertainty in the parameters listed above propagates to a prediction uncertainty in the maximum radius of the contaminant boundary of 234 to 308 m and 234 to 302 m, for tritium and strontium-90, respectively. Although the uncertainty in the input parameters is large, the prediction uncertainty in the contaminant boundary is relatively small. The relatively small prediction uncertainty is primarily due to the small transport velocities such that large changes in the uncertain input parameters causes small changes in the contaminant boundary. This suggests that the model is suitable in terms of predictive capability for the contaminant boundary delineation

Evaluation of Potential Hydrocarbon Transport at the UC-4 Emplacement Hole, Central Nevada Test Area

Emplacement hole UC-4 was drilled in 1969 at the Central Nevada Test Area and left filled with drilling mud. Surface characterization samples collected from abandoned mud pits in the area yielded elevated concentrations of total petroleum hydrocarbon, thereby raising a concern that the mud-filled emplacement hole may be leaching hydrocarbons into alluvial aquifers. This study was initiated to address this concern. An analytical solution for flow near a wellbore was used to calculate the amount of time it would take for a contaminant to move through the mud-filled well and into the surrounding aquifer. No hydraulic data are available from the emplacement hole; therefore, ranges of hydraulic conductivity and porosity were used in 100 Monte Carlo realizations to estimate a median travel time. Laboratory experiments were performed on samples collected from the central mud pit to determine the hydrocarbon release function for the bentonite drilling mud. The median contaminant breakthrough took about 12,000 years to travel 10 m, while the initial breakthrough took about 300 years and the final breakthrough took about 33,000 years. At a distance of about 10 m away from the emplacement hole, transport velocity is dominated by the hydraulics of the aquifer and not by the emplacement hole hydraulics. It would take an additional 45,500 years for the contaminant to travel 800 m to the U.S. Department of Energy land exclusion boundary. Travel times were primarily affected by the hydraulic conductivity and porosity of the drilling mud, then by the hydraulic conductivity, porosity and hydraulic gradient of the alluvial aquifer, followed by the hydrocarbon release function