Jenny B. Chapman

Soil‐water flux in the Southern Great Basin, United States: Temporal and spatial variations over the last 120,000 years

The disposal of hazardous and radioactive waste in arid regions requires a thorough understanding of the occurrence of soil-water flux and recharge. Soil-water chemistry and isotopic data are presented from three deep vadose zone boreholes (>230 m) at the Nevada Test Site, located in the Great Basin geographic province of the southwestern United States, to quantify soil-water flux and its relation to climate. The low water contents found in the soils significantly reduce the mixing of tracers in the subsurface and provide a unique opportunity to examine the role of climate variation on recharge in arid climates. Tracing techniques and core data are examined in this work to reconstruct the paleohydrologic conditions existing in the vadose zone well beyond the timescales typically investigated. Stable chloride and chlorine 36 profiles indicate that the soil waters deep in the vadose zone range in age from approximately 20,000 to 120,000 years. Secondary chloride bulges that are present in two of the three profiles support the concept of recharge occurring at or near the last two glacial maxima, when the climate of the area was considerably wetter and cooler. The stable isotopic composition of the soil water in the profiles is significantly more depleted in heavy isotopes than is modern precipitation, suggesting that recharge under the current climate is not occurring at this arid site. Past and present recharge appears to have been strongly controlled by surface topography, with increased incidence of recharge where runoff from the surrounding mountains may have been concentrated. The data obtained from this detailed drilling and sampling program shed new light on the behavior of water in thick vadose zones and, in particular, show the sensitivity of arid regions to the extreme variations in climate experienced by the region over the last two glacial maxima.

A Model-Averaging Method for Assessing Groundwater Conceptual Model Uncertainty

This study evaluates alternative groundwater models with different recharge and geologic components at the northern Yucca Flat area of the Death Valley Regional Flow System (DVRFS), USA. Recharge over the DVRFS has been estimated using five methods, and five geological interpretations are available at the northern Yucca Flat area. Combining the recharge and geological components together with additional modeling components that represent other hydrogeological conditions yields a total of 25 groundwater flow models. As all the models are plausible given available data and information, evaluating model uncertainty becomes inevitable. On the other hand, hydraulic parameters (e.g., hydraulic conductivity) are uncertain in each model, giving rise to parametric uncertainty. Propagation of the uncertainty in the models and model parameters through groundwater modeling causes predictive uncertainty in model predictions (e.g., hydraulic head and flow). Parametric uncertainty within each model is assessed using Monte Carlo simulation, and model uncertainty is evaluated using the model averaging method. Two model-averaging techniques (on the basis of information criteria and GLUE) are discussed. This study shows that contribution of model uncertainty to predictive uncertainty is significantly larger than that of parametric uncertainty. For the recharge and geological components, uncertainty in the geological interpretations has more significant effect on model predictions than uncertainty in the recharge estimates. In addition, weighted residuals vary more for the different geological models than for different recharge models. Most of the calibrated observations are not important for discriminating between the alternative models, because their weighted residuals vary only slightly from one model to another.

Using Markov Chain Monte Carlo to quantify parameter uncertainty and its effect on predictions of a groundwater flow model

A statistical Bayesian framework is used to solve the inverse problem and develop the posterior distributions of parameters for a density-driven groundwater flow model. This Bayesian approach is implemented using a Markov Chain Monte Carlo (MCMC) sampling method. Three sets of data pertaining to the location of the freshwater-seawater transition zone exist for the site, including chemistry data, hydraulic head data and newly collected magnetotelluric (MT) data. A sequential conditioning approach is implemented where the chemistry data and MT-converted salinity are combined as a single data set and are used to first condition the parameter distributions. The head data are subsequently used as a second conditioning data set where the posterior distribution developed by the first conditioning is used as a prior for this second conditioning. Results of this analysis indicate that conditioning on the available data sets yields dramatic reduction of uncertainty compared to unconditioned simulations, especially for the recharge-conductivity ratio. This ratio controls the location of the transition zone, and the conditioning results in a smaller range of variability compared to the distribution used in previous modelling of the site. Using the conditioned distributions to solve the density-driven flow problem in a stochastic framework (i.e., model parameters are randomly sampled from the posterior distributions) results in a range of output flow fields that is much narrower than the previous model. The ensemble mean of these solutions and the uncertainty bounds expressed by the mean+/-one standard deviation lie within the uncertainty bounds of the original model. For the case study shown here, the effect of conditioning data is dominant over the effect of prior information.

Measuring the specific surface area of natural and manmade glasses: effects of formation process, morphology, and particle size

The specific surface area of natural and manmade solid materials is a key parameter controlling important interfacial processes in natural environments and engineered systems, including dissolution reactions and sorption processes at solid/fluid interfaces. To improve our ability to quantify the release of trace elements trapped in natural glasses, the release of hazardous compounds trapped in manmade glasses, or the release of radionuclides from nuclear melt glass, we measured the specific surface area of natural and manmade glasses as a function of particle size, morphology, and composition. Volcanic ash, volcanic tuff, tektites, obsidian glass, and in situ vitrified rock were analyzed. Specific surface area estimates were obtained using krypton as gas adsorbent and the BET model. The range of surface areas measured exceeded three orders of magnitude. A tektite sample had the highest surface area (1.65 m 2 g 1 ), while one of the samples of in situ vitrified rock had the lowest surface area (0.0016 m 2 g 1 ). The specific surface area of the samples was a function of particle size, decreasing with increasing particle size. Different types of materials, however, showed variable dependence on particle size, and could be assigned to one of three distinct groups: (1) Samples with low surface area dependence on particle size and surface areas approximately two orders of magnitude higher than the surface area of smooth spheres of equivalent size. The specific surface area of these materials was attributed mostly to internal porosity and surface roughness. (2) Samples that showed a trend of decreasing surface area dependence on particle size as the particle size increased. The minimum specific surface area of these materials was between 0.1 and 0.01 m 2 g 1 and was also attributed to internal porosity and surface roughness. (3) Samples whose surface area showed a monotonic decrease with increasing particle size, never reaching an ultimate surface area limit within the particle size range examined. The surface area results were consistent with particle morphology, examined by scanning electron microscopy, and have significant implications for the release of radionuclides and toxic metals in the environment. # 2002 Elsevier Science B.V. All rights reserved.

Isotopic investigation of infiltration and unsaturated zone flow processes at Carlsbad Cavern, New Mexico

Stable isotopes of hydrogen and oxygen together with tritium were used to investigate infiltration and unsaturated zone flow at Carlsbad Cavern. Water samples were collected from drips and pools every 4 months for a year. Seepage flow dominates infiltration at the cavern, homogenizing the isotopic composition of individual and seasonal precipitation events early along the vertical flowpath. Within the relatively narrow range in the stable isotopic ratios of cave drip water (σD of −53 tc −44%.σ18O of −8.3 to −85%.), isotopic changes on two different time-scales were identified. A longer-term variation, probably driven by changes in the relative amount of heavy-isotope-enriched summer precipitation, is evident in the slight enrichment of drips collected at shallower depths in the cavern (above 1100 m elevation). This suggests vertical travel times to the main cave rooms of the order of decades, consistent with observed tritium activities (from less than 4.6 to 16.9 tritium units (T.U.)). Given these residence times and the narrow range in isotopic composition, a seasonal trend in the isotopic composition of cave water (coincident at all levels in the cavern) is probably related to seasonal changes in the cave climate. Active air circulation driven by differences in air temperature between the cave and outside during the winter increases the evaporation rate from cave pools and causes enrichment in heavy isotopes. This signal may be transmitted to infiltrating water via isotope exchange before the water drips into the cave, consistent with the temporary storage of drip water in limited zones of saturation around the cave rooms. These zones may develop in response to capillary-driven surface tension before infiltrating water drips into the unsaturated-flow barriers represented by the large, air-filled cave passages.

Modeling ground water flow and radioactive transport in a fractured aquifer.

Three-dimensional numerical modeling is used to characterize ground water flow and contaminant transport at the Shoal nuclear test site in north-central Nevada. The fractured rock aquifer at the site is modeled using an equivalent porous medium approach. Field data are used to characterize the fracture system into classes: large, medium, and no/small fracture zones. Hydraulic conductivities are assigned based on discrete interval measurements. Contaminants from the Shoal test are assumed to all be located within the cavity. Several challenging issues are addressed in this study. Radionuclides are apportioned between surface deposits and volume deposits in nuclear melt glass, based on their volatility and previous observations. Surface-deposited radionuclides are released hydraulically after equilibration of the cavity with the surrounding ground water system, and as a function of ground water flow through the higher-porosity cavity into the low-porosity surrounding aquifer. Processes that are modeled include the release functions, retardation, radioactive decay, prompt injection, and ingrowth of daughter products. Prompt injection of radionuclides away from the cavity is found to increase the arrival of mass at the control plane but is not found to significantly impact calculated concentrations due to increased spreading. Behavior of the other radionuclides is affected by the slow chemical release and retardation behavior. The transport calculations are sensitive to many flow and transport parameters. Most important are the heterogeneity of the flow field and effective porosity. The effect of porosity in radioactive decay is crucial and has not been adequately addressed in the literature. For reactive solutes, retardation and the glass dissolution rate are also critical.

Description of hydrogeologic heterogeneity and evaluation of radionuclide transport at an underground nuclear test

Realistic models of contaminant transport in groundwater demand detailed characterization of the spatial distribution of subsurface hydraulic properties, while at the same time programmatic constraints may limit collection of pertinent hydraulic data. Fortunately, alternate forms of data can be used to improve characterization of spatial variability. We utilize a methodology that augments sparse hydraulic information (hard data) with more widely available hydrogeologic information to generate equiprobable maps of hydrogeologic properties that incorporate patterns of connected permeable zones. Geophysical and lithologic logs are used to identify hydrogeologic categories and to condition stochastic simulations using Sequential Indicator Simulation (SIS). The resulting maps are populated with hydraulic conductivity values using field data and Sequential Gaussian Simulation (SGS). Maps of subsurface hydrogeologic heterogeneity are generated for the purpose of examining groundwater flow and transport processes at the Faultless underground nuclear test, Central Nevada Test Area (CNTA), through large-scale, three-dimensional numerical modeling. The maps provide the basis for simulation of groundwater flow, while transport of radionuclides from the nuclear cavity is modeled using particle tracking methods. Sensitivity analyses focus on model parameters that are most likely to reduce the long travel times observed in the base case. The methods employed in this study have improved our understanding of the spatial distribution of preferential flowpaths at this site and provided the critical foundation on which to build models of groundwater flow and transport. The results emphasize that the impacts of uncertainty in hydraulic and chemical parameters are dependent on the radioactive decay of specific species, with rapid decay magnifying the effects of parameters that change travel time.

Potential use of time domain reflectometry for measuring water content in rock

Abstract Quantifying water movement through bedrock materials requires a technique which can accurately measure water content at precise locations within the medium. Time Domain Reflectometry (TDR) is widely used to measure the water content of soil and has a measurement sensitivity which is tightly confined to the area immediately surrounding the probes. This study applies TDR to the measurement of water content in rocks. The technique was evaluated using sandstone and welded tuff in a series of laboratory experiments. A block of each rock type was instrumented with a series of evenly spaced TDR probes. Each block was first saturated with water and then allowed to air dry. The water content was determined gravimetrically using the TDR over a period of several weeks. The results indicate that the TDR technique can be used to determine the water content of rock with an accuracy comparable with that reported for the technique used in soils.

Modeling Groundwater Flow and Transport of Radionuclides at Amchitka Island's Underground Nuclear Tests: Milrow, Long Shot, and Cannikin

Since 1963, all United States nuclear tests have been conducted underground. A consequence of this testing has been the deposition of large amounts of radioactive material in the subsurface, sometimes in direct contact with groundwater. The majority of this testing occurred on the Nevada Test Site (NTS), but a limited number of experiments were conducted in other locations. One of these locations, Amchitka Island, Alaska is the subject of this report. Three underground nuclear tests were conducted on Amchitka Island. Long Shot was an 80-kiloton-yield test conducted at a depth of 700 meters (m) on October 29, 1965 (DOE, 2000). Milrow had an announced yield of about 1,000 kilotons, and was detonated at a depth of 1,220 m on October 2, 1969. Cannikin had an announced yield less than 5,000 kilotons, and was conducted at a depth of 1,790 m on November 6, 1971. The purpose of this work is to provide a portion of the information needed to conduct a human-health risk assessment of the potential hazard posed by the three underground nuclear tests on Amchitka Island. Specifically, the focus of this work is the subsurface transport portion, including the release of radionuclides from the underground cavities and their movement through the groundwater system to the point where they seep out of the ocean floor and into the marine environment. This requires a conceptual model of groundwater flow on the island using geologic, hydrologic, and chemical information, a numerical model for groundwater flow, a conceptual model of contaminant release and transport properties from the nuclear test cavities, and a numerical model for contaminant transport. Needed for the risk assessment are estimates of the quantity of radionuclides (in terms of mass flux) from the underground tests on Amchitka that could discharge to the ocean, the time of possible discharge, and the location in terms of distance from shoreline. The radionuclide data presented here are all reported in terms of normalized masses to avoid presenting classified information. As only linear processes are modeled, the results can be readily scaled by the true classified masses for use in the risk assessment. The modeling timeframe for the risk assessment was set at 1,000 years, though some calculations are extended to 2,000 years. This first section of the report endeavors to orient the reader with the environment of Amchitka and the specifics of the underground nuclear tests. Of prime importance are the geologic and hydrologic conditions of the subsurface. A conceptual model for groundwater flow beneath the island is then developed and paired with an appropriate numerical modeling approach in section 2. The parameters needed for the model, supporting data for them, and data uncertainties are discussed at length. The calibration of the three flow models (one for each test) is then presented. At this point the conceptual radionuclide transport model is introduced and its numerical approach described in section 3. Again, the transport parameters and their supporting data and uncertainties are the focus. With all of the processes and parameters in place, the first major modeling phase can be discussed in section 4. In this phase, a parametric uncertainty analysis is performed to determine the sensitivity of the transport modeling results to the uncertainties present in the parameters. This analysis is motivated by the recognition of substantial uncertainty in the subsurface conditions on the island and the need to incorporate that uncertainty into the modeling. The conclusion of the first phase determines the parameters to hold as uncertain through the main flow and transport modeling. This second, main phase of modeling is presented in section 5, with the contaminant breakthrough behavior of each test site addressed. This is followed by a sensitivity analysis in section 6, regarding the importance of additional processes that could not be supported in the main modeling effort due to lack of data. Finally, the results for the individual sites are compared, the sensitivities discussed, and final conclusions presented in section 7.

Uncertainty Analysis of Radionuclide Transport in a Fractured Coastal Aquifer with Geothermal Effects

Groundwater flow and radionuclide transport at the Milrow underground nuclear test site on Amchitka Island are modeled using two-dimensional numerical simulations. A multi-parameter uncertainty analysis is adapted and used to address the effects of uncertainties associated with the definition of the modeled processes and the values of the parameters governing these processes. In particular, we focus on the effects on radioactive transport of uncertainties associated with conduction and convection of heat relative to the uncertainties associated with other flow and transport parameters. These include recharge, hydraulic conductivity, fracture porosity, dispersivity and strength of matrix diffusion. The flow model is conceptualized to address the problem of density-driven flow under conditions of variable salinity and geothermal gradient. The conceptual transport model simulates the advection–dispersion process, the diffusion process from the high-velocity fractures into the porous matrix blocks, and radioactive decay.For this case study, the uncertainty of the recharge-conductivity ratio contributes the most to the output uncertainty (standard deviation of mass flux across the seafloor). The location of the freshwater–saltwater transition zone changes dramatically as this ratio changes with the thickness of the freshwater lens and the location of the seepage face changing as well. In the context of radionuclide transport from the nuclear test cavity that is located in the area where the transition zone is uncertain, travel times of radionuclide mass from the cavity to the seepage face along the seafloor are significantly impacted. The variation in transition zone location changes the velocity magnitude at the cavity location by a large factor (probably an order of magnitude). When this effect is combined with porosity and matrix diffusion uncertainty, the uncertainty of transport results becomes large. Although thermal parameters have an effect on the solution of the flow problem and also on travel times of radionuclides, the effect is relatively small compared to other flow and transport parameters.