Amargosa Desert Research Site Research Articles

Spatial Fingerprinting of Biogenic and Anthropogenic Volatile Organic Compounds in an Arid Unsaturated Zone

Core Ideas 17 years of VOC data show variations around and far from a mixed‐waste landfill. Principal components analysis fingerprints landfill and ambient VOCs. Biogenic VOCs occur throughout the deep unsaturated zone and outnumber landfill VOCs. Anthropogenic and biogenic sources at a single site complicate analysis of VOC transport. Subsurface volatile organic compounds (VOCs) can pose risks to human and environmental health and mediate biological processes. Volatile organic compounds have both anthropogenic and biogenic origins, but the relative importance of these sources has not been explored in subsurface environments. This study synthesized 17 yr of VOC data from the Amargosa Desert Research Site in Nevada with the goal of improving understanding of spatial and temporal variations that distinguish sources of VOCs from a landfill and from ambient sources including biogenic VOCs (bVOCs). Gas samples were collected from 1999 to 2016 from an array of shallow sample points (0.5‐ and 1.5‐m depth) and from vertical profiles at three deep boreholes: two (109 m deep) near the border of a waste facility (33 and 100 m distant), and one (29 m deep) in a remote area 3 km to the south. Samples were analyzed for target VOCs and a subset was analyzed for non‐target VOCs to enumerate a greater variety of potential bVOCs. Principal components analysis of the target and non‐target VOCs provided an assessment of the spatial variability of VOCs originating from the landfill site and from ambient sources. Ambient VOCs occurred at all sample sites across a range of depths, and most were consistent with biogenic origins, indicating for the first time the presence of bVOCs in the deep unsaturated zone. Because some VOCs have both anthropogenic and biogenic sources, discrimination of sources can be important for estimating the extent and migration of anthropogenic plumes in arid unsaturated zones.

Unsaturated Zone CO2, CH4, and δ13C‐CO2 at an Arid Region Low‐Level Radioactive Waste Disposal Site

Core Ideas CO2–C isotopic composition discriminates sources and shows mixing. The shallow root zone is key in shallow CO2 and CH4 production and cycling. Differences in CO2 and CH4 distributions are explained by interactions with pore water. Elevated tritium, radiocarbon, Hg, and volatile organic compounds associated with low‐level radioactive waste (LLRW) at the USGS Amargosa Desert Research Site (ADRS) have stimulated research on factors and processes that affect contaminant gas distribution and transport. Consequently, we examined the sources, mixing, and biogeochemistry of CO2 and CH4, two additional important species in the unsaturated zone at ADRS. In spring 2015 and 2016, shallow unsaturated zone gas samples were collected from the 1.5‐m depth both inside and outside the LLRW disposal area. Samples also were collected from two 110‐m‐deep multilevel gas‐sampling boreholes and a distant background site. These samples were analyzed for CO2 mole fraction (xCO2) and C isotopic composition (δ13C‐CO2) and CH4 mole fraction (xCH4). Graphical analysis of the results indicates mixing of CO2 characteristic of the root zone (δ13C −18 to −19‰), deep soil gas of the capillary fringe (−13 to −15‰), and CO2 produced by microbial respiration of organic matter disposed in the LLRW trenches (−22 to −25‰). Distribution of CH4 overall reflects atmospheric sources and production in anaerobic microzones in the LLRW area and methanotrophy in the undisturbed shallow subsurface outside the LLRW area. Although xCH4 reflecting lateral transport from the LLRW area is decreasing with time in the deep profiles, deep unsaturated zone xCO2 has changed little in recent decades. The results imply that CH4 and δ13C‐CO2 may serve as good tracers of anthropogenic effects in the unsaturated zone even when CO2 primarily reflects natural processes.

Steady state fractionation of heavy noble gas isotopes in a deep unsaturated zone

AbstractTo explore steady state fractionation processes in the unsaturated zone (UZ), we measured argon, krypton, and xenon isotope ratios throughout a ∼110 m deep UZ at the United States Geological Survey (USGS) Amargosa Desert Research Site (ADRS) in Nevada, USA. Prior work has suggested that gravitational settling should create a nearly linear increase in heavy‐to‐light isotope ratios toward the bottom of stagnant air columns in porous media. Our high‐precision measurements revealed a binary mixture between (1) expected steady state isotopic compositions and (2) unfractionated atmospheric air. We hypothesize that the presence of an unsealed pipe connecting the surface to the water table allowed for direct inflow of surface air in response to extensive UZ gas sampling prior to our first (2015) measurements. Observed isotopic resettling in deep UZ samples collected a year later, after sealing the pipe, supports this interpretation. Data and modeling each suggest that the strong influence of gravitational settling and weaker influences of thermal diffusion and fluxes of CO2 and water vapor accurately describe steady state isotopic fractionation of argon, krypton, and xenon within the UZ. The data confirm that heavy noble gas isotopes are sensitive indicators of UZ depth. Based on this finding, we outline a potential inverse approach to quantify past water table depths from noble gas isotope measurements in paleogroundwater, after accounting for fractionation during dissolution of UZ air and bubbles.

Field‐Scale Sulfur Hexafluoride Tracer Experiment to Understand Long Distance Gas Transport in the Deep Unsaturated Zone

A natural gradient SF6 tracer experiment provided an unprecedented evaluation of long distance gas transport in the deep unsaturated zone (UZ) under controlled (known) conditions. The field‐scale gas tracer test in the 110‐m‐thick UZ was conducted at the U.S. Geological Survey's Amargosa Desert Research Site (ADRS) in southwestern Nevada. A history of anomalous (theoretically unexpected) contaminant gas transport observed at the ADRS, next to the first commercial low‐level radioactive waste disposal facility in the United States, provided motivation for the SF6 tracer study. Tracer was injected into a deep UZ borehole at depths of 15 and 48 m, and plume migration was observed in a monitoring borehole 9 m away at various depths (0.5–109 m) over the course of 1 yr. Tracer results yielded useful information about gas transport as applicable to the spatial scales of interest for off‐site contaminant transport in arid unsaturated zones. Modeling gas diffusion with standard empirical expressions reasonably explained SF6 plume migration, but tended to underpredict peak concentrations for the field‐scale experiment given previously determined porosity information. Despite some discrepancies between observations and model results, rapid SF6 gas transport commensurate with previous contaminant migration was not observed. The results provide ancillary support for the concept that apparent anomalies in historic transport behavior at the ADRS are the result of factors other than nonreactive gas transport properties or processes currently in effect in the undisturbed UZ.

Volatile Organic Compounds in the Unsaturated Zone from Radioactive Wastes

Volatile organic compounds (VOCs) are often comingled with low-level radioactive wastes (LLRW), but little is known about subsurface VOC emanations from LLRW landfills. The current study systematically quantified VOCs associated with LLRW over an 11-yr period at the USGS Amargosa Desert Research Site (ADRS) in southwestern Nevada. Unsaturated-zone gas samples of VOCs were collected by adsorption on resin cartridges and analyzed by thermal desorption and GC/MS. Sixty of 87 VOC method analytes were detected in the 110-m-thick unsaturated zone surrounding a LLRW disposal facility. Chlorofluorocarbons (CFCs) were detected in 100% of samples collected. Chlorofluorocarbons are powerful greenhouse gases, deplete stratospheric ozone, and are likely released from LLRW facilities worldwide. Soil-gas samples collected from a depth of 24 m and a horizontal distance 100 m south of the nearest waste-disposal trench contained >60,000 ppbv total VOCs, including >37,000 ppbv CFCs. Extensive sampling in the shallow unsaturated zone (0-2 m deep) identified areas where total VOC concentrations exceeded 5000 ppbv at the 1.5-m depth. Volatile organic compound concentrations exceeded background levels up to 300 m from the facility. Maximum vertical diffusive fluxes of total VOCs were estimated to be 1 g m yr. Volatile organic compound distributions were similar but not identical to those previously determined for tritium and elemental mercury. To our knowledge, this study is the first to characterize the unsaturated zone distribution of VOCs emanating from a LLRW landfill. Our results may help explain anomalous transport of radionuclides at the ADRS and elsewhere.

Evaluating Coupled Water, Vapor, and Heat Flows and Their Influence on Moisture Dynamics in Arid Regions

The flow and thermal driven processes are crucial for the soil water dynamics in the vadose zone in arid regions. The understanding of the soil moisture and heat distribution is important for assessing the fate and transport of contaminants for the waste-burial site in a desert setting. A coupled water, vapor, and heat flow model was used in this study for evaluating the interactions of these processes and their influence on the moisture dynamics in the Amargosa Desert Research Site (ADRS) in southern Nevada. The treatment of preferential flow was also included in the model. Data from a previously conducted tracer experiment were used for estimating initial parameters of preferential flux and the parameters in pedotransfer functions (PTFs), while the Levenberg-Marquardt algorithm was used to optimize various parameters in the model calibration on the basis of the observation data. After the model calibration, three-year simulations (1998–2000) were conducted for evaluating the spatiotemporal variability of soil moisture and temperature in the site. The simulation results show that the performance of thermal model with vapor and preferential flow is close to those of isothermal model without vapor and preferential flow as compared with the observed records in ADRS, while critical physical mechanisms could be well described with the thermal model. The vapor flow for the water budget on the upper boundary is insignificant while it influences the water content distribution in the soil profile; the wetting front depth of preferential flow and water budget has increased in rain days; however, the high evaporation counteracts it in the annual water budget. The thermal diffusion of water vapor and thermal liquid fluxes were affected by temperature fluctuation and soil water saturation status. Comparison between the flux components shows that the thermal driven flux has more weight on the water movement in the soil profile. Results indicate that the coupled flow and thermal model could well describe the natural condition in arid regions and is more efficient in simulating mutual effects among various processes.

Transport of Tritium Contamination to the Atmosphere in an Arid Environment

Soil–plant–atmosphere interactions strongly influence water movement in desert unsaturated zones, but little is known about how such interactions affect atmospheric release of subsurface water‐borne contaminants. This 2‐yr study, performed at the U.S. Geological Survey's Amargosa Desert Research Site in southern Nevada, quantified the magnitude and spatiotemporal variability of tritium (3H) transport from the shallow unsaturated zone to the atmosphere adjacent to a low‐level radioactive waste (LLRW) facility. Tritium fluxes were calculated as the product of 3H concentrations in water vapor and respective evaporation and transpiration water‐vapor fluxes. Quarterly measured 3H concentrations in soil water vapor and in leaf water of the dominant creosote‐bush [Larrea tridentata (DC.) Coville] were spatially extrapolated and temporally interpolated to develop daily maps of contamination across the 0.76‐km2 study area. Maximum plant and root‐zone soil concentrations (4200 and 8700 Bq L−1, respectively) were measured 25 m from the LLRW facility boundary. Continuous evaporation was estimated using a Priestley–Taylor model and transpiration was computed as the difference between measured eddy‐covariance evapotranspiration and estimated evaporation. The mean evaporation/transpiration ratio was 3:1. Tritium released from the study area ranged from 0.12 to 12 μg d−1 and totaled 1.5 mg (8.2 × 1010 Bq) over 2 yr. Tritium flux variability was driven spatially by proximity to 3H source areas and temporally by changes in 3H concentrations and in the partitioning between evaporation and transpiration. Evapotranspiration removed and limited penetration of precipitation beneath native vegetation and fostered upward movement and release of 3H from below the root zone.

Transport of elemental mercury in the unsaturated zone from a waste disposal site in an arid region

Mercury contained in buried landfill waste may be released via upward emission to the atmosphere or downward leaching to groundwater. Data from the US Geological Survey’s Amargosa Desert Research Site (ADRS) in arid southwestern Nevada reveal another potential pathway of Hg release: long-distance (102 m) lateral migration of elemental Hg (Hg0) through the unsaturated zone. Gas collected from multiple depths from two instrumented boreholes that sample the entire 110-m unsaturated zone thickness and are located 100 and 160 m away from the closest waste burial trench exhibit gaseous Hg concentrations of up to 33 and 11 ng m−3, respectively. The vertical distribution of gaseous Hg in the borehole closest to the disposal site shows distinct subsurface peaks in concentration at depths of 1.5 and 24 m that cannot be explained by radial diffusive transport through a heterogeneous layered unsaturated zone. The inability of current models to explain gaseous Hg distribution at the ADRS highlights the need to advance the understanding of gas-phase contaminant transport in unsaturated zones to attain a comprehensive model of landfill Hg release.

Field data validation of an algorithm for computing obsidian effective hydration temperature

This paper describes an analysis to validate the effective hydration temperature (EHT) algorithm for obsidian [Rogers, A.K., 2007. Effective hydration temperature of obsidian: a diffusion-theory analysis of time-dependent hydration rates. J. Arch. Sci. 34, 656-665], using hourly temperature data from the Amargosa Desert Research Site near Beatty, NV. These data were used as input to a numerical model of the temperature-dependent diffusion process, and EHT was calculated yearly and for the aggregate. The same temperature data were processed to extract input parameters for the climatic model reported and used as input for EHT computation. EHT as computed from recorded hourly data was found to be within 1 °C of that computed from the climatic model. It is also shown that computed EHT differences between sites are not sensitive to whether air or surface temperature data are used, as long as they are used consistently; however, surface temperature data must be used if depth corrections are to be made.

Modeling Tritium Transport Through a Deep Unsaturated Zone in an Arid Environment

Understanding transport of tritium (3H) in unsaturated zones is critical to evaluating options for waste isolation. Tritium typically is a large component of low‐level radioactive waste (LLRW). Studies at the U.S. Geological Survey's Amargosa Desert Research Site (ADRS) in Nevada investigate 3H transport from a closed LLRW facility. Two boreholes are 100 and 160 m from the nearest waste trench and extend to the water table at 110 m. Soil‐water vapor samples from the deep boreholes show elevated levels of 3H at all depths. The objectives of this study were to (i) test source thermal and gas‐advection mechanisms driving 3H transport and (ii) evaluate model sensitivity to these mechanisms and to selected physical and hydraulic properties including porosity, tortuosity, and anisotropy. A two‐dimensional numerical model incorporated a non‐isothermal, heterogeneous domain of the unsaturated zone and instantaneous isotopic equilibrium. The TOUGH2 code was used; however, it required modification to account for temperature dependence of both the Henry's law equilibrium constant and isotopic fractionation with respect to tritiated water. Increases in source temperature, pressure, and porosity enhanced 3H migration, but failed to match measured 3H distributions. All anisotropic simulations with a source pressure component resembled, in shape, the upper portion of the 3H distribution of the nearest borehole. Isotopic equilibrium limited migration of 3H, while effects of radioactive decay were negligible. A 500 Pa pressure increase above ambient pressure in conjunction with a high degree of anisotropy (1:100) was necessary for simulated 3H transport to reach the nearest borehole.

CO2 dynamics in the Amargosa Desert: Fluxes and isotopic speciation in a deep unsaturated zone

Natural unsaturated‐zone gas profiles at the U.S. Geological Survey's Amargosa Desert Research Site, near Beatty, Nevada, reveal the presence of two physically and isotopically distinct CO2 sources, one shallow and one deep. The shallow source derives from seasonally variable autotrophic and heterotrophic respiration in the root zone. Scanning electron micrograph results indicate that at least part of the deep CO2 source is associated with calcite precipitation at the 110‐m‐deep water table. We use a geochemical gas‐diffusion model to explore processes of CO2 production and behavior in the unsaturated zone. The individual isotopic species 12CO2, 13CO2, and 14CO2 are treated as separate chemical components that diffuse and react independently. Steady state model solutions, constrained by the measured δ13C (in CO2), and δ14C (in CO2) profiles, indicate that the shallow CO2 source from root and microbial respiration composes ∼97% of the annual average total CO2 production at this arid site. Despite the small contribution from deep CO2 production amounting to ∼0.1 mol m−2 yr−1, upward diffusion from depth strongly influences the distribution of CO2 and carbon isotopes in the deep unsaturated zone. In addition to diffusion from deep CO2 production, 14C exchange with a sorbed CO2 phase is indicated by the modeled δ14C profiles, confirming previous work. The new model of carbon‐isotopic profiles provides a quantitative approach for evaluating fluxes of carbon under natural conditions in deep unsaturated zones.

Constraining the Inferred Paleohydrologic Evolution of a Deep Unsaturated Zone in the Amargosa Desert

Natural flow regimes in deep unsaturated zones of arid interfluvial environments are rarely in hydraulic equilibrium with near-surface boundary conditions imposed by present-day plant–soil–atmosphere dynamics. Nevertheless, assessments of water resources and contaminant transport require realistic estimates of gas, water, and solute fluxes under past, present, and projected conditions. Multimillennial transients that are captured in current hydraulic, chemical, and isotopic profiles can be interpreted to constrain alternative scenarios of paleohydrologic evolution following climatic and vegetational shifts from pluvial to arid conditions. However, interpreting profile data with numerical models presents formidable challenges in that boundary conditions must be prescribed throughout the entire Holocene, when we have at most a few decades of actual records. Models of profile development at the Amargosa Desert Research Site include substantial uncertainties from imperfectly known initial and boundary conditions when simulating flow and solute transport over millennial timescales. We show how multiple types of profile data, including matric potentials and porewater concentrations of Cl−, δD, δ18O, can be used in multiphase heat, flow, and transport models to expose and reduce uncertainty in paleohydrologic reconstructions. Results indicate that a dramatic shift in the near-surface water balance occurred approximately 16000 yr ago, but that transitions in precipitation, temperature, and vegetation were not necessarily synchronous. The timing of the hydraulic transition imparts the largest uncertainty to model-predicted contemporary fluxes. In contrast, the uncertainties associated with initial (late Pleistocene) conditions and boundary conditions during the Holocene impart only small uncertainties to model-predicted contemporaneous fluxes.

Constraining the Inferred Paleohydrologic Evolution of a Deep Unsaturated Zone in the Amargosa Desert

Natural flow regimes in deep unsaturated zones of arid interfluvial environments are rarely in hydraulic equilibrium with near‐surface boundary conditions imposed by present‐day plant–soil–atmosphere dynamics. Nevertheless, assessments of water resources and contaminant transport require realistic estimates of gas, water, and solute fluxes under past, present, and projected conditions. Multimillennial transients that are captured in current hydraulic, chemical, and isotopic profiles can be interpreted to constrain alternative scenarios of paleohydrologic evolution following climatic and vegetational shifts from pluvial to arid conditions. However, interpreting profile data with numerical models presents formidable challenges in that boundary conditions must be prescribed throughout the entire Holocene, when we have at most a few decades of actual records. Models of profile development at the Amargosa Desert Research Site include substantial uncertainties from imperfectly known initial and boundary conditions when simulating flow and solute transport over millennial timescales. We show how multiple types of profile data, including matric potentials and porewater concentrations of Cl−, δD, δ18O, can be used in multiphase heat, flow, and transport models to expose and reduce uncertainty in paleohydrologic reconstructions. Results indicate that a dramatic shift in the near‐surface water balance occurred approximately 16000 yr ago, but that transitions in precipitation, temperature, and vegetation were not necessarily synchronous. The timing of the hydraulic transition imparts the largest uncertainty to model‐predicted contemporary fluxes. In contrast, the uncertainties associated with initial (late Pleistocene) conditions and boundary conditions during the Holocene impart only small uncertainties to model‐predicted contemporaneous fluxes.