Evaluation of Electromagnetic Induction as a Noninvasive Technique for Monitoring Water Movement into and Beneath Waste Disposal Facilities

The purpose of this study was to evaluate the use of electromagnetic induction to noninvasively monitor water content in waste disposal facility cover soils. We compared apparent electrical conductivity measurements monitored with the EM38 ground conductivity meter with water content monitored with a neutron probe at 20 locations over an 18-month period from August 1998 to January 2000. Two cover designs were monitored: a gel/asphalt barrier at 1.3 m depth and a capillary barrier at 2.0 m depth. The EM38 instrument was operated in both the vertical and horizontal dipole modes with the instrument resting on the ground surface and all data were normalized to 25°C. Linear regression techniques were applied to analyze the survey data. Water content to a depth of 0.75 m was correlated with horizontal dipole mode data and water content to depths of 1.1 m and 1.5 m was correlated with vertical dipole mode data. Initially higher water content values decreased by an average of 0.10 m^3/m^3 in the top 0.75 m and an average of 0.07 m^3/m^3 in the top 1.5 m over the course of the study. The regression model of the EM38 vertical dipole mode data with water content to the 1.5 m depth for all locations monitored on the capillary barrier design resulted in a standard deviation of 0.016 m^3/m^3. Horizontal dipole mode data correlated with water content to the 0.75 m depth had a standard deviation of 0.022 m^3/m^3 for all locations on both barrier designs. Models at individual survey locations generally exhibited much smaller standard deviations, ranging from 0.005 to 0.018 m^3/m^3 and averaging 0.010 m^3/m^3. The smaller standard deviations and general similarity of regression slope values of the models at individual locations indicate that this technique is more accurate as an indicator of changes in water content than as an indicator of the absolute value of water content at a given location. Sources of variability were attributed to horizontal and vertical variation in soil salinity, the vertical distribution of water at the time of a particular survey, and subtle differences in topsoil thickness and surface roughness. Results indicate that electromagnetic induction is useful for evaluating infiltration. The EM technique resulted in standard deviation values for water content similar to those of the neutron probe method but is capable of monitoring larger areas much more rapidly and at a lower cost.Bureau of Economic Geolog

Numerical Modeling of Preferential Flow at the Pantex Plant Area

The objective of this study was to analyze preferential flow based on observations from four ponding tests conducted in the vicinity of the Pantex Plant. According to soil profiles exposed after the field ponding tests (Xiang et al., 1993), we derived the hydraulic conductivity of an equivalent homogeneous soil as that of the actual heterogeneous soil. Models of four different types of subsurface flow were considered. These include flow through the soil matrix, through root tubules, between ped faces, and along soil-filled cracks. The results of numerical simulations of matrix flow were similar to field observations of matrix flow from ponding tests. To evaluate preferential flow, different values of hydraulic conductivity were used. The simulations show that preferential flow results in an increased rate of water movement because of the higher equivalent hydraulic conductivity.Bureau of Economic Geolog

Evaluation of Subsurface Flow in Fissured Sediments in the Chihuahuan Desert, Texas

Fissures are surface features, or gullies, some of which are underlain by sediment-filled fractures. A previous study of subsurface flow beneath a fissure showed higher water fluxes beneath the fissure, which was attributed to infiltration of ponded water in the fissure. This study was conducted to investigate the vertical and lateral extent of increased flow associated with fissured sediments, to compare subsurface flow beneath fissures of different maturity, and to examine different techniques for evaluating flow in fissured zones. Boreholes were drilled directly beneath four fissures and at distances of 10 m and 50 m from the fissures, and soil samples were analyzed for various soil physics parameters and environmental tracer distribution. Electromagnetic induction was used to map apparent conductivity in transects perpendicular to the fissures. Fissures had higher water potentials and lower chloride concentration than surrounding sediments. Zones of high flux were restricted to the area directly beneath some fissures, whereas others also had high fluxes in the profiles 10 m distant from the fissure. Water potential and chloride fronts were found beneath two of the fissures in the upper 20-m zone, which indicates that most of the flow occurred in this zone. Water flux estimates, based on the position of the chloride front and an assumed age of the fissures of 50 yr, ranged from 28 to 48 mm yr^-1. High tritium levels were found throughout the fissured profiles (to maximum depth of 26.4 m) and in some cases in the profiles 10 m distant from the fissure, indicating post-1952 water. The occurrence of high tritium levels beneath the chloride front in one fissure indicates that some of the water is flowing preferentially. Minimum estimates of water flux based on the tritium data ranged from 28 to 120 mm yr^-1. Stable isotopes of oxygen and hydrogen were less enriched beneath the fissure, which is consistent with higher fluxes beneath the fissure. Plant water potentials were of limited use in delineating fissure flow. Apparent conductivities were higher across two fissures, whereas the other two fissures did not show any variation in apparent conductivity. The higher conductivity in some fissures is attributed to higher water content. Multiple independent lines of evidence indicate that subsurface water fluxes are higher beneath fissures.Bureau of Economic Geolog

Uncertainty in evapotranspiration from land surface modeling, remote sensing, and GRACE satellites

International audienceProliferation of evapotranspiration (ET) products warrants comparison of these products. The study objective was to assess uncertainty in ET output from four land surface models (LSMs), Noah, Mosaic, VIC, and SAC in NLDAS-2, two remote sensing-based products, MODIS and AVHRR, and GRACE-inferred ET from a water budget with precipitation from PRISM, monitored runoff, and total water storage change (TWSC) from GRACE satellites. The three cornered hat method, which does not require a priori knowledge of the true ET value, was used to estimate ET uncertainties. In addition, TWSC or total water storage anomaly (TWSA) from GRACE was compared with water budget estimates of TWSC from a flux-based approach or TWSA from a storage-based approach. The analyses were conducted using data from three regions (humid-arid) in the South Central United States as case studies. Uncertainties in ET are lowest in LSM ET ( 5 mm/mo), moderate in MODIS or AVHRR-based ET (10-15 mm/mo), and highest in GRACEinferred ET (20-30 mm/month). There is a trade-off between spatial resolution and uncertainty, with lower uncertainty in the coarser-resolution LSM ET ( 14 km) relative to higher uncertainty in the finer-resolution ( 1-8 km) RS ET. Root-mean-square (RMS) of uncertainties in water budget estimates of TWSC is about half of RMS of uncertainties in GRACE-derived TWSC for each of the regions. Future ET estimation should consider a hybrid approach that integrates strengths of LSMs and satellite-based products to constrain uncertainties

Ground referencing GRACE satellite estimates of groundwater storage changes in the California Central Valley, USA

International audience[1] There is increasing interest in using Gravity Recovery and Climate Experiment (GRACE) satellite data to remotely monitor groundwater storage variations; however, comparisons with ground-based well data are limited but necessary to validate satellite data processing, especially when the study area is close to or below the GRACE footprint. The Central Valley is a heavily irrigated region with large-scale groundwater depletion during droughts. Here we compare updated estimates of groundwater storage changes in the California Central Valley using GRACE satellites with storage changes from groundwater level data. A new processing approach was applied that optimally uses available GRACE and water balance component data to extract changes in groundwater storage. GRACE satellites show that groundwater depletion totaled $31.0 6 3.0 km 3 for Groupe de Recherche de Geodesie Spatiale (GRGS) satellite data during the drought from October 2006 through March 2010. Groundwater storage changes from GRACE agreed with those from well data for the overlap period (April 2006 through September 2009) (27 km 3 for both). General correspondence between GRACE and groundwater level data validates the methodology and increases confidence in use of GRACE satellites to monitor groundwater storage changes

Unsaturated Zone Studies at the Proposed Low-Level Radioactive Waste Disposal Facility, Eagle Flat Basin, Texas

Hydrologic studies were conducted to characterize unsaturated zone processes at the proposed low-level radioactive waste disposal site and surrounding area in southern Hudspeth County, Texas. The study area is in northwest Eagle Flat basin, which is within the Basin and Range Physiographic Province. Fractured Cretaceous bedrock crops out to the southeast of the site. The thickness of the basin-fill sediments at the proposed site ranges from 164 feet (50 m) to approximately 656 feet (200 m). Northwest Eagle Flat basin is an internally drained basin that drains through the ephemeral Blanca Draw into Grayton Lake playa. The climate in the study area is subtropical arid and the long-term average annual rainfall is 12.6 inches (320 mm). Unsaturated zone studies were conducted in ephemeral stream and interstream geomorphic settings. In addition to studies of areas typical of these settings, the impact of pseudofissures, an earth fissure, and borrow pits on shallow zone unsaturated processes was also investigated. To evaluate unsaturated zone processes, 57 boreholes were drilled in the various geomorphic settings for collection of soil samples and installation of monitoring equipment. Soil samples were analyzed in the laboratory for particle size, water content, water potential, and chloride concentration. Water potential data are used to evaluate the direction of the driving force for water movement. Chloride concentration data provide information on water fluxes because chloride concentrations are inversely proportional to water flux; low chloride concentrations indicate high water fluxes because chloride is flushed through the soil, whereas high chloride concentrations indicate low water fluxes because chloride is concentrated by evapotranspiration. In addition to laboratory analyses, a monitoring program was initiated by installing neutron probe access tubes in the different geomorphic settings to monitor water content. Field psychrometers were installed to a depth of 60.7 feet (18.5 m) in the interstream setting to monitor water potential and temperature. Hydraulic conductivity was also measured in the field using permeameter tests, constant-head borehole infiltration tests, and multistep constant-head borehole infiltration tests.Bureau of Economic Geolog

Estimation of Gas Permeabilities for the Maricopa Site, Arizona

Upward and downward migration of gases from waste-disposal facilities is a critical issue for low-level radioactive waste disposal. Gaseous radionuclides in low-level waste include H-3, C-14, and Rn-222. Upward migration of gases to the surface can be important, particularly during operation of the facility (Kozak and Olague, 1994). High tritium values (for example 1,100 TU at 24 m depth, 162 TU at 109 m depth) have been found adjacent to the Beatty site, Nevada, that cannot readily be explained by liquid or combined liquid and vapor transport (Prudic and Striegl, 1995; Striegl et al., 1996). Because disposal practices at Beatty varied in the past and included disposal of as much as 2,000 m3 of liquid waste, further research in tritium movement at Beatty is warranted. Transport mechanisms for gases include not only diffusion but also advection. Analysis of gas transport is important at many low-level waste disposal facilities as shown by the intensive program to monitor concentrations and concentration gradients of gaseous radionuclides proposed for the California low-level radioactive waste disposal facility (Harding Lawson & Assoc., 1991). Performance assessment calculations require information on parameters related to gas transport to predict long-term migration of gases in the subsurface. The purpose of this study is to evaluate different techniques of estimating gas transport parameters and monitoring subsurface gas migration. The objective of this study is to examine different techniques for evaluating gas permeability. Pneumatic pressure tests will be conducted to estimate vertical and horizontal air permeabilities at different levels. In addition, permeabilities will be calculated from atmospheric breathing data that will include evaluation of subsurface response to barometric pressure fluctuations. Computer simulations suggest that air from the surface can move several meters into the ground during typical barometric pressure cycles (Massmann and Farrier, 1992). Gas ports will be installed at different depths in two boreholes to evaluate atmospheric pumping. The results of this study will provide valuable information on subsurface gas transport processes and the various techniques to obtain data on parameters required for simulation of such processes. These data will be required for performance assessment calculations.Bureau of Economic Geolog

Investigation of Pollution in a Karst Aquifer Utilizing Optical Brightener

Optical brightener is an additive to laundry detergents and is found contaminating groundwater. Its concentration may rapidly and inexpensively be determined by fluorescence techniques, and because its source is human wastewater, its presence in groundwater serves as a direct indication of pollution from septic tanks, sewer leaks, and landfills. A total of 105 wells and springs in an area within the Inner Bluegrass Karst Region near Lexington, Kentucky, were described and sampled. Analyses were made for optical brightener (430 samples), total coliform (91), fecal coliform (93), and fecal streptococci (90). As many as 20 optical brightener and 4 bacterial samples were analyzed from a single site during the period from May 20, 1984 to June 17, 1985. Data were also collected on spring discharges, well water.levels, and other site characteristics. Statistical analysis of the relationship between optical brightener and the bacterial indices showed low correlations for both springs and wells, in ·contrast to an earlier study. Although time constraints have precluded a thorough analysis of the data, the difference between the results of the two studies appear to be related to differing site populations and analytic and statistical procedures. The data further suggest that the low correlations between optical brightener and the bacterial indices may be a result of bacterial contamination being largely derived from animal waste and other non-human sources, and that optical brightener may be a more reliable indicator of human contamination

Evaluation of Unsaturated Flow in Fissured Sediments in the Chihuahuan Desert, Texas

Localized flow in fissured sediments in arid settings has important implications for waste disposal in these regions. Fissures are surface features or gullies that are underlain by partially open or sediment-filled fractures. The objectives of this study were to compare unsaturated flow beneath different fissures, investigate the vertical and lateral extent of increased flow associated with fissured sediments, and examine different techniques for evaluating flow in fissured zones. Boreholes were drilled directly beneath four fissures and at distances of 10 and 50 m from the fissures. Sediment samples were analyzed for hydraulic parameters such as water content and water potential and environmental tracers such as Cl, 36Cl, 3H, 2H, and 18O. A trench was dug beneath one fissure for detailed sampling. Electromagnetic induction was used to measure apparent electrical conductivity in transects perpendicular to the fissures. Unsaturated flow is relatively high beneath fissures, as evidenced by higher water potentials and lower chloride concentrations there than in surrounding sediments. The lateral extent of high water flux was restricted to the zone directly beneath one fissure but extended to profiles 10 m from two other fissures. The profiles 50 m distant from all fissures had low water fluxes, as indicated by low water potentials and high maximum chloride concentrations. The vertical extent of high water fluxes was restricted to the upper 10 to 20 m, as shown by water potential and chloride fronts within the upper 10 m zone beneath one fissure and by chloride fronts in the upper 20 m zone beneath and 10 m distant from another fissure. Additional evidence for localized water flux was provided by high tritium levels, less-enriched 2H and 18O, and higher plant water potentials in fissured sediments than in nonfissured sediments. Apparent electrical conductivity was higher in two of the four fissures. Multiple independent lines of evidence indicate that subsurface water fluxes are higher at shallow depths beneath fissures; however, the various techniques differ in their effectiveness in delineating higher water fluxes beneath fissures. Multiple profiles drilled in one fissure indicate that there is large variability in flow along this fissure that is attributed to topographic variations and degree of ponding.Bureau of Economic Geolog

Electrical Conductivity and Gamma Ray Response to Clay, Water, and Chloride Content in Fissured Sediments, Trans-Pecos, Texas

Near-surface conductivity profiles determined using surface and borehole electromagnetic induction instruments were compared with each other and with variations in several important hydrological parameters, including clay content, water content, and chloride content in unsaturated sediments in fissured settings. Time-domain electromagnetic soundings were acquired at 10 boreholes in the Eagle Flat, Red Light Bolson, Hueco Bolson, and Ryan Flat areas in the arid Trans-Pecos region of West Texas. These boreholes were logged with induction and passive radiation probes to determine conductivity profiles and natural gamma ray activity. At the Red Light Bolson and Hueco Bolson fissures, the gamma logs were sensitive to clay content and followed the conventional trend of increasing count rates with increasing clay content. At Eagle Flat, gamma count rates were not much higher in the clay fraction than they were in the silt and sand fraction; thus, the gamma log underestimated the variability in clay content. At the Ryan Flat fissure, gamma count rates were higher than for the other fissure sites and were higher for the sand and silt fraction than for the clay fraction. This suggests that the sedimentary grains making up these deposits are volcanogenic and that the coarse fraction contains a larger percentage of K-bearing minerals than the clay fraction.Bureau of Economic Geolog