Recharge Rates for the Major Aquifers

The attached table contains recharge rates for 8 major aquifers including the Carrizo-Wilcox, Gulf Coast, High Plains, Edwards-Trinity, Trinity, Seymour, Cenozoic Pecos Alluvium, and Hueco-Mesilla Bolson aquifers. Recharge rates for the Edwards aquifer can be found in Slattery et al., 1998, and in annual reports published by the Edwards Aquifer Authority (e.g., EAA, 2000). Recharge data were compiled from reports published by the Texas Water Development Board, U.S. Geological Survey, and other publications. The table lists the study areas (counties or general area), underlying aquifers, recharge rates (units of mm/yr, inches/yr, or total recharge in acre-feet/yr), data sources, and techniques used to estimate recharge. Additional notes are provided in some cases. The full reference citations are listed separately.Bureau of Economic Geolog

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

Assessment of Groundwater Contamination, in Situ Treatment, and Disposal of Treatment Residuals in the Vicinity of Lubbock, Texas

This report presents an assessment of groundwater contamination potential, in situ treatment technologies for drinking water supply, and assessment of injecting ex situ treatment residuals into the source aquifer in the vicinity of Lubbock, Texas. The study area surrounding Lubbock includes the following nine counties: Lubbock, Lamb, Hale, Floyd, Crosby, Garza, Lynn, Terry, and Hockley Counties. Many public water supply wells in the region exceed the maximum contaminant levels (MCLs) for the following water quality constituents of concern (COCs): nitrate (NO3), arsenic (As), fluoride (F), selenium (Se), uranium (U), and radium-226 (Ra-226), and radium-228 (Ra-228). The study was divided into three main tasks: (1) stratification of naturally occurring contaminants in groundwater; (2) potential for in situ treatment; and (3) potential for disposal of treatment residuals in drinking water. Task 1 was conducted by the Bureau of Economic Geology, and tasks 2 and 3 were conducted by CH2M Hill under subcontract to the Bureau of Economic Geology.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

Review of the State of Art: Ground Water Under the Direct Influence of Surface Water Programs

The objective of this study was to: 1. Evaluate the state of the art for groundwater under the direct influence (GWUDI) of surface water programs in the US and other countries. 2. Evaluate the existing groundwater under the influence (GUI) program in Texas with respect to hydrogeologic parameters and microbial indicators, including total and fecal coliform and microscopic particulate analyses (MPA). 3. Make recommendations to the Texas program based on a synthesis of best practices and elements from all reviewed programs. The driving force behind these evaluations is the fact that particles entrained in water can make the disinfection process ineffective because pathogens can be shielded within microscopic debris (LeChevallier et al., 1981). Further, some particles themselves, such as Giardia and Cryptosporidium cysts, can be pathogenic even if not encased in debris. Hence, groundwater sources that contain surface water-linked debris (algae, nematodes, etc.) that receive only disinfection have been implicated in many waterborne diseases.Bureau of Economic Geolog

Push-pull experiments to evaluate in Situ arsenic remediation in the Ogallala aquifer

Arsenic concentrations exceed the U.S. Environmental Protection Agency maximum contaminant level (MCL) for drinking water (10 μg/L) in 220 Public Water Systems in Texas. Conventional treatments for arsenic removal are ex-situ treatments, which are costly and produce high concentration arsenic-contaminated sludge which must be disposed of properly. A pilot study to assess an in-situ arsenic remediation treatment was conducted in the city of Andrews wellfield, Andrews County, Texas. This procedure involved adding dissolved iron to the aquifer through water wells. Under oxidizing conditions, the dissolved iron precipitates as iron oxides near the well. Water pumped from the well following iron precipitation flows through the iron-enriched zone and arsenic is adsorbed onto the iron oxides. Two experiments were conducted in which 24 g and 72 g of Fe were injected into an experimental well. Prior to the experiments, arsenic concentration in the well water was 43 μg/L. Following the first experiment, arsenic concentration in produced water stabilized at 25 μg/L, and after the second experiment arsenic values stabilized at 20 μg/L, representing an approximate 50% reduction in produced arsenic under the test conditions. These preliminary results suggest that there is potential for treating groundwater arsenic contamination in small public water systems using in situ treatment based on addition of iron to the wells. The reconnaissance tests showed a 50% reduction in arsenic concentration; however, additional testing is required to determine if arsenic concentrations can be reduced to less than the MCL of 10 mg/L and the frequency and volume (mass) of treatment injections required to maintain these low arsenic concentrations needs to be determined for full demand production rates.Bureau of Economic Geolog

Estimation of Groundwater Recharge in Texas Related to Aquifer Vulnerability to Contamination

This study aimed to estimate groundwater recharge rates for major porous media aquifers in Texas, essential for assessing aquifer vulnerability to contamination. Recharge rates were estimated through one-dimensional unsaturated flow modeling and limited field studies in the Southern High Plains and Seymour aquifers. Field studies involved measuring soil physics and environmental tracers in soil samples from boreholes in various land use settings. Numerical modeling of unsaturated flow utilized long-term climatic data, soils data, and vegetation data. Simulated recharge rates varied from 54 mm/yr in west Texas to 720 mm/yr in east Texas, positively correlated with precipitation. High recharge rates in east Texas suggest climate is not the limiting factor, with soil texture and vegetation playing crucial roles in reducing recharge. Layered soil profiles led to locally variable recharge estimates, but areally weighted averages were less variable and positively correlated with precipitation. Vegetation significantly reduced recharge rates in all cases, with simulations including vegetation and layered soil profiles resulting in recharge rates ranging from 0.2 to 114 mm/yr. The 30-year average recharge rates across simulated stations in the state were positively correlated with precipitation, suggesting long-term precipitation as a predictor of recharge. This comprehensive approach provides valuable insights into groundwater recharge dynamics, crucial for assessing aquifer vulnerability to contamination.Bureau of Economic Geolog