Sulfate and chloride concentrations in Texas aquifers

Median sulfate and chloride concentrations in groundwater were calculated for 244 Texas counties from measurements at 8236 water wells. The data were mapped and analyzed with a geographic information system (GIS). Concentration clusters for both solutes were highest in north-central, west, and south Texas. Thirty-four counties had median sulfate levels above the secondary standard of 250 mg/L, and 31 counties registered median chloride concentrations above 250 mg/L. County median concentrations ranged from < 1.5 to 1,953 mg/L for sulfate, and from 6 to 1,275 mg/L for chloride. Various factors contribute to high sulfate and chloride levels in Texas aquifers, including mineral constitutents of aquifers, seepage of saline water from nearby formations, coastal saltwater intrusion, irrigation return flow, and oil/gas production. Ten counties in central and northeast Texas lack data and warrant additional monitoring.     

Conjunctive Vadose and Saturated Zone Monitoring for Subsurface Contamination

A comprehensive subsurface monitoring program should include contaminant detectors in both the vadose and saturated zones. Vadose zone detectors can provide an early warning of an impending groundwater contamination problem, and also yield information relevant to placing groundwater monitoring wells. Moisture probes, gas monitoring wells, and pore-liquid samplers deployed in the vadose zone complement groundwater detection wells. The objective(s) of a monitoring program, spatial-scales, and hydrogeology are important considerations for designing subsurface monitoring networks. Often, these networks are used to detect potential releases or characterize existing contamination beneath land-based waste storage facilities. A case study in Santa Barbara, California, U.S.A., illustrates the utility of vadose zone monitoring in characterizing a gasoline contamination problem and guiding the placement of groundwater monitoring wells.     

Groundwater Monitoring Strategies for Variable Versus Constant Contaminant Loading Functions

Contaminant plumes were derived for constant and variable loading functions at locations within a landfill. Annually, the alternative loading functions injected the same volume of contaminated water. Mass transport modeling was used to evaluate the detection efficiencies of 25 monitoring transects, spaced evenly between the landfill and a downgradient compliance boundary. Respectively, the most efficient transects (requiring the fewest monitoring wells) for constant and variable loading were located at 60–64 and 40 percent of the distance to the compliance boundary. The mean detection efficiency was 29 percent higher for variable loading, but the variation in detection efficiency was similar for constant and variable loading. At the most efficient transects, the minimum number of detection wells was 20 percent lower for variable loading. Given the influence of source loading on monitoring efficiency, alternative loading functions should be considered when designing detection monitoring networks in aquifers.     

A Method for Designing Upgradient Groundwater Monitoring Networks

A graphical heuristic was devised for locating upgradient groundwater monitoring wells near landfills. Utilizing computer-simulated contaminant plumes, the heuristic considers the direction of groundwater flow relative to the shape of a landfill, the location of the downgradient migration boundary used for configuring detection wells, and uniformity of spatial coverage. The heuristic positions upgradient wells far enough from a landfill to avoid contamination, but close enough to measure ambient water quality near the landfill. It can be adapted to nonuniform flow fields, nonlinear migration boundaries, and irregularly shaped landfills. An application to a rectangular landfill, oriented at various angles to the direction of groundwater flow, demonstrates the utility of the approach.     

Pesticide Use and Residual Occurrence in Thailand

This paper addresses pesticide use, environmental problems, and regulations in Thailand. Annually, Thailand imports several thousand metric tons of herbicides, fungicides, and insecticides. Agricultural goods are among the country's primary exports. Over the past decade, Thailand's agricultural sector has shifted from labor- to machine-intensive farming practices. Pressures to sustain high crop yields have led to heavy usage of pesticides. Residues, especially organochlorine and organophosphate compounds, have been found in soil, water, and agricultural products throughout the country. Occupational exposure and suicide are the main causes of pesticide poisoning to Thailand's residents. Recognizing the growing problem, Thailand's government has enacted environmental laws and education programs aimed at minimizing adverse effects of pesticides.     

Performance assessment of hanging funnel‐and‐gate structures designed by reverse particle tracking for capturing polluted groundwater

The objective of this study was to evaluate the capability of partially penetrating (hanging) funnel‐and‐gate structures, designed using reverse flow trajectories, for capturing plumes of contaminated groundwater. Linear capture structures, comprised of two slurry cutoff walls on either side of a permeable gate, were positioned perpendicular to regional groundwater flow in a hypothetical unconfined aquifer. A four‐step approach was used for each of two simulated settings: (1) a numerical mass transport model generated a contaminant plume originating from a source area; (2) a particle‐tracking model projected groundwater flow paths upstream from a treatment gate; (3) the structure was widened and deepened until bounding path lines contained the plume; and (4) mass transport simulation tested the ability of the structure to capture the plume. Results of this study suggest that designing funnel‐and‐gate structures using reverse particle tracking may result in too small a structure to capture a contaminant plume. This practice generally ignores effects of hydrodynamic dispersion, which may enlarge plumes such that contaminants move beneath or around a capture structure. This bypassing effect may be considerable even for low values of dispersivity. Particle‐tracking approaches may also underestimate the amount of time required to reduce contaminant concentrations to acceptable levels. © 2007 Wiley Periodicals, Inc.     

Spontaneous combustion of shale spoils at a sanitary landfill

Shale deposits excavated from the Upper Cretaceous, Eagle Ford Formation for a sanitary landfill near Dallas, Texas spontaneously combusted. The shale is dark gray in color, with 3-4% fractional organic carbon, and no lignite seams. Gradual sifting and segregation of fine particles, having high surface area per unit volume, along with moisture trapped in the pile as it accumulated led to spontaneous combustion. Hot summer temperatures compounded the problem by heating the pile and preventing heat from efficiently venting to the atmosphere. Maximum temperatures exceeded 960 degrees F (516 degrees C) at several hot spots located approximately 4 m beneath the pile surface. The ongoing combustion problem, which has lasted for approximately 1 year, eventually will be extinguished by spreading the deposits in thin layers, and compacting them to reduce air circulation and segregation of fines.     

A Method for Designing Detection Monitoring Networks in Layered Aquifers with Non-uniform Flow

An integer programming method was devised to locate detection monitoring wells in layered aquifers. The method is applicable to aquifers with non-uniform groundwater flow, and it does not require that a compliance boundary be linear or perpendicular to the direction(s) of groundwater flow. In each layer, monitoring sites are defined along curvilinear transects that parallel equipotential lines. The model can be formulated to allocate wells to the transect with the highest detection efficiency, or to establish multiple lines of defense against contaminant migrating to a compliance boundary. Detection efficiencies of alternative monitoring transects are calculated from parameters obtained via numerical modeling of contaminant transport. These parameters include the narrowest plume that could traverse a monitoring transect, and the zone width of potential contaminant migration at the transect. Problem formulations are compact, and computational requirements are low relative to alternative approaches for designing detection monitoring networks in aquifers. An application to a glacial outwash aquifer demonstrates the utility of the method.     

Optimizing environmental monitoring networks with direction-dependent distance thresholds

In the direction-dependent approach to location modeling developed herein, the distance within which a point of demand can find service from a facility depends on direction of measurement. The approach is effective for environmental location problems in which an underlying process with a prevailing gradient (e.g., wind or water flow) influences the interaction between sites in a modeled field. The utility of the approach is illustrated through an application to groundwater remediation.     

Designing funnel‐and‐gate groundwater remediation systems near property corners

Numerical models were used to simulate alternative funnel‐and‐gate groundwater remediation structures near property corners in hypothetical homogeneous and heterogeneous unconfined aquifers. Each structure comprised a highly permeable central gate (hydraulic conductivity = 25 m/d) and soil‐bentonite slurry walls (hydraulic conductivity = 0.00009 m/d). Gates were perpendicular to regional groundwater flow and approximately 5 m from a contaminant plume's leading tip. Funnel segments collinear to the central gate reached property boundaries; additional funnel segments followed property boundaries in the most hydraulically upgradient direction. Structures were 1 m thick and anchored into the base of the aquifer. Two structures were simulated for each aquifer: one with a 3.0‐m‐long central gate and funnels on either side; and a second with a 1.5‐m‐long central gate, funnels on either side, and 0.75‐m‐long end gates. Funnels were lengthened in successive simulations, until a structure contained a contaminant plume. Results suggest that, for the same total gate length, one‐gate structures may facilitate more rapid remediation, up to 44 percent less time in trials conducted in this study, than multiple‐gate structures constructed near property corners. However, in order to effectively contain a plume, one‐gate structures were up to 46 percent larger than multiple‐gate structures. © 2011 Wiley Periodicals, Inc.