Oil production, agriculture, and groundwater quality in the southeastern Gulf Coast Aquifer, Texas.

Associations between groundwater quality and land use were evaluated in the southeastern Gulf Coast Aquifer, Texas. Data from 19234 oil/gas wells and 256 water wells were mapped with a geographic information system (GIS) and statistically analyzed. Water wells near oil/gas wells had significantly higher levels of chloride, bromide, and total dissolved solids (TDS). Bromide-chloride ratios were also higher at water wells near oil/gas wells. Shallower water wells had significantly higher chloride, bromide, TDS, and nitrate concentrations. Nitrate concentrations were higher beneath cropland compared to other land uses. Results of this study suggest that oil/gas production and agriculture have impacted water quality in the Gulf Coast Aquifer.

Automated sampling of stormwater runoff in an urban watershed, north-central Texas.

Automated samplers were used to collect urban runoff in Pecan Creek, Denton, Texas. Runoff from four storm events was sampled at four stations. Concentrations of some heavy metals (cadmium and arsenic), as well as pesticides (atrazine and diazinon), exceeded maximum contaminant levels for drinking water, but were within ranges typical for urban runoff. Calcium and phosphorous concentrations were high compared to previous studies, which was attributed to soils, building materials, and fertilizer applications in the study area. First flush samples were more concentrated than composite samples for most of the constituents analyzed, but statistically different for only five constituents. Concentrations of four constituents differed significantly among stations, and six differed significantly among storms, due to variations in land use, channel composition, and storm conditions. Overall, the automated samplers were an effective way to sample urban runoff in Pecan Creek.

Effects of brine injection wells, dry holes, and plugged oil/gas wells on chloride, bromide, and barium concentrations in the Gulf Coast Aquifer, southeast Texas, USA.

Data from 1,122 brine injection wells, 24,515 dry holes, 20,877 plugged oil/gas wells, and 256 water wells were mapped with a geographic information system (GIS) and statistically analyzed. There were 9, 107, and 58 water wells within 750 m of a brine injection well, dry hole, or plugged oil/gas well, respectively. Computed median concentrations were 157 mg/l for chloride, 0.8 mg/l for bromide, and 169 microg/l for barium. The maximum chloride concentration was 2,384 mg/l, close to 10 times the secondary drinking water standard. Shallow water wells and water wells near plugged oil/gas wells had significantly higher chloride and bromide levels.

Water hardness and sodium trends in Texas aquifers.

Median hardness and sodium levels in groundwater were calculated for 244 Texas counties from measurements at 7728 water wells. The data were mapped and analyzed with a geographic information system (GIS). County median hardness levels varied widely, from 4-2304 mg L-1. More than 60% of the counties had hardness medians above 180 mg L-1. County medium sodium concentrations ranged from 6-1170 mg L-1, with more than 90% of those values exceeding 20 mg L-1. There was a significant positive correlation between hardness and sodium concentrations in six of Texas' nine major aquifers. A significant negative correlation between hardness and sodium was observed in two aquifers. Several factors control hardness and sodium variations in Texas aquifers including rock/sediment composition, groundwater chemical evolution, and seepage from nearby formations. Probable human controls include agricultural return flow and pumping-induced saltwater intrusion.

Effects of groundwater velocity on sampling intervals for contaminant-detection networks in aquifers.

This study evaluated how groundwater velocity affects the sampling interval of a groundwater monitoring network and its ability to intercept contaminant plumes before reaching a buffer zone boundary. A computer simulation model tested the detection capability of a groundwater-monitoring network in different groundwater velocity settings. A 0.4-m/d velocity registered a maximum sampling interval of four months. By comparison, decreasing the velocity to 0.04 m/d resulted in a 3-year maximum sampling interval. Groundwater velocity exerts a strong control on the maximum sampling interval of contaminant detection networks in aquifers. Assigning an arbitrary sampling interval, that does not take into account a site's hydrogeology, could waste financial resources or allow contaminants to reach a buffer zone boundary without being detected.

Effective contaminant detection networks in uncertain groundwater flow fields.

A mass transport simulation model tested seven contaminant detection-monitoring networks under a 40 degrees range of groundwater flow directions. Each monitoring network contained five wells located 40 m from a rectangular landfill. The 40-m distance (lag) was measured in different directions, depending upon the strategy used to design a particular monitoring network. Lagging the wells parallel to the central flow path was more effective than alternative design strategies. Other strategies allowed higher percentages of leaks to migrate between monitoring wells. Results of this study suggest that centrally lagged groundwater monitoring networks perform most effectively in uncertain groundwater-flow fields.

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.

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.

Effects of compliance boundary location on contaminant detection networks in aquifers.

Groundwater monitoring networks were derived for 15 alternative compliance boundaries, located from 10 to 150 meters downgradient of a landfill. For each compliance boundary, a mass transport model was used to define the linear monitoring transect, perpendicular to groundwater flow, requiring the fewest detection wells. The distance (dt) to the optimal monitoring transect was consistently 0.40 to 0.75 times the distance to the compliance boundary (dc). Compliance boundaries located near a landfill provide capability for early detection, but also require a substantial number of closely spaced wells. As dc increases, the minimum number of wells (No) required along the optimal transect decreases. However, the rate of decrease (No/dc) is progressively smaller in the downgradient direction. And there is a value for dc, in this example 70 meters, beyond which the decrease in No is negligible.

A method for monitoring ground water quality near waste storage facilities.

A practical optimization approach developed in this paper derives effective monitoring configurations for detecting contaminants in ground water. The approach integrates numerical simulation of contaminant transport and mathematical programming. Well sites identified by the methodology can be monitored to establish the occurrence of a contaminant release before a plume migrates to a regulatory compliance boundary. Monitoring sites are established along several horizons located between the downgradient margin of a contaminant source and a compliance boundary. A horizon can form an effective line of defense against contaminant migration to the compliance boundary if it is spanned (covered) by a sufficient number of sites to yield a well spacing that is equal to or less than a maximum value established by numerical modeling. The objective function of the integer programming model formulation expresses the goals of: (1) covering a maximum number of siting horizons, and (2) allocating wells to the single most effective horizon. The latter is determined from well spacing requirements and the width of the zone of potential contaminant migration traversed by the horizon. The methodology employs a highly tractable linear programming model formulation, and the user is not required to predefine a set of potential well sites. These attributes can facilitate its implementation in practice.