David J. Wachal

Mapping landslide susceptibility in Travis County, Texas, USA

A geographic information system (GIS) was used to construct a landslide hazard map for Travis County, Texas. The County is experiencing rapid growth, and development has encroached into unstable terrain that is vulnerable to landslides. Four layers of data were superimposed to create the landslide hazard map. Slope was given the most emphasis, followed by geology, vegetation, and proximity to faults. The final map shows areas of low, medium, and high landslide susceptibility. Areas of high susceptibility occupy stream and reservoir banks, rock escarpments, and agricultural land. The landslide hazard map can be a useful geologic criterion for land use planning. Planners can use the map to allocate appropriate land uses to unstable terrain, and to identify existing structures at risk from landslide activity. The methods presented in this paper can be adapted to other counties in the U.S. and elsewhere. Results of this study suggest that geographic information systems can effectively compile and overlay several data layers relevant to landslide hazards.

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.

Managing subsurface property hazards: reactive soils and underground storage tanks

Abstract Fuels stored in underground tanks often contaminate urban soil and water. A geographic information system (GIS) mapped 1068 underground fuel tanks in Denton County, Texas, USA. Tank locations were compared with soil shrink–swell and corrosivity potentials. Higher percentages of reported leaking tanks were observed in expansive and corrosive soils. This study illustrates the utility of GIS for overlaying layers of data that affect underground tank integrity. Such overlays could find appropriate locations for new tanks, survey existing tanks for potential leak hazards, and assess hazards posed by leaking tanks based upon soil permeability.

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

Associations between groundwater quality and land usewere evaluated in the southeastern Gulf Coast Aquifer,Texas. Data from 19 234 oil/gas wells and 256 water wellswere mapped with a geographic information system (GIS) andstatistically analyzed. Water wells near oil/gas wells hadsignificantly higher levels of chloride, bromide, and totaldissolved solids (TDS). Bromide-chloride ratios were alsohigher at water wells near oil/gas wells. Shallower waterwells had significantly higher chloride, bromide, TDS, andnitrate concentrations. Nitrate concentrations were higherbeneath cropland compared to other land uses. Results ofthis study suggest that oil/gas production and agriculturehave impacted water quality in the Gulf Coast Aquifer.

The Application of WEPP to Natural Gas Exploration and Production Sites

Natural gas development in the United States has been increasing in recent years, with over 30,000 new sites added per year. Natural gas exploration and production (NGE&P) sites disturb several acres of land, increasing the potential for accelerated soil losses due to land cover modifications, increased slopes, and flow concentration. The contribution of sediment from multiple NGE&P sites in a watershed may potentially warrant inclusion in a sediment TMDL source assessment. Models are often used to assist in source assessments. WEPP was chosen to model sediment yields from NGE&P sites because both the spatial (field scale) and temporal (single storm event) scales were applicable and because WEPP was specifically developed for a variety of different land use/land cover conditions including disturbed sites. This research project was designed to evaluate the ability of WEPP to adequately predict sediment yields from NGE&P sites in north central Texas. Sediment yields were measured for a total of fifteen storm events at three sites and compared to WEPP modeled sediment yields using the Nash-Sutcliffe model efficiency coefficient. Data from three storm events were used to calibrate the model, and data from the remaining twelve events were used for model validation. The model produced a validation efficiency coefficient of 0.71. Results of this research demonstrate that WEPP is effective for field scale modeling of sediment yields from NGE&P sites and that WEPP could be used to develop sediment TMDLs under these land use conditions.

Sediment Impacts from Natural Gas Exploration and Production Sites

Oil and gas development, exploration, and production requires construction of a well site, access roads, and pipelines, all of which have the potential for accelerating erosion due to land cover modifications, increased slopes, and flow concentration. The objective of this paper was to characterize storm water runoff from gas well sites in North Central Texas. Flow proportioned storm water samples were collected over a one-year period at the edge of three sites using small weirs and automated sampling equipment. Total Suspended Solids (TSS) concentrations ranged from 32 to 14,350 mg/L with the median Event Mean Concentration (EMC) of 30 storms equaling 2,624 mg/L. Annual sediment yields ranged from 15.7 to 29.7 t/ha. Measured annual yields compared well to modeled yields, and both TSS concentrations and overall yields were well within the ranges previously reported for typical construction site studies. The similarity between the measured annual yield and the modeled annual yield indicates that erosion models could be a useful tool for estimating potential storm water impacts from gas well sites and for evaluating site management alternatives. Also, the similarity to typical construction sites suggests that gas well sites should be regulated and operated in a manner similar to typical construction sites.