GROUNDWATER FLOW IN THE VICINITY OF A PETROLEUM CONTAMINATION SITE OVER THE EDWARDS AQUIFER, AUSTIN, TEXAS

Leaking petroleum storage tanks from gas stations are one of the most common sources of contamination of groundwater in the US and Texas. In 1992 a 2,900 gallon gasoline and diesel release occurred at the Big Wheel Truck Stop located on the highly sensitive Edwards Aquifer in Austin, Texas. The fate and transport of the contaminants in the aquifer was never fully realized in the subsequent site studies required by the State of Texas. We hypothesize that by applying an accurate conceptual aquifer model to the historic site investigation data, combined with recent hydrogeologic data, one can realistically constrain the fate and transport of the released contaminants. Key aspects of that conceptual model should include the karstic nature of the Edwards Aquifer and the proximity of the site to a major fault zone.

Results of our evaluation suggest that the petroleum hydrocarbons likely behaved similar to groundwater tracing studies performed in the region, with approximate minimum flow velocities of 400-500 ft/d. Contaminants at wells down-gradient and off-site from the spill quickly decreased in concentration in a matter of days as the detached plume moved rapidly past the wells, or became diluted. Soil and epikarst horizons were likely a source for remobilization of contaminants with subsequent recharge events. After remediation of the soil in 1996, it took more than 5 years for groundwater contaminant concentrations at the site to reach target levels. By using an accurate conceptual model at this site, limited historic site contamination data provides insight into how future petroleum contaminants could behave in the karstic Edwards Aquifer recharge zone. A secondary finding of the evaluation was that the Mount Bonnell fault, running through the site, appears to behave as a barrier to inter-aquifer flow in the study area. This has implications for water resource management of the Edwards and Trinity Aquifers.