Evaluation of the Potential for Cross Contamination of the Edwards Aquifer from Dissolved Contaminants in the Shallow Groundwater Zone in the Vicinity of Kelly AFB via Faults and Wells

Shallow groundwater in alluvial sediments in southwest San Antonio has been impacted by urban and industrial activities. The shallow groundwater zone overlies an interval characterized as an aquitard, which in turn overlies the Edwards aquifer. The Edwards aquifer is a highly transmissive hydrologic unit that supplies drinking water to the city of San Antonio, as well as diverse other users downstream. The purpose of this study is to assess the risk that contaminated water can move across the aquitard from the shallow groundwater zone into the Edwards aquifer. The study area is defined by a polygon that includes the Kelly Air Force Base, east Kelly area, and the contaminant plumes mapped by Miller (2000).

The most significant conditions with potential to transmit contaminants from the shallow groundwater zone to the Edwards exist at times when a hydrologic gradient is present between the two zones and at places where a pathway having significant permeability connects them. In this study, we assessed the hydraulic gradient between the shallow groundwater zone and the Edwards aquifer. We analyzed faults and water wells as potential pathways having significant permeability through the aquitard.

The gradient between the Edwards and the shallow groundwater in the study area varies spatially and temporally. Likelihood of a downward hydraulic gradient is suggested over the northwestern half of the study area during the lowest 25% of water levels in the Edwards aquifer. During average and high water levels in the Edwards aquifer, there is no downward gradient between the shallow groundwater and the Edwards in the study area.