The hydrologic function of small sinkholes in the Edwards Aquifer recharge zone

Abstract

Thin soils on Cretaceous karst bedrock typify the Edwards aquifer recharge zone in central and south Texas U.S.A. Most of the recharge occurs in major streambeds in this region, but abundant evidence of active karstic dissolution suggests that some of the recharge is occurring in the uplands. The City of San Antonio uses the karstic Edwards aquifer as its main municipal water source, and is undergoing rapid development in the uplands of the aquifer recharge zone. What is the risk to water quality of development over typical small-scale yet abundant upland karst features? This project is designed to determine more precisely what constitutes a significant recharge feature in the context of state law by directly measuring infiltration rates of typical upland karst features. The hydrologic function of the soil and bedrock system in small sinkholes, identified by their morphological characteristics, and background areas is determined by large-scale constant head infiltration tests, microtopographic and soil thickness surveys, dye tracing and subsequent excavation of features, and imaging the subsurface with GPR. Measurements made with a large-scale single ring infiltrometer compare typical upland karst features to paired control plots. The average infiltration per unit head for sinkholes is slightly higher than background in areas with similar soils (0.30 1/hr for sinkholes and 0.27 1/hr for background), though both are within the range for local soils. Results from infiltrometer experiments indicate the thin clay soils typical in the uplands dominates the infiltration process, yet recharge via these features under natural conditions is greater than background when ponding occurs due to their maintained microtopography. As the thin soils typical in the uplands of the Edwards aquifer recharge zone dominate the infiltration process in these small sinkholes, as shown by ring infiltrometer experiments, the risk of recharging poor quality water via these small features from developed areas sufficient to pose a threat to aquifer water quality is little greater than background