INFILTRATION AT SMALL SINKHOLES IN THE UPLANDS OF THE EDWARDS AQUIFER RECHARGE ZONE OF CENTRAL AND SOUTH TEXAS

The City of San Antonio uses the karstic Edwards aquifer, developed in the Cretaceous limestones in Central and South Texas, as its main municipal water source. Areas to the north of San Antonio, part of the environmentally sensitive recharge zone, are undergoing development. Development in the recharge zone of any karst aquifer can pose a risk to water quality. Mass balance calculations indicate that most of the Edwards aquifer recharge occurs where major streams cross the aquifer’s recharge zone. However, the uplands contain abundant evidence of active karst dissolution including sinkholes, caves, and solution-enlarged fractures and are where the majority of houses, streets, and water and sewer lines are constructed. What is the risk of development over these typical upland karst features for water quality?

The Edwards Rules are designed to reduce water quality risks during development in the recharge zone by identifying fast and direct flowpaths then implementing BMPs to mitigate the impact of development. This project determines how these rules succeed via directly testing infiltration rates of small, subtle karst features by inducing recharge on them.

Constant head infiltration measurements compare typical upland karst features to paired control plots with a large diameter single ring infiltrometer to provide direct evidence of the net hydrologic function of the contained soil and bedrock system. Selected typical karst features include soil-floored sinkholes, sinkholes with cobble-filled drains, and an excavated solution cavity.

The excavated solution cavity and cobble-filled sinkhole exhibit infiltration rates that exceed 5 and 50 times background, respectively. The soil-lined sinkholes exhibited slightly lower infiltration rates than control plots, 0.061 and 0.097 in/hr (1.55 and 2.46 mm/hr) respectively, though both are within the range of infiltration rates for soils in the area. Maintenance of a depression suggests active karst processes, like soil sapping, occur but 30% to 40% clay in the clay loam soil appears to retard infiltration. Future work will consist of excavating one or more soil-filled features to confirm karst origin then, test the infiltration rate of the resulting exposed area to determine effects caused by soil disturbances.