The Hydrologic Function of the soil and Bedrock System at Upland Sinkholes in the Edwards Aquifer Recharge Zone of South-Central Texas

Small subtle karst features are ubiquitous in the uplands of the Edwards aquifer recharge zone of south-central Texas. The maintained surface expression of sinkholes, e.g. depressed area, of a number of sinkholes suggests these features are actively undergoing karst processes of soil piping or soil sapping. Much of the urban development around San Antonio and Austin is currently occurring in these upland areas; therefore, it is important to determine the hydrologic function of these features to minimize there potential impact on structures as well as to mitigate water quality concerns. Several artificial recharge experiments were conducted at typical small, soil-lined, upland sinkholes. Artificial recharge experiments using a constant-head ring infiltrometer indicated the infiltration rates at these sinkholes are not significantly higher; but are actually somewhat lower, than control plots. Both rates are within the range of infiltration for the 30% to 40% clay soils in the area. Volumetrically, these features likely recharge more than background due to microtopography. In order to further discover what impacts modification of these features might have on water quality in the Edwards aquifer, several follow-up tests were conducted. Repeat GPR tests were used to determine subsurface geometry of soil and rock and area of wetting distribution. Dyed water was ponded and allowed to infiltrate the features, which were subsequently excavated to determine the distribution of dye in the soil to locate preferential pathways. Once complete, the excavated features were subjected to additional constant head infiltration experiments to determine the infiltration rates of the underlying epikarst. The results of the follow-up experiments were compared with previous experiments to determine epikarst storage and the role soil has on infiltration rates of features with modified soil cover.