Time series resistivity analyses of unsaturated karst terrain, Edwards limestone, San Antonio, Texas

The karstic Edwards aquifer in the San Antonio, Texas area is recharged mostly from losing streams but also from infiltration of water across upland, interstream areas. One possible method for mapping where recharge occurs in the upland interstream areas is time series electrical resistivity. The purpose of this study is to better define where and how recharge occurs in the upland interstream areas of this karst terrain. The research design examined whether time series resistivity mapping could be used to track hydrologic activity in the recharge zone and evaluate how changes in resistivity might be used to quantify changes in water content.; The field area includes jointed and karstic limestone overlying Headquarters Cave at Camp Bullis located on the northwest side of San Antonio. Spatial variations in resistivity are associated with (a) stratigraphy, (b) karst features, and (c) water content changes in the unsaturated zone. Temporal fluctuations in resistivity, however, would mainly depend on how water content changes through time in the near-surface environment.; Because changes in resistivity are associated mainly with changes in water content, recording resistivity values in a time series tracks temporal changes in water content at each of the subsurface data points. Zones of high variance in resistivity might, for example, indicate greater wetting and drying of the pore space that could indicate where recharge is taking place through preferential flow paths in this upland setting.; Using a 48 electrode dipole-dipole array (1 m spacing) the electrodes were left in place throughout the study to ensure the same electrode locations were used in every survey. The sampling interval was every two weeks for 10 months. The data was interpreted through pseudosections and statistical analyses.; The pseudosections provided visual confirmation of the wetting and drying of the near surface (0.5m), the epikarst. In the epikarst the time-series survey showed a seasonal variation in resistivity measurements with respect to water content. Transport pathways were visible not only as possible vertical water flow pathways, but also horizontally through the epikarst. Specifically, the survey identified the location of two near surface fracture features which may act as flowpaths for infiltration waters.