THE 21ST CENTURY ART OF FINDING GROUNDWATER IN KARST

In the mid 19^th century, Jean-Baptiste Paramelle developed a method of finding groundwater on the dry karst limestone plateaus of southwestern France. He used sinkholes, especially aligned sinkholes in dry valleys to determine the location of shallow groundwater flow. GIS technology makes it possible to evaluate Paramelle’s method anywhere by asking the question: do aligned sinkholes, and by extension, subsidence areas correspond to locations of high productivity water wells in karst terrain? The University of Texas Bureau of Economic Geology Miscellaneous Geologic Map No. 39 of the New Braunfels Texas Quadrangle, which incorporates field-checked sinkhole and subsidence features, shows 166 sinkholes and subsidence areas within a 60 mi (97 km) x 30 mi (64 km) faulted karst area near New Braunfels Texas. The Texas Water Development Board Submitted Well Driller Reports database lists 2395 wells in the study area. GIS analysis revealed seven wells ranging in depth from 600 to 1240 feet (183 to 387 m) located within a 100 m buffer zone around sinkhole and subsidence areas and 18 wells ranging in depth from 440 to 1240 feet (134 to 387 m) located within a 200 m buffer zone around sinkholes and subsidence areas. Wells located within 100 m buffer zones have higher mean well yields (53 gpm or 289 m³/day) than wells within 200 m buffer zones (33 gpm or 179 m³/day) and wells that lie outside 200 m buffer zones (24 gpm or 130 m³/day). Many sinkholes and subsidence areas within 100 m and 200 m buffer zones are in close proximity to faults. None of the 103 highest yielding wells (between 100 and 1000 gpm /545 and 5451 m³/day) in the study area lie within 200 m of karst or subsidence features, indicating that other factors contribute to these very high well yields. A larger sample size would improve future studies of the relationship between well yields and sinkholes/subsidence areas in karst.