DELINEATING FLOWPATHS IN THE UNSATURATED ZONE OF A KARST AQUIFER: SR ISOTOPE CONSTRAINTS

Understanding flowpaths in karst aquifers is a principal challenge in these complex systems. Variations in the strontium isotope composition of cave dripwaters, soils, and limestone bedrock permit us to infer sources of dissolved ions and flowpaths in karst waters. In this model, dripwaters with a chemical signature similar to that of the soils indicate conduit flowpaths and relatively low extents of interaction with the limestone; whereas waters with a chemical signature similar to that of the limestone bedrock indicate higher residence times in the limestone via more diffuse flowpaths. We apply this flow route model to analyzing the evolution of dripwater in a cave in the recharge zone of the Edwards aquifer of central Texas. The aquifer is comprised of lower Cretaceous marine limestones and dolostones, overlain by thin mollisols. Two contrasting dripwater sites have been monitored on a monthly basis; one site has a relatively fast drip rate that varies with effective precipitation (0.011-0.28 mL/sec), whereas a second site has a lower and less variable drip rate (0.007-0.016 mL/sec). Dripwater ⁸⁷Sr/⁸⁶Sr values fall between the average soil value of 0.7088 and the average limestone bedrock value of 0.7076, indicating interaction with both sources. At the faster drip site, dripwater ⁸⁷Sr/⁸⁶Sr values are closer to the soil values and vary temporally with effective precipitation and to a lesser extent with drip rate. This indicates predominantly conduit flow during wet periods and a larger component of diffuse flow with higher bedrock residence times during dry periods. At the slower drip site ⁸⁷Sr/⁸⁶Sr values vary only within analytical uncertainty and are closer to limestone bedrock ⁸⁷Sr/⁸⁶Sr values, indicating longer residence time year-round and less responsiveness to effective precipitation. Sr isotope data in conjunction with other geochemical tracers such as Mg/Ca and Sr/Ca ratios may also provide insights into controls of local-scale variability in soil composition and lithology. These approaches can be used to better understand the dynamics of karst unsaturated zone hydrology, and also paleohydrology via Sr isotope analysis of speleothems.