CONTROLS ON THE GEOCHEMICAL EVOLUTION OF VADOSE WATER IN A KARST TERRAIN IN CENTRAL TEXAS

Significant spatial variability in drip water chemistry at Natural Bridge Caverns, in the Edwards aquifer region of central Texas, yields insight to factors controlling the geochemical evolution of water in the vadose zone of karst terrains. Geochemical evolution of water is controlled by the chemistry of the soil and rock that water infiltrates through as well as the extent of interaction of water with its host carbonate rock. On the Edwards Plateau, thin silicate soils (typical ⁸⁷Sr/⁸⁶Sr = 0.7089) overlie Cretaceous limestone (typical ⁸⁷Sr/⁸⁶Sr = 0.7076), making a useful setting to evaluate the influence of soil and rock chemistry and extent of rock-water interaction on the geochemical evolution of vadose water. As the extent of rock-water interaction increases, vadose water will become enriched in Mg and Sr relative to Ca and acquire a lower ⁸⁷Sr/⁸⁶Sr value. Drip waters at seven sites over a 450 m extent of the cave have low Mg/Ca (0.003 - 0.4) and Sr/Ca (0.00001 – 0.0006) ratios and high ⁸⁷Sr/⁸⁶Sr (0.7085 - 0.7091) relative to five sites in a portion of the cave over a 200 m extent which have distinct water compositions of higher Mg/Ca (0.005 - 2.5) and Sr/Ca (0.0002 – 0.002) ratios and lower ⁸⁷Sr/⁸⁶Sr values (0.7083 - 0.7084). The chemistry of the soil (⁸⁷Sr/⁸⁶Sr = 0.7086 - 0.7093) and rock (⁸⁷Sr/⁸⁶Sr = 0.7075 - 0.7079) overlying the rest of the cave is not distinct from the soil (⁸⁷Sr/⁸⁶Sr = 0.7091 - 0.7092) and rock (⁸⁷Sr/⁸⁶Sr = 0.7076 - 0.7077) overlying the portion of the cave with unique drip water chemistry. This suggests that the unique drip water chemistry results from more extensive rock-water interaction. Extent of rock-water interaction is a function of water's residence time in the subsurface above the cave. Residence time is controlled by type of flow route, conduit or diffuse, and the flux of water through the subsurface. Possible explanations of why more rock water interaction occurs in the portion of the cave with unique drip water chemistry are that more diffuse flow routes are followed because 1) the capacity of diffuse flow routes is higher due to thicker bedrock and/or less fracturing, or 2) diffuse routes are the predominant routes followed due to fewer fractures or limited recharge. As brush overlies the portion of the cave with unique drip water chemistry, the latter explanation is consistent with the hypothesis that removing brush increases recharge to underlying aquifers.