INFLUENCE OF STREAM RECHARGE ON SPRING DISCHARGE HYDROGRAPHS AND GEOCHEMISTRY

Although recharge to karst systems commonly is considered to be instantaneous through point recharge features or diffuse through soil or epikarst, some karst systems receive a substantial amount of recharge through the beds of ephemeral streams that might flow for hours to weeks. Here we investigate the influence of stream recharge on spring discharge hydrographs and aqueous geochemistry of a karst spring in the Barton Springs segment of the Edwards aquifer in central Texas. Continuous discharge records for recharging streams and Barton Springs since 2000 were used in conjunction with measurements of major ions, pesticides, and volatile organic compounds collected for the same period. The data indicate that recharge through streambeds can strongly affect the spring discharge hydrograph recession for several weeks following rainfall. They also indicate that recharge through streambeds has a greater control on spring geochemistry than does aquifer water level, in some cases explaining as much as 90 percent of the variance in concentration. The apparent relation between recharge and spring geochemistry facilitates improved identification of recharge and aquifer end members for use in geochemical modeling. Our results highlight the high degree of interaction between the surface-water and groundwater systems and lead to an improved conceptualization of transport dynamics in karst systems that receive substantial recharge through streambeds.