QUANTIFYING CONTROLS ON KARST SPRING AND GROUNDWATER QUALITY DURING A TRANSITION FROM EXTREME DROUGHT TO WETTER THAN AVERAGE CONDITIONS

Understanding how groundwater systems respond to changing hydrologic conditions is critical for groundwater protection and land-use management, especially in karst terrains where surface water can rapidly affect groundwater quality. We analyzed the evolution of groundwater compositions as conditions transitioned from extreme drought to wet conditions, and quantified controls on groundwater compositions throughout this evolution. Surface, spring, and groundwater in the Barton Springs segment of the Edwards aquifer (central Texas, USA) were sampled (every 3-4 weeks) over 18 months (Nov 2008-Mar 2010) and analyzed for major ions. Inverse modeling (using PHREEQC) was used to quantify geochemical reactions and source-water mixing that could account for observed spring and groundwater compositions. Results indicate variations in Edwards groundwater compositions during drought conditions are dominantly controlled by variable mixing with a saline zone source (3-6% of the composition) and another groundwater source (23-25%) with anomalously high Sr concentrations. Following the onset of rainfall and the resulting increases in streamflow, recharging surface water rapidly became the dominant control on groundwater composition. Surface water was on average 63% of the groundwater composition after 4 weeks and 57% of spring water composition after 6 weeks. As wet conditions continued, surface water represented as much as 100% of the groundwater composition and 85% of spring water composition. Geochemical modeling was also used to quantify the contribution of individual surface water sources contributing to groundwater compositions under both dry and wet conditions. Modeling results indicate that i) losing streams contributed as much as 27% of spring discharge during dry conditions and as much as 83% during wet conditions, ii) direct recharge (through karst features such as sink holes) contributed as much as 74% of spring discharge during wet conditions, and iii) municipal water was as much as 15% of spring discharge during dry conditions and as much as 5% during wet conditions. This study is unique in quantifying the controls on the evolution of a karst groundwater system after a prolonged drought to wetter than average conditions.