IDENTIFICATION OF WATER MIXING SOURCES IN A KARST SYSTEM DURING A STORM EVENT USING STABLE ISOTOPES AND MAJOR IONS

Orangeville Rise is a major karst spring located in southern Indiana. It is the main discharge of an aquifer comprised of the Mississipian limestones belonging to Ste. Genevieve and St. Louis Formation. Previous dye tracing studies defined a 112 km2 watershed situated north of the spring. Large conduits have developed in the carbonate aquifer due to dissolution processes that create a myriad of sinkholes characterizing the Mitchell Plain. Such karst structures determine turbulent flow after intense rain events from the recharge area to the spring. Orangeville Rise was monitored before and during an intense storm event (20mm of rain in 2 hours) which occurred on the 10th of November 2002. The discharge increased almost immediately and reached its maximum on the hydrograph (from less than 1 to 6 m3/s) in 24 hours. Applying linear mixing models to the chemistry of the spring water, the hydrograph separation technique allowed us to quantify contribution percentages to the total discharge. Stable isotopes (oxygen-18, deuterium, sulphur-34, nitrogen-15 and carbon-13) and major ions were used to track three different water sources (rainwater, epikarst and phreatic water). The hydrograph separation shows that 40% of the total discharge is to be attributed to rainwater entering the system during the event; vadose water supplies another 40 % throughout the entire flood and phreatic water represents the remaining part. The hydrograph separation also presents an initial piston effect produced by the rain on the groundwater and the vadose water located in the shallow subsurface. The arrival of the highest rain contribution (>50%) occurred when the total discharge approached pre-storm values.