A SHORT-TERM MONITORING OF A KARST SYSTEM IN THE MITCHELL PLAIN, INDIANA: IDENTIFICATION OF WATER MIXING SOURCES 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. Stable isotopes (oxygen-18, deuterium and carbon-13) and major ions were used to track three different water sources (rainwater, epikarst and phreatic water). 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. Fresh rain water entering the system supplies about 40% of the spring discharge throughout the entire storm event. Moreover timing the peak arrival of different contributions, we were able to define the circulation within the aquifer and to identify shallow and deep flow systems. Using deuterium as a conservative tracer to track rainwater, the hydrograph separation shows that the rain water produced an initial piston effect on the groundwater or on the vadose water present in the shallow subsurface. The arrival of the highest rain contribution (>50%) occurred when the total discharge approached to pre-storm values.