OXYGEN ISOTOPE STUDIES OF MAJOR KARST SPRINGS ON THE MITCHELL PLAIN (USA) AND THE TRNOVSKI GOZD KARST PLATEAU (SLOVENIA)

Joint hydrochemical and stable isotope research on spring discharge response to storm events was conducted on the Orangeville Rise (USA) and the Hubelji spring (Slovenia). An indirect method, a two – component oxygen isotope storm hydrograph separation technique was used to divide the storm (new) and the prestorm (old) component during major precipitation events. The studies showed similar results even though the discharge, size of the recharge areas, rainfall and structure were quite different.

The groundwater basin recharging the Orangeville Rise is 125 km² , with 1140 mm precipitation per year. It has gently dipping strata (10 meters/mile) and local relief of 100 meters. Discharge varies from 0.06 to 6 m³/s. The catchment area (50 to 80 km² ) of the Hubelji is a high karst plateau (900 m, a.s.l.) with an average rainfall of 2450 mm. The predominant tectonic element is an extensive and complex overthrust of Jurassic limestones over flysch deposits. Discharge varies from 3 to 60 m³/s.

The oxygen isotope storm hydrograph separations in these two dissimilar karst basins show that the majority of the water discharging from the springs during the storm was prestorm (old) water. The average contribution of the prestorm water component from Orangeville Rise and Hubelji were 77% and 79% respectively. Storm water appeared in both springs generally during the hydrograph recession with the maximum contribution 16 to 18 hours after the peak discharge. Results indicate that the prestorm water discharging at the springs has a large unsaturated (epikarst) component.