Identifying subsurface storage compartments contributing to karst spring discharge through the use of major-ion chemistry and oxygen and deuterium isotopes in the upper Lost River drainage basin, south-central Indiana

Karst aquifers are highly heterogenous due to the presence of integrated conduit systems housed within limestone that is often dense and possesses low permeability. The dramatic variability can produce a complicated scenario for groundwater recharge, storage and transmission within karst aquifers. This research investigates the rainfall-discharge response of karst springs in the upper Lost River drainage basin of south-central Indiana, in order to identify how recharge influences subsurface water flow in these carbonate aquifers.;To account for the large volume of pre-storm water during peak flow, water held in storage must travel quickly through conduits to the springs. Storage compartments which could contribute this water include phreatic and vadose conduits. Pre-storm water flowing from the springs during discharge recession may be derived from soil moisture and the epikarst.;In a sideline investigation of pesticide occurrence in the Lost River basin, it was found that they can be quickly mobilized from cultivated fields to surface streams and karst springs during high flow conditions. However, pesticides were not detected in baseflow discharge at springs, or in any domestic water wells sampled during the course of the study. These results may be misleading, however, due to the high detection limits of analytical procedures used.;Major-ion and oxygen and deuterium isotopes were analyzed in rainfall and storm discharge from the large perennial springs, Orangeville Rise and Rise of the Lost River during two storm events. Isotopic data was used in a two component mixing model to identify pre-storm water and new rainwater within storm discharge at the springs. Following each storm, discharge increased rapidly at the springs. Ionic concentrations and isotopic composition of peak storm flow indicated that the bulk of discharge was derived from pre-storm water, and the time of maximum rainwater contribution was not until four to twenty-four hours after peak flow. Rainwater contribution to the storm hydrographs at both springs was 20 to 30%.