Delineating the karstic flow system in the upper Lost River drainage basin, south central Indiana: using sulphate and δ34SSO4 as tracers

A karstic flow system in the upper Lost River drainage basin in south central Indiana, USA, was investigated using SO4 concentration and δ34SSO4 as tracers. The flow system was characterized as vadose flow and phreatic diffuse flow. Vadose-flow samples were collected from 7 epikarstic outlets after storm events. Phreatic diffuse flow samples were collected from the Orangeville Rise, the major emergence point for the drainage basin, during the base flow periods. Discharge from the Orangeville Rise was constant during the base flow periods but showed large variations in flow rate (0.3–11.7 m3/s), SO4 concentration (11–220 mg/l), and δ34SSO4 (+5.2 to +15.0‰) after storm events, due to the mixing of rain, vadose flow, and phreatic diffuse flow in the conduits that feed the Orangeville Rise. Sulphate concentrations and δ34SSO4 were unique in vadose flow (SSO4: 13–24 mg/l; δ34SSO4: +1.9 to +3.8‰) and phreatic diffuse flow (SO4: 220 mg/l; δ34SSO4: +15.0‰). Mean SO4 concentration of rainwater in the study area was measured as 1.8 mg/l. Using a 3-component mixing model for water in the karstic conduits, the mixing ratios of rain (16.5%), vadose flow (58.5%), and phreatic diffuse flow (25.0%) components were calculated in the Orangeville Rise discharge. These mixing ratios attained using SO4 concentration as a tracer indicated the important role of the vadose zone as a water storage area in karst aquifers.