A four-component mixing model for water in a karst terrain in south-central Indiana, USA. Using solute concentration and stable isotopes as tracers

The study area lies in a highly karstified carbonate terrain in south central Indiana. Sinkholes, conduits, and caves form large secondary pathways for the subsurface flow. As a result, the discharge from a main emergence point for the subsurface flow system, the Orangeville Rise, quickly responds to the storm events and shows wide variations in flow rate, water chemistry, and stable isotopic compositions. These responses are attributed to the mixing of water in secondary pathways. In the study area, recharge occurs through the thick, mantled karst plain and the sinkhole plains, and the role of soil layer and epikarst in the recharge process is of great importance. Rain (DIC: 2 HCO3− mg/l, δ13CDIC: −7‰), soil water (DIC: 544 HCO3− mg/l, δ13CDIC: −14.7‰), epikarstic water (DIC: 224 HCO3− mg/l, δ13CDIC: −13.6‰), and phreatic diffuse flow water (DIC: 299 HCO3− mg/l, δ13CDIC: −11.8‰) generally showed unique and constant dissolved inorganic carbon (DIC) and δ13CDIC values over time. Using DIC and δ13CDIC as tracers, a four-component mixing model was established for the karstic flow system. By constructing the discharge hydrograph separation curves, the mixing ratio of each component, rain (10.6%), soil (3.1%), epikarstic (52.3%), and phreatic (34.0%) water, was determined for the Orangeville Rise discharge over the testing period of 104 h after the storm event of 10/4/90. Vadose water occupied 55.4% of spring discharge and this demonstrates the importance of the unsaturated zone, especially the epikarst, in the karstic flow systems.