ABSTRACT
Samples of infiltrating soil water through a 30- foot (~ 10m ) unsaturated and saturated section of the unconsolidated material above a Karst terrain in southern Indiana were collected via an array of suction lysimeters and wells to study the annual variation of stable carbon isotopes. Soil waters were collected monthly from 9 suction lysimeters installed at various depths in the soil matrix to the top of the underlying bedrock. Groundwater samples from a nearby water well installed into the top portion of the limestone aquifer ( epikarst ) were also collected. All samples were analyzed for stable carbon isotopes, major ions, pH. Eh, conductivity, and temperature. Depth and time variations in stable carbon isotopes through the unsaturated zone were studied.
A preliminary evaluation of the data shows a clear and consistent shift towards higher C13/C12 ratio as soil water infiltrates. The carbon isotonic composition ranged from - 35 ‰ (PDB) in soil water samples collected near the surface to approximately - 14.7 ‰ in the epikarst zone. The study also shows significantly high HCO3 concentrations in the deeper soil zone indicating continued interaction between soil CO2 and the soil water. The annual variation in C13/C12 ratio through the thick soil zone to the epikarst zone in this study is believed to be influenced by multiple factors. Among them, the soil CO2, organic content, available CaCO3 and chemical composition of the soil are thought to be the most important
