SULFUR, CARBON, OXYGEN, AND HYDROGEN ISOTOPE STUDIES OF FRESHWATER SPRING AND MINERAL SPRING FLOW ON THE KARST MITCHELL PLAIN AND THE CRAWFORD UPLAND OF SOUTHERN INDIANA

Water chemistry and stable isotope (oxygen, hydrogen, carbon and sulfur) suggest that two flow systems exist in the karst terrain of Southern Indiana: (1) a shallow flow system, which is dominated by surface water that infiltrates through the soil profile to the epikarst then flows laterally to major vertical joints and fractures to the springs;  and (2) a deeper flow system driven by topographic cells forcing water downward where it flows through and dissolve evaporate minerals from the St. Louis limestone.  The water then flows upward and discharges at topographic lows. 

Sulfur isotopic values δ³⁴S (SO₄) for springs in the study area range from +1.1 to +22.1 ^o/oo.  δ³⁴S of HS^- range from –0.2 to –38.4.  The fresh water springs have lighter δ³⁴S (SO₄) values approximately 10 ^o/oo.  The mineral springs are approximately 14 ^o/oo  similar to values of the local St. Louis gypsum (14 to 16 ^o/oo).  Heavier δ³⁴S (SO₄)  values (18 to 19 ^o/oo) in water such as Pluto spring are due to bacterial fractionation.  Orangeville rise a major fresh water spring has a low flow δ³⁴S (SO₄) of approximately +16 ^o/oo and a sulfate concentration of 160 mg/l.  The δ³⁴S (SO₄) and sulfate concentration decreases to +9 ^o/oo and 40 mg/l respectively at high flow.  This suggests the mixing of shallow epikarst water and deeper mineral waters.

The ranges of δ¹⁸O and δ²H in the spring waters are -7.0 to -8.0 ^o/oo and –42 and –49 ^o/oo respectively.  Waters in the study area become enriched in ¹³C with increased mineral content.  Freshwater springs are depleted in ¹³C (δ¹³C_DIC = -14 to -12 ^o/oo).  The mineral springs are more enriched in ¹³C (δ¹³C_DIC = +1.8 ^o/oo) near equilibrium with the limestone bedrock.