DELINEATING MINERAL AND FRESH WATER FLOW PATHS IN A KARST TERRAIN USING OXYGEN, HYDROGEN AND INORGANIC CARBON ISOTOPES AND ION CHEMISTRY

The groundwater flow path and source of the dissolved constituents of fresh and mineral spring waters were studied using major ion chemistry coupled with the stable isotopes of oxygen, hydrogen and inorganic carbon.  Flow paths are established when meteoric water recharges in the Crawford Upland and the Mitchell plain.  Gravity driven convection cells within the Crawford Upland force the water from high points to the discharge point at the base of hills in entrenched streams.  Fractures and bedding planes provide pathways for water to travel deeper in the system eventually reaching the St. Louis gypsum unit.  The dissolution of gypsum and possibly other evaporite minerals create the mineral-rich waters that flow upward along fractures to discharge in entrenched streams.  Waters of intermediate composition are minerals waters that have been slightly diluted by shallower waters. 

Oxygen and hydrogen isotopic ratios were measured to study the water flow through the carbonate rocks.  The d¹⁸O and d²H values of local meteoric water range from -4 to –8 ^o/oo and -21 to –54 ^o/oo respectively.  The d¹⁸O and d²H of the spring waters range from +6.1 to   -8.0 ^o/oo and -37.4 to -49.2 ^o/oo respectively.  The spring water values are within the range of the local meteoric water.  When plotted with the meteoric water line the waters plot slightly above the line.  Deviations from the meteoric water lines are probably due to hydration of the anhydrite along the flow path. 

Inorganic carbon is dissolved in karst waters by atmospheric CO₂ (»-7 ^o/oo), plant root respiration (-25 to –30 ^o/oo in study area), oxidation of organic carbon by bacteria (-25 ^o/oo) and dissolution of carbonate rock (+4.4 to -0.82 ^o/oo).  The d¹³C_DICvalues of the water samples range from -14.4 to -0.1 ^o/oo.  The freshwater springs are more depleted in ¹³C with d¹³C_DIC values ranging between -11.9 and -14.4 ^o/oo,  As the mineral content of the springs increases they become more enriched in ¹³C and approach the d¹³C_DIC value of the regional carbonates (1.8 ^o/oo).  The mineral springs range between -8.4 and -3.0 ^o/oo and the mine water sample is most enriched at -0.1 ^o/oo.