GSA Annual Meeting, November 5-8, 2001

Paper No. 0
Presentation Time: 4:45 PM

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


CANNON, Dana L., Geological Sciences, Indiana Univ, 1001 East Tenth Street, Bloomington, IN 47405, KROTHE, Noel C. and LOHEIDE II, Stephen P., dacannon@indiana.edu

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 d13CDICvalues of the water samples range from -14.4 to -0.1 o/oo.  The freshwater springs are more depleted in 13C with d13CDIC 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 d13CDIC 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.