CHEMISTRY OF SULFIDE-RICH KARST SPRINGS IN OTSEGO AND SCHOHARIE COUNTIES, NEW YORK

The chemistry of 6 sulfide springs in the Helderberg Group of Schoharie and Otsego Counties, NY, is being measured to determine the nature of the groundwater that feeds them. To provide for sulfur isotope analysis, sulfide is precipitated as Ag₂S by adding AgNO₃, and sulfate is precipitated as BaSO₄ by adding BaCl₂ after sparging of H₂S with N₂ gas. Radium isotopes are also trapped in the precipitates. Other isotopes and dissolved solids are analyzed directly form water samples. Isotopic analyses are performed at the U.S. Geological Survey, Reston, Va. About 50% of the project has been completed.

Groundwater in this area flows through interbedded carbonate and sulfate rocks of Silurian-Devonian age. Interaction among them causes precipitation of calcite and enhanced solubility of gypsum and dolomite. On average, the springs are supersaturated with calcite, slightly undersaturated with dolomite, and approximately saturated with gypsum (mean SI values: SIc=+0.33, SId=-0.078, SIg=+0.098, where SI=log [IAP/K]). Mg and SO₄ molalities are at least 1.5 times greater than could be achieved by saturation with either gypsum or dolomite alone. Reaction-path modeling suggests that about 0.004M of calcite is precipitated in the groundwater owing to dedolomitization. Mean PCO₂ is 0.011 atm, typical of well water in the region, and the mean temperature is 9^oC, about 1-2^o higher than that of nearby shallow well water. ²²⁸Ra/²²⁶Ra ratios are close to 1.0, which, together with high activities of each isotope, indicate long-term contact with both detrital and carbonate strata. Spring samples cluster tightly around the point δ¹⁸O=-11‰, δD=-75‰, midway between the seasonal extremes for local precipitation, which indicates mixing of water from diverse infiltration events. δ¹⁸O is slightly enriched relative to the meteoric water line, apparently because of lengthy contact with carbonates. Hydrogen sulfide varies widely, from 3.6 to 390 mg/L. Whether it forms at depth by bacterially mediated sulfate reduction cannot be verified until analysis of δ³⁴S is completed. Tritium values (~11 TU) indicate a mean groundwater age of less than 10 years.