EFFECTS OF SOIL RECHARGE AND DISCHARGE REGIMES ON THE MODERN DISTRIBUTION OF SULFATE, OXIDIZED DURING THE HYPSITHERMAL INTERVAL, IN AN UNCONFINED GLACIOFLUVIAL AQUIFER

A shallow unconfined glaciofluvial aquifer with stratified sulfate concentrations was observed to have largest sulfate concentration in the lower aquifer. A silt layer underlying the aquifer was identified as the modern sulfate source for the lower aquifer. The original source of sulfate was pyrite in shale comprising part of the aquifer grain matrix. It was determined that an oxidizing event occurred in which sulfide was oxidized to sulfate and placed within the silt layer. A semi-generic process model using soil mapping units to define recharge and discharge regimes indicated that the oxidizing event occurred in the lower aquifer and underlying silt, several thousand years ago, likely during the Hypsithermal Interval. Isotopic diffusion models, residual water chemistry, and times for soil development support these conclusions. Thereafter the silt served as a "conserving" layer, slowly feeding sulfate into the lower aquifer and the underlying till aquitard. Rates of sulfate depletion in the silt layer and lower aquifer were affected by the thickness of the silt layer, and the local recharge-discharge characteristics of the aquifer. Soil Great Group taxa were found to be indicators of local flow system properties, and thereby served as general indicators of modern sulfate concentrations and retention in the silt and underlying till as deep 35 m below the water table. Sulfate in the EVA was shown to be of historical origin and the long-term trend is of declining sulfate.