Origin and Distribution of Sulfate in a Fractured Till in Southern Alberta, Canada

Abstract

Glacial till in a 22‐km2 area of the Interior Plains Region of southern Alberta, Canada consists of an upper brown gypsiferous weathered zone (up to 20 m thick) and a lower discontinuous grey nonweathered zone (up to 25 m thick). Although the total sulfur content of the two till zones is the same (0.34%), the mean total sulfate content of the weathered till (4.0 mg g−1) exceeds that of the nonweathered till (0.8 mg g−1). Similarities suggest that all SO42− in the nonweathered till and 20% of the sulfate in the weathered till was derived from sulfate‐rich bedrock fragments incorporated in the till during deposition. Laboratory experiments and δ18O and δ34S analyses of total SO42− and the sulfur forms in the two zones show that the majority of the sulfate in the weathered till was derived from the oxidation of organic S at some time after the till was deposited. Mass balance and flux calculations indicate that this oxidation occurred under partially saturated conditions. Groundwater flow simulations suggest that the water table was much lower and that a thick partially saturated zone was possible during the drier Altithermal period (11,000 to 3000 years B.P.). The following chemical processes account for the major ions that are found in the groundwater but were generated when the partially saturated zone existed. The oxidation of reduced S produced SO42− and H+. The H+ reacted with carbonate minerals under elevated PCO2, with Ca2+ loss by exchange for Na+ and gypsum precipitation. Volume shrinkage in the weathered till resulting from these geochemical processes may have enhanced the development of fractures and caused the associated increased hydraulic conductivity.