High arsenic concentrations and enriched sulfur and oxygen isotopes in a fractured-bedrock ground-water system

Ground water with high arsenic concentrations (up to 26.6 μmol L− 1) has sulfate enriched in 34S and 18O in the fractured-bedrock, ground-water system of the Kelly's Cove watershed, Northport, Maine, USA. The ranges of sulfur and oxygen isotope values in aqueous sulfate, δ34S[SO4] and δ18O[SO4], at the Kelly's Cove watershed are + 3.4 to + 4.9‰ and − 2.0 to + 6.7‰, respectively. These isotope values are strikingly similar to those of the Goose River, Maine watershed which has δ34S[SO4] and δ18O[SO4] ranges of + 3.7 to + 4.6 ‰ and − 2.6 to + 7.5‰, respectively. In both systems, high arsenic concentrations occur with high δ34S[SO4] and δ18O[SO4] values, yet redox conditions and underlying rock types are quite different.The isotope values of sulfide minerals, δ34S[min], from four bedrock cores vary over short distances and range from − 5.1 to + 7.5‰. The δ34S[SO4] values are controlled by the δ34S[min] values with minor input of atmospheric SO4. The much narrower range in δ34S[SO4] values than δ34S[min] values is probably due to sufficient ground-water mixing at a scale greater than the δ34S[min] variability. The δ34S[SO4] values are about 2‰ higher than the average δ34S[min] value and fall within the range of δ34S[min] values, indicating only minor fractionation due to bacterial reduction of SO4.The highest δ18O[SO4] values were measured in the downgradient, confined, arsenic-rich ground water. High δ18O[SO4] values there cannot be due to aeration by atmospheric oxygen, but may arise from reoxidation of reduced SO4 products. The enrichment factors of δ18O in SO4 compared to H2O, + 7.2 to + 15.5‰, in the Kelly's Cove ground water and the negligible 34S enrichment is very similar to those derived from experimental data of anaerobic sulfide oxidation in the presence of Mn and Fe oxides.Sea level at the Kelly's Cove watershed was approximately 80 m above present sea level about 13 000 years before present, imposing reducing conditions on that area of the watershed. Sea level dropped approximately 60 m below present sea level about 11 000 years before present, allowing for possible oxidation of sulfide minerals and precipitation of arsenic in ferric oxyhydroxides during aeration of the ground-water system. Under present redox conditions, there is evidence that bacteria reduction of ferric oxyhydroxides releases arsenic. The fractionation of 18O in the SO4 during anaerobic oxidation of sulfide in the presence of Mn and Fe oxides and subsequent release of arsenic during Mn and Fe oxide reduction may explain the relationship between high arsenic concentrations and elevated 18O[SO4] at Kelly's Cove.