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

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• 作者：Gail Lipfert ; William C. Sidle ; Andrew S. Reeve ; Robert A. Ayuso ; Adrian J. Boyce
• 关键词：Arsenic ; Oxygen isotopes ; Sulfur isotopes ; Fractured bedrock ; Ground water ; Anaerobic oxidation
• 刊名：Chemical Geology
• 出版年：2007
• 出版时间：15 August 2007
• 年：2007
• 卷：242
• 期：3-4
• 页码：385-399
• 全文大小：800 K

文摘

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.