SULFUR ISOTOPES FOR TRACING ARSENIC IN THE KOPE FORMATION AND GROUNDWATER SUPPLIES, SOUTHWEST OHIO

  Naturally occurring arsenic in groundwater supplies is a widespread environmental/public health problem.  Most groundwater in the Midwest is produced from buried valley aquifers (Thomas, 2003).  The Mason and the Lebanon Correctional Institute (LCI) drinking water distribution systems, in southwest Ohio, obtain their water from the Shaker Creek “buried valley” Aquifer and these systems have elevated arsenic levels.  Arsenic is released from solids into the groundwater because of dissolution of arsenic-bearing sulfides, especially pyrite during redox potential changes (Moore et al., 1988).  Where sulphide-bearing rocks are exposed to high oxidation environments, pyrite (FeS₂) and other sulphide minerals, which may contain arsenic, can contribute considerable amounts of sulphate and arsenic to groundwaters (Clark and Fritz, 1997).  Sources of sulphate in groundwater can be detected with sulfur isotopes.  Thus, the purpose of this study was to analyze groundwater, glacial outwash aquifer fill material and bedrock samples for sulphur isotopes in order to determine the location of arsenic in groundwater.  The results for sulphate in groundwater ranged from δ ³⁴S -4 to -16 ‰VCDT.  Pyrite in the bedrock was found to be -5 ‰ VCDT, compared to glacial outwash pyrite, which was –38 to-41 ‰ VCDT.  All these values indicate depletion in δ ³⁴S and sulfate ions (SO₄) in the groundwater are predominately derived from oxidation of pyrite from the bedrock.  The depletion in δ ³⁴S bedrock is indicating that the main contributor of arsenic in the groundwater is the bedrock. To prove this, bedrock samples were analyzed by Inductively Coupled Plasma (ICP) showing a peak of arsenic of 40 ppm and glacial outwash aquifer material ranged from 2-15 ppm.  Thus, it is likely that the source of the arsenic is from the bedrock.  We suspect that this zone is the source of the arsenic in the groundwater.