Geologic constraints on arsenic in groundwater with applications to groundwater modeling

In the 1980s, high concentrations of arsenic were measured in the Fox River Valley, eastern Wisconsin.  Research suggested mineralization occurring at the contact of a middle Ordovican St. Peter (Ancell Group) and Platteville formations (Sinnipee Group) as a source of the contamination.  The stratigraphic units of the Ancell Group (St. Peter and Glenwood formations) and the Sinnipee Group (Platteville, Decorah, and Galena formations) were measured, described, and defined, and possible geologic sources of the arsenic contamination were investigated.  Quarries and drill cores were examined to construct a stratigraphic framework of the region, define lithostratigrahic facies and hydrostratigraphic units, and locate mineral deposits that may be a source of arsenic contamination.  Field and laboratory investigations of mineral deposits were performed to determine the presence of arsenic.  Residential well data were used to determine if a statistical correlation exists between the suspected source of arsenic contamination and areas of known contamination.  Stratigraphy of the Ancell and Sinnipee Groups is divided into three packages:  the sandstones and minor shales of the St. Peter and Glenwood formations (Package A); the dominantly clay-free dolostones of the Platteville Formation (Package B); and the dolostones, argillaceous dolostones, and clays of the Galena Formation (Package C).  Each of these packages is divided into lithostratigraphic units A1-A2, B1-B6, and C1-C5 based on argillaceousness, texture, and fossil content.  Six hydrostatigraphic units are defined based on estimatations of argillaceousness, porosity, and permeability.  Mineralization was dominantly calcine and sulfides [pyrite (FeS) and less commonly sphalerite (ZnS) and marcasite (FeS₂)].  The mineralization was observed in three geologic settings across the study area:  joint faces, porosity, and a cement horizon.  The only volumetrically signification mineralization observed within the study area occurs as a sulfide-bearing secondary cement horizon immediately below the base of the Platteville Formation which we have termed the Sulfide Cement Horizon (SCH).  Whole rock analyses of samples from several locations show concentrations of arsenic up to 212 ppm.  SEM/Microprobe analysis found that the horizon is characterized by a complex digenetic succession of large and massive pyrite crystals.  The SCH and water table generally intersect in a region approximately correlative with a region of high arsenic concentrations.  Private well data show high arsenic concentrations where the water table lies at or immediately below the SCH.  Fluctuating levels may have increased the leaching of arsenic from the SCH, possibly through exposure of the SCH to an oxygenated environment and consequent formation of sulfuric acid by oxidation of pyrite.  More research is needed to understand the geochemical reactions between the mineralization and groundwater, and how private well use affects these reactions, before recommendations for remediating the contamination problem can be made.