IN SEARCH OF A SULFATE MINERAL SOURCE FOR NATURALLY-OCCURRING, ELEVATED LEAD IN SOUTHERN INDIANA LIMESTONE AQUIFERS

In Indiana, a database of groundwater samples collected by the Indiana Geological and Water Survey from homeowner wells, includes over 400 locations where trace element analyses were determined along with major ion and field chemistry. Factor analysis indicates a strong correlation between calcium, sulfate and lead in well water samples, with the strongest correlation and highest lead concentrations coming from a region where groundwater supply is predominantly Middle Mississippian limestone aquifers of either the Blue River or Sanders Groups. The St. Louis Limestone of the Blue River Group is known to contain gypsum and anhydrite deposits, which attain economic importance down-dip from the lead-containing groundwater region. A set of 27 samples extracted from 11 rock cores from locations across southern Indiana were identified by X-ray diffraction as containing some combination of gypsum and anhydrite in the St. Louis Limestone. Lead content of these mineral phases was determined by X-ray fluorescence and dissolution chemistry analysis.

Lead concentrations in all 27 samples were too low to register by X-ray fluorescence, and of the 25 samples analyzed by chemical dissolution only 7 registered above the analytical method detection limit of 0.00010 mg/L, ranging from 0.00010 to 0.00048 mg/L. This calculates to a lead content in a soluble calcium sulfate phase of 50 to 438 ppb. The majority of samples having no detectable lead were calculated to contain less than 50 ppb lead in gypsum or anhydrite. Lead values in water well samples from the lead-bearing groundwater region range from 0.017 to 0.026 mg/L. Given the amount of sulfate in solution, a corresponding lead content of gypsum or anhydrite would require approximately 10,000 ppb to generate the level of lead measured in wells from this region. With no available core from groundwater in the lead-bearing region this cannot be verified, but elevated lead concentrations correlating to calcium and sulfate suggest the presence of lead mineralization that has undergone mobilization processes. Previous researchers have suggested an as yet undiscovered Mississippi Valley-type ore deposit as a possible source.