EFFECT OF CHLORINATION ON ARSENIC RELEASE FROM SULFIDE MINERALS IN THE ST PETER SANDSTONE AQUIFER, EASTERN WISCONSIN

High arsenic concentrations (>100 ug/L) have been measured in wells completed in the Ordovician St. Peter sandstone aquifer of eastern Wisconsin. The primary source of arsenic is As-bearing sulfide minerals within the aquifer. There is a concern that periodic disinfection of wells by chlorination may facilitate arsenic release to groundwater by increasing the rate of sulfide mineral oxidation. During typical well disinfection procedures, aquifer solids exposed along the uncased portions of wells remain in direct contact with chlorine disinfection solutions for up to twenty-four hours.

This study will focus on abiotic processes that mobilize arsenic from the solid phase during controlled exposure to chlorination solutions, using spectroscopic techniques to characterize St. Peter sulfide minerals and alteration products that form on them. Sulfide bearing core samples of St. Peter sandstone will be sliced and exposed to chlorine solutions similar to those used in well disinfection. Before and after exposure to the chlorine solutions, sulfide surface features will be imaged using scanning electron microscopy. Transmission electron microscopic diffraction analysis will be used to identify resulting alteration products. Electron microprobe analysis will provide quantitative geochemical data about sulfide minerals and their alteration products. The chlorine solutions will be collected after contact with the sulfide minerals and analyzed to determine the amount of arsenic released to solution. Experimental results will help identify and evaluate potential short- and long-term impacts of chlorine-induced chemical oxidation of As-bearing sulfides on well water quality.