Naturally Occurring Arsenic in Sandstone Aquifer Water Supply Wells of Northeastern Wisconsin

Concentrations of arsenic, ranging from 1.0 to 12000 micrograms per liter (ug/L), were detected in groundwater over a broad geographic region in Outagamie and Winnebago Counties in Wisconsin. The hydrologic and geochemical properties of the area were examined and the source of arsenic determined to be natural. Groundwater collected from two geologic formations, the St. Peter Sandstone and the overlying Platteville/Galena Dolomite, were found to be the principle sources of the elevated arsenic concentrations. These two formations supply most of the drinking water to a large portion of eastern Wisconsin.

Arsenic levels of 5 ug/L or greater were found in 622 of 1943 water supply wells sampled in Outagamie and Winnebago Counties. When arsenic was detected at or above 5 ug/L, the concentrations generally ranged between 5 ug/L and 16 ug/L. These levels are below the NR 140 Wisconsin Administrative Code enforcement standard (ES) of 50 ug/L for groundwater but above or equal to the 5 ug/L preventative action limit (PAL). The enforcement standard of 50 ug/L for arsenic was exceeded in 68 of 1943 water supply wells sampled in Outagamie and Winnebago Counties. Arsenic was also detected in water supply wells at concentrations below 5 ug/L.