High levels of arsenic are commonly observed in well water in the Fox River valley area of eastern Wisconsin. The primary source of arsenic in this study area is a sulfide-mineral-rich horizon located at the top of the St. Peter sandstone. The highest concentrations of arsenic in groundwater are from wells open to the top of the St. Peter; in these wells, the static water level likely intersects the mineralized zone in the rock. This suggests that a primary mechanism for arsenic release to groundwater is introduction of oxygen at the borehole and subsequent oxidation of arsenic-bearing sulfide minerals.

We have designed a field-based experiment to further investigate the geologic and geochemical conditions that result in the release of arsenic to well water in this region. Rock cores were collected at three locations within a 1500 ft2 area to assess the distribution of arsenic-bearing minerals within the affected aquifer at the scale of the field site. Two of the three borings have been converted to monitoring wells. Preliminary results show that in addition to the mineralized zone at the top of the sandstone, sulfide mineralization occurs throughout the St. Peter sandstone in bands and nodules. Whole-rock arsenic concentrations range from about 500 mg/kg at the top of the St. Peter to about 80 mg/kg in the nodules and bands of sulfide minerals. In areas where mineralization is not present, arsenic concentrations are less then 10 mg/kg. Water samples show reducing conditions, with dissolved oxygen less than 0.2 mg/L and ORP measurements on the order of -200 mV. Arsenic concentration in formation water is less than the laboratory detection limit of 3.7 ug/L. A sample of borehole water, collected prior to purging the well, contained arsenic at 8 ug/L.

Ongoing research includes additional baseline water quality monitoring; a pumping test to determine whether cyclical water-level fluctuations across the arsenic-rich horizon, which periodically expose the formation to atmospheric oxygen, cause an arsenic release; and the use of standard groundwater sampling filters and ultra-filtration techniques to investigate the role of colloid transport in the aquifer. The effect of well chlorination (used by homeowners to treat iron bacteria problems) on arsenic levels will also be evaluated.