Mechanisms of Arsenic Release to Ground Water from Naturally Occurring Sources, Eastern Wisconsin

Arsenic concentrations up to 12,000 μg/l have been measured in ground water from a sandstone aquifer in the Fox River valley in eastern Wisconsin, USA. In addition to a sulfide-bearing secondary cement horizon (SCH), which is present at the top of the aquifer, sulfide mineralization is also present throughout the aquifer. Within the SCH, arsenic occurs in pyrite and marcasite, and in iron hydroxides, but not as a separate arsenopyrite phase. Geologic, hydrogeologic, and geochemical data were used to characterize the arsenic source and the predominant geochemical process that controls its release to ground water. Several lines of evidence suggest that oxidation of sulfides is the cause of high (>100 μg/l) concentrations of arsenic in ground water, including 1) the presence of the arsenic-bearing sulfides in the aquifer; 2) water chemistry data that show a positive correlation between arsenic, iron, and sulfate and negative correlation between arsenic and pH; and 3) similar sulfur isotopic signatures in sulfides of the SCH and dissolved sulfate in ground water. We propose that atmospheric oxygen, introduced to the SCH through well boreholes, provides an oxidant to the system. This hypothesis is supported by the occurrence of high arsenic concentrations where water levels within the well intersect the SCH. However, the data do not unequivocally show sulfide oxidation to be the cause of the moderate (10−100 μg/l) and low (<10 μg/l) arsenic concentrations measured in ground water in the study area. The variability in thickness of the SCH and the concentration of arsenic within the sulfides, as well as the local availability of oxygen to the SCH, likely contribute to the spatial variability of ground water arsenic concentrations.