High arsenic concentrations (up to 12,000 mg/l) have been measured in groundwater from a confined sandstone aquifer in eastern Wisconsin. The source of the arsenic is a sulfide-bearing secondary cement horizon (SCH) that has variable thickness, morphology, and arsenic concentrations. Arsenic occurs in pyrite and marcasite as well as in iron oxyhydroxides but not as a separate arsenopyrite phase. The heterogeneity of arsenic content of the SCH results in significant variation in arsenic concentrations in groundwater, even in wells in close proximity that have similar construction.