Arsenic species and chemistry in groundwater of southeast Michigan

Groundwater samples, taken from 73 wells in 10 counties of southeast Michigan in 1997 had arsenic concentrations in the range of 0.5 to 278 μg/l, the average being 29 μg/l. About 12% of these wells had arsenic concentrations that exceeded the current USEPAʹs maximum contaminant level of 50 μg/l. Most (53–98%) of the arsenic detected was arsenite [As(III)] and other observations supported the arsenic species distribution (low redox potential and DO). In shallow groundwater (<15 m), arsenic concentrations are low likely due to the formation of insoluble ferrosoferric hydroxide complex. In deep groundwater (>15 m), the concentration of arsenic is possibly controlled by reductive dissolution of arsenic-rich iron hydroxide/oxyhydroxide and dissolution of arsenic sulfide minerals.