Arsenic in upper Midwest ground water: Occurrence and geochemical mobilization mechanisms
In the upper Midwest, USA, elevated arsenic concentrations in ground water are associated with the lateral extent of northwest provenance Late Wisconsinan drift. Inside the sediment footprint, 23.8% of 7,101 wells exceed 10 μg/l arsenic; outside the footprint only 4.3% of 4,333 wells have arsenic above 10 μg/l. Both glacial drift aquifers and shallow bedrock aquifers overlain by northwest provenance Late Wisconsinan sediment are significantly affected by arsenic contamination. Evidence suggests that the distinct physical characteristics of northwest provenance Late Wisconsinan drift—its fine-grained matrix and entrained organic carbon that fosters biological activity—cause the geochemical conditions necessary to mobilize arsenic via reductive mechanisms. This analysis demonstrates the scientific and economic value of using existing large but imperfect data sets to observe and characterize regional-scale environmental problems.
Within the drift footprint, public water system wells are significantly less affected by arsenic contamination than domestic and monitoring wells. Arsenic contamination is also more prevalent in domestic wells with short screens set in proximity to an upper confining unit, such as till. Wells with long screens set a distance from an upper confining unit are at lower risk of exposure to reductive geochemical conditions conducive to arsenic mobilization. Public water system construction practices such as exploiting a thick, coarse aquifer and installing a long well screen yield good water quantity for public water system wells. Coincidentally, these construction practices also often yield low arsenic water.
Arsenic concentrations in some public water system wells fluctuate around 10 μg/l in a predictable manner. Results indicate that two reductive arsenic mobilization mechanisms, reductive dissolution and reductive desorption, are responsible for the observed arsenic concentration variability. Routine quarterly and annual sampling did not reveal the pattern to arsenic concentration variability: systematic frequent sampling did. There was also some random variability in arsenic concentrations in public water system wells. This research illustrates that additional sampling—above and beyond minimal compliance sampling—can be of tremendous benefit if the results reveal reasons for variable parameter concentrations.