Glacial Sediment Causing Regional‐Scale Elevated Arsenic in Drinking Water

Abstract

In the upper Midwest, USA, elevated arsenic concentrations in public drinking water systems are associated with the lateral extent of northwest provenance late Wisconsin-aged drift. Twelve percent of public water systems located within the footprint of this drift (212 of 1764) exceed 10 microg/L arsenic, which is the U.S. EPA's drinking water standard. Outside of the footprint, only 2.4% of public water systems (52 of 2182) exceed 10 microg/L arsenic. Both glacial drift aquifers and shallow bedrock aquifers overlain by northwest provenance late Wisconsin-aged sediment are affected by arsenic contamination. Evidence suggests that the distinct physical characteristics of northwest provenance late Wisconsin-aged drift--its fine-grained matrix and entrained organic carbon that fosters biological activity--cause the geochemical conditions necessary to mobilize arsenic via reductive mechanisms such as reductive desorption and reductive dissolution of metal oxides. This study highlights an important and often unrecognized phenomenon: high-arsenic sediment is not necessary to cause arsenic-impacted ground water--when "impacted" is now defined as >10 microg/L. This analysis also demonstrates the scientific and economic value of using existing large but imperfect statewide data sets to observe and characterize regional-scale environmental problems.

Solid-phase arsenic speciation in aquifer sediments: A micro-X-ray absorption spectroscopy approach for quantifying trace-level speciation

“…For As in glacial aquifers, this principle has been demonstrated through a statistical comparison of As concentrations in 1000s of wells on a regional-scale with screened intervals with varying proximity to an aquitard. Wells that were screened near the aquitard were more likely to have elevated well-water As concentrations than wells with screens farther from the aquitard (Erickson and Barnes, 2005a). These findings generated the first tractable explanation for the geographic distribution of elevated As in wells completed in confined glacial aquifers: the aquifer-aquitard contact zone is a hot-spot for As mobilization from the solidphase to the aqueous-phase (Erickson and Barnes, 2005b).…”

“…The crustal average is about 5.1 mg kg À1 (Rudnick and Gao, 2003). The highest As concentration found in any of the sediments measured for this study is 12.1 mg kg À1 , the lowest concentration was 2.6 mg kg À1 , the average concentration was 6.6 mg kg À1 and the median concentration was 6.8 mg kg elevated-As wells is strongly heterogeneous with respect to geography and well depth and previous research based on well-water chemistry and well-construction records suggested that the aquitard/aquifer interface was the likely source of As from solids to waters (Erickson and Barnes, 2005a). Wells with screened intervals close to an aquitard are more likely to have elevated well-water As concentrations than wells with screens farther from the aquitard; Erickson and Barnes, 2005b).…”

The quality of our Nation's waters: water quality in the glacial aquifer system, northern United States, 1993-2009

“…Concentrations of arsenic in glacial sediments reflect the lithology (bedrock type) over which the major glacial ice lobes flowed (30) and the type of bedrock underlying the current location of glacial sediments.…”

“…(29) The presence or absence of arsenicbearing metal oxides is related to the source of the sediments, redox conditions in the aquifer, and the original arsenic-bearing minerals. Concentrations of arsenic in glacial sediments reflect the lithology (bedrock type) over which the major glacial ice lobes flowed (30) and the type of bedrock underlying the current location of glacial sediments. (12) For example, in areas where the glacial sediments are derived from arsenic-rich marine shales and redox conditions favor the release of arsenic to groundwater, the groundwater tends to have high arsenic concentrations.…”

Water-quality assessment of the Cambrian-Ordovician aquifer system in the northern Midwest, United States

“…3 for location) west of the confluence of the Minnesota and Mississippi Rivers. High arsenic concentrations in the regionally unconfined area of Minnesota may be associated with the overlying Des Moines glacial lobe (Erickson and Barnes, 2005). The extent of the Des Moines glacial lobe shown in figure 33 is from Schumann (1993).…”