Ground-water-quality data collected as part of 12 U.S. Geological Survey National Water-Quality Assessment studies during 1996-2001 were analyzed to (1) document arsenic occurrence in four types of glacial deposits that occur in large areas of the Midwest, (2) identify hydrogeologic or geochemical factors associated with elevated arsenic concentrations, and (3) search for clues as to arsenic source(s) or mechanism(s) of mobilization that could be useful for designing future studies.
Arsenic and other water-quality constituents were sampled in 342 monitor and domestic wells in parts of Illinois Indiana Ohio Michigan and Wisconsin. Arsenic was detected (at a concentration >1 µg/L) in one-third of the samples. The maximum concentration was 84 µg/L, and the median was less than 1 µg/L. Eight percent of samples had arsenic concentrations that exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level (MCL) of 10µg/L.
Samples were from four aquifer typesconfined valley fill, unconfined valley fill, outwash plain, and till with sand lenses. Highest arsenic concentrations were found in reducing waters from valley-fill deposits. In confined valley fill, all waters were reducing and old (recharged before 1953), and almost half of samples had arsenic concentrations greater than the MCL. In unconfined valley fill, redox conditions and ages were varied, and elevated arsenic concentrations were sporadic. In both types of valley fill, elevated arsenic concentrations are linked to the underlying bedrock on the basis of spatial relations and geochemical correlations.
In shallow (<50 ft) till with sand lenses, arsenic was detected in oxic or mixed waters, but concentrations were rarely greater than the MCL. In shallow outwash-plain deposits, arsenic concentrations greater than the MCL were detected in waters that were reducing and young (recharged after 1953).
Although arsenic concentrations were significantly higher in deep wells (>150 ft), all deep wells were from a distinctive aquifer type (confined valley fill). It is not known whether wells at similar depths in other aquifer types would produce waters with similarly high arsenic concentrations.
Correlations of arsenic with fluoride, strontium, and barium suggest that arsenic might be related to epigenetic (Mississippi Valley-type) sulfide deposits in Paleozoic bedrock. Arsenic is typically released from sulfides by oxidation, but in the current study, the highest arsenic concentrations in glacial deposits were detected in reducing waters. Therefore, a link between epigenetic sulfides and elevated arsenic concentrations in glacial deposits would probably require a multi-step process.
Purpose and scope
Description of the study area
Summary of arsenic occurrence in well networks
Lower Illinois River Basin
Upper Illinois River Basin
Lake Erie-Lake Saint Clair Drainages
Great and Little Miami River Basins
Relation of arsenic to selected hydrogeologic factors
Till with sand lenses
Relation of arsenic to selected water-quality properties and constituents
Estimated redox conditions
Other properties and chemical constituents
Dissolved organic carbon
Sodium minus chloride
Observations about arsenic in midwestern glacial deposits
Summary and conclusions
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