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Arsenic in midwestern glacial deposits— Occurrence and relation to selected hydrogeologic and geochemical factors

By Mary Ann Thomas


Water-Resources Investigations Report 03-4228
Columbus, Ohio 2003

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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 types—confined 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.

Table of Contents



Purpose and scope

Description of the study area

Previous studies


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

Well depth

Well type

Aquifer type

Till with sand lenses

Outwash plain

Valley fill

Bedrock geology

Land use

Relation of arsenic to selected water-quality properties and constituents

Redox-sensitive constituents



Estimated redox conditions

Ground-water age

Other properties and chemical constituents



Dissolved organic carbon



Sodium minus chloride






Observations about arsenic in midwestern glacial deposits

Summary and conclusions

References cited


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