High concentrations of naturally occuring arsenic are present in ground water in midwestern United States including Illinois, Missouri, and Ohio. Several studies in the Midwest have related high arsenic concentrations to redox conditions in alluvial aquifers and buried bedrock valley aquifers (Korte, 1991; Gulen and others, 1979). The most common source of arsenic in ground water is in situ dissolution or leaching of arsenic-containing minerals, usually because of a change in redox conditions. Some sources of arsenic in ground water are the result of oxidation of arsenic sulfides caused by mining (Bottomly, 1984) or a shift from oxidizing to reducing conditions, which result in the mobilization of sorbed or co-precipitated arsenic as documented in Ohio (Matisoff and others, 1982). Typically, the aquifers are relatively high in clay content and relatively low yield, and for that reason many of the aquifers are only used by self-served population. In alluvial aquifers, the mechanism by which the arsenic is mobilized begins with the deposition of iron oxides and associated arsenic during streamflow while the alluvium is being deposited. As the alluvium is buried, it eventually becomes subject to slow ground water movement. The conditions become more reducing, so ferric oxides are reduced to soluble ferrous oxides, resulting in mobilization of sorbed arsenic (Korte, 1991).

The ground water in the buried Mahomet Bedrock Valley aquifer (MBV) is the major water supply in the Lower Illinois River Basin. Arsenic was found to be present at elevated levels in the deep glacial-drift aquifer in the buried bedrock valley, up to 84 micrograms per liter (mg/L). The current drinking water standard is 50 mg/L. A subset of deep aquifer wells with elevated arsenic concentrations was resampled in 1997 to determine the arsenate and arsenite concentrations. Arsenite is more toxic than arsenate and is detected in higher concentrations. Total dissolved arsenic concentrations consisted of 50 to 100 percent arsenite. Total arsenic concentrations did not vary significantly from 1996 to 1997.

Arsenic source is most likely to be bedrock. The source of the arsenic is not clear, but several hypotheses have been presented in literature. Panno and others (1991) state that it may be from recharge into the buried glacial aquifer from the Pennsylvanian bedrock or deeper. Another hypothesis is that the pyrite in the coal and black shales of the Pennsylvanian were exposed prior to glaciation and then eroded (oxidized). Then burial by clay-rich glacial drift caused reducing conditions which resulted in the reduction of absorbed arsenate to arsenite which is more mobile (Panno and others, 1994).

Several studies have shown that arsenate is the major species of total arsenic in ground water (Welch and others, 1988; Holm and Curtiss, 1988), but a study on the alluvial aquifer in Missouri found that essentially all the arsenic in wells was arsenite (Korte, 1991). Comprehensive geochemical ground water analyses, such as arsenic speciation, is much more likely to reveal the source of arsenic contamination than total arsenic determinations alone (Holm and Curtiss, 1988, p.3). Arsenic speciation is of interest from a public health stand point because arsenite is generally more toxic than arsenate. Arsenic speciation also will help determine the source of arsenic contamination in the Lower Illinois River basin and will provide well owners or communities with more knowledge on how to effectively treat the arsenic porblem. The method of most effective remediation depends on the arsenic species in the ground water.

Selected wells were resampled to verify low-level pesticide detections and determine the extent of high concentrations of dissolved arsenic. A set of the first Study Unit Survey (SUS1) wells (less than 10) were resampled that had atrazine or other herbicide detections. The aquifer underlies 100 to 200 feet or more of clayey till, and the pesticide detections are at or below the reporting limit for the analytes. These detections were verified with additional environmental and quality assurance samples.

The first year's total dissolved arsenic were compared with the two sets of arsenic data we received from Frontier Geochemical (arsenite and arsenate speciation by hydride generation/ion exchange chromatography- atomic absorbtion, total arsenic and selected heavy metals by IP/MS and John Garbarino's NWQL methods development lab for As speciation). The preliminary data indicated that most to all the dissolved arsenic in the Mahomet aquifer was arsenite- the more reduced and potentially most toxic form. The data from the Land-Use Survey (LUS) wells indicated that arsenic was not generally as high in the shallow ground water and the arsenic problem may be restricted to deep (pre-Illinoian) glacial aquifers. Based on these data we sampled more SUS1 wells to determine the extent of high arsenite in the Mahomet aquifer and the other bedrock valley aquifers. Integrated samples of the aquifer material were collected from the core library at the Illinois State Geological Survey (ISGS). The solid-phase samples were submitted to geologic Division for bulk chemistry, X-ray diffraction, X-ray fluorescence, heavy mineral analysis, and potentially for selective extraction for arsenic species. The USGS also talked with the ISGS about solid-phase analyses as well. Further arsenic sampling and analyses was coordinated with Alan Welch.