Arsenic in Glacial Drift Aquifers and the Implication for Drinking Water—Lower Illinois River Basin

Abstract

The lower Illinois River Basin (LIRB) covers 47,000 km2 of central and western Illinois. In the LIRB, 90% of the ground water supplies are from the deep and shallow glacial drift aquifers. The deep glacial drift aquifer (DGDA) is below 152 m altitude, a sand and gravel deposit that fills the Mahomet Buried Bedrock Valley, and overlain by more than 30.5 m of clayey till. The LIRB is part of the USGS National Water Quality Assessment program, which has an objective to describe the status and trends of surface and ground water quality. In the DGDA, 55% of the wells used for public drinking-water supply and 43% of the wells used for domestic drinking water supply have arsenic concentrations above 10 micrograms/L (a new U.S. EPA drinking water standard). Arsenic concentrations greater than 25 micrograms/L in ground water are mostly in the form of arsenite (AsIII). The proportion of arsenate (AsV) to arsenite does not change along the flowpath of the DGDA. Because of the limited number of arsenic species analyses, no clear relations between species and other trace elements, major ions, or physical parameters could be established. Arsenic and barium concentrations increase from east to west in the DGDA and are positively correlated. Chloride and arsenic are positively correlated and provide evidence that arsenic may be derived locally from underlying bedrock. Solid phase geochemical analysis of the till, sand and gravel, and bedrock show the highest presence of arsenic in the underlying organic-rich carbonate bedrock. The black shale or coal within the organic-rich carbonate bedrock is a potential source of arsenic. Most high arsenic concentrations found in the DGDA are west and downgradient of the bedrock structural features. Geologic structures in the bedrock are potential pathways for recharge to the DGDA from surrounding bedrock.

Arsenic in Illinois ground water — Community and private supplies

“…Possible arsenic sources may be determined by establishing a relation between arsenic (Warner, 2001). A comparison of private supplies in this same area found concentrations to be similar for comparable depths (Warner, 2001).…”

“…The deep portions of the glacial and alluvial aquifer system are defined as the deep glacial drift aquifer ( fig. 4) by Warner (2001). In the deep portions of the glacial and alluvial aquifer system, filtered arsenic concentrations for ground-water samples from private wells ranged from less than 1.0 to 84 ^ig/L, with a median concentration of 1.5 \igfL', whereas the range of unfiltered arsenic concentrations for ground water from community-water supplies ranged from less than 1 to 110 jig/L, with a median of 1.0 ug/L (Warner, 2001 fig.…”

“…The USGS National Water-Quality Assessment (NAWQA) program has collected data from 225 monitoring and private wells screened in the glacial and alluvial aquifer system. The glacial and alluvial aquifer system, as defined in this report, Willman and Frye, 1970;Hansel and Johnson, 1996;and Warner, 2001). …”

“…Factors such as ground-water age, depth to water, and land use affect the fate and transport of constituents introduced by humans, but have limited effects on the concentration of arsenic in ground water. Arsenic data from community and private supplies were compared in Illinois and other parts of the country (Warner, 2001;Bruce and Oelsner, 2001 Figure 5. Arsenic concentrations in community-supply well samples in Illinois, 1978Illinois, -2001 tions of the glacial and alluvial aquifer system in Illinois were similar for a data set containing less than 100 samples (Warner, 2001).…”

“…Arsenic data from community and private supplies were compared in Illinois and other parts of the country (Warner, 2001;Bruce and Oelsner, 2001 Figure 5. Arsenic concentrations in community-supply well samples in Illinois, 1978Illinois, -2001 tions of the glacial and alluvial aquifer system in Illinois were similar for a data set containing less than 100 samples (Warner, 2001). The deep portions of the glacial and alluvial aquifer system are defined as the deep glacial drift aquifer ( fig.…”