DISTRIBUTION OF ARSENIC IN THE MAHOMET AQUIFER OF CENTRAL ILLINOIS, USA

The Mahomet Buried Bedrock Valley Aquifer is one of the largest and most important aquifers in Illinois, providing approximately 670,000 people in central Illinois with drinking water. Arsenic (As) has been measured in water samples collected from over 1000 public, private, and monitoring wells in the aquifer for several research projects in the past 20 years. Arsenic has been found in groundwater in many parts of the aquifer, with highest concentrations generally found in the western part.

The spatial variability of aqueous As is substantial. For example, in one study As concentrations below 3 µg/L were found less than one mile from concentrations greater than 90 µg/L in wells of similar depth. Well depth, however, does appear to be a key variable. A recent study by the USGS suggests that the source of As in the central part of the aquifer may be from underlying Pennsylvanian bedrock (Warner, 2001). In the western part of the aquifer, however, greatest concentrations appear to be found in the shallowest parts of the aquifer. In the mid-1980s, over 500 private wells were sampled, and more than one third had As concentrations greater than 50 µg/L; public wells in the area have significantly lower concentrations. Private wells are usually finished near the top of the aquifer, while public wells are screened much deeper to insure uninterrupted supplies. It appears that in this part of the aquifer materials in the confining layer may be the major source of As or organic material from the confining layer may reduce iron oxide in the upper aquifer and release As . An additional finding in the western part of the aquifer was that As concentrations in private wells were lower in an unconfined section of the aquifer, suggesting removal of As from solution due to input of oxygen-rich water. An ongoing study is attempting to determine the connection among the geological, hydrologic, and geochemical factors affecting As concentrations in several parts of the aquifer.

Warner, K.L., 2001. Arsenic in glacial drift aquifers and the implication for drinking water–lower Illinois River Basin. Ground Water 39(3):433-442.