Spatial Variability of Arsenic in Glacial Aquifers

Recent research by the Illinois State Water Survey and others found that arsenic levels in private wells can be difficult to predict, even in areas where elevated arsenic levels are known to occur. Sampling to date has generally been over large areas, with sample points a kilometer or more apart, evaluated at a county or township scale. However, there was one cluster in which a well with 190 μg/L As was located 200-400m from wells with 2 and textless0.5 μg/L As (the detection limit). In another cluster, two wells with textless0.5 and 44.0 μg/L were on adjoining properties textasciitilde100m apart. In the present work, three sets of private wells were selected, in glacial aquifers where previous sampling has shown elevated arsenic in close proximity. In each area, the wells are all within a radius of 1-2 kilometers. The distance between wells is on the order of 10s to 100s of meters. Between 10 and 20 wells in each area are being sampled. Even on this smaller scale, arsenic concentrations ranged from undetectable to textasciitilde100 μg/L. The sampling results and their implications will be discussed.