Elevated arsenic (As) concentrations were recently measured in water samples sent in by well-owners near Tolono, IL. In response to this, we conducted a study to determine the extent of the As contamination and evaluate conditions that might be causing the elevated concentrations. Samples from 54 wells were evaluated, 17 of which we sampled directly, the rest from the ISWS water quality database. Of these, 25 had As concentrations > 10 µg/L (the current drinking water standard), and eight had concentrations > 50 µg/L. There did not appear to be any discernible spatial pattern for elevated As levels, except for a small subdivision southwest of Tolono where the original elevated samples were found.
There was, however, a relationship between well depth and As concentrations, in that the samples with the highest As concentrations were from wells between about 165 and 180 feet deep. These results suggest that a specific sand layer within the Glasford Formation is the location of the highest As concentrations. Previous studies in aquifers in Illinois suggested that oxidation-reduction (redox) conditions were the primary control of As concentrations in groundwater, especially influenced by the amount of dissolved organic carbon (DOC). However, there were no strong correlations between As and redox conditions in the Tolono samples, probably because redox conditions were relatively consistent. This was probably because DOC concentrations were very high (> 5 mg/L) in all measured samples.
Samples collected from inside houses revealed that treatment, primarily water softening, did not reduce As concentrations significantly, which is consistent with other studies.