Arsenic Geochemistry and Distribution in the Mahomet Aquifer, Illinois

At the time of this study (2001–2004), the U.S. Environmental Protection Agency (USEPA) proposed lowering the maximum contaminant level (MCL) of arsenic from 50 to 10 µg/L for drinking water because it was a suspected carcinogen. Now arsenic is considered a known carcinogen, and the drinking water MCL is 10 µg/L because studies have confirmed that arsenic causes cancer when humans and other mammals are exposed to chronic low doses. Although arsenic had been found in the Tazewell County area of the Mahomet Aquifer, a drinking water source for both private and public wells, little was known about the extent of naturally occurring arsenic in the aquifer.

The researchers collected samples from 50 private wells in Tazewell and Champaign Counties and 11 municipal water treatment plants. Using a special method (HPLC (IC)-ICPMS) developed by ISTC’s senior chemist, John Scott, total arsenic and arsenic species levels (As(III) and As(IV)) in samples were analyzed.

The team found that arsenic levels were below the proposed MCL (10 µg/L) for 90% and 55% of wells in Champaign and Tazewell Counties, respectively. The remaining 10% of wells in Champaign County were between the existing MCL of that time and the proposed MCL, while in Tazewell County, 35% of wells had levels between the two MCLs. Tazewell County also had arsenic levels of between 50 and 200 µg/L in 10% of its wells.

The predominant arsenic species found in private and public wells was As(III), which is the more toxic form of inorganic arsenic. However, it is well known that arsenic sorbs strongly to iron; therefore, a simple iron filter on home wells will reduce arsenic levels in wells with high concentrations.

All arsenic levels in raw water from the public wells were below 50 µg/L (the former U.S. EPA MCL). In addition, all but one of the water treatment plants had an iron removal method that also removed arsenic because of its iron-sorbing affinity. Three water treatment plants produced water with arsenic levels below the new MCL level, and five plants had arsenic levels between 10 and 20 µg/L. Two of the five plants had dissolved arsenic levels below the new MCL. Researchers recommended improved filtration methods to remove the arsenic. For the other plants, the team recommended using improved treatment processes to meet the new U.S. EPA MCL.

Mahomet Aquifer Map