North-Central Section - 50th Annual Meeting - 2016

Paper No. 23-6
Presentation Time: 10:00 AM


GAHALA, Amy, U.S. Geological Survey, DeKalb, IL 60115,

In 2010, 46 monitoring wells open to the sand and gravel aquifers of McHenry County, Illinois were sampled for major ions and nutrients. Five monitoring wells from the National Water Quality Assessment Program (NAWQA) were also included in the total wells sampled. Sixteen of these wells, less than 25 feet deep, were also sampled for volatile organic compounds (VOCs) and eight were sampled for pesticides. The results of the 2010 sampling effort were compared to results of the residential well sampling that was completed in 1979-1980 (Nicholas and Krohelski, 1984) and six residential wells were also resampled in 2015 and compared to the previous residential well sampling. These data were compared to previous data to assess changes in chlorides through time. Additionally, chloride-bromide (Cl/Br) ratios (Mullaney and others, 2009) in the water were used to evaluate potential sources of the chloride concentrations detected in the shallow sand and gravel aquifers and results will be discussed.

Monitoring wells near roadsides and near urban areas had the highest chloride and sodium concentrations, reflecting the influence of road salt. Chloride concentrations were near or above the U.S. Environmental Protection Agency (USEPA) drinking water secondary maximum contaminant level (SMCL) of 250 milligrams per liter (mg/L) and the aquatic life chronic toxicity level of 230 mg/L in 6 of the 46 wells sampled. Nitrates were highest in agricultural areas with the concentration in one well at the USEPA maximum contaminant level (MCL) of 10 mg/L. Seventy-five percent (6 out of 8) of the wells detected pesticides (atrazine and prometon). Forty-three percent (7 out of 16) of the wells had detections of VOCs (trichloroethane and trichloromethane) above the reporting level but below their respective USEPA MCLs.

The average chloride concentration from the 1979-1980 data was 11.7 mg/L. The six residential-wells that were resampled in 2015 had an average chloride concentration of 25.1 mg/L in 1979. Average chloride concentrations increased to 57.4 mg/L. The average chloride concentration from the 2010 monitoring wells was 58.9 mg/L, similar to the average change identified from the 2015 residential well sampling.