During 1991-94, the U.S. Geological Survey (USGS) conducted a study of agricultural chemicals in ground water of the midcontinental United States (Kolpin and others, 1994; Kolpin and Thurman, 1993; Kolpin, 1998). For this study, referred to as the Midcontinent Herbicide Initiative, ground-water samples from 303 wells in 12 States were analyzed for nutrients, herbicides, and herbicide-transformation products; 68 wells in the network were located in Illinois. Details regarding well-selection criteria and sampling methodology are given in Kolpin and Burkart (1991). Analytical results and ancillary well-construction, land-use, hydrogeologic, and water-quality data are given in Kolpin and others (1993; 1996). During October 2000-February 2001, 55 of the available wells in the network were resampled for the constituents analyzed for during 1991-94, as well as additional herbicides and transformation products for which analytical procedures were not available previously. The analytical results from the resampling during 2000-2001 are given below.

Water samples were collected using field-parameter stabilization and volumetric purge criteria to ensure samples were representative of the aquifer water quality. Samples from water-supply wells were collected before water treatment (chlorination, for example). Samples collected for analysis of nutrients were filtered through a 0.45 micron acetate filter. Samples collected for analysis of herbicides and their transformation products were filtered through a 0.7 micron baked glass-fiber filter. All samples were chilled immediately to 4 degrees Celsius for handling and shipment. Laboratories received the samples within 1-4 days of sample collection. Nutrient samples were analyzed by the USGS National Water-Quality Laboratory in Lakewood, Col., using automated colorimetric procedures described by Fishman and Friedman (1989). Analyses for herbicides and their transformation products were made at the USGS Organic Geochemistry Research Laboratory in Lawrence, Kans. Using solid-phase extraction techniques, parent herbicides, the atrazine products deethylatrazine and deisopropylatrazine, and the cyanazine product cyanazine-amide were analyzed by gas chromatography/mass spectrometry (GC/MS) (Zimmerman and Thurman, 1999; Kish and others, 2000). Chloroacetanilide herbicide products (ethanesulfonic (ESA) and oxanilic acid (OXA) compounds) were analyzed by liquid chromatography/mass spectrometry (LC/MS) (Zimmerman and others, 2000; Lee and others, 2001). Glyphosate was analyzed by immunoassay using a proprietary method (Elizabeth Scribner, U.S. Geological Survey, oral commun., 2001).