Herbicides and degradates in shallow aquifers of Illinois: spatial and temporal trends

In Illinois, 55 water wells completed in unconsolidated and bedrock aquifers were sampled during October November 2000 to explore the spatial and temporal trends in herbicide residues. The wells represented a subset of wells sampled initially in 1991. While herbicide compounds were detected at concentrations >0.05 mu g/l in samples from 56% of the wells, no concentrations exceeded current EPA drinking-water standards. The six most-frequently detected compounds were: metolachlor ethanesulfonic acid (ESA), alachlor-ESA, metolachlor oxanilic acid (OA), desethylatrazine, acetochlor-OA, and alachlor-OA. For the parent compounds and degradates analyzed in both 1991 and 2000, the frequency of detection as a group decreased from 25 to 18%, and total concentrations decreased from 8.90 to 1.81 mu g/l. Herbicide residues were detected more frequently in unconsolidated aquifers than in bedrock aquifers.