OCCURRENCE OF SELECTED HERBICIDES AND HERBICIDE DEGRADATION PRODUCTS IN IOWA GROUND-WATER, 1995

Herbicide compounds were prevalent in ground water across Iowa, being detected in 70% of the 106 municipal wells sampled during the summer o f 1995, Herbicide degradation products were three of the four most fre quently detected compounds for this study. The degradation product ala chlor ethanesulfonic acid was the most frequently detected compound (6 5.1%), followed by atrazine (40.6%), and the degradation products deet hylatrazine (34.9%), and cyanazine amide (19.8%). The corn herbicide a cetochlor, first registered for widespread use in the United States in March 1994, was detected in a single water sample, No reported herbic ide compound concentrations for this study exceeded current U.S. Envir onmental Protection Agency's maximum contaminant levels or health advi sory levels for drinking water, although the herbicide degradation pro ducts examined have get to have such levels established. The occurrenc e of herbicide compounds had a significant, inverse relation to well d epth and a significant, positive relation to dissolved-oxygen concentr ation. It is felt that both well depth and dissolved oxygen are acting as rough surrogates to ground-water age, with younger ground water be ing more likely to contain herbicide compounds. The occurrence of herb icide compounds was substantially different among the major aquifer ty pes across Iowa, being detected in 82.5% of the alluvial, 81.8% of the bedrock/ karst region, 40.0% of the glacial-drift, and 25.0% of the b edrock/nonkarst region aquifers. The observed distribution was partial ly attributed to variations in general ground-water age among these aq uifer types. A significant, inverse relation was determined between to tal herbicide compound concentrations in ground water and the average soil slope within a 2-km radius of sampled wens. Steeper soil slopes m ay increase the likelihood of surface runoff occurring rather than gro und-water infiltration-decreasing the transport of herbicide compounds to ground water. As expected, a significant positive relation was det ermined between intensity of herbicide use and herbicide concentration s in ground water.