WATER-SUPPLY IMPLICATIONS OF HERBICIDE SAMPLING

The temporal distribution of the herbicides alachlor, atrazine, cyanaz ine, and metolachlor was documented from September 1991 through August 1992 in the Platte River at Louisville, Neb., the drainage of the Cen tral Nebraska Basins. Lincoln, Omaha, and other municipalities withdra w groundwater for public supplies from the adjacent alluvium, which is hydraulically connected to the Platte River. Data were collected, in part, to provide information to managers, planners, and public utiliti es on the likelihood of water supplies being adversely affected by the se herbicides. Three computational procedures-monthly means, monthly s ubsampling, and quarterly subsampling-were used to calculate annual me an herbicide concentrations. When the sampling was conducted quarterly rather than monthly, alachlor and atrazine concentrations were more l ikely to exceed their respective maximum contaminant levels (MCLs) of 2.0 mu g/L and 3.0 mu g/L and cyanazine concentrations were more likel y to exceed their health advisory level of 1.0 mu g/L. The US Environm ent Protection Agency has established a tentative MCL of 1.0 mu L for cyanazine; data indicate that cyanazine is likely to exceed this level under most hydrologic conditions.