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.