Occurrence and flux of selected pesticides in surface water of the upper snake River Basin, Idaho and western Wyoming

During May and June 1994, 37 water samples were collected at 31 sites in the upper Snake River Basin and analyzed for 83 pesticides and pesticide metabolites. EPTC, atrazine, and the atrazine metabolite deethylated atrazine were the most frequently detected and were found in 30, 20, and 13 of the samples, respectively. Fifteen additional pesticides were detected at least once. All the compounds detected were at concentrations of less than 1 microgram per liter. Total annual applications of EPTC and atrazine within subbasins and their instantaneous instream fluxes have a logarithmic relation with coefficients of determination (R2 values) of 0.55 and 0.62, respectively. At the time of sampling, the median daily flux of EPTC was about O. 0001% of the annual amount applied in a subbasin, whereas the median daily flux of atrazine was between 0.001 and 0.01%. The difference in fluxes between EPTC and atrazine probably results from differences in their physical properties and in the method and timing of application.