Herbicide loads and unit area loads are calculated for these two watersheds. Acetochlor was introduced to the market in 1994 as a replacement for corn herbicides such as alachlor. These sites were selected because of the extensive agricultural activity within the watersheds and because they both are long-term USGS streamflow stations. Atrazine use was prohibited on 94 percent of the Yahara watershed whereas only 4 percent of the Pecatonica River watershed was under atrazine use prohibition. It was expected that the effect of atrazine use prohibitions would be reflected in the unit area loads of atrazine to surface waters.
Farmers apply most of the herbicides used in the watersheds to corn and soybeans.
The drainage area and land uses for each watershed are shown in table 1. The amounts of pesticide active ingredient applied in each watershed are shown in table 2. Location of sampling sites for herbicides. These QA samples, collected and processed near the beginning and at the end of the sampling periods, contained no detectable concentrations of the herbicides analyzed in this study. Acetochlor and cyanazine were detected in 59 percent of the samples.