ATRAZINE, ALACHLOR, AND CYANAZINE IN A LARGE AGRICULTURAL RIVER SYSTEM

Atrazine, alachlor, and cyanazine exhibited maximum concentrations of about 1000-6000 ng/L in the Minnesota River in 1990 and 1991, resultin g from precipitation and runoff following the application period. Tran sport of these herbicides to the river occurs via overland flow or by infiltration to the drainage networks. Suspended sediment, SO42-, and Cl- concentrations were used as indicators of transport mechanisms. Th e atrazine metabolite, DEA, was present in the river throughout the ye ar. The ratio of DEA to atrazine concentration was used to calculate a n apparent first-order soil conversion rate of atrazine to DEA. Half l ives of 21-58 d were calculated for 1990 and 1991, respectively. The l onger conversion rate in 1991 results from rapid flushing from the soi l and minimum exposure to soil microorganisms. Total flux of herbicide to the river was 1-6.5 t, with over 60% of this loading occurring dur ing the month of June. Loading to the river accounts for less than 1.5 % of applied herbicide.