RAPID CHANGES IN CONCENTRATION OF HERBICIDES IN CORN FIELD SURFACE DEPRESSIONS

Although herbicide concentrations in runoff water leaving treated fiel ds have long been evaluated reports of in-field concentrations in wate r that may infiltrate before runoff begins, are lacking. To characteri ze this potential contribution to ground water contamination, lire tre ated one plowed and two no-till corn fields with 2 and 3 Ib per acre ( 2.24 and 3.36 kg/ha), respectively, of atrazine and alachlor. We used a simulator to rain on plots within the fields 1, 2, 4, 8, 16 and 32 d ays after herbicide application and collected water that accumulated i n surface depressions 0, 5, 10, 15, 20, and 30 minutes after initial p onding. Highest atrazine concentrations were found in t = 0 samples on day 1, ranging up to 2 mg/L in the no-till and 10 mg/L in the plowed areas. At 5 and 30 minutes later, concentrations averaged 2/3 and 1/4 of the t = 0 values, respectively. Alachlor concentrations were 1/2 to 1/3 of atrazine values in the same samples. By day 32 initial concent rations of both herbicides were about 10% of those an day I dnd contin ued to decrease during the 30-minute sampling period. The rapid decrea ses in herbicide concentration indicate that significant infiltration of herbicide in preferential flow paths si probably limited to the ear ly parts of the first few storms following application.