Tile effluent quality and chemical losses under conventional and no tillage. 2. Atrazine and metolachlor

The long-term effect of no-tillage (NT) and conventional tillage (CT) on the concentration and loss of the atrazine, deethylatrazine and metolachlor in tile effluent was studied over a 40-month period in four loam soil, corn (Zea mays L.) fields of approximately 3 ha each. Atrazine and deethylatrazine were detected at low concentrations in most of the 773 samples collected between January 1991 and early May 1994. Under both tillage treatments, atrazine concentrations were mostly below the USEPA advisory of 3 micrograms/L while concentrations of atrazine plus deethylatrazine were generally below 5 micrograms/L, the Canadian interim Maximum Acceptable Concentration (IMAC) for the sum of atrazine and its metabolites. Concentrations exceeding these limits were mainly observed during rainfall-induced flow events within a few days or weeks following herbicide application. The flow-weighted average concentrations of atrazine during flow events were significantly (p 0.05) higher under NT than CT. Metolachlor was also detected at low concentrations, and in relatively few flow events. Concentrations were always well below the USEPA advisory of 100 micrograms/L or the Canadian IMAC of 50 micrograms/L. Annual loss of herbicides in tile effluent ranged from 0.02% to 0.34% of the amount applied. Most loss occurred during spring flow events when tile flow was the highest. Atrazine and deethylatrazine losses under the NT treatment were significantly (p 0.05) higher than those under the CT treatment