GROUNDWATER QUALITY UNDER CONVENTIONAL AND NO-TILLAGE - II - ATRAZINE, DEETHYLATRAZINE, AND METOLACHLOR

The effect of no tillage (NT) and conventional tillage (CT) on the con tamination of shallow groundwater by atrazine 2-chloro-4-ethylamino-6- isopropylamino-s-triazine) deethylatrazine (2-chloro-4-amino-6-isoprop ylamino-s-triazine), and metolachlor -(ethyl-6-methylphenyl)-N-(2-meto xy-1-methylethyl) acetamide) at nominal depths of 1.2, 1.8, 3.0, and 4 .6 m was studied during four successive crop-years in four corn (Zea m ays L.) fields with loam soil. Atrazine and deethylatrazine were almos t always present in the shallow groundwater, Atrazine concentrations w ere below the U.S. Environmental Protection Agency (USEPA) lifetime he alth advisory (LHA) of 3 mu g/L in more than 99% of the 920 groundwate r samples from the two tillage treatments. Atrazine concentrations, al though low, were significantly higher under NT than CT (P < 0.05) at t he 1.8 and 3.0 m depths. Deethylatrazine concentrations were also low but were generally higher than those of the parent compound. They a er e significantly higher under NT than CT at the 1.2 and 1.8 m depths (P < 0.1), as well as at 3.0 m (P < 0.01), Metolachlor was detected in o nly 25% of the samples and concentrations were well below the USEPA LH A of 100 mu g/L. There was no tillage treatment effect on metolachlor concentration. Results from this study suggest a low potential for ser ious pollution of shallow groundwater from application of atrazine and metolachlor at agronomically recommended rates in tile-drained corn f ields in cold humid regions under both CT and NT practices. However, a trazine leaching appears to be greater under NT than under CT.