Groundwater quality under conventional and no tillage. II. Atrazine, deethylatrazine, and metolachlor

The effect of no tillage (NT) and conventional tillage (CT) on the contamination of shallow groundwater by atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), deethylatrazine (2-chloro-4-amino-6-isopropylamino-s-triazine), and metolachlor (2-chloro-N-(ethyl-6-methylphenyl)-N-(2-metoxy-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 mays L.) fields with loam soil. Atrazine and deethylatrazine were almost always present in the shallow groundwater. Atrazine concentrations were below the U.S. Environmental Protection Agency (USEPA) lifetime health advisory (LHA) of 3 micrograms/L in more than 99% of the 920 groundwater samples from the two tillage treatments. Atrazine concentrations, although low, were significantly higher under NT than CT (P 0.05) at the 1.8 and 3.0 m depths. Deethylatrazine concentrations were also low but were generally higher than those of the parent compound. They were 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 only 25% of the samples and concentrations were well below the USEPA LHA of 100 micrograms/L. There was no tillage treatment effect on metolachlor concentration. Results from this study suggest a low potential for serious pollution of shallow groundwater from application of atrazine and metolachlor at agronomically recommended rates in tile-drained corn fields in cold humid regions under both CT and NT practices. However, atrazine leaching appears to be greater under NT than under CT