EFFECT OF TILLAGE REVERSAL ON HERBICIDE LEACHING TO GROUNDWATER

Studies have demonstrated greater pesticide leaching to groundwater under well established no-till (NT) than under conventional-till (CT). Increased leaching in NT is thought to be caused by preferential transport through macropores. The time required for preferential pathways to develop or dissipate when tillage of well established NT and CT are reversed is unclear. Therefore, a 3-year field study was conducted to determine the effect of reversing the tillage of 7-year-old NT and CT plots on the leaching of atrazine (6-chloro-N-ethyl-N'-(methylethyl)-1,3,5-triazine-2,4-diamine), alachlor (2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide), and cyanazine (2-[[4-chloro-6-(ethylamino)-1,3,5-triazine-2-yl]amino]-2-methylpropanenitrile) to groundwater. Groundwater samples were taken monthly before (January 1992 to April 1993) and after (May 1993 to December 1994) tillage reversal and analyzed for the herbicides. Atrazine concentrations in groundwater ranged from 0.15 to 8.9 µg L-1 and 0.07 to 4.9 µg L-1 under NT and CT, respectively, from January 1994 to July 1993 (before tillage reversal to three months after). Concentrations averaged 2.5 times (1.1 to 5) higher under NT than CT at each sampling. Atrazine levels were identical for both tillages from September 1993 to before herbicide application in May 1994. From June through December 1994, atrazine levels were again higher under NT than CT, but differences were smaller than before tillage reversal. Alachlor and cyanazine concentrations were consistently higher under NT than CT for all 3 years and decreased to nondetectable levels within 3 months of application. Results confirm that NT increases herbicide leaching compared with CT and that several years are required for preferential pathways to develop under NT.