EFFECT OF TILLAGE REVERSAL ON HERBICIDE LEACHING TO GROUNDWATER

Studies have demonstrated greater pesticide leaching to groundwater un der well established no-till (NT) than under conventional-till (CT), I ncreased leaching in NT is thought to be caused by preferential transp ort through macropores,The time required for preferential pathways to develop or dissipate when tillage of well established NT and CT are re versed is unclear, Therefore, a S-year field study was conducted to de termine the effect of reversing the tillage of 7-year-old NT and CT pl ots on the leaching of atrazine -ethyl-N'-(methylethyl)-1,3,5-triazine -2,4-diamine alachlor o-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetami de and cyanazine 1,3,5-triazine-2-yl]amino]-2-methylpropanenitrile) to groundwater, Groundwater samples were taken monthly before (January 1 992 to April 1993) and after (May 1993 to December 1994) tillage rever sal and analyzed for the herbicides. Atrazine concentrations in ground water ranged from 0.15 to 8.9 mu g L(-1) and 0.07 to 4.9 mu g L(-1) un der NT and CT, respectively, from January 1994 to July 1993 (before ti llage reversal to three months after). Concentrations averaged 2.5 tim es (1.1 to 5) higher under NT than CT at each sampling, Atrazine level s were identical for both tillages from September 1993 to before herbi cide application in May 1994, From June through December 1994, atrazin e levels were again higher under NT than CT, but differences were smal ler than before tillage reversal, Alachlor and cyanazine concentration s were consistently higher under NT than CT for all 3 years and decrea sed to nondetectable levels within 3 months of application, Results co nfirm that NT increases herbicide leaching compared with CT and that s everal years are required for preferential pathways to develop under N T.