WATER-QUALITY - ATRAZINE AND ALACHLOR LOSSES FROM SUBSURFACE TILE DRAINAGE OF A CLAY LOAM SOIL

Herbicide losses in subsurface drainage from agricultural land is not well understood. Research was conducted to assess atrazine [6-chloro-N'-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and alachlor [2-chloro-N-(12,6-diethylphenyl)-N-(methoxymethyl) acetamide] losses in subsurface tile drainage during and after long-term use on a Webster clay loam soil (Typic Haplaquoll) at Waseca, MN, under typical agricultural management conditions. Individual subsurface drainage systems were installed in 1975 to plots cropped continuously to corn (Zea mays L.) under moldboard plow and no-tillage systems. Annual alachlor and atrazine use was initiated in 1974 and 1975, respectively. Alachlor use was continued through 1991 while atrazine use was terminated after the 1988 application. Tile drainage water was collected and analyzed for atrazine and alachlor beginning in October 1985. Alachlor was detected in only 2% of the samples from 1985 through 1991. Atrazine was detected in 97% of the samples from 1985 through 1990. Atrazine detection frequency declined to 78% in 1991. Atrazine concentration in October 1985 was 0.56 mug L-1. Average atrazine concentration in tile drainage water in 1986 was 1.0 mug L-1 and estimated loss by leaching was 4.1 g ha-1. Average atrazine concentrations in 1987 and 1988 were 1.24 and 0.84 mug L-1, respectively, but due to limited tile flow, atrazine losses were less than 1 g ha-1 each year. Twelve to 18 mo after the last atrazine application, average atrazine concentration in tile water did not decline. However, from 24 to 30 mo after the final atrazine application, average concentration dropped to 0.42 mug L-1 and during the following growing season average concentration declined to 0.29 mug L-1. There was little fluctuation in atrazine concentration during any year. Tillage systems had minimal impacts on atrazine concentration or loss in tile drainage water. This research indicates that low concentrations of atrazine may contaminate tile drainage water during and after long-term use and may persist for several years after use is stopped. Contamination of drainage from similar use of alachlor appears minimal.