SORPTION AND DEGRADATION OF ALACHLOR IN SOIL AND AQUIFER MATERIAL

Herbicides may contaminate groundwater when used over shallow aquifers , Alachlor o-N-methoxymethyl-N-(2,6-diethylphenyl)-acetamide] is widel y used in corn (Zea mays L.) and soybean (Glycine mar L.) for grass co ntrol in eastern South Dakota where depth to groundwater ranges from < 2 to >20 m. Sorption and degradation of alachlor, two processes that c ontrol ifs fate in the environment, were measured in laboratory batch equilibrium and incubation studies. Surface soil and sediment samples from above and below the water table (similar to 6 m below the soil su rface) were collected following aseptic procedures. Alachlor sorption to the surface A horizon soil (silty clay Loam) was about 13 times gre ater than sorption to the 2C horizon soil (sand and gravel). About 2.5 % of the alachlor was completely mineralized after 112 d in surface so il, whereas <0.5% was mineralized in the 2C horizon soils. The additio n of C (lyophilized algae biomass) increased ring mineralization and p olar metabolite formation in all samples. These data indicate that bac terial populations in the Big Sioux aquifer are sufficiently diverse t o degrade complex organic molecules. Carbon was the factor most limiti ng alachlor metabolism in subsurface sediments.