Sorption and degradation of alachlor in soil and aquifer material

Herbicides may contaminate groundwater when used over shallow aquifers. Alachlor [2-chloro-N-methoxymethyl-N-(2,6-diethylphenyl)acetamide] is widely used in corn (Zea mays L.) and soybean (Glycine max L.) for grass control in eastern South Dakota where depth to groundwater ranges from 2 to 20 m. Sorption and degradation of alachlor, two processes that control its 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 (approximately 6 m below the soil surface) were collected following aseptic procedures. Alachlor sorption to the surface A horizon soil (silty clay loam) was about 13 times greater than sorption to the 2C horizon soil (sand and gravel). About 2.5% of the alachlor was completely mineralized after 112 d in surface soil, whereas 0.5% was mineralized in the 2C horizon soils. The addition of C (lyophilized algae biomass) increased ring mineralization and polar metabolite formation in all samples. These data indicate that bacterial populations in the Big Sioux aquifer are sufficiently diverse to degrade complex organic molecules. Carbon was the factor most limiting alachlor metabolism in subsurface sediments