ALACHLOR AND ATRAZINE DEGRADATION IN A NEBRASKA SOIL AND UNDERLYING SEDIMENTS

Degradation reduces ground water contamination from soil-mobile herbic ides, but degradation rates vary among soils and with depth, Alachlor and atrazine degradation were determined in Herd silt loam (fine-silty , mixed, mesic Pachic Haplustolls) surface soil (0-15 cm), subsurface soil (45-120 cm), and underlying sediment (150-240 cm) from a terrace of the Platte River near Shelton Nebraska. Herbicide solution containi ng C-14-ring-labeled alachlor or atrazine was added at 100 or 1000 ng g(-1) to soil adjusted to -50 kPa water content and incubated at 22 de grees C up to 200 days. (CO2)-C-14 evolution was monitored to determin e mineralization with time, and soil was extracted and combusted to de termine residual C-14-labeled herbicide and bound residue formation. R espective first-order half-lives of alachlor and atrazine ranged from 8 and 11 days in surface soil to 49 and 248 days in deep soil. Soil-bo und (unextractable) residue from alachlor and atrazine typically incre ased with time and ranged from 54 and 46% of applied C-14 in surface s oil to 2 and 3% of C-14 applied in deep soil. Stepwise regression indi cated that rate of degradation of both herbicides in these soils could be estimated from sorption, NH;: or NO3- + NO2- and orthophosphate co ntent. Although atrazine is usually persistent, with little or no mine ralization of the s-triazine ring, enhanced degradation was observed i n two of the surface soils, where more than 60% of the herbicide was m ineralized in 28 days. Lower deethylatrazine concentrations in soil ex hibiting enhanced atrazine degradation suggested transformation to mor e labile polar degradates. A decrease in the polar degradation product s and bound residue fractions as mineralization increased indicated th eir utilization during atrazine degradation in soil.