Alachlor and atrazine degradation in a Nebraska soil and underlying sediments

Degradation reduces ground water contamination from soil-mobile herbicides, but degradation rates vary among soils and with depth. Alachlor and atrazine degradation were determined in Hord 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 containing¹⁴C-ring-labeled alachlor or atrazine was added at 100 or 1000 ng g⁻¹to soil adjusted to −50 kPa water content and incubated at 22°C up to 200 days.¹⁴CO₂evolution was monitored to determine mineralization with time, and soil was extracted and combusted to determine residual¹⁴C-labeled herbicide and bound residue formation. Respective 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-bound (unextractable) residue from alachlor and atrazine typically increased with time and ranged from 54 and 46% of applied¹⁴C in surface soil to 2 and 3% of¹⁴C applied in deep soil. Stepwise regression indicated that rate of degradation of both herbicides in these soils could be estimated from sorption, (Equation presented) or (Equation presented), and orthophosphate content. Although atrazine is usually persistent, with little or no mineralization of the s-triazine ring, enhanced degradation was observed in two of the surface soils, where more than 60% of the herbicide was mineralized in 28 days. Lower deethylatrazine concentrations in soil exhibiting enhanced atrazine degradation suggested transformation to more labile polar degradates. A decrease in the polar degradation products and bound residue fractions as mineralization increased indicated their utilization during atrazine degradation in soil.