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Aerobic and Anaerobic Degradation of Alachlor in Samples from a Surface-to-Groundwater Profile

USDA-ARS, Southern Weed Science Lab., P.O. Box 350, Stoneville, MS 38776;

D. C. Obenhuber and R. D. Wauchope

USDA-ARS, Southeast Watershed Res. Lab., P.O. Box 946, Tifton, GA 31793.

^* Corresponding author.

ABSTRACT

Estimates of pesticide degradation rates in subsoils are needed to improve models predicting pesticide movement to groundwater. Biodegradation rates of the herbicide alachlor [2-chloro-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] in surface soil, vadose zone, and aquifer samples collected from a single site near Plains, GA were determined in the laboratory under aerobic and anaerobic conditions. Degradation was described by first-order kinetics during 126 d of incubation. Under aerobic conditions the halflife (t_1/2) of alachlor in the surface soil (t_1/2 = 23 d) was less than in the vadose zone (t_1/2 = 73 to 285 d) and aquifer samples (t_1/2 = 320 to 324 d). Alachlor in anaerobic samples degraded less rapidly in the surface (0 to 0.6 m) and the next deepest (0.6 to 2.4 m) subsoil than under aerobic conditions (t_1/2 = 100 and 144 d, respectively). Degradation in anaerobic aquifer samples was very slow (t_1/2 = 337 to 553 d). Addition of organic nutrients enhanced aerobic degradation in subsurface soils and one aquifer sample, indicating that nutrient availability limits biodegradation. Total aerobic microbial populations ranged from 6.6 x 10³ to 2.5 x 10⁶ cells per gram of soil in the subsoils and aquifer samples, but were not correlated with aerobic or anaerobic degradation rates. The lower degradation rates in vadose zone and aquifer materials may be due to less microbial activity or the absence of alachlor degraders.