Evaluation of alachlor degradation under transport conditions

Degradation is an important process that influences pesticide fate in the environment. In this study, we present a new methodology to evaluate pesticide degradation under transport conditions using miscible displacement techniques. The degradation of alachlor [2-chloro-2',6'-diethyl-N-(methoxmethyl)acetanilide] in soils amended with alfalfa, manure, or sludge, as well as in the unamended soil, was examined with both static incubation and column leaching experiments. The first-order degradation rate coefficient in the latter experiments was calculated numerically based on both the shape of the alachlor breakthrough curve (BTC) and the quantity of alachlor residues remaining in soil after leaching. Alachlor BTCs were all asymmetric, indicating existence of nonequilibrium sorption during transport. In both experimental conditions, degradation rates were most rapid in alfalfa- and manure-amended soils, but were inhibited by sludge amendment. Degradation rate coefficients measured in the column experiments, however, were at least twice as fast as those measured in the incubation experiments, indicating that nonequilibrium transport-favored alachlor degradation. Based on our results and those reported elsewhere, we conclude that pesticide degradation is transport-dependent. Thus, degradation rate parameters measured under transport conditions, which are more representative of field conditions, may differ substantially from incubation experiments and should be used in modeling for prediction purposes.