Evaluation of alachlor degradation under transport conditions

Degradation is an important process that influences pesticide fate in the e nvironment. In this study, we present a new methodology to evaluate pestici de degradation under transport conditions using miscible displacement techn iques. The degradation of alachlor [2-chloro-2',6'-diethyl-N-(methoxmethyl) acetanilide] in soils amended with alfalfa, manure, or sludge, as well as i n the unamended soil, was examined with both static incubation and column l eaching experiments. The first-order degradation rate coefficient in the la tter experiments was calculated numerically based on both the shape of the alachlor breakthrough curve (BTC) and the quantity of alachlor residues rem aining in soil after leaching. Alachlor BTCs were all asymmetric, indicatin g existence of nonequilibrium sorption during transport. In both experiment al conditions, degradation rates were most rapid in alfalfa- and manure-ame nded soils, but were inhibited by sludge amendment. Degradation rate coeffi cients measured in the column experiments, however, were at least twice as fast as those measured in the incubation experiments, indicating that noneq uilibrium transport-favored alachlor degradation. Based on our results and those reported elsewhere, we conclude that pesticide degradation is transpo rt-dependent. Thus, degradation rate parameters measured under transport co nditions, which are more representative of field conditions, may differ sub stantially from incubation experiments and should be used in modeling for p rediction purposes.