Model predictions of fate and transport of organic solutes in soils and groundwater are sensitive to assumptions concerning rates of microbial degradation. We studied the independent effects of residence time (RT) and pore water velocity (ν) onthe degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) by employing a series of unsaturated soil column experiments (continuous pulse concentration = 1 mg L{sup}-1) with varying column lengths andν. While 2,4-D degradation under batch conditions wasbest described by a logistic rate expression, analysis of the 2,4-D breakthrough curves (BTCs) showed that (i) observed 2,4-D degradation rates were consistent with a first-order kinetic model and (ii) a single set of independently determined rateparameters from batch experiments could not describe 2,4-D degradation for all transport conditions. Apparent first-order degradation rate constants obtained from column data were found to be independent of column RT, but increased with decreasingν,especially atν<1 cm h{sup}-1. Variations in apparent degradation rate constants with changingν may be due to effects ofν on microbial attachment and distribution, local opportunity times required for maximum 2,4-D degradation, or nutrient desorptionrates from the soil solid phase. Results from this study emphasize the difficulty in obtaining accurate model predictions using a single set of degradation rate parameters for all transport conditions, and the need to develop a better understanding ofcoupled processes involving contaminant degradation and transport.
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