Pore water velocity and residence time effects on the degradation 2,4-D during transport

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 (v) on the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) by employing a series of unsaturated soil column experiments (continuous pulse concentration = 1 mg L^-1) with varying column lengths and v. While 2,4-D degradation under batch conditions was best 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 rate parameters 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 decreasing v, especially at v < 1 cm h^-1. Variations in apparent degradation rate constants with changing v may be due to effects of v on microbial attachment and distribution, local opportunity times required for maximum 2,4-D degradation, or nutrient desorption rates 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 of coupled processes involving contaminant degradation and transport.