PORE-WATER VELOCITY AND RESIDENCE TIME EFFECTS ON THE DEGRADATION OF 2,4-D DURING TRANSPORT

Model predictions of fate and transport of organic solutes in soils an d groundwater are sensitive to assumptions concerning rates of microbi al degradation. We studied the independent effects of residence time ( RT) and pore water velocity (v) on the degradation of 2,4-dichlorophen oxyacetic acid (2,4-D) by employing a series of unsaturated soil colum n 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 degradatio n rates were consistent with a first-order kinetic model and (ii) a si ngle set of independently determined rate parameters from batch experi ments could not describe 2,4-D degradation for all transport condition s. Apparent first-order degradation rate constants obtained from colum n data were found to be independent of column RT, but increased with d ecreasing v, especially at v < 1 cm h(-1). Variations in apparent degr adation rate constants with changing v may be due to effects of v on m icrobial 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 deg radation and transport.