Natural attenuation of herbicides: Importance of growth-based degradation

A field injection experiment in a sandy, aerobic aquifer showed that two phenoxy acids MCPP (mecoprop) and dichlorprop were degraded within 1 m downgradient of the injection wells after an apparent lag period. The plume development and microbial measurements indicated that microbial growth governed degradation. The results were satisfactorily described by a newly developed three-dimensional reactive solute transport model including growth based degradation kinetics. The model was applied at a larger scale to simulate the effect of growth kinetics on a release of phenoxy acids. The results revealed an efficient removal of phenoxy acids close to the source. However, a significant mass escaped during the initial growth phase and the resulting plume was only slowly degraded as short exposure time and low concentrations limited the growth of specific degraders and thereby degradation. The observations may be important for application of natural attenuation as a remedy in field scale systems.