Natural Biological Attenuation of Phenoxy Herbicides in Groundwater: Dow AgroSciences Paritutu Site, New Zealand

Abstract

Groundwater beneath a manufacturing site previously used for herbicide production has been shown to contain low levels of chlorinated phenols and phenoxy herbicides. The importance of biological processes in the natural attenuation of the groundwater contaminants was examined as part of an ongoing investigation. Analysis of the groundwater chemistry indicated that the aquifer is essentially aerobic in the area of interest. Laboratory microcosm experiments demonstrated that the naturally occurring microorganisms rapidly degraded a mixture of the predominant organic contaminants under conditions that simulate those in the aquifer. The time required for 50% degradation ranged from 7 to 27 days for 2,4-dichlorophenoxyacetic acid (2,4-D) and 9 to 49 days for 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). The rapid biodegradation rates were consistent with the results of microbiological analyses, which demonstrated that a substantial proportion of the culturable bacteria were capable of growth on 2,4-D as a sole carbon source. Results of gene probe assays suggested the numbers of bacteria with the potential to degrade 2,4-D were one to two orders of magnitude higher than were detected using plate counts. Computer model simulations illustrated that biodegradation would be expected to significantly contribute to the attenuation of 2,4-D and 2,4,5-T in the aquifer. On the basis of the various lines of evidence and the distances the groundwater must travel, the groundwater contaminants would be expected to naturally biodegrade to below levels of concern before the plume reaches potential environmental receptors.