Effect of exposure history on microbial herbicide degradation in an aerobic aquifer affected by a point source.

The effects of in situ exposure to low concentrations (micrograms per liter) of herbicides on aerobic degradation of herbicides in aquifers were studied by laboratory batch experiments. Aquifer material and groundwater were collected from a point source with known exposure histories to the herbicides mecoprop (MCPP), dichlorprop, BAM, bentazone, isoproturon, and DNOC. Degradation of the phenoxy acids, mecoprop and dichlorprop, was observed in five of six sampling points from within the plume. Mecoprop was mineralized, and up to 70% was recovered as 14CO2. DNOC was degraded in only two of six sampling points from within the plume, and neither BAM, bentazone, nor isoproturon was degraded in any sampling point. A linear correlation (R2 > or = 0.83) between pre-exposure and amount of herbicide degraded within 50 days was observed for the phenoxy acids, mecoprop and dichlorprop. An improved model fit was obtained from using Monod degradation kinetics compared to zero- and first-order degradation kinetics. An exponential correlation (R2 > or = 0.85) was also found between numbers of specific phenoxy acid degrading bacteria and pre-exposure. Combination of these results strongly indicates that the low concentration exposure to phenoxy acids in the aquifer resulted in the presence of acclimated microbial communities, illustrated bythe elevated numbers of specific degraders as well as the enhanced degradation capability. The findings support application of natural attenuation to remediate aerobic aquifers contaminated by phenoxy acids from point sources.