Monitored natural attenuation at a coal-tar impacted aquifer in Northwestern Iowa: coupled biogeochemical and molecular microbiological approaches for establishing secondary and tertiary lines of evidence

Natural attenuation is being investigated at a coal-tar polluted aquifer in Northwestern Iowa. It is a complex process to identify intrinsic biodegradation of polycyclic aromatic hydrocarbons (PAHs) because several hundred potential substrates comprise coal-tar mixtures. Molecular microbiological techniques (whole-cell hybridizations and microautoradiography (MICRO-FISH)) are interfaced with biogeochemical monitoring and modeling to identify PAH-degrading bacteria and ascertain their intrinsic activity. Through this approach, the potential for natural attenuation of specific PAHs are explored.;Plume-scale modeling yielded mass transformation rates that compared favorably to estimates based on geochemical mass balance approaches. Microbial populations were three orders of magnitude greater in polluted than non-affected sediments suggesting growth on coal-tar constituents in situ. Mineralization of naphthalene and phenanthrene in aerobic and anaerobic incubations was documented supporting these observations. Whole-cell hybridizations revealed that Actinobacteria, gamma-Proteobacteria, Bacteriodetes, and beta-Proteobacteria dominated the intrinsic microbial community.;MICRO-FISH was applied to the polluted sediments and established the activity of beta-Proteobacteria, gamma-Proteobacteria, and Actinobacteria in growth on phenanthrene and naphthalene. The presence of beta-Proteobacteria in the intrinsic microbial community was overshadowed by Actinobacteria and gamma-Proteobacteria indicating that bacteria of the later two were the primary PAH-degraders in situ. Cell-specific naphthalene mass transformation rates based on MICRO-FISH compared well to estimates using degradation rate coefficients of plume-scale models. Mass transformation rates for phenanthrene were one to two orders of magnitude greater than model-predicted values, possibly due to model fitting with limited data and/or dosing large masses of 14C-PAH in order to elicit an autoradiographic response for MICRO-FISH.;These studies suggest that enrichment of microbial phylotypes associated with polluted versus nearby pristine conditions may not be a reliable marker for PAH-degrading microbes. Further, enrichment of microbial phylotypes in laboratory-scale incubations may not accurately reflect PAH-degrading microbes identified by MICRO-FISH. The results of this study support natural attenuation of PAH compounds in this polluted aquifer, and suggest that direct evidence of intrinsic degradation of PAHs such as that presented herein may be necessary to demonstrate natural attenuation at contaminated sites. MICRO-FISH may be an effective tool for establishing tertiary lines of evidence useful for supporting intrinsic bioremediation of at least low-ring coal-tar PAHs