Characterization of Anaerobic Natural Attenuation of Petroleum Hydrocarbons

Detailed characterization of the physical, chemical, and biological processes contributing to or limiting monitored natural attenuation at petroleum hydrocarbon contaminated sites is critical in determining if natural attenuation is a feasible approach to meet regulatory cleanup standards. A former service station site where concentrations of benzene in groundwater persist above maximum contamination limits despite several efforts of engineered remediation and natural attenuation was characterized using conventional and emerging field and analytical techniques. Boring log data, profiles of hydraulic conductivity, and estimates of groundwater flow suggest that hydraulically connected silty fine sands within a silt and clay matrix are the primary pathway for flushing of contaminants. Groundwater table fluctuations combined with concentrations and mass estimates of petroleum hydrocarbons in groundwater and sediments suggest that the contaminated zone may contain immobile non-aqueous phase liquid creating source-zone conditions by continuous dissolution of contaminants; back diffusion of contaminants from the low permeability silt and clay matrix to the higher permeability silty fine sands may also be occurring. Conventional geochemical data suggested that natural sulfate-reducing conditions at the site had the highest potential to serve as the dominant mechanism for natural attenuation of petroleum hydrocarbons. However, microbiological and stable isotope data coupled with our site conceptual model suggest that methanogensis may in fact be the dominant natural attenuation mechanism. Based on our characterization of the site, previously recommended groundwater recirculation/mixing to achieve accelerated natural sulfate reduction may not be physically feasible if source-zone conditions are present in the low permeability media which is not conducive to mixing of aqueous sulfate amendments. Moreover, the presence of benzene-degrading sulfate-reducing bacteria at the study site has not been confirmed giving rise to considerable uncertainty to the feasibility of accelerated natural sulfate reduction. Lastly, sulfate addition to the source zone may negatively interfere with the ambient methanogenic conditions under which biodegradation of petroleum hydrocarbons is occurring. Therefore, further characterization efforts are needed to confirm the presence of benzene-degrading sulfate-reducing bacteria at the study site and to better define the distribution of source-zone conditions in both low and high permeable media. Further characterization is also needed to better quantify the saturated and unsaturated-zone pathways for biodegradation byproducts of carbon dioxide and methane to determine if optimization of these loss mechanisms can significantly enhance natural attenuation.