Laboratory Evaluation of Polycyclic Aromatic Hydrocarbon Biodegradation at a Former Tar Plant Site.

A former tar plant site in the southeastern U.S. is one of hundreds across the country that is contaminated with polycyclic aromatic hydrocarbons (PAHs). The 2007 CERCLA Priority List of Hazardous Substances ranks PAHs as the eighth most prominent hazardous substance at National Priority List sites. However, benzo(a)pyrene is the only PAH that is regulated under the Safe Drinking Water Act, with a maximum contaminant level of 0.2 mug/l. As with most sites, monitored natural attenuation was the preferred bioremediation approach for this site, assuming it could be shown that the PAHs were undergoing biodegradation. Field data from the site suggested that the contaminated groundwater was stable, i.e., the PAHs were not migrating. However, the field data was not sufficiently conclusive to warrant monitored natural attenuation as a remedy. In such cases, laboratory studies may be conducted to fill in gaps left by the field data. The focus of this thesis was to evaluate PAH biodegradation in samples taken from the site using microcosms. The research objectives were as follows: (1) To evaluate the potential for aerobic and anaerobic biodegradation of PAHs at the tar plant site using microcosms prepared with crushed carbonate bedrock and groundwater from two locations; (2) To determine the distribution of products formed during biodegradation of [14C]naphthalene, including 14CO2 and 14C-labeled soluble metabolites; and (3) To evaluate the potential of sulfate to enhance in situ anaerobic biodegradation of naphthalene. Since naphthalene was present at considerably higher levels than the other PAHs (acenaphthene, fluorene and phenanthrene), its fate was evaluated in more depth via the use of [14C]naphthalene. Fractured rock and groundwater samples were collected from two locations: location 1 was heavily contaminated with PAHs (i.e., 7.5 mg/L concentrations in the groundwater); and location 2 was less contaminated (i.e., less than approximately 1 mg/L in the groundwater). Two sets of microcosms were evaluated. Set I was designed to compare the extent of PAH biodegradation under aerobic and anaerobic conditions, for both locations. Set II microcosms were designed to enhance anaerobic degradation by biostimulation with sulfate. For the Set I microcosms, four treatments were prepared: (i) live microcosms, designed to simulate in situ conditions; (ii) live microcosms with approximately 0.45 muCi of [14C]naphthalene; (iii) killed controls, to determine if activity observed in the live microcosms was a consequence of biotic or abiotic processes; and iv) killed controls with approximately 0.45 muCi of [ 14C]naphthalene. Each treatment was prepared in triplicate, yielding 12 microcosms per redox condition per location, or a grand total of 48 microcosms. Biodegradation of naphthalene, acenaphthene, fluorene and phenanthrene was observed under aerobic conditions for both locations 1 and 2. The first order decay rate for naphthalene was greater than or equal to 0.19 d -1 for location 1 and approximately 0.053 d-1 for location 2. The decrease in naphthalene coincided with a decrease in the percentage of oxygen in the headspace of the live treatments. Nevertheless, delivering oxygen to the contaminated areas of this site would be challenging, due in part to the high costs associated with aeration and the low solubility of oxygen. (Abstract shortened by UMI.)