| Poor waste management practices at former manufactured gas plant (MGP) sites have left behind a legacy of soil, groundwater, and surface water contamination. MGP waste residues contain a number of hazardous compounds, including polycyclic aromatic hydrocarbons (PAHs), which require effective remediation strategies to mitigate environmental and health impacts. In situ bioremediation is a lower cost alternative for sites where conventional remediation strategies (e.g., excavation and landfill disposal) are either cost-prohibitive or infeasible; biological strategies are sometimes combined with more aggressive treatments such as chemical oxidation, to reduce treatment times. To realize the potential for in situ bioremediation at MGP sites, a series of continuous-flow columns packed with contaminated MGP soil were operated for over two years, representing treatment by persulfate oxidation, treatment by biostimulation, and a control. Changes in PAH distribution and bioavailability, soil- and aqueous-phase PAH concentrations, and the quantity and activity of the indigenous microbial community and known PAH-degrading bacteria were monitored over time.;Persulfate oxidation did adversely impact the overall microbial community and specific PAH-degrading bacteria; however, recovery of PAH degraders occurred well after the general microbial community. These findings suggest that the use of total bacterial quantity as a surrogate for the recovery of contaminant degraders may be inappropriate for evaluating the compatibility of chemical treatment with subsequent bioremediation.;Biostimulation resulted in significant PAH removal (up to 80%). Spatial and temporal variations in soil PAH concentration and PAH-degrader abundance were strongly correlated to dissolved oxygen advancement, suggesting that oxygen was the limiting factor in PAH removal. Bacterial transport was also implicated as a factor in the establishment of PAH-degrading bacteria ahead of the oxygen front.;Density-separation of the MGP soil revealed that a majority of PAH mass was associated with carbonaceous particles. Desorption of PAHs from this soil fraction was substantially reduced after biostimulation, although a small portion remained bioavailable. Fast-desorbing fractions in the original MGP soil, quantified by a two-site desorption model, were found to be poor predictors of PAH bioavailability under long-term biostimulation. Overall, this research highlights the importance of physical and biological assessment tools for the evaluation and implementation of in situ bioremediation at MGP sites. |
