MICROCOSM STUDIES OF SUBSURFACE PAH-DEGRADING BACTERIA FROM A FORMER MANUFACTURED-GAS PLANT

A study was conducted to evaluate the potential for natural in situ biodegradation of polycyclic aromatic hydrocarbons (PAH's) in the subsurface at the site of a former manufactured gas plant. Fifty-seven samples of unconsolidated subsurface sediments were aseptically obtained from five boreholes across the site. Bacteria capable of aerobically degrading PAH's without an acclimation period were detected throughout shallow (2.7 m) and deep (24.7 m) areas of the subsurface in both relatively clean (<20 mu g L(-1) naphthalene) and contaminated (4400 mu g L(-1) naphthalene) zones. Significant (p < 0.05) quantities of naphthalene (8+/-3% to 43+/-7%) and/or phenanthrene (3+/-1% to 31+/-3%) were mineralized in sediment-groundwater microcosms during 4 weeks of aerobic incubation at 22 degrees C. Three samples out of 11 were able to aerobically mineralize significant quantities of benzene (6+/-2% to 24+/-1%). Of 11 samples tested for anaerobic mineralization, naphthalene biodegradation (7+/-1% to 13+/-2%) in the presence of NO3- was observed in two samples. Compound removals were first order with respect to substrate concentration during the first 10-15 days of incubation. Compound biodegradation plateaued in the later stages of incubation (15-40 days), most likely from diminishing bioavailability and nutrient and oxygen depletion. Population densities in the sediments were typically low, with viable aerobic counts ranging from 0 to 10(5) CFU gdw(-1), viable anaerobic counts ranging from 0 to 10(4) CFU gdw(-1),and total counts (AODC) usually 10-fold greater than viable counts. Total counts exhibited a strong (p<0.01) positive correlation with sample grain size. Viable aerobic and anaerobic populations commonly occurred in the same sample, suggesting the presence of facultative anaerobes. Bacteria were metabolically active in samples from groundwaters with low pH (3.7) and high naphthalene concentrations (11,000 mu g L(-1)). Data from these enumeration and microcosm studies suggest that natural in situ biodegradation is occurring at the site.