Field and Laboratory Evaluation of the Impact of Tall Fescue on Polyaromatic Hydrocarbon Degradation in an Aged Creosote-Contaminated Surface Soil

A field study was initiated in 1997 to assess the ability of tall fescue grass to remediate an aged creosote-contaminated surface soil. Field monitoring was combined with aerobic microcosm experiments, microbial enumerations, and plant tissue analysis to determine the impact of tall fescue on the degradation of six polycyclic aromatic hydrocarbons (PAHs), acenaphthene, fluorene, phenanthrene, fluoranthene, pyrene, and chrysene, and to elucidate the mechanisms of remediation. Fescue grass had a beneficial impact on the degradation of all PAHs except phenanthrene. Mean concentrations of the three-ring PAHs, acenaphthene and fluorene, were lower in fescue cells compared to unvegetated cells after 36 months. In microcosms with soil from fescue cells, acenaphthene had a significantly higher degradation rate and lower final concentration after 180 days than in microcosms prepared with soil from unvegetated cells. Mean concentrations of the four-ring PAHs, fluoranthene, pyrene, and chrysene, were statistically similar in the field study; however, the 10th and 20th percentile concentrations were lower in fescue cells during all sampling periods. Microcosm studies showed increased degradation of fluoranthene and pyrene in soil samples taken from tall fescue rhizosphere compared to unvegetated soil and abiotic controls. Degradation of four-ring PAHs was enhanced in the shallow zones (10-15 cm below ground surface) of vegetated cells. The root mass was approximately 35% greater in shallow zones than in medium depth zones (15-21 cm below ground surface). Microbial populations, on solid mineral media plates with pyrene and chrysene as the sole carbon source were two times higher in soils from tall fescue plots than from unvegetated soils, suggesting that the increased PAH degradation was a result of increased microbial activity in the rhizosphere. Gas chromatography/mass spectrometry analysis of fescue shoots indicated that no uptake or translocation of PAHs or PAH degradation intermediates into the shoots was occurring.