Application of natural attenuation for the control of petroleum hydrocarbon plume: Mechanisms and effectiveness evaluation

The objective of this study was to evaluate the effectiveness and mechanisms of natural attenuation (NA) on the remediation of petroleum-hydrocarbon contaminated groundwater caused by gasoline and diesel fuel leakage. During the effectiveness investigation, site groundwater samples were collected and analyzed for the variations in geochemical indicators, microbial diversity, dominant bacteria, biodegradation rates, biodegradation capacity, and the trend of contaminant degradation. Results show that the total BTEX (benzene, toluene, ethylbenzene, and xylenes) concentrations dropped from 9.5 mg/L at the source zone to 0.06 mg/L at a 120-m downgradient line. The calculated BTEX biodegradation capacity (20 mg/L) was much higher than the BTEX concentrations (9.5 mg/L) within the most contaminated area. This indicates that the spilled BTEX could be completely biodegraded through different intrinsic microbial processes using available electron acceptors. Results from polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE), and nucleotide sequence analyses reveal that the petroleum-hydrocarbon plume caused the bacterial variation in microbial communities, and more than 27 different petroleum-hydrocarbon degrading bacteria were observed in site groundwater indicating that BTEX could be biodegraded by the intrinsic bacterial consortium. Results also imply that the occurrence of intrinsic biodegradation was the major cause of the limited spreading of the BETX plume and reduction of contaminant concentrations. Results from Mann-Kendall test reveal that BTEX concentrations were decreasing in all monitor wells, and the BTEX first-order NA rate was 5.4 x 10(-4) 1/d along the transport path. The observed NA processes within the plume included the following: (1) depletion of electron acceptors (e.g., dissolved oxygen, nitrate, sulfate), (2) production of the degradation products (e.g., ferrous iron, sulfide, methane, CO2), (3) decreased BTEX concentrations along the groundwater flow direction, (4) a decreased trend of BTEX concentrations in monitor wells, (5) increased alkalinity, (6) increased microbial populations, (7) deceased pH in the spill source area, and (8) preferential removal of certain BTEX components along the transport path. Results indicate that NA can effectively contain the plume, and intrinsic biodegradation mechanisms were the major causes for contaminant removal. Results will be useful in applying NA as an alternative option for the management of petroleum-hydrocarbon contaminated sites. (C) 2013 Elsevier B.V. All rights reserved.