Application of monitored natural attenuation to remediate petroleum-hydrocarbon contaminated groundwater

Contamination of groundwater by petroleum-hydrocarbons is a serious environmental problem. Monitored natural attenuation (MNA) is a passive remedial approach to degrade and dissipate contaminants in groundwater. In this study, a full-scale natural bioremediation investigation was conducted at a gasoline spill site. Results show that the concentrations of major contaminants [benzene, toluene, ethylbenzene, and xylenes (BTEX)] dropped to below detection limit before they reached the downgradient monitor well. This indicates that natural biodegradation process was the major cause of the contaminant reduction. In this study, 1,2,4-trimethylbenzene (1,2,4-TMB) was used as tracer to calculate the biological decay rates of BTEX. The calculated biological first-order decay rates for ethylbenzene, m+p-xylene, toluene, benzene, and o-xylene were 1.5A10-3, 1.2A10-3, 7.0A10-4, 6.7A10-4, and 1.2A10-4, respectively. Mann-Kendall test was applied to analyze the trend of contaminant variations. Results show that the S-value of four monitor wells (CT-4, CT-41, CT-42, and CT-7) were -0.52, -1.57, -0.52, and -1.22, respectively. The negative S values reveal that the all contaminants tended to decrease. This indicates that the hydrocarbon plume at this site is not expanding, and has been contained effectively by the natural attenuation mechanisms. Evidences for the occurrence of natural attenuation include the following: (1) depletion of dissolved oxygen, nitrate, and sulfate; (2) production of dissolved ferrous iron, sulfide, and CO2; (3) decreased BTEX concentrations and BTEX as carbon to TOC ratio along the transport path; (4) increased alkalinity and microbial species; (5) limited spreading of the BTEX plume; and (6) preferential removal of certain BTEX components along the transport path. Results also show that the biodegradation capacity (46.02 mg/L) for BTEX and 1,2,4-TMB was much higher than the detected contaminants within the plume. Results indicate that natural attenuation can effectively contain the plume, and biodegradation processes played an important role on contaminant removal.