Control of BTEX migration by intrinsic bioremediation at a gasoline spill site

A full-scale and detailed intrinsic bioremediation investigation was conducted at a gasoline spill site in Dublin, North Carolina, USA. Due to the appearance of nonaqueous phase liquid (NAPL) hydrocarbons beneath the former spill location, dissolved BTEX (benzene, toluene, ethylbenzene, and xylene isomers) are being continuously released from NAPL into the groundwater with a total BTEX concentration of 60 mg/l. At this spill site, a cropland extends from the mid-plume area to the downgradient edge of the plume. Approximately 15 mg/l of nitrate as nitrogen was detected in groundwater beneath the cropland due to the usage of fertilizer. During the three-year study, the following tasks were conducted: (1) groundwater analysis; (2) microbial enumeration; and (3) mass flux and decay rate calculations. Results show that BTEX concentrations dropped to below detection limit (BDL) before they reached the downgradient monitor, well located 110 m from the spill location. Groundwater and microbial analyses indicate that iron reduction was the dominant biodegradation process between the source and mid-plume area. However, nitrate spill in the cropland area switched the degradation pattern to denitrification, and also changed the preferential removal of certain BTEX components. Under iron-reducing conditions, toluene and o-xylene declined most rapidly followed by m+p-xylene, benzene, and ethylbenzene. Within the denitrifying zone, toluene and m+p-xylene had very rapid degradation, followed by ethylbenzene, o-xylene, and benzene. The mass flux calculations show that up to 93.1% of the BTEX was removed within the iron-reducing zone, and 5.6% of the BTEX was degraded within the nitrate spill zone. The remaining 1.3% was removed within the oxidized zone at the downgradient edge of the plume. Toluene had the highest first-order decay rate (0.16%) detected in the iron-reducing zone and benzene had the lowest rate (0.07%) within the denitrification area. Results reveal that the mixed intrinsic bioremediation processes (iron reduction, denitrification, methanogenesis, aerobic biodegradation) have effectively contained the plume, and iron reduction played an important role on the BTEX removal.