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

A full-scale and detailed intrinsic bioremediation investigation was conduc ted at a gasoline spill site in Dublin, North Carolina, USA. Due to the app earance of nonaqueous phase liquid (NAPL) hydrocarbons beneath the former s pill location, dissolved BTEX (benzene, toluene, ethylbenzene, and xylene i somers) are being continuously released from NAPL into the groundwater with a total BTEX concentration of 60 mg/l. At this spill site, a cropland exte nds from the mid-plume area to the downgradient edge of the plume. Approxim ately 15 mg/l of nitrate as nitrogen was detected in groundwater beneath th e cropland due to the usage of fertilizer. During the three-year study, the following tasks were conducted: groundwater analysis; microbial enumeratio n; and (3) mass flux and decay rate calculations. Results show that BTEX co ncentrations dropped to below detection limit (BDL) before they reached the downgradient monitor, well located 110 m from the spill location. Groundwa ter and microbial analyses indicate that iron reduction was the dominant bi odegradation process between the source and mid-plume area. However, nitrat e spill in the cropland area switched the degradation pattern to denitrific ation, and also changed the preferential removal of certain BTEX components . Under iron-reducing conditions, toluene and o-xylene declined most rapidl y followed by m + p-xylene, benzene, and ethylbenzene. Within the denitrify ing zone, toluene and m + p-xylene had very rapid degradation, followed by ethylbenzene, o-xylene, and benzene. The mass flux calculations show that u p 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 iro n-reducing zone and benzene had the lowest rate (0.07%) within the denitrif ication area. Results reveal that the mixed intrinsic bioremediation proces ses (iron reduction, denitrification, methanogenesis, aerobic biodegradatio n) have effectively contained the plume, and iron reduction played an impor tant role on the BTEX removal. (C) 2000 Elsevier Science Ltd. All rights re served.