IN-SITU AIR INJECTION, SOIL VACUUM EXTRACTION AND ENHANCED BIODEGRADATION - A CASE-STUDY IN A JP-4 JET FUEL CONTAMINATED SITE

The U.S. Environmental Protection Agency (U.S. EPA) and the U.S. Coast Guard (USCG) conducted a joint demonstration of in situ remediation o f a JP-4 jet fuel spill at the USCG Support Center in Elizabeth City, North Carolina. The jet fuel was trapped beneath a clay layer that ext ended from the surface to a depth of 1.5 m. The water table was 2.0 m below land surface, and jet fuel extended from a depth of 1.0 to 3.5 m . Air was injected under pressure to depress the water table and bring the entire spill into the unsaturated zone, where hydrocarbons could be removed by volatilization and biodegradation. The injected air teas recovered through soil vacuum extraction (SVE) at the treatment area. To document actual removal of hydrocarbons core samples were acquired in August 1992 before air injection, and September 1994 at the end of the demonstration. The spill originally contained 3600 kg of JP-4. Be tween the core sampling events, only 55 % of the total petroleum hydro carbons were removed, but more than 98 % of benzene was removed. The i nitial goal was to reduce the concentration of total petroleum hydroca rbons (TPH) to concentrations less than 100 mg/kg soil. This was not a ccomplished within 18 months of operation. During the period of operat ion, ground water was monitored for the concentration of benzene, tolu ene, ethylbenzene, and the xylene isomers (BTEX), and methyl tertiary butyl ether (MTBE). The concentration of BTEX and MTBE in the subsurfa ce was reduced to a very low level, but concentrations of benzene and MTBE in ground water did not meet the EPA drinking water standards in the most heavily impacted wells. The effluent gas from SVE was monitor ed for the concentration of total hydrocarbon vapors. Eased on analysi s of soil cores, 2000 kg of TPH was removed during the demonstration. About 1700 kg of hydrocarbon was collected through the SVE system, acc ounting for most of the actual removal. The rate of biodegradation was estimated from the rate of oxygen consumption in the unsaturated zone when the air injection pumps were turned off. Within the area contain ing nonaqueous phase liquid (NAPL), the biodegradation rate based on o xygen consumption ranged from 0.72 to 13 mg hydrocarbon/kg soil/day, w ith an average of 3 mg hydrocarbon/kg soil/day. The mass balance revea led that the contribution of biodegradation was apparently overestimat ed from oxygen consumption in the soil gas.