Field-scale biofiltration of gasoline vapors extracted from beneath a leaking underground storage tank

Approximately 15000 L of unleaded gasoline were released into the surrounding vadose zone from a leaking underground storage tank. Initial remediation was by soil vapor extraction and combustion which soon became cost prohibitive, as added propane was required to reach the combustion limit of the extracted vapors. As a cost effective alternative, a field-scale compost based biofilter was used in conjunction with soil vapor extraction to remediate the vadose zone. The biofilter was constructed on site using 4:1 diatomaceous earth:composted horse manure. Results of a five month study showed that the biofilter removed approximately 90% of total petroleum hydrocarbons (TPH) and >90% of the BTEX compounds (benzene, toluene, ethylbenzene, xylene), achieving the stringent permit requirements set at either 90% TPH reduction or less than 1.36 kg per day of volatile organic compounds (VOC's) released to the atmosphere. The biofilter showed the capacity to readily adapt to changing environmental conditions such as increased contaminant loading, and variations in temperature and moisture. The bacterial population in the biofilter was uniformly diverse throughout the biofilter, suggesting that a consortium of bacteria was needed for efficient biodegradation. The cost of biofilter set up and operation saved 90% in the first year alone of the operating expenses incurred by soil vapor extraction and combustion.