Aromatic hydrocarbon biodegradation with mixtures of O-2 and NO3- as electron acceptors

BTEX and polycyclic aromatic hydrocarbons have been detected in the groundw ater and subsurface sediments at the site of a former manufactured gas plan t in Maryland. As part of a bioremediation feasibility study at this site, laboratory experiments were conducted to understand the effect of mixed oxy gen/nitrate electron acceptor conditions on biodegradation of aromatic hydr ocarbons. Three separate oxygen concentration ranges provided different res ults in terms of BTEX, naphthalene, and phenanthrene removal and oxygen use . At initial levels ranging from 0 to 1 mg O-2/L, only toluene was degraded . Naphthalene degradation was observed in addition to toluene removal at in itial oxygen levels ranging from 1.5 to 2.0 mg O-2/L. This range represents a threshold level below which naphthalene could not be degraded by this mi crobial enrichment. At oxygen levels greater than or equal to 7 mg O-2/L, s ome removal of all aromatic compounds was observed. Biodegradation with 7 m g O-2/L and 30 mg O-2/L was comparable, although the initiation of compound removal was significantly delayed, and denitrification was completely inhi bited with 30 mg O-2/L. Denitrification in conjunction with toluene removal was observed at all initial oxygen levels except oxygen saturation (30 mg O-2/L). These results support a decision to provide a moderate level of oxy gen (7 mg O-2/L) to the subsurface in conjunction with nitrate to ensure re moval of all compounds, to minimize any acclimation period associated with high oxygen concentrations, allow for denitrification to occur and thereby conserve oxygen, and to minimize plugging of the aquifer with excessive mic robial growth.