Enhanced bioremediation of dissolved aromatic hydrocarbons within a gasoline source area using nitrate and oxygen as electron acceptors

Abstract

Controlled releases of API 91-01 gasoline within 4m2 sheeting-piling cells, laboratory studies and numerical simulations were used to investigate the enhanced bioremediation of dissolved aromatic hydrocarbons (BTEXTMB) derived from a gasoline source area under mixed electron-acceptor conditions. The main objective of the research was to evaluate nitrate-based bioremediation as a remedial technology in a gasoline source area under highly-controlled experimental conditions. Gasoline was released into two treatment cells (70L per cell) in the Borden aquifer to create gasoline-contaminated source areas below the water table, and then water amended with different combinations of electron acceptors was flushed vertically through the cells. One cell received a mixture of NO3 and microaerophilic O2 (ca. 2mg/L dissolved O2), and the other cell, a control, received microaerophilic O2 only. Aromatic-hydrocarbon and electron-acceptor utilization as well as other geochemical indicators of biotransformation were then monitored during both flushing and static periods over a 13 months period. Laboratory microcosm experiments and microbial characterization studies were also performed with both pristine and contaminated aquifer material to augment the findings based on in situ observations.

Although the consumption of electron acceptors and production of metabolites suggested that microbial activity had been stimulated in the treatment cells, field and laboratory data indicated overall that nitrate-based bioremediation was not an effective source-area remedial technology under the conditions established in this study. Nitrate utilization was slow relative to the residence time in the treatment cell, and preferential utilization of the labile aromatic hydrocarbons was not apparent. Based on laboratory data, the addition of microaerophilic O2 may have led to the degradation of compounds that otherwise would have been recalcitrant under anaerobic, denitrifying conditions, but the effect in situ appeared to be small relative to the mass of gasoline hydrocarbons in the cells. Consequently, mass losses did not appear to be enhanced in the cell treated with microaerophilic O2 and NO3 relative to the unremediated control, and effluent breakthrough curves were consistent with concentration trends expected to result from abiotic gasoline dissolution. Despite the negative results obtained in this highly-contaminated system, the data suggested that mixed electron acceptors might be more effective during the latter stages of an enhanced bioremediation project when source-area concentrations were lower, or for downgradient plume control using a reactive wall or other semi-passive remedial technology.