Effect of nitrate-based bioremediation on contaminant distribution and sediment toxicity-column study

A laboratory column study was set up to evaluate changes in contaminant distribution and sediment toxicity following nitrate‐based bioremediation and to correlate toxicity reduction with loss of fuel components. Glass columns were packed with sediment from an aquifer that had been contaminated with JP‐4 jet fuel and were remediated using feed solution containing 20 mg/L NO3‐N. Column influents and effluents were monitored for BTEXTMB (benzene, toluene, ethylbenzene, xylenes, trimethyl‐benzenes), electron acceptors, nutrients, and dissolved gases. Duplicate columns were sacrificed after 1, 4, and 7 months, and core material was analyzed for chemical constituents. In addition, core material was evaluated for toxicity using FETAX, a developmental toxicity test employing frog embryos. After 1 month of operation, total mass of BTEXTMB dropped from 51.8 ± 7.3 mg to 29.8 ± 2.9 mg (42% reduction) in the column sediments, with 1.45 ± 0.06 mg eluting in the column effluents and 20.6 ± 3.0 mg being unaccountable, presumably due to biodegradation. Based on stoichiometry of denitrification, nitrate consumption and nitrite and nitrous oxide production were sufficient to account for the observed loss. In contrast, JP‐4 levels only dropped from 2,070 ± 260 mg to 1,750 ± 22 mg (15% reduction). Despite the similar distribution of contaminants in the two columns of this first column pair, FETAX mortality remained unchanged at 93.3% in the first column and dropped from 93.3% to 11.7% in the second. Toxicity reduction could not be therefore directly attributed to either BTEXTMB or JP‐4 levels. After 7 months, total mass reductions were 93.1 ± 1.1% and 35 ± 1.6% for BTEXTMB and JP‐4, respectively, and FETAX mortality and malformation had declined to less than 10%. These data show that closure standards requiring complete removal of residual hydrocarbon may overestimate the time required to counteract sediment toxicity, and that nitrate‐based bioremediation may be a viable treatment alternative for fuel‐contaminated aquifers.