Combined laboratory/field study on the use of nitrate for in situ bioremediation of a fuel-contaminated aquifer

A pilot demonstration project was conducted at Eglin Air Force Base, FL, to compare the extent of bioremediation of a fuel-contaminated aquifer using sprinkler application with and without nitrate addition on two adjacent 30 m x 30 m cells. Target compound groups included both BTEXTMB (benzene, toluene, ethylbenzene, xylenes, and trimethylbenzenes) and the JP-4 jet fuel. Bioremediation performance was monitored through the use of groundwater quality measurements as well as periodic core analyses. Operation began April 1994, and an interim performance evaluation was conducted August 1994. The final performance evaluation was conducted May 1995. Minimal remediation occurred during the first four months. Water quality analyses showed that the nitrate cell subsurface was actively denitrifying, but lysimeter samples indicated that much of the nitrate was consumed within the rhizosphere above the fuel-contaminated interval. A 9 m x 9 m plot inside each cell was therefore stripped of vegetative cover and covered with a weed barrier to enhance nitrate transfer into the subsurface. After an additional 8 months operation, lysimeter samples showed increased nitrate transfer to the contaminated interval beneath the nitrate cell stripped plot, and there was higher fractional removal of contaminant groups beneath the stripped plots as well. On the basis of core data, BTEXTMB was reduced by 66 +/- 1% in both treatment cells, equivalent to a mass loss of 106 and 21 kg in the nitrate cell and control cell, respectively. Monitoring well data provided evidence of sulfate reduction in the control cell but not in the nitrate cell. In addition, post-test treatability studies using Gore material from both cells demonstrated removal of alkylbenzenes under denitrifying and iron-reducing conditions, with different profiles for each cell. One year after completion of the project, BTEXTMB reductions in downgradient monitoring wells remained consistent with the core data. Collective laboratory and field data indicated that contaminant reduction occurred as a result of anaerobic bioremediation as well as soil washing and that different anaerobic processes predominated in the control cell due to circulation of endogenous electron accepters.