Nitrate reduction by denitrifying bacteria within a porous medium

Abstract

Acid processing of uranium ore resulted in aquifer contamination (nitrate, sulfate, and uranium) of the aquifer associated with the Navajo Sandstone formation at Tuba City, Arizona. The objectives of this study focused on the use of bioprocesses to remediate this aquifer contamination. The bench-scale objective of this study was to evaluate the ability of an indigenous microbial consortium to bioremediate nitrate contamination. The reduction of nitrate typically results in the production of nitrite, which under most conditions is further reduced to dinitrogen gas or ammonia. However, under some conditions inhibitory concentrations of nitrite may accumulate. Sandstone-packed columns fed with aquifer-relevant concentrations of nitrate were used to assess denitrification rates by indigenous bacteria. To enumerate denitrifying consortia used in the column experiments, most probable number (MPN) techniques were used. Sandstone-packed column influent and effluent data for nitrate, nitrite, carbon substrate and biomass concentrations were collected over time. These data were used to assess nitrate reduction rates within a sandstone column. This research demonstrates that with indigenous bacteria with stable conditions nitrate is reduced to dinitrogen forming only minimum levels of nitrite which should not inhibit sulfate-reducing bacteria (SRBs). The results of these studies indicate that bacterial denitrification has good potential as a remediation strategy for nitrate-contaminated groundwater to levels below the established regulatory limits of 44 mg/L. Field tests applications are planned for the Department of Energy UMTRA site in Tuba City, Arizona, using an extensive grid of injection and pumping wells.