Sustained ability of a natural microbial community to remove nitrate from groundwater

Abstract

Microbial-mediated nitrate removal from groundwater is widely recognized as the predominant mechanism for nitrate attenuation in contaminated aquifers and is largely dependent on the presence of a carbon-bearing electron donor. The repeated exposure of a natural microbial community to an electron donor can result in the sustained ability of the community to remove nitrate; this phenomenon has been clearly demonstrated at the laboratory scale. However, in situ demonstrations of this ability are lacking. For this study, ethanol (electron donor) was repeatedly injected into a groundwater well (treatment) for six consecutive weeks to establish the sustained ability of a microbial community to remove nitrate. A second well (control) located up-gradient was not injected with ethanol during this time. The treatment well demonstrated strong evidence of sustained ability as evident by concomitant ethanol and nitrate removal and subsequent sulfate removal upon consecutive exposures. Both wells were then monitored for six additional weeks under natural (no injection) conditions. During the final week, ethanol was injected into both treatment and control wells. The treatment well demonstrated sustained ability as evident by concomitant ethanol and nitrate removal whereas the control did not. Surprisingly, the treatment well did not indicate a sustained and selective enrichment of a microbial community. These results suggested that the predominant mechanism(s) of sustained ability likely exist at the enzymatic- and/or genetic-levels. The results of this study demonstrated that the in situ ability of a microbial community to remove nitrate can be sustained in the prolonged absence of an electron donor. Moreover, these results implied that the electron-donor exposure history of nitrate-contaminated groundwater can play an important role nitrate attenuation.