Long-term monitoring and evaluating biological activity of in situ anaerobic reductive dechlorination at a highly recharged and TCE-contaminated aquifer

Abstract

Few studies have evaluated how rainfall recharges affect biological dechlorination of trichloroethene (TCE). In this study, a sequence of three well-to-well tests (Phases I, II, and III) were performed for 100d to investigate the variation of biological dechlorination of TCE at a highly recharged and TCE-contaminated aquifer. Test solutions containing different nutrients were delivered into the aquifer through a nitrogen gas spray. In Phase I, where only formate was supplied, the degradation of TCE to cis-dichloroethene (cis-DCE) was negligible. However, when sulfate as well as formate was introduced during Phase II, the reductive dechlorination of TCE occurred. During Phase III, fumarate was also provided as an alternative electron donor. High cis-DCE production rates were observed with complete degradation of formate and sulfate at 65th day monitoring, the end of dry period in Phase III. Since then, there were heavy rainfalls that accounted for approximately 36% of total annual precipitation in this area. Although oxygenated rain was rapidly infiltrated into the bioactive zone, the consumption of formate and sulfate were not significantly affected, with the DO as low as <0.2mg/L. Nonetheless, the dechlorination activity of TCE was decreased as evidenced by 70% reduction in the cis-DCE production rate compared to that before the heavy rainfall. In 10d after heavy rainfall ceased, the dechlorination activity was partially recovered (approximately 60%), while sulfate and formate consumptions were fully recovered. These results suggest that the dechlorination activity of TCE at the tested aquifer was sustained even in the presence of heavy rainfall recharges. This was probably due to the growth of the dechlorinating microbes on solid surfaces by forming biofilms.