Evaluation of natural attenuation-potential and biogeochemical analysis in nitrate contaminated bedrock aquifers by carbon source injection

In this study, the natural attenuation potential and biogeochemical analysis of nitrate contaminated bedrock aquifers by injection of carbon sources was evaluated. The denitrification capacity was assessed by injecting different carbon sources (succinate, acetate, fumarate) into the groundwater. Acetate was identified as the optimum source of electron donors for microbial metabolic processes, as it improved the effect of nitrate removal and microbial activity in the groundwater. In addition, when acetate was injected with a C/N ratio = 2.1:1, the ratio of denitrifying bacteria was the greatest (C/N 2.1 (2.1%) > C/N 4.2 (1.9%) > C/N 7.0 (0.9%) > control (0.7%)). Reflecting the geochemical characteristics of the bedrock aquifer environment, acetate was injected into groundwater at the research site to activate biological heterotrophic denitrification. As a result, the nitrate reduction rate was 0.377 g-N/day (YP-3), while the rate in groundwater unaffected by acetate was significantly lower, at 0.028 g-N/day (YP-4) over the same reaction time. In particular, the ratio of Dechloromonas denitrificans sp., which is a representative denitrification bacteria involved in anaerobic reduction of nitrate, increased (before injection: 0.0089%, after injection: 1.3067%). Expression of the nosZ gene, which is involved in the denitrification pathway (N₂O → N₂), increased from 4.82 Log (gene copies L⁻¹) to 9.71 Log (gene copies L⁻¹). Together, these results demonstrate that denitrification in bedrock aquifers can be activated by injection of carbon sources and identified the genetic reason for that denitrification.