Nitrate removal rates, isotopic fractionation, and denitrifying bacteria in a woodchip-based permeable reactive barrier system: a long-term column experiment

This study evaluated the effectiveness of using organic carbon materials (i.e., woodchips) to remove nitrate from groundwater. The results of our flow-through column experiment, which was conducted over 1.6 years, suggested that denitrifying bacteria reduce nitrate by using it as an electron acceptor and woodchips as an electron donor. The nitrate removal rates were sufficiently high (0.39–1.04 mmol L⁻¹ day⁻¹) during the operation of the column. Denitrification process was supported by fractionation of nitrogen and oxygen isotopes (δ¹⁵N and δ¹⁸O), with the δ¹⁵N and δ¹⁸O values enriched from 7.4‰ and 22.3‰ to 21.2‰ and 30.4‰, respectively. Enrichment factors (ε) for ¹⁵ N and ¹⁸O were calculated using the Rayleigh fractionation model, with values of − 13.2‰ for ε¹⁵N and − 7.1‰ for ε¹⁸O. The fractionation ratio of ¹⁵ N to ¹⁸O was 1.9:1, confirming denitrification. The most abundant bacterial genera in the soil used for inoculation were Enterobacter (86.7%), Nitrospira (1.8%), and Arthrobacter (1.5%), while those in the column effluent were Macrococcus (37.1%), Escherichia (14.7%), and Shigella (14.6%), indicating that bacterial communities changed in response to geochemical conditions in the column. This study suggests that nitrate in groundwater can be effectively removed using woodchip-based passive treatment systems and that information on isotopic fractionation and denitrifying bacteria can be key tools to understand denitrification.