Nitrate attenuation with rising groundwater levels: An integrated assessment using isotope tracers and microbial signatures

Nitrate contamination of groundwater has long been a source of concern. Most field studies conducted in areas with rising groundwater levels have found an increase in nitrate concentration owing to nitrate leaching, but they have not investigated the possible biogeochemical processes. In this study, dual nitrate isotopes and microbial signatures were combined to assess and quantify nitrate attenuation in the North China Plain with an apparent groundwater-level recovery trend over the past decade. Significant denitrification was revealed using the dual nitrate isotopes on the temporal scale. High-throughput sequencing provided microbial functional and taxonomic evidence, showing the relative abundances of denitrification-related functional genes negatively related to the nitrate concentration. Nitrate attenuation was strengthened in the aquifer as the groundwater levels rose, with a larger temporal enrichment factor (ε) value of −25‰ compared with the spatial ε value of −5‰. This study is the first to reveal the mechanisms of nitrate attenuation in aquifers undergoing groundwater-level recovery with an integrated assessment using nitrate isotope tracers and microbial signatures. In identifying the main nitrate attenuation processes, it is important to consider the scale and domain of interest and to conduct sufficient spatial and temporal monitoring to capture transient conditions.