Nitrogen biogeochemical reactions during bank filtration constrained by hydrogeochemical and isotopic evidence: A case study in a riverbank filtration site along the Second Songhua River, NE China

Abstract

River eutrophication and nitrogen pollution poses a potential threat to the groundwater quality in riverbank filtration systems. The river filtration process is often accompanied by complex redox reactions. Therefore, identification of nitrogen biogeochemical processes is essential to scientifically characterize the cause of NO3− attenuation and NH4+ accumulation in groundwater, optimize the design of groundwater pumping wells, and design protection strategies in sudden environmental pollution accidents. Hydrogeochemical and stable isotope tracing techniques were employed in this study at a typical riverbank filtration site located in Songyuan, NE China, to explore the main geochemical reactions controlling the migration and transformation of nitrogen along the groundwater flow path during riverbank filtration. The results indicate that mixing, adsorption, organic nitrogen mineralization, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) represent the major geochemical reactions. Denitrification primarily occurs within 10–20 m from the riverbed surface along the filtration path and is the primary reaction accounting for NO3− attenuation, whereas DNRA typically occurs between 1.5 and 6 m from the riverbed surface along the filtration path and is more active in the wet season, characterized by high temperatures (>15–17 °C), low dissolved oxygen (DO, < 2–4 mg/L), and high carbon load i.e., organic carbon (OC):NO3− > 10–15. This work confirms that DNRA is an important nitrogen geochemical reaction during riverbank filtration, and emphasizes its role in NH4+ enrichment.