Model the evolutionary pattern of N species and pool size in groundwater continuum by utilizing measured source and sink rates of nitrate and ammonium.

Nitrate and ammonium are primary nitrogen (N) contaminants in groundwater and effective restoration strategies depend on understanding the interactions of N transformation processes along redox gradients. Utilizing the N tracing technique, we assess nitrate removal rates, focusing on denitrification and anammox in a N-rich groundwater of the Hetao Basin, a typical semiarid region in western China. Results showed that N removal rate (0.36-22.01 µM N d) was composed mainly of denitrification (73 ± 18 %), with rates increasing from upstream oxidizing environment to downstream reducing areas. In reducing downstream, both denitrification and anammox adhered to substrate-driven Michaelis-Menten kinetics. Integrating data on all source and sink rates of nitrate and ammonium pools (denitrification, anammox, dissimilatory nitrate reduction ammonia, nitrification, mineralization), we constructed a N-transfer-dynamics model based on chemical stoichiometry. This model effectively captured the observed spatial N transfer patterns and highlighted that the balance of oxidants and biodegradable organic N inputs influences N species retention and removal in groundwater. Our combined experimental and modeling approach underscores the importance of reducing organic N and/or adding oxidants to mitigate groundwater N pollution. These findings provide crucial insights for optimizing high N groundwater remediation strategies and potentially inform for wastewater management practices.