INT-02 Marine Nitrogen Cycle
Transition of source/sink processes and fate of ammonium in groundwater along with redox gradient
Li-Li Han* , State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China
He-Lin Wang, State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, China
Liang-Hao Ge, State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China
Min Nina Xu, State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou, China
Jin-Ming Tang, State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China
Li Luo, State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou, China
Ping Li, State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, China
Shuh-Ji Kao, State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China; sjkao@xmu.edu.cn

Ammonium (NH4+) retention/removal processes in groundwater are of great interest due to the continuous increase of loading of nitrogenous compounds. However, the transition of multiple co-occurred transformation processes determining the fate of NH4+ in groundwater along with redox gradient remains underexplored. We selected high nitrogen (N) groundwater system in western Hetao Basin, China for study aiming to identify and quantify NH4+ source and sink processes, including mineralization, dissimilatory nitrate reduction to ammonia (DNRA), nitrification, and anammox, to better understand the dynamics of NH4+. Based on redox-sensitive parameters, i.e., oxidation-reduction potential (ORP), NH4+, nitrate (NO3−), etc., the groundwater system was classified into three zones from up to downstream, namely zone I (oxidizing), zone II (moderately reducing), and zone III (strong reducing). By using the 15N tracing technique we found NH4+ was produced mainly by mineralization with < 2% by DNRA throughout the studied area. Mineralization increased downstream according to augmenting the supply of biodegradable N-containing compounds, which created a strong redox gradient to host a serial co-occurring reaction chain. In zone I, NH4+ was mainly transferred to NO3− via nitrification, yet in zone II and III NH4+ was mainly transferred to N2 via anammox. The averaged NH4+ production/consumption ratio (P/C) for zone I, II, and III are 0.7, 6.9, and 51.1, respectively. Obviously, only under aerobic conditions NH4+ purification ability can exceed supply, suggesting that NH4+ will accumulate unlimitedly and be retained in strong reducing groundwater. Such an unfavorable situation would deteriorate over space and time as increasing human activities unless extra oxidants can be supplied artificially. The results provide mechanistic insights to quantitatively comprehend the dynamics and fate of NH4+ in groundwater, shedding light on groundwater NH4+ mitigation.