Transition of source/sink processes and fate of ammonium in groundwater along with redox gradients.

Ammonium (NH) retention/removal processes in groundwater are of great interest because of the continuous increase in nitrogenous compound loading due to anthropogenic activities. However, the transition of multiple co-occurring transformation processes that determine the fate of NH in groundwater along a redox gradient remains underexplored. We selected a high nitrogen (N) groundwater system in the western Hetao Basin, China, to identify and quantify NH source and sink processes, including mineralization, dissimilatory nitrate reduction to ammonium (DNRA), nitrification, and anammox, to better understand the dynamics of NH. Based on redox-sensitive parameters, that is, the oxidation-reduction potential (ORP) and NH and nitrate (NO) contents, etc., the groundwater system was classified into three zones from upstream to downstream: zone I (oxidizing), zone II (moderately reducing), and zone III (strongly reducing). Using the N tracing technique, we found that NH was mainly produced by mineralization while < 2% was produced by DNRA throughout the study area. Mineralization increased downstream because the supply of biodegradable N-containing compounds was augmented, which created a strong redox gradient to host a serial reaction chain. In zone I, NH was mainly transferred to NO via nitrification, whereas in zones II and III, NH was mainly transferred to N via anammox. The average NH production/consumption ratios (P/C) in zones I, II, and III were 0.7, 6.9, and 51.1, respectively. Obviously, the NH purification ability can only exceed the supply under aerobic conditions, thus suggesting that NH will accumulate without limitation and be retained in strongly reducing groundwater. The situation of NH accumulation would deteriorate over space and time in groundwater as human activities increase without an additional artificial supply of oxidants. The results provide mechanistic insights for quantitatively comprehending the dynamics and fate of NH in groundwater, shedding light on groundwater NH mitigation techniques.