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

Ammonium (NH4+) 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 NH4+ 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 NH4+ source and sink processes, including mineralization, dissimilatory nitrate reduction to ammonium (DNRA), nitrification, and anammox, to better understand the dynamics of NH4+. Based on redoxsensitive parameters, that is, the oxidation-reduction potential (ORP) and NH4+ and nitrate (NO3- ) 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-15 tracing technique, we found that NH4+ 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, NH4+ was mainly transferred to NO3- via nitrification, whereas in zones II and III, NH4+ was mainly transferred to N-2 via anammox. The average NH4+ production/consumption ratios (P/C) in zones I, II, and III were 0.7, 6.9, and 51.1, respectively. Obviously, the NH4+ purification ability can only exceed the supply under aerobic conditions, thus suggesting that NH4+ will accumulate without limitation and be retained in strongly reducing groundwater. The situation of NH4+ 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 NH4+ in groundwater, shedding light on groundwater NH4+ mitigation techniques.

Automated summary

Ammonium (NH4+) retention/removal processes in groundwater are important due to human activities increasing nitrogenous compound loading. However, the transformation processes determining NH4+ fate in groundwater under different redox conditions have not been well studied. In a high nitrogen groundwater system in China, NH4+ source and sink processes were identified and quantified, revealing the dominance of mineralization and nitrification in oxidizing zones, and anammox in reducing zones. The NH4+ production/consumption ratio increased downstream, indicating NH4+ accumulation in strongly reducing groundwater where aerobic conditions are lacking. These findings provide insights for understanding NH4+ dynamics and suggest mitigation techniques for groundwater NH4+.