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

Li-Li Han; Helin Wang; Lianghao Ge; Min Nina Xu; Jin-Ming Tang; Li Luo; Ping Li; Shuh-Ji Kao

doi:10.1016/j.watres.2023.119600

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

Water Res. 2023 Mar 1:231:119600. doi: 10.1016/j.watres.2023.119600. Epub 2023 Jan 13.

Authors

Li-Li Han¹, Helin Wang², Lianghao Ge¹, Min Nina Xu³, Jin-Ming Tang¹, Li Luo³, Ping Li⁴, Shuh-Ji Kao⁵

Affiliations

¹ State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China.
² State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, China; Hubei Key Laboratory of Yangtze Catchment Environmental Aquatic Science, China University of Geosciences, Wuhan, China.
³ State Key Laboratory of Marine Resources Utilization in South China Sea, Hainan University, Haikou, China.
⁴ State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, China; Hubei Key Laboratory of Yangtze Catchment Environmental Aquatic Science, China University of Geosciences, Wuhan, China. Electronic address: pli@cug.edu.cn.
⁵ State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China; State Key Laboratory of Marine Resources Utilization in South China Sea, Hainan University, Haikou, China. Electronic address: sjkao@xmu.edu.cn.

PMID: 36680827
DOI: 10.1016/j.watres.2023.119600

Abstract

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 redox-sensitive 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 ¹⁵N 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 N₂ 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.

Keywords: Ammonium; Groundwater; Mineralization; Nitrification; Nitrogen isotope; Redox gradient.

MeSH terms

Ammonium Compounds*
Denitrification
Groundwater*
Humans
Nitrates / analysis
Nitrogen
Oxidation-Reduction

Substances

Nitrates
Ammonium Compounds
Nitrogen