Succession of nitrogen processes along a redox gradient in highly nitrogen-polluted groundwater in Hetao Basin, China
Li-Li Han 1,2, He-lin Wang 3, Liang-Hao Ge 1,2, Li Luo 4, Ping-Li 3,
Shuh-Ji KAO 1,4*
1. State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China.
2. College of Ocean and Earth Sciences, Xiamen University, Xiamen, China.
3. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, China.
4. State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou, China.
Abstract: Groundwater nitrogen (N) pollution is of great concern threatening human health. However, multiple rates of N removal and retention during the succession of various nitrogen processes along with redox gradient in groundwater remains unclear. According to PCA analysis basing on redox sensitive parameters, including oxidation reduction potential (ORP), ammonium (NH4+), nitrate (NO3-), nitrite (NO2-) etc., we classified the highly N polluted groundwater system in western Hetao Basin, China, into three zones, namely zone Ⅰ (oxidizing, NO3- > 1600 µM/L), zone Ⅱ (moderately oxidizing), and zone Ⅲ (reducing, NH4+ >134 µM/L), respectively. By using the 15N isotope-labelling method, we measured rates simultaneously for all known N processes, including nitrogen fixation (NFR), mineralization (MNR), nitrification (NR), dissimilatory nitrate redox to ammonia (DNRA), anammox (ANR) and denitrification (DNR). Overall, NH4+ was produced mainly via mineralization, DNRA occupied less than 10%, and both rates increased as ORP reduced. In zone Ⅰ, NH4+ was consumed mainly by nitrification, which is larger than mineralization production, leading to a reduction of NH4+ While in zone II and III, NH4+ was consumed by anammox, which is lower than mineralization, resulting in an accumulation of NH4+ . As for NO3- pool, nitrification was the major source, denitrification plus anammox were the major sink, with greater occupation by denitrification. As the ORP decreased, nitrification reduced concomitantly, while both sink process increased. In zone I, NO3- pool could increase or decrease according to relative source and sink. While in zone II and III, NO3- sink higher than source resulting in a consumption. However, organic matter mineralization consumes oxygen to reduce NOX- production for subsequent nitrogen removal. Furthermore, it’s worth noting that nitrogen fixation may as an important dissolved nitrogen source have been overlooked, especially for oxidizing groundwater. Our findings reveal the NO3- pollution may persist over 60 years in oxidizing state while NH4+ may accumulate (up to 300 µM) within weeks and persist in reducing state until oxygen introduced.
Keywords: Groundwater, Redox environment, 15N isotope-labelling, Nitrate, Ammonia, Nitrogen cycle processes.
