Hydrogeological controls on ammonium enrichment in shallow groundwater in the central Yangtze River Basin

• Published in: The Science of the total environment 2020-11, Vol.741, p.140350-140350, Article 140350
• Main Authors: Liang, Ying, Ma, Rui, Wang, Yanxin, Wang, Shuo, Qu, Le, Wei, Wenhao, Gan, Yiqun
• Format: Article
• Language: English
• Subjects: Ammonium; Groundwater; Seasonal hydrogeological conditions; The Central Yangtze River basin
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2020.140350

The controlling processes of excessive ammonium in surface water and groundwater in the central Yangtze River Basin remain unclear. In this study, monitoring of water levels and temporal-spatial distributions of major N compounds were implemented at the large Jiangshan plain and at the local site scale in the central Yangtze River Basin. The results indicate that the recharge, movement and transformation of ammonium were controlled by hydrogeological conditions. Manure and sewage from anthropogenic activities were identified as the main source of nitrogen compounds. The nitrogen loading into aquifers were governed by water table and groundwater flow. After entering subsurface soils, nitrification and dissimilatory nitrate reduction to ammonium (DNRA) were proposed as the ammonium consumption and production mechanisms, respectively, by combining the concentrations of ammonium‑nitrogen and nitrate‑nitrogen with the corresponding isotopic compositions. These microbially mediated processes controlling transport and transformation of nitrogen compounds were influenced by the seasonally varying groundwater flow regime that changed the redox conditions in the aquifers. In the subsurface environments, ammonium was converted to nitrate when sufficient oxygen supply was available, and this process was reversed under anoxic conditions along the groundwater flow path. A conceptual model for the reactive transport of nitrogen compounds jointly controlled by the vertical groundwater flows and biogeochemical processes was proposed, which provides new insights into the genesis of high ammonium groundwater. [Display omitted] •N compounds in surface water and groundwater in the central Yangtze River basin were sourced from manure and sewage.•Seasonal hydrogeological variations could contribute to the enrichment of ammonium in shallow groundwater.•High concentration of nitrate in oxic zone derived from nitrification in low groundwater level period.•Ammonium could be produced via dissimilatory nitrate reduction to ammonium under anoxic condition.