Spatial Variability of Nitrate and Ammonium in Pleistocene Aquifer of Central Yangtze River Basin

Abstract

It becomes increasingly important and challenging for nitrogen pollution prevention to identify key controls for spatial variability of nitrogen in groundwater that could be affected by multiple factors, including anthropogenic input, groundwater flow, and local geochemistry. This study characterized spatial variability of both nitrate and ammonium in the Pleistocene aquifer of central Yangtze River Basin and assessed the effect of various factors in controlling nitrate and ammonium levels based on multiple statistical approaches (correlation, geostatistics, multiple liner regression). The results indicate that nitrate is mostly influenced by Cl − that represents anthropogenic input, while Eh representing local redox state is a secondary variable influencing nitrate concentrations. The groundwater with elevated nitrate concentrations are estimated to occur mainly in areas with higher-permeability near-surface sediments which can facilitate more anthropogenic nitrate transport and less nitrate removal owing to more oxidized state. Ammonium is mostly correlated to Eh, followed by dissolved organic carbon (DOC), but only DOC improves significantly the accuracy of co-kriging prediction model. The groundwater with elevated ammonium concentrations are estimated to occur mainly in areas with more organic-rich sediments within or around the aquifer which can facilitate more ammonium release owing to natural organic matter consumption accompanying strong reducing conditions. The regional groundwater flow is not a factor significantly controlling nitrate or ammonium levels owing to flat topography and sluggish lateral flow.

Novel Insights into Dissolved Organic Matter Processing Pathways in a Coastal Confined Aquifer System with the Highest Known Concentration of Geogenic Ammonium

“…The Quaternary aquifer system in the PRD, south China, which was the target of this study, and that in the Central Yangtze River Basin (CYRB), central China, are most representative of coastal and inland alluvial−lacustrine groundwater systems with high geogenic ammonium. 10,[15][16][17]20,29,30 The reported concentrations of geogenic ammonium in the groundwater of the PRD and CYRB were up to 390 and 33.5 mg/L. 10,16 It is interesting to see the substantial difference in ammonium concentration levels between the coastal and the inland alluvial−lacustrine aquifer systems.…”

“…In recent years, the enrichment of naturally occurring (geogenic) ammonium (NH 4 + ) as a “specific nitrogen contamination” has been widely detected in groundwater systems in various geological settings, − among which the coastal aquifer system was the most focused. In the Pearl River Delta (PRD) in China, the Red River Delta in Vietnam, , the Bengal Basin in Bangladesh, the Po River Delta in Italy, and the Gualdalquivir Basin in Spain, naturally occurring high ammonium concentrations, on the order of tens to hundreds of milligrams per liter (as N), have been found in Quaternary aquifer systems.…”

“…In the Pearl River Delta (PRD) in China, the Red River Delta in Vietnam, , the Bengal Basin in Bangladesh, the Po River Delta in Italy, and the Gualdalquivir Basin in Spain, naturally occurring high ammonium concentrations, on the order of tens to hundreds of milligrams per liter (as N), have been found in Quaternary aquifer systems. By contrast, the concentration levels of geogenic ammonium in inland alluvial–lacustrine or glacial drift aquifer systems are generally lower. − However, as an abundant subsurface nitrogen pool that was highly neglected in the past, the detailed mechanisms responsible for the formation of the naturally high ammonium in coastal groundwater systems are not well understood.…”

Enrichment of Geogenic Ammonium in Quaternary Alluvial–Lacustrine Aquifer Systems: Evidence from Carbon Isotopes and DOM Characteristics

“…The alluvial Jianghan Plain (JHP) as part of central Yangtze River basins has experienced extensive lacustrine deposits since the Holocene, which are characterized by low permeability and generally regarded as aquitard layers 24,25 that can effectively prevent the transport of ammonium from anthropogenic activities on the land surface to the underlying Pleistocene aquifers. High levels of ammonium up to 29.75 mg/L (N) have been detected in the Pleistocene aquifers of the JHP, 26,27 which is far above the limit in drinking water as 0.5 mg/L (N) in China. Abnormally high levels of NH 4 -N in groundwater mainly occurred close to the meandering bank of the middle reaches of the Yangtze River.…”

“…Abnormally high levels of NH 4 -N in groundwater mainly occurred close to the meandering bank of the middle reaches of the Yangtze River. 27 The elevated concentrations of ammonium in the Pleistocene aquifer of the JHP have been attributed to organic matter mineralization based on δ 15 N-NH 4…”