Dissolved organic matter characterization in high and low ammonium groundwater of Dongting Plain, central China.

High levels of ammonium in groundwater is a potential threat to drinking water security and ecological status. The role of dissolved organic matter (DOM) in mobilization of natural ammonium in groundwater is crucial but the intrinsic link between them has still been poorly understood. This study used high-pressure size exclusion chromatography (HPSEC) and fluorescence excitataion-emission-matrix spectra (EEMs) with parallel factor analysis (PARAFAC) to elucidate the influence of DOM characteristics in groundwater systems having contrastive ammonium levels in Dongting Plain, central Yangtze River. The results indicate that NH4+-N concentration in groundwater of western plain (0-16.75 mg/L) are much higher compared with southern plain (0-1.5 mg/L). The groundwater in western plain is in a more reductive environment and characterized by larger molecular weight (MW) of DOM and lower polydispersity (ρ), whereas DOM with relatively small molecular weight and high polydispersity is detected in the south with a more oxidative condition. The groundwater in western plain is characterized by lower fluorescence index (f450/500) and biological index (BIX), and dominated by the high molecular weight terrestrial humic-like component and larger amounts of microbial humic-like components. Protein-like is the main component in groundwater of southern plain with higher f450/500 and BIX. The ammonium concentration in groundwater correlates well with molecular weight and increases significantly with the content of high molecular weight terrestrial humic-like component, indicating that mobilization of ammonium is more closely associated with the terrestrial organic matter of high molecular weight. This study further enriches the theory on mobilization of ammonium in Quaternary alluvial-lacustrine aquifer systems and provides theoretical basis for the local water supply security.

Sources and enrichment of phosphorus in groundwater of the Central Yangtze River Basin.

In addition to the anthropogenic sources for elevated concentrations of phosphorus (P) in groundwater systems, the importance of geogenic enrichment of P warrants attention. To assess factors controlling the sources and enrichment processes of P in Quaternary aquifers, 355 groundwater samples were collected in the Jianghan Plain of central Yangtze River Basin. In the phreatic aquifer, the total dissolved phosphorus (TDP) concentrations range from below detection limit (BDL) to 2.56 mg/L, with 6.25% of samples exceeding 1 mg/L; in the confined aquifer, TDP concentrations range from BDL to 4.31 mg/L, with 22.18% of samples exceeding 1 mg/L. Factor analysis and correlation were used to determine major factors controlling P enrichment in the groundwater. Elevated levels of P in the confined aquifer are related to reductive dissolution of P-rich Fe(III)-(hydr)oxides (FeOOH) as well as organic phosphorus (OP) mineralization. The SEDEX sequential extraction procedure was applied to core samples from two boreholes with high and low P levels in groundwater, respectively, to characterize phosphorus speciation in aquifer sediments. Bioavailable P, particularly exchangeable P, in sediments with high P groundwater are significantly higher than those with low P groundwater. The content of Fe-bound P is higher than that of residual organic P (Res-OP) in sediments from both boreholes, indicating the greater contribution of reductive dissolution of P-rich FeOOH to geogenic P enrichment in groundwater than OP mineralization. Using the Redfield ratio, groundwater samples collected from the confined aquifer can be divided into three groups, with 65% of the samples falling into the group closely related to reduction of FeOOH. The present research provides new insights into the enrichment of geogenic P in groundwater systems, which are not only applicable in the Jianghan Plain, but also to other similar alluvial aquifers in floodplains and delta regions worldwide.