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

Abstract

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.