Sediment geochemistry and its influence on chromium enrichment in porewater from a deep aquifer in the Baiyangdian Basin, China

摘要

Purpose High chromium (Cr) in groundwater is a potential environmental issue worldwide that seriously threatens people's health. However, which Cr forms and species in aquifer solids control Cr mobility in the sedimentary aquifer is poorly understood. We hypothesize that Cr(VI) species, Cr(III) oxidation potential, and the potential of Cr(VI) release in aquifer sediments regulate dissolved Cr in aquifer systems. In this study, we have chosen the Baiyangdian Basin as the study area to investigate this hypothesis. Materials and methods Thirty-one samples were collected from a borehole with a depth of 600 m in the study area for geochemical analysis. In addition to physicochemical characteristics of sediments, the Cr(VI) release potential and Cr(III) oxidation potential in sediments were analyzed. Results and discussion Results revealed that total Cr contents in sediments ranged from 4 to 192 mu g g(-1) and total hexavalent chromium Cr(VI) contents accounted for 39.5 (average) of total Cr contents. The partition coefficients (K-d) of Cr, the ratio of adsorbed Cr in sediments to dissolved Cr in depth-matched porewater, were evidently higher in shallow aquifer systems (0.04-0.15 L g(-1)) than in deep aquifer systems (0.02-0.10 L g(-1)) hosting high Cr(VI) groundwater. Cr(VI) in sediments can be naturally generated and released into groundwater, especially at depths between 300 and 430 m below land surface with high concentrations of dissolved Cr(VI). Strong positive correlations were found between dissolved Cr in porewater and the potential of trivalent chromium Cr(III) oxidation or the potential of Cr(VI) release in sediments, indicating that Cr(III) oxidation and Cr(VI) release were conducive to the enrichment of dissolved Cr in porewater. Sediments in deep aquifers with weakly alkaline pH were also favorable for Cr(III) oxidation and Cr(VI) release. Conclusions Cr tended to be enriched in sediments in the relatively reducing shallow aquifer, while in the oxic deep aquifer, Cr was enriched in porewaters. Concentrations of dissolved Cr in porewater were positively related with adsorbed Cr forms, the oxidation potential of Cr(III), and the release potential of original Cr(VI) in aquifer sediments. This study provides new insights into mechanisms of Cr mobility in non-mafic, deep sedimentary aquifers.