High Hexavalent Chromium Concentration in Groundwater from a Deep Aquifer in the Baiyangdian Basin of the North China Plain

Abstract

Hexavalent chromium (Cr­(VI)) is known to occur naturally in shallow oxic groundwater, typically from aquifers associated with mafic and ultramafic formations, but information on the occurrence of Cr­(VI) in deep groundwater from large sedimentary basins is limited. This study shows that groundwater from the Baiyangdian Lake Basin (BYB), home to the future second capital city of China, had high Cr concentration (>10 μg/L, up to 86 μg/L) in the deep aquifer (>150 m), while shallow groundwater had lower Cr concentration (<10 μg/L). Chromium occurred predominantly as Cr­(VI) (>95%). Shallow groundwater was characterized by higher Mn and Fe concentrations relative to deep groundwater, likely indicating more reducing conditions. Sequential extraction experiments from aquifer sediments suggest that Cr­(VI) may derive from silicate weathering and that Mn oxides in the aquifer play a major role in the formation of Cr­(VI) in groundwater. Inverse correlations between Mn and Cr­(VI) suggest that reductive dissolution of Mn oxides constrains Cr­(VI) mobilization in the shallow groundwater, while oxic–suboxic conditions in the deep aquifer limit Mn solubility, which enhances oxidation of Cr­(III) to Cr­(VI) and promotes desorption of Cr­(VI) under alkaline conditions. This study demonstrates the potential geogenic occurrence of high Cr­(VI) concentration in deep groundwater from a nonmafic, large sedimentary basin containing Mn oxides in the aquifer sediments.

Abundant Fe(III) Oxide‐Bound Arsenic and Depleted Mn Oxides Facilitate Arsenic Enrichment in Groundwater From a Sand‐Gravel Confined Aquifer

“…Other Pleistocene aquifers hosting As‐poor groundwater in the Hetao Basin (Gao, Jia, et al., 2020), Baiyangdian Basin (Guo et al., 2020), Bangladesh (Dhar et al., 2011; Stollenwerk et al., 2007), Cambodia (Gillispie et al., 2016), and Vietnam (Eiche et al., 2008; Neidhardt et al., 2021) had around one to two orders of magnitude higher contents of Mn oxides (up to 10.8 mmol/kg; Table S4 in Supporting Information ) than those in the deep confined aquifers of the study area. High Mn oxide contents may result in the predominant Mn oxide reduction in these aquifers with dissolved Mn concentrations averaged at 20 μmol/L in Vietnam (Eiche et al., 2008) and 36 μmol/L in Bangladesh (Stollenwerk et al., 2007).…”

Overlooked Role of Chromium(V) and Chromium(IV) in Chromium Redox Reactions of Environmental Importance

“… The primary oxidizing agent of Cr­(III) in alkaline and aerobic conditions is MnO x from the birnessite mineral; this is due to its large surface area, high degree of crystallinity, strong electronegativity, and ability to regenerate via the oxidation of Mn­(II) by microorganisms and/or dissolved oxygen. ,− The oxidation of Mn­(II) yielding Mn­(III)/(IV) oxides at alkaline and oxic conditions was shown to accelerate Cr­(VI) production; this was the dominant pathway of Cr­(III) oxidation in the long-term analysis (1 year) compared to oxidation by MnO 2 or O 2 (Figure ). , The accumulation of MnO x in groundwater and subsequent oxidation of Cr­(III) to Cr­(VI) has occurred across the globe, creating drinking water health risks, especially in systems without adequate Cr treatment. − The following reaction indicates the oxidation of Cr­(III) by MnO x species: …”

Effects and bioaccumulation of Cr(III), Cr(VI) and their mixture in the freshwater mussel Corbicula fluminea

“…Chromate ions also show limited reactivity towards environmental particles, which usually bear a net negative charge at pH values above 4 (Warren and Haack, 2001). Chromate ions can therefore undergo long-distance transport and migrate into groundwater (Guo et al, 2020;Izbicki et al, 2015;Oze et al, 2007).…”