Occurrence and formation of high fluoride groundwater in the Hengshui area of the North China Plain

Abstract

Although high F⁻ groundwater was observed in the Hengshui of the North China Plain, the depth dependence of groundwater F⁻ and its relation to aquifer sediments remains unknown. Both shallow groundwater and deep groundwater were collected for chemical and isotopic characterization. Sediments were taken from different depths down to 130 m for total element analysis and F⁻ form determination with sequential extraction procedure. Results show that F⁻ concentration ranges from 0.37 to 3.28 mg/L, and from 0.28 to 3.6 mg/L in shallow and deep groundwater, respectively. High F⁻ water from shallow aquifers are of Cl–SO₄–Na–Mg, HCO₃–Cl–SO₄–Na and SO₄–Cl–Na–Mg types, while HCO₃–SO₄–Cl–Na and Cl–SO₄–Na types are from deep aquifers. High F⁻ waters are mainly found at the depths of 50, 200 and 300 m. Along the flow path, groundwater shows an increasing trend in F⁻ concentration from the northwest to the southeast. Isotopes of ¹⁸O and D in deep groundwater are more depleted in comparison with shallow groundwater, suggesting that shallow and deep groundwater replenish through different ways with longer retention time of deep groundwater. δ¹³C of dissolved inorganic carbon are between −11.9 and −8.8 ‰ in shallow groundwater and between −10.6 and −7.5 ‰ in deep groundwater, respectively, which indicates that groundwater DIC comes from both rock weathering and biodegradation of organic matter. Fluoride content in the sediment ranges from 140 to 1690 mg/kg, showing a decreasing trend with depth. Good correlations between F⁻ and Al₂O₃ and Fe₂O₃ are observed, demonstrating that F⁻ contents in sediments are significantly influenced by the minerals containing Fe₂O₃ and Al₂O₃ in terms of adsorption. Sequential extraction procedure shows that exchangeable F⁻ form (F1) and Fe–Mn oxides-bound F⁻ (F3) generally decrease with depths, while organic matter or sulfide-bound F⁻ (F4) keeps relatively stable and carbonate-bound F⁻ (F2) exhibits highly variable. The relationships between F⁻ concentration and Fe or Mn imply that F⁻ in F1 is more inclined to be scavenged by Fe and Mn oxides/hydroxides which have a stronger affinity for F⁻. Although a variety of hydrogeochemical processes affected F⁻ concentrations, dissolution–precipitation is a vital process in the study area. In comparison with shallow groundwater, cation exchange may exhibit more significantly effect on F⁻ enrichment in deep groundwater. Competitive adsorption of HCO₃ ⁻ and OH⁻ with F⁻ leads to the release of F⁻ from aquifer matrix into solution, which increases groundwater F⁻ concentration. Evaporation is another control on F⁻ concentration, especially in shallow groundwater.