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.