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

摘要

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 ClSO4-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 O-18 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. delta C-13 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.