Enrichment mechanism of fluoride and iodine in saline groundwater in the lower flood plain of the Yellow River, northern China

To identify the enrichment mechanism of fluoride and iodine in saline groundwater and reveal the impact of salinization on it, hydrogeochemical studies were conducted in different zones from the lower flood plain of the Yellow River, China. The results showed that high-F and high-I groundwater is mainly distributed in the pale-o-Yellow River alluvial plain (Zone 2) and alluvial-lacustrine plain (Zone 3) rather than the piedmont allu-vial-proluvial plain (Zone 1), with exceedances of groundwater guidelines reaching up to 57.69 %-80.77 %. The high-F and high-I groundwater is characterized by high total dissolved solids (TDS) concentrations, which ranged between 475 and 7002 mg/L, with 47 % and 77 % of groundwater samples from Zone 2 and Zone 3 having TDS over 1000 mg/L, respectively. Groundwater F- mainly originated from geogenic sources of fluorite dissolution and silicate weathering, and its release was promoted by alkaline conditions, cation exchange processes, and the competitive adsorption of HCO3 -. With increasing TDS concentration in the groundwater, F- concentration showed a trend of first increasing and then slightly decreasing. Furthermore, the Cl/Br molar ratio in ground-water and Gibbs diagram indicated that water-rock interactions could lead to F- enrichment in slightly saline water (TDS < 3000 mg/L), and strong evaporation would restrict F- concentration in saline water (TDS > 3000 mg/L) due to the limitation of fluorite solubility. In contrast, no close correlation between I- and TDS was observed, indicating that salinization of groundwater is not the dominant reason for I- enrichment. In Zone 2 and Zone 3, large amounts of alluvial-lacustrine sediments with abundant organic matter are probably the reason for high-I groundwater, which is supported by the significantly positive correlation between I- and Br-. The reduction of iodate and nitrate could be the direct cause of elevated I- concentration in groundwater. In addition, results of factor analysis further confirm contrasting behaviors of fluoride and iodine in saline groundwater.