Fluoride-leaching simulation of aquifer sediment and its influence on groundwater fluoride levels along coastal plains

Abstract

Seawater intrusion and drinking-water fluorosis are frequently documented along coastal plains. Groundwater is characterized by high OH−, Na+, total dissolved solids, and low Ca2+ because of seawater intrusion, and such conditions favor sediment fluoride-leaching and fluorosis. But the geological process of seawater intrusion has not been noticed when high-fluoride groundwater along coastal areas is discussed. The groundwater and sediments in a typical seawater intrusion and fluorosis area are gained, and fluoride-leaching simulation experiments are performed. Sediment fluoride levels are equal to or lower than average sediment fluoride levels in China and Shandong province, but strong fluoride-leaching in aquifers is observed. Compared with the supplied water from the non-intruded neighboring area, the local groundwater has higher fluoride levels, together with higher pH, total dissolved solids, HCO3−, Cl−, Br−, SO42−, Na+, K+, and low Ca2+ because of seawater intrusion. Aquifer sediment fluoride-leaching ability increases with an increase in seawater (brine water) mixing ratios, and NaCl or NaHCO3 levels, but with a decrease in CaCl2 levels. This directly confirms that seawater (brine water) intrusion promotes sediment fluoride-leaching, and the high pH, Na+, HCO3−, and low Ca2+ levels caused by seawater intrusion are responsible for the high-fluoride groundwater along coastal plains.

Design of an Exploratory Experiment in Teaching for Engineering Education Accreditation: Fluoride Geochemical Simulation during Water–Rock Interactions under the Effect of Seawater Intrusion

“…As shown in Figure 2, the amounts of leached fluorine from soil were ranked as follows: seawater > 1:1 freshwater and seawater > freshwater; brine water > 1:1 freshwater and brine water > freshwater. These results intuitively confirm that fluorine leaching from soil increases with increasing proportions of seawater or brine water, which directly reveals the effect of seawater (brine water) intrusion on groundwater fluoride levels [37]. The following conclusions were reached by comparing fluorine leaching in different types of solutions.…”

“…The leached fluoride amount in 1 mol/L NaCl and Na-HCO3 solutions was greater than that in 0.1 mol/L NaCl and NaHCO3, indicating that the increased NaCl and NaHCO3 levels promoted fluorine leaching from the soil. However, the leached fluoride in the 1 mol/L CaCl2 solution was less than that in the 0.1 mol/L CaCl2 solution, indicating that the increased CaCl2 levels restrict fluoride leaching from the soil [37]. CaF2 is an insoluble compound, and its solubility deeply governs fluoride levels in water [27,31].…”