Geochemical process of groundwater fluoride evolution along global coastal plains: Evidence from the comparison in seawater intrusion area and soil salinization area

High-fluoride groundwater along coastal plains is frequently documented. The typical geological processes of seawater intrusion and soil salinization both result in conditions of high Na+, HCO3−, Cl−, TDS (Total Dissolved Solid), and the previous related researches argue high-fluoride groundwater is favored in such conditions, together with low Ca2+. But the fluorosis in soil salinization area contrarily does not occur although more seawater mixes, and the region is characterized by higher Na+, Cl−, TDS. Thus, groundwater in Kenli District of soil salinization area and Buzhuang Town of seawater intrusion area was analyzed and compared, to reveal the potential groundwater fluoride evolution. Generally, the groundwater fluoride levels in Kenli District (0.19–1.27 mg/L) are within but those in Buzhuang Town (0.65–9.87 mg/L) are beyond the safe limit. Geochemical properties indicate a direct cation exchange in Kenli District but a reverse cation exchange in Buzhuang Town. The reverse cation exchange results in the decreasing of groundwater Ca2+(Mg2+) and under-saturation of CaF2, MgF2, CaCO3, MgCO3, CaSO4‧2H2O, MgSO4 minerals, which permits more fluoride dissolving and causes high-fluoride groundwater in seawater intrusion areas. However, the direct cation exchange increases groundwater Ca2+(Mg2+) levels to the saturation of CaF2, CaCO3 and MgCO3 minerals, and the high Ca2+ restricts the fluoride dissolution and causes low fluoride groundwater in soil salinization areas. Moreover, the calculated Ca2+, Mg2+ originating from cation exchange (△(Ca2++Mg2+)C) occupies a large proportion because of the high Na+ levels in Kenli District, and is positively correlated with the degree of mixed seawater. The difference of cation exchange processes and orientations in the two areas is confirmed to be the key to groundwater fluoride levels. Moreover, extraordinarily high levels of groundwater Na+ is found to contrarily decrease groundwater fluoride levels because of cation exchange, which is different from the previous observation that Na+ always enhances groundwater fluoride. A new insight into the function of Na+ on groundwater fluoride evolution was proposed. These findings provide some information about groundwater fluoride evolution along coastal plains. •enli District has lower groundwater fluoride than Buzhuang Town although it has higher Na+ and HCO3- levels.•Degree and orientation of cation exchange are the key to groundwater fluorine levels along coastal zones.•H