Hydrogeochemistry of fluoride in shallow groundwater of the abandoned Yellow River delta, China

Fluorosis is the most widespread and serious endemic disease in the abandoned Yellow River flooding delta of East China. One of the important causes of fluorosis is drinking high-fluorine groundwater. In this study, 313 groundwater samples were collected in the central part of the flooding delta to address the chemistry of high-fluoride groundwater, and 33 core samples were collected from one borehole of 150 m depth to analyse the vertical distribution characteristics of total fluorine and water-soluble fluorine. The fluorine concentration in groundwater ranges from 0.2 to 6.7 mg/L, and 72.8% of the samples have fluorine above the China maximum permissible limit of 1.0 mg/L for drinking water. These 313 samples can be divided into nine hydrochemical subtypes, and over 77% of the samples belong to the bicarbonate types. High-fluorine groundwater (over 3.0 mg/L) is generally alkaline water with high HCO3− and low Ca2+. The concentration of water-soluble fluorine decreases gradually with the increase of formation depth, and that in vertical sediment is negatively correlated with Ca2+ and Cl−, but positively correlated with HCO3−. According to the calculation by PHREEQC package, MgF+ and CaF+ are the dominant species controlling the endemic fluorosis of the study area.