Assessing the controls of the fluoride concentrations in the groundwater of Gothenburg area

Abstract: The human uptake of fluoride can promote healthy teeth in low concentration (<1.5 mg/l) or cause fluorosis when concentration is high (>1.5 mg/l). Drinking water is a major route to which we humans are expose to fluoride intake. The work presented in this thesis was carried out along the west coast of the Gothenburg area exploring the sources of fluoride in groundwater, the controls the fluoride groundwater concentration and its risks for human health. To this end, we used the water analysis from 2537 private wells and evaluated the relation of their fluoride content to the geological units. Higher concentrations of fluoride in groundwater were related to basement rocks especially granitic rocks, whereas the dissolution of the fluoride-bearing biotite, hornblende, and fluorite were mainly responsible for the fluoride accumulation in groundwater. The concentration of fluoride in the collected samples ranged between 0 mg/l to 16 mg/l with most of the samples ranging mainly between 0 - 8.9 mg/l with a small number of samples ranging between 10 - 16 mg/l. At least 17% of the household were at risk of mild to severe fluorosis upon consumption of water that contain fluoride concentration higher than the 1.5mg/l limit. A high number of private wells located in the Bohus granite showed to contain high fluoride concentration. This high fluoride concentration was shown to be associated with high radioactive measurements of uranium and thorium because of their late crystallisation during the magma fractionating process. Moreover, calculations of saturation indices for fluoride-bearing minerals using PHREEQC revealed that fluorite was the only mineral at or close to saturation. This showed that the maximal fluoride concentration in groundwater in the Gothenburg area is likely solubility controlled by the mineral fluorite in all different rock units.