Geochemical controls on fluoriferous groundwaters of the Pliocene and the more recent aquifers: The case of Aigion region, Greece

Abstract

High fluoride concentrations (>8 mg/L) in the groundwater of the Plio-Pleistocene sediments are rare; however, this is the case around Aigion town, where teeth fluorosis has been detected since the 80s. Aiming to investigate the origin and the mobility mechanism of fluorine in groundwater and sediments a hydrogeological and geochemical research has been conducted. The hydrogeological research revealed that the Na-HCO3 water type of boreholes aligned along a fault and hosted in the confined aquifers display the higher fluoride content. The unconfined aquifer is mostly dominated by Na-Ca-Mg-HCO3-SO4 water, which displays much lower fluoride concentrations. The most permeable sectors of this aquifer host fresh water of Ca-HCO3 type. The geochemical research revealed significant amount of fluorine in the base of a lignite sequence hosted in the Plio-Pleistocene sediments. The Na-HCO3 water type represents a deep water circulation, proved by its elevated temperature and isotopic composition, carrying a significant load of fluorine and trace elements. During its uprise through faults and the Plio-Pleistocene sediments, part of its load is being deposited on the base of the lignite beds. Leaching experiments and mineralogical determinations in lignite samples revealed enrichment in fluorine at the bottom of the studied lignite sequence, where silicate minerals are dominant. This is due to either, the presence of F-bearing minerals or the presence of organic matter which can also absorb/adsorb fluorine. At the upper part of the sequence, where carbonates prevail, calcite is the major phase indicating the circulation of Ca-ΗCO3-rich fresh water. The Na-Ca-Mg-HCO3-SO4 water type could be considered as mixed water spending longer time in sediments of relatively low permeability and thus, dissolving fluorine and trace elements from clay minerals and organic matter.