Silurian-Ordovician (S-O) aquifer system is an important drinking water source of central and western Estonia. The fluoride and boron contents of groundwater in aquifer system vary considerably. The fluoride concentration in 60 collected groundwater samples ranged from 0.1 to 6.1 mg/l with a mean of 1.95 mg/l in the study area. Boron content in groundwater varied from 0.05 mg/l to 2.1 mg/l with a mean value of 0.66 mg/l. Considering the requirements of EU Directive 98/83/EC and the Estonian requirements for drinking water quality, the limit value for fluoride (1.5 mg/l) and for boron (1.0 mg/l) is exceeded in 47 and 28 % of wells, respectively. Groundwater with high fluoride and boron concentrations is found mainly in western Estonia and deeper portion of aquifer system, where groundwater chemical type is HCO3-Cl-Na-Mg-Ca, water is alkaline, and its Ca(2+) content is low. Groundwater of the study area is undersaturated with respect to fluorite and near to equilibrium phase with respect to calcite. The comparison of TDS versus Na/(Na + Ca) and Cl/(Cl + HCO3) points to the dominance of rock weathering as the main process, which promotes the availability of fluoride and boron in the groundwater. The geological sources of B in S-O aquifer system have not been studied so far, but the dissolution of fluorides from carbonate rocks (F = 100-400 mg/kg) and K-bentonites (F = 2,800-4,500 mg/kg) contributes to the formation of F-rich groundwater.

The chemical composition of groundwater was studied and plausible reasons for its peculiarities were evaluated on the basis of long-term groundwater monitoring data. Fluoride and boron anomaly was distinguished in the Silurian-Ordovician aquifer system in western Estonia. The chemical type of groundwater controls the dissolution of fluorides and boron in water. Their concentrations are highest in deep groundwater having long residence time in the host rocks. The dissolution of carbonate rocks and clayey K-bentonite beds are the sources of F in groundwater. Barium anomaly in the Cambrian-Vendian aquifer system occurs in northeastern Estonia, where the intrusion of saline groundwater from the weathered crystalline basement contributes to the formation of Ba-rich groundwater. The overexploitation of fresh groundwater has resulted in the increase in groundwater salinity, however, detectable intrusion of seawater into the Cambrian-Vendian aquifer system is ruled out by the stable isotope signature.

The objective of this study was to examine the possible natural sources of fluorides and boron in SilurianOrdovician (S-O) aquifer system, as the anomaly of these elements has been distinguished in groundwater of western Estonia. Water-rock interactions, such as dissolution and leaching of the host rock, are considered to be the main source of high fluoride and boron concentrations in groundwater. Altogether 91 rock samples were analysed to determine if high F -and B levels in groundwater could be attributed to certain aquifer forming rock types. Fluorine and boron contents in limestones and dolomites vary from 100 to 500 mg/kg and 5 to 20 mg/kg, reaching up to 1,000 and 150 mg/kg in marlstones, respectively. K-bentonites, altered volcanic ash beds, are rich in fluorine (400-4,500 mg/kg) and boron (50-1,000 mg/kg). Thus, clay-rich sediments, providing ion-exchange and adsorption sites for F -and B, are the probable sources of both elements in S-O aquifer system in western Estonia.

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High natural fluoride (up to 6.1 mg/l) and boron (up to 2.1 mg/l) concentrations in groundwater are recorded in western Estonia, where a carbonaceous Silurian-Ordovician aquifer system is the main source of drinking water. The aim of this study is to examine the dissolution of boron and fluoride from different carbonate rocks using batch dissolution tests. The total boron and fluorine concentration within carbonate rocks varies between 100-1000 and 5-150 mg/kg, respectively. Generally, boron and fluoride concentrations increase as the amount of terrigenous material in the rock increases. Laboratory leaching experiments showed that the amount of leached boron and fluoride was proportional to the content of these elements in the rock samples. Further, boron and fluoride concentrations in leachates were positively correlated with the amount of terrigenous material in rocks. Both boron and fluoride concentrations in leachates continually rose over 49 days of leaching. Long-term water-rock interaction is responsible for the high boron and fluoride contents in groundwater of western Estonia.