Supervisor: TÜ ÖMI vanemteadur Enn Karro, PhD
Opponent: Timo Tarvainen, Soome Geoloogia Uurimiskeskus, vanemteadur, PhD
Health problems related to drinking water in Estonia are mainly caused by the high levels of fluoride and boron in groundwater. Low concentrations of fluoride (below 0.5 mg/l) in drinking water have a beneficial effect on dental health by reducing the incidence of caries, and long-term exposure to excessive fluoride (above 1.5 mg/l) may cause dental and skeletal fluorosis. Excessive concentrations of boron (above 1.0 mg/l) in drinking water lead to malfunctioning of the reproductive system, low fetal weights; also, incidences of metabolistic disorders and acute neurological effects have been observed. The purpose of this study is to examine distribution, hydrochemical behavior and geological sources of fluorides and boron in Silurian-Ordovician aquifer system, which is an important drinking water source in Estonia. High fluoride (up to 6.1 mg/l) and boron (up to 2.1 mg/l) concentrations are mostly recorded in western Estonia and deeper portion of the aquifer system, where groundwater chemical type is HCO3-Cl-Na-Mg-Ca, water is alkaline, and its Ca2+ content is low. The aquifer system consists of diverse limestone and dolomite with K-bentonite interlayers composed of altered volcanic material. Leaching of the host rock are considered to be the main source of high fluoride and boron concentrations in groundwater. Fluorine and boron contents in limestones and dolomites vary from 100 to 500 mg/kg and 5 to 20 mg/kg, reaching up to 1000 mg/kg and 150 mg/kg in marlstones, respectively. K-bentonites are also rich in fluorine (400-4500 mg/kg) and boron (50-1000 mg/kg). Generally, the increase of the clay fraction is followed by the increase in the fluorine and boron contents in the carbonate rocks. Further, batch dissolution tests with different carbonate rocks were performed in order to examine the dissolution of fluoride and boron. The results of the tests showed that the amount of leached fluoride and boron is proportional to the content of these elements in the rock samples and higher amounts of both elements were rather leached out from rock samples rich in clay minerals. Thus, clay rich sediments, providing ion-exchange and adsorption sites for fluoride and boron, are the probable sources of fluoride and boron in Silurian-Ordovician aquifer system in western Estonia.