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LITHOLOGICAL AND HYDROCHEMICAL FACTORS CONTROLLING THE OCCURRENCE OF HIGH FLUORIDE AND IRON LEVELS IN ESTONIAN AQUIFER SYSTEMS
Abstract
The drinking water demand of most Estonian communities is covered by groundwater abstracted from the sedimentary rocks. Due to the climate and small population, EstoniaпїЅs fresh water resources are sufficient. However, Estonia faces a number of problems with natural groundwater quality in some areas. Fluoride concentrations up to 7 mg/l are recorded in western Estonia and deeper portion of the Silurian-Ordovician aquifer system, which consists of diverse limestone and dolomite. Fluorine contents in carbonate rocks vary from 70 to 1030 mg/kg and the increase of the clay fraction is followed by the increase in the fluorine content in the carbonate rocks. Batch dissolution tests showed that the amount of leached fluoride is proportional to the fluorine content in rock samples. Total iron concentrations as high as 26 mg/l have been determined in terrigenous Middle Devonian aquifer system in places. Those high iron values are caused by high Fe2+ contents, which points to the domination of reducing conditions in groundwater. FeO and Fe2O3 contents in aquifer forming rocks increase by the increasing of the silt and clay content of the terrigenous rocks. However, the highest leached Fe concentrations (up to 1.5 mg/l) were detected from the sandy sedimentary rocks, where large pores enhance the circulation of water and intensive leaching of iron.
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References11
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