Chemical peculiarities of the Silurian–Ordovician and Cambrian–Vendian aquifer systems in Estonia an overview of hydrochemical.docx

E. Karro et aL:Che mical peculiarities of groundwaterChemical peculiarities of the Silurian - Ordovician and Cambrian- Vendianaquifer systems in Estonia: an overview of hydroche mical studiesEnn Karro, Andres Marandi , Rein Vaikmae , and Marge Uppinaa bbail Department of Geology, University of Tartu Vanemuise 46 51014 Tartu Estonia; ena karro ut andre&marandi utemargel3ut.ee'Institute of Geology at Tallinn University of Technology, Ehitajate tee 5 19086 Tallinn, Estonia; rein Vciikmae ttu eeReceived 18 June 2009, accepted 21 September 2009A bstract The chemical composition of ground water was studied and plausible reasons for its peculiarities were evaluated on thebasis of' longten ground water m o nitoring data Fluoride and boron anomaly was distinguished in the Silnian-Ordovician aquifersyste in western Estonia The cheiccil type of ground water controls the dissolution of fluorides cind boron in water Theirconcentrcitions cire highest in deep groundwater having long residence ti me in the hosl rocks. The dissolution ol'caonate rocksand clayey K-bentonite beds are the sources of F in groundwater Barium anomaly in the Cambrian- Vendian aquifer systemoccurs in northeastern Estonia, where the intrusion of saline ground water from the weathered crystalline basement contributes tothe formation of Badch groundwater The overexploitation of fi'esh ground water has resulted in the increase in ground watersalinit¾ howeve detectable intsion of seawater into the Cambnan- Vendian aquifer system is led out by the stable isotopesignatureKey words: fluoride boron, isotopes, bariumt Silurian- Ordovician aquifer system, Cambrian- Vendian aquifer system.347INTRODUCTIONGround water is one of the most important, naturalresources in Estonia as it provides two thirds of thedrinking and domestic water supply; It is the drinkingwater source for most of Estonid stowns and settlementexcept Tallinn and Narva where ground water resourcesare limited and therefore surface water is mainly usedApproximately 75% of the inhabitants use the publicwater supply; the rest obtain their water fi m shallowdrilled or dug wells During the last 15 years the waterconsumption has reduced by more than a half and iscurrently 50 million m3 per year (Narusk & Jtirimagi2008). The leason for this is primarily decrease inindustrial production, and increase in the price of water;resulting in a more sustainable usage of drinking waterby the population and the reduction of leakage fromthe water supply systems. Owing to decreasing waterconsumption, more attention can be paid to water qualityproblems.The che mical in position of ground water abstractedfor drinking purposes is generally closely related tolocal geology. The ground water chemistry is controlledamong other factory by the lithological composition ofwater-bearing rocks and geochemical processes thatoccur in the saturated zone Besides, the water quality isinfluenced by the intrusion of present-day seawater intocoastal areas and pumping-induced upward inigration ofdeeper saline ground water in deeprseated aquifer systems.These processes lead to an increase in the ntent ofcelain dissolved chemical 印ecies> as well as to theoverall change in chemical wata' composition Thus, thegeochemical peculiarities of water-bearing rocks andpumping-induced ground water deterioration may locallyrestrict the use of ground water as a source of drinkingwater suppl½ The purpose of this paper is to summarizeand present, the main outcomes of the hydrochemicalstudies perfbnned by the authors in 2001 - 09.GEOLOGICAL SETTING ANDHYDROGEOLOGYEstonia is situated in the northwestern part of the EastEuropean Platform. Its sedimentary beds, lying on thesouthern slope of the Baltic Shield, are declined south-wards at about 2-4 m per kilo metre The Estoniancrystalline Palaeoproterozoic basement is overlain byNeoproterozoic (Vendian) and Palaeozoic (Cambrian,Ordovician, Silurian, and Devonian) sedi mentary rks(Fig 1) covered by Quaternary deposits (Raukas &Teedumae 1997).11 ydrogeologically Estonian sedi mentary rocks forma typical artesian basia where five aquifer systems(Middle Devonian Middle-Lower Devonian, Silurian-Ordovician, Ordovician- Cambrian, and Cambrian-Fig L (ci) Scheicitic map show ing the localion of Estonia (b) Geological map of Estonia with the position of the line ol* the crosssection (c) Nolh-south cross section of major hydrogeological units of Estonia 1, Voronka aquifer; 2, Kotlin aquitard; 3 GdovaquiferVendian) are isolated from each other by imperviousbeds (Fig 1). The aquifer systems differ fix) n each otherin distribution, bedding conditions hydciulic parameterand chemical mposition Quaternary deposits consistingpredo minantly of glacial till and glaciolacustrine andglaciofluvial xnd form the uppermost aquif