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Aqueous Geochemistry of the Cambrian–Vendian Aquifer System in the Tallinn Intake, Northern Estonia

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Aqueous Geochemistry of the Cambrian–Vendian Aquifer System in the Tallinn Intake, Northern Estonia GEOLOGIJA. 2005. Nr. 51. P. 50–56 ©50 Lietuvos mokslø akademija, 2005 Robert Mokrik, Lehte Savitskaja, Leonid Savitski © Lietuvos mokslø akademijos leidykla, 2005 Hidrogeologija • Hydrogeology • Ãèäðîãåîëîãèÿ Aqueous geochemistry of the Cambrian–Vendian aquifer system in the Tallinn intake, northern Estonia Robert Mokrik, Mokrik R., Savitskaja L., Savitski L. Aqueous geochemistry of the Cam- brian–Vendian aquifer system in the Tallinn intake, northern Estonia. Lehte Savitskaja, Geologija. Vilnius. 2005. No. 51. P. 50–56. ISSN 1392-110X. Identification of seawater intrusion is based on intake observation Leonid Savitski results in the groundwater chemistry and isotopes. Indications of the sa- line front of marine origin can be obtained from the cation exchange process which is reflected in the groundwater chemical composition by Ca–Cl and Na–HCO3 types of groundwater. The Na–Ca exchange evi- dences are accompanied by sulfate reduction reaction, dissolution and deposition of carbonates, which are also confirmed by the distribution and evolution of content of stable and radioisotopes in the groundwater of the Cambrian–Vendian aquifer system on the Tallinn intake, northern Estonia. Key words: groundwater chemistry, seawater intrusion, cation exchange, chloride and bromide ratio, stable isotopes, Baltic Basin Received 29 March 2005, accepted 16 May 2005 Robert Mokrik. Department of Hydrogeology and Engineering Geology of Vilnius University, M. K. Èiurlionio 21, LT-2009, Vilnius, Lithuania. E- mail: robert. [email protected] Lehte Savitskaja and Leonid Savitski. Geological Survey of Estonia, Ka- daka tee 82, 12618, Tallinn, Estonia. E-mail: [email protected] INTRODUCTION exploitation of the Tallinn intake resulted in saliniza- tion not only in the Cambrian–Vendian sandstones As a result of intensive exploitation, a vast depres- but also in the basement water, which kept brackish sion cone exceeding 100 km has formed at the Tal- groundwater because of seawater intrusion. The sa- linn intake situated in the coastal area of northern linity in the Tallinn intake had been discussed ear- Estonia. The lowest altitude of the potentiometric lier by many authors, but the seawater intrusion was level (more than 30 metres b.s.l) is concentrated at never assessed from the point of view of cation ex- the intake center. In 1977, by observation wells of change processes formed by Ca–Cl and Na–HCO3 the Tallinn intake, a fast increase in total dissolved types of groundwater. Based on hydrochemical and solids (TDS) was detected. Chemical and isotopic isotopic data, it is concluded that the recent seawa- data indicated that many factors control the rise in ter intrusion into the aquifer system previously pala- groundwater salinity. The resources of coastal ground- eorecharged by glacial water is generally accompa- water of the Cambrian–Vendian aquifer system are nied by ion exchange which caused the present fresh limited and consist mainly of natural reserves for- and saline groundwater distribution in the Tallinn med during the last glaciation. The long-time over- intake (northern Estonia). Aqueous geochemistry of the Cambrian–Vendian aquifer system in the Tallinn intake, northern Estonia 51 Increased salinity since 1977 has been observed sis within the aquifers and between the flow system in many wells drilled into the Cambrian–Vendian boundaries. The objectives of the Cambrian–Vendian aquifer system at Kopli Peninsula site of the Tallinn aquifer system study were: (1) to delineate the ex- intake. In earlier studies, several attempts have been tent of fresh to saline water in the various hydroge- made to determine the origin of salinization. One ological units and in basement rocks, (2) to analyze opinion is that salinization is attributed to many sour- the human impact on groundwater chemistry and on ces, such as brackish groundwater upcoming from the flow system in the coastal intake, and (3) to the basement and seawater intrusion (Mokrik et al., assess the current groundwater isotope distribution 1987). According others, the detectable intrusion of and to identify the trends of these changes. modern seawater into the aquifer system should be Equilibrium thermodynamics of groundwater was ruled out (Savitski and Belkina, 1985; Yezhova et interpreted using the PHREEQUE Interactive 2.8 al., 1996; Karro et al., 2004). Thus, the origin of computer code program which calculates mineral sa- fresh groundwater in the Cambrian–Vendian aquifer turation states for an aqueous solution by the che- system as well as the brackish basement water is not mical analysis and physical parameters relative to car- clear. bonate minerals and other species. METHODS AND PURPOSE DISCUSSION The basic approach of the study was to collect and Problems and objectives analyze previously published reports and articles and In 1977–1978, hydrochemical observations of ground- to use the new interpretative data on groundwater water in Kopli Peninsula on the Tallinn intake fixed chemistry to define their origin and assess the hu- an increase in water salinity (Fig. 1). Here ground- man impact on water geochemistry in the Tallinn water contained higher concentrations of TDS, chlo- intake. All these data were incorporated into water ride, sulfate, sodium, calcium and other constituents chemistry database analysis to draw up a more com- relative to the area farther from the coast line and prehensive vision of groundwater formation and to buried valleys. In the other sites, the background provide an improved estimate to a qualitative analy- chloride concentration of fresh groundwater was about 20–150 mg/l. The most intensive growing of TDS was observed in the Kopli site boreholes 632 and 625, where it reached up to 1.1 g/l at a growth rate 16–70 mg/l per year. In the basement borehole 798, chloride content in 1995–2004 has been gro- wing more rapidly (from 826 to 2367 mg/l). Seawa- ter percolation occurs through the buried valleys, which are largely developed on the coastal and seaside submarine parts and con- nected with the sea hydrau- lically. It is difficult to iden- tify sea water intrusion he- re, because many chemical Fig. 1. Map of the study area long Tallin intake, northern Estonia. types of water spread and 1 – observation borehole; 2 – potentiometric contour, m b.s.l.; 3 – location scheme of the isotopic homogeneity is the Kopli Peninsula; 4 – contour of the buried valley. On the location scheme: 1 – disturbed. Salinization of observation borehole, numerator – number of the well, denominator – Cl/Br ratio; 2 aquifers is a result of fresh – Cl/Br contour; 3 – boundary of the buried valley and seawater mixing and is 1 pav. Talino vandenvietës (Ðiaurës Estija) tirtø plotø schema: coupled with the ion ex- 1 – stebëjimo græþinys, 2 – pjezometrinis paviršius, 3 – Kopli pusiasalio schema, 4 – change process. The water– paleoárëþis. Kopli pusiasalio schemos sutartiniai þenklai: 1 – stebëjimo græþinio ëminys; rock interaction processes skaitiklyje – græþinio numeris, vardiklyje – Cl/Br santykis; 2 – Cl/Br santykio izolinija; are often masked by salini- 3 – paleoárëþis zation in the boreholes set- 52 Robert Mokrik, Lehte Savitskaja, Leonid Savitski ting up to the basal level of aquifers. Previous stu- changed against Ca2+ ions adsorbed on the clays and, dies on the ground of the stable isotopes and Br/Cl vice versa, the reverse process takes place with re- ratios have shown that groundwater salinity in the freshening, when fresh water flushes in saline seawa- Tallinn intake is influenced only by basement water ter (Appelo, Geirnaert, 1983; Appelo, Postma, 1993). and has no relation with the seawater intrusion pro- As a result of the sea and fresh water mixing pro- cess (Savitski and Belkina 1985; Yezhova et al., 1996; cess via Na–Ca cation exchange reaction, seawater Karro et al., 2004). Another opinion is that the se- becomes saline of Ca-Cl type and fresh water of awater intrusion has influenced the salinity increase Na–HCO3 type. When Ca–HCO3 type groundwater (Gregorauskas et al., 1988; Mokrik, 1997). The lat- mixes with Na–Cl type water, the calcite dissolution ter opinion was based on the observation of changes is driven by subsaturation due to loss of Ca2+ from 2– 2+ – of the characteristic coefficient SO4 + Mg / Cl . solution and formation of NaHCO3 and CaCl2 type This coefficient value in the Gulf of Finland water groundwater (Appelo, Postma 1993): equals to 3 · 10–1, in the fresh groundwater of the Cambrian–Vendian aquifer system on Kopli Peninsu- 2NaCl + Ca(HCO3)2 = 2NaHCO3 + CaCl2. (1) la site from 2 · 10–1 to 2.5 · 10–1, and in the base- –2 –1 ment water ranges between 5 · 10 – 1.5 · 10 . If In the Na–HCO3 type groundwater, characteristic is – on the investigated site there is a suction of seawa- an increase of HCO3 , when water pH becomes high ter salinity, the values of the characteristic coeffi- because of the evident carbonate dilution process. cient must increase by seawater intrusion, together The Ca–Cl type water is the result of carbonate pre- with growth in groundwater TDS values. And vice cipitation driven by enrichment of solution after ca- versa, if there is a suction of saline water from the tion exchange. Dissolution of carbonates moves the crystalline basement, together with the increase in mixture on the Piper diamond towards the Na+ +K+ – 2+ 2+ – groundwater TDS its value will decrease. – HCO3 and Ca + Mg – HCO3 corners. At the The low 14C concentration (below 3–4 pmC) could beginning of Na–Ca cation exchange intensities re- suggest that the age of groundwater corrected by 13C sulted increase in the Ca2+ content and that effects is about 26–34 ka BP, indicating that a recharge be- a lowering of pH and solution due to subsaturated cause of the cold climate conditions took place be- with respect to dolomite and calcite, which move 2– fore the Late Weichselian, during the Denekamp In- the changes in groundwater chemistry into SO4 + terstadial time, when the Estonian territory was not Cl– and Ca2+ + Mg2+ corner.
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