Bulletin of the Geological Society of Finland, Vol. 75 (1–2) pp. 17–27 Mapping of potentially hazardous elements in the Cambrian– Vendian aquifer system, northern Estonia Enn Karro* and Andres Marandi Institute of Geology, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia Abstract Communities and towns in northern Estonia rely upon either surface waters or ground- water from Cambrian–Vendian aquifer system with limited fresh water resources. The composition of groundwater is controlled, among other factors, by the lithologic compo- sition of water-bearing rocks and sediments. The peculiarities of the rock matrix may local- ly restrict the use of groundwater as a source of water supply. The distribution of fl uorine, barium and arsenic has been studied in northern Estonian water supply wells. The results of the hydrogeochemical mapping enable to delimit the barium anomaly in the Cambri- an–Vendian aquifer system. Vertical distribution of studied compounds and hydraulic con- nection between water-bearing Vendian sediments with underlying crystalline basement refers the weathered basement rocks as the plausible source of barium and fl uorine in groundwater. The elevated concentrations of barium and fl uorine detected in groundwa- ter from North Estonian wells may become a source of growing concern in Estonia. The results of hydrochemical research should be transformed to instructions how to avoid or eliminate the quality problems in the risk areas. Key words: groundwater, aquifers, fl uorine, barium, arsenic, bedrock, Cambrian, Ven dian, Estonia 1. Introduction First regional study of micro compounds in United States (US EPA) has set the maximum the Cambrian-Vendian aquifer system in Es- concentration limit (MCL) for barium in the tonia was performed in 1994, but did not in- drinking water at 2.0 mg/l. World Health Or- clude barium (Savitskaja & Viigand, 1994). Ac- ganisation has published a guideline value 0.7 cording to later fi ndings (Otsa & Tamm, 1997; mg/l for barium (WHO, 1996). According to Tamm, 1998), barium concentration exceeded EPA, if people are exposed to barium at levels the limit value of the previous Estonian drink- above the MCL for relatively short periods of ing water standard 0.7 mg/l (Joogivesi, 1995) time, gastrointestinal disturbances and muscu- in several locations, with maximum concentra- lar weakness may result. A lifetime exposure at tions (9 mg/l) found in the water supply of Koh- levels above MCL may lead to high blood pres- tla-Järve town. sure diseases. If barium levels were found to be New drinking water standard (Joogivee, consistently above the MCL, the water suppliers 2001), which is compatible with the Drink- had to use water treatment methods for barium ing Water Directive of European Union (98/ removal (EPA, 1995). 83/EEC) does not limit barium concentration. Fluorine is an important microelement, However, it cannot be supposed that the EU needed by man and higher animals as a constit- standard does not consider barium being tox- uent of teeth and bones. Unlike most of the oth- ic. The Environmental Protection Agency of the er essential elements, a large portion of fl uorine is ingested from drinking water. Health prob- * Corresponding author lems arising from excess fl uoride intake (dental e-mail: [email protected] fl uorosis) have been recorded in Estonia (Saava, 18 Karro, E. and Marandi, A. In study area (Fig. 1), the Vendian, Cam- Helsinki brian and Ordovician rocks are the only sedi- St. 59o00’ mentary rocks covering the crystalline basement Tallinn Petersburg Stockholm w E which lies approximately in the depth of 150 m (Fig. 2). The crystalline basement consisting Baltic Sea mostly of gneisses and biotite gneisses (Koisti- o 58 00’ nen et al., 1996) is in its upper 10–150 m thick portion fractured and weathered. Weathering 150 km profi les are composed predominantly of kao- 22o00’ Riga 27o00’ linite, illite, chlorite and montmorillonite, de- Fig. 1. Location of the study area and the line of hydro- pending on original bedrock composition and geological cross-section (W-E) intensity of weathering. Weathered basement rocks are overlaid by water-bearing Vendian and Cambrian silt- and 1998; Russak et al., 2002). On the other hand, sandstones, which form the Cambrian–Vendian in most of the North-Estonian wells F- concen- aquifer system. In eastern part of Estonia Ven- trations in drinking water are below the lim- dian sedimentary rocks are divided by clay lay- it value (Savitskaja & Viigand, 1994). Howev- er of Kotlin Formation (V kt) into the Voronka 2 er, the occurrence and hydrochemistry of fl uo- (V vr) and Gdov (V gd) aquifers (Fig. 2). The 2 2 rine and arsenic in the Cambrian–Vendian aq- upper, the Voronka aquifer, consists of quartzose uifer system is incompletely studied so far, and sand- and siltstones having hydraulic conduc- needs more detail approach in terms of sam- tivity of 0.6 to 12.5 m/d and transmissivity of pling density as well as hydrogeochemical in- 100–150 m2/d. The lower, the Gdov aquifer, is terpretations. formed of mixed-grained sand- and siltstones, The aim of this study is to map the areal dis- the conductivity of which varies 0.5–9.2 m/d tribution of Ba2+, As and F-, and fi nd the ge- while the transmissivity is about 300 m2/d (Per- ological sources for the probable anomalies of ens & Vallner, 1997). these compounds in the Cambrian–Vendian aq- The Cambrian–Vendian aquifer system is uifer system in northern Estonia. The results of covered by clays of Lower Cambrian Lonto- the current study contribute to the understand- va Formation, which form laterally continuous ing of the reasons of natural groundwater con- Lontova aquitard. This aquitard has a strong tamination and help to work out the strategies isolation capacity, as its transversal conductiv- for water supply. ity is predominantly 10-7–10-5 m/d (Perens & Vallner, 1997). Lontova clays have the thick- 2. General geological and hydrogeo- ness of 60–90 m in northeastern Estonia (Fig. logical setting 2). Westwards from Tallinn the Lontova Forma- tion is gradually replaced by interbedding clay Estonia, covering 45 000 km2 of land area, is and sandstone deposits of the Voosi Formation, situated in the northwestern part of the East- which attain a thickness of 90 m in southwest- European Platform. Structurally, its sedimenta- ern Estonia. On the western Estonian islands, ry beds, lying on the southern slope of the Fen- the Vendian deposits have also been pinched out noscandian Shield, are declined southwards at and the water-bearing terrigenous rocks consist about 3-4 metres per kilometre. Crystalline Pal- only of Cambrian sand- and siltstones with in- aeoproterozoic basement is overlain by Neopro- terlayers of clay. terozoic (Vendian) and Palaeozioc (Cambrian, The Cambrian-Vendian aquifer system is dis- Ordovician, Silurian and Devonian) sedimenta- tributed throughout Estonia, except the Lokno- ry rocks covered by Quaternary deposits. Mõniste uplift area in southern Estonia. Water- Mapping of potentially hazardous elements in the Cambrian-Vendian aquifer system, northern Estonia 19 (m) a.s.l. (m) a.s.l. Q 50 50 D2 nr 0 O 0 -50 -50 O-Cm -100 -100 Cm ln 1 V vr -150 2 -150 Cm-V V2 kt -200 -200 V2g d -250 PP -250 0 50 100 150 200 250 km Quaternary sediments Carboniferous aquifer Terrigenouserrigenous aquifer Aquitard Crystalline basement Linear zones of disturbances Fig. 2. Hydrogeological cross-section of North-Estonia. Q: Quaternary sediments, D2nr: Narva aquitard, O: Ordovi- cian aquifer system, O-Cm: Ordovician–Cambrian aquifer system, Cm1ln: Lontova aquitard, Cm-V: Cambrian–Ven- dian aquifer system, V2vr: Voronka aquifer, V2kt: Kotlin aquitard, V2gd: Gdov aquifer, PP: Paleoproterozoic crystalline basement. Line of section shown in Fig. 1. bearing Cambrian and Vendian sedimentary groundwaters dominate (Karise, 1997; Mokrik, rocks as well as clays of the Lontova Formation 1997; Perens et al., 2001). pinch out in the bottom of Finnish Gulf, about The Cambrian-Vendian aquifer system is 20 km northward from the coastline. used for the water supply in northern Estonia Intensive erosional processes in pre-Qua- only, because in the southern part of country the ternary, inter-, late- and postglacial times have water-bearing layers are too deep and the salini- formed erosional valleys, cutting through the ty of water is high. In northern Estonia however, Lontova aquitard (Tavast, 1997). These an- the supply is very signifi cant, amounting to 10– cient buried valleys are fi lled with Quaternary 13% of the Estonian groundwater consump- sediments, mostly by till, but also the marine, tion. In Ida-Virumaa and Harjumaa counties glaciofl uvial and glaciolacustrine sediments the share of Cambrian-Vendian groundwater are presented. The valleys have north-west to consumption is about 75 % and in Lääne-Viru- south-east direction, thus being approximately maa almost 50 % (Savitskaja, 1999). The saline perpendicular to the North-Estonian coastline. groundwater resources in South-Estonia can be In places, where groundwater is intensively ab- used as mineral water (Karise, 1997). stracted, valleys provide the Cambrian-Vendian aquifer system with fresh meteoric water. 3. Materials and methods The variations in the groundwater salinity and the associations of chemical compounds During 2001 and 2002, 47 groundwater sam- dominating in water can be followed both in ples were collected from North Estonia. The vertical and horizontal direction. Fresh ground- sampling points were selected according to the waters of Na-Ca-HCO -Cl and Ca-Na-Cl- following criteria: 1) assurance of a good areal 3 HCO type with total dissolved solids (TDS) representation and 2) sampling of both Gdov 3 value, mainly below 1.0 g/l (0.4–1.1 g/l), are and Voronka aquifers at every sampling point, characteristic in northern Estonia (Savitskaja if possible.