The Viimsi peninsula is located north-east of Tallinn, capital of Estonia. The Cambrian-Vendian (Cm-V) aquifer system is a sole source of drinking water in the area. Historically, the groundwater exploitation has led to freshening of groundwater in the peninsula, but in recent years an increase in chloride concentrations and enrichment in δ18O values has been detected, but in recent years hydrochemical parameters indicate an increasing influence of a saline water source. The exact origin of this saline water has remained unclear. The aim of the current study is to elucidate whether the increase in Cl− concentrations is related to seawater intrusion or to the infiltration of saline water from the underlying crystalline basement. To identify the source of salinity, chemical composition of the groundwater and the isotope tracers (e.g., δ18O and radium isotopes) were studied in the Viimsi peninsula in the period from 1987 to 2018. Our results show that chemical composition of Cm-V groundwater in the peninsula is clearly controlled by three-component mixing between glacial palaeogroundwater, saline water from the underling crystalline basement and modern meteoric water. The concentrations of Ra are also significantly affected by the mixing, but the spatial variation of radium isotopes (226Ra and 228Ra) suggests the widespread occurrence of the U in the surrounding sedimentary sequence. Our hypothesis is that, in addition to U originating from the crystalline basement, some U could be associated with secondary U deposits in sedimentary rocks. The formation of these secondary U deposits could be related to glacial meltwater intrusion in the Pleistocene. Although the results suggest that the infiltration of saline groundwater from the underlying crystalline basement as the main source of salinity in the study area, the risk of seawater intrusion in the future cannot be ruled out. It needs to be highlighted that the present groundwater monitoring networks may not be precise enough to detect the potential seawater intrusion and subsequent changes in water quality of the Cm-V aquifer system in the Viimsi peninsula.

In this study, modelled groundwater actual flow times in intermediate and deep aquifers, covered by regional scale impermeable aquitards, were compared with 4He and 81Kr age dating results. To improve the reliability of the steady state 3D groundwater flow model, the isotopic ages of deep groundwater were compared to the MODPATH modelled travel times. The highest helium values in groundwater reservoirs coincide with fault zones in the crystalline basement and sedimentary cover near Rapakivi granite massifs. Insights into isotope-geochemical anomalies of the Baltic Artesian Basin intermediate and deep groundwater support their main distribution peculiarities in the flow path towards the Baltic Sea coast lowland and seabed depression as the regional groundwater discharge area.

The anomalous isotope-hydrogeochemistry phenomena in the groundwater of Estonian Cambrian-Vendian (Ediacaran) and Ordovician-Cambrian aquifer systems were formed in the Late-Middle Pleistocene. In the periglacial environment, in northern and northwestern Estonia, these aquifer systems with fracture porose crystalline basement are connected to hydraulically joint unit characterized by high radioactivity groundwater. A significant alteration of groundwater occurred by series of isotope and chemistry facies fractionation. In this study, uranium isotopes activity ratio (234U/238U), 4He content, isotope-hydrogeochemistry and adjusted 14C ages are coupled for a new prospect of the estimation of northern Baltic Basin groundwater evolution. Analyzing radiocarbon and 4He groundwater residence time results and uranium isotope activity ratio distribution suggests a prolonged periglacial environment in which groundwater evolved. Stable isotope ratios of δ18O and δ2H correlation and hydrochemical composition changes support the cryogenic origin of groundwater. Pleistocene glaciations cyclically affect groundwater in multiple ways: permafrost isotope-geochemistry partitioning; periodically changing reversed flow directions of recharge and discharge areas; oscillations of the sea, river system, and periglacial lakes level, surface and sub-permafrost water mixing via taliks and fractured basement rocks. These processes lead to forming the sequence of isotope-hydrogeochemistry types and specific zoning; in general, two separate groundwater fractions – brackish in the lower part and freshened above. An extensive groundwater exploitation on the northern coast sites influenced a sharp dysfunction in the groundwater body, destabilizing the natural equilibrium state formed in the Holocene and Pleistocene.

Samalavičius V., Mokrik R. Tritium activity trend formation in groundwater of Quaternary aquifer system, south-eastern Lithuania. Geo logija. Geografija. 2016. T. 2(4). ISSN 2351-7549.Radioactive decay and mixing leads to tritium activity decrease in groundwater. In south-eastern Lithuania, the unconfined layer and precipitation water tritium activity decrease rate is dominated by radioactive decay for the period of 1980-2013. Confined aquifers are vastly affected by mixing; therefore, it is hard to predicted tritium activity values. To achieve it, the monitoring type of sampling must be applied in order to calculate tritium activity gradient value. Moreover, gradient values in different sampling points along the water pathway must be similar. It is a necessary condition when comparing theoretical decay curves with actually calculated. Only the shallowest confined aquifer (ag II-III md-gr) in this study met these conditions. An additional method for modelled data interpretation was tritium peak analysis. According to the modelled tritium peak, a few possible dating water age alternatives exist. The data analysis suggests that no certain water age could be determined. The tritium peak analysis could be a helpful method to gain more comprehensive and valuable hydrogeological information.

Pietų Lietuvoje esantis cenomanio-apatinės kreidos vandeningasis sluoksnis pasižymi savitomis plačiai paplitusiomis sūraus požeminio vandens anomalijomis. Šis sluoksnis formuojasi vandeniui pertekant per gilumines lūžių zonas iš giliau slūgstančių triaso ir permo vandeningųjų sluoksnių. Sūraus vandens intruzijos išreikštos didesnės chloridų koncentracijos kupolais, aptinkamos iškrovos vietose – Šešupės ir Nemuno upių slėniuose, dažnai siekia net gruntinio vandens sluoksnius. Sūraus vandens kontūras juda link vandenviečių, blogina požeminio vandens eksploatacinių išteklių kokybę ir riboja jų kiekį.