Intrusion of Saline Water into a Coastal Aquifer Containing Palaeogroundwater in the Viimsi Peninsula in Estonia

Abstract

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.

Calibrated geochemical ages of the Baltic Artesian Basin groundwater

“…The freshwater found in this area (especially the Cm-V and O-Cm aquifers) is depleted with δ 18 O and is ten fold older, as based on radiocarbon (20-30 ka), than the rest of the BAB at the same depth (50-300 m). (Mokrik and Vaikmäe, 1988;Yezhova et al, 1996;Mokrik, 1997;Vaikmäe et al, 2001;Raidla et al, 2012Raidla et al, , 2019aBabre et al, 2016;Sterckx et al, 2017Sterckx et al, , 2018Pärn et al, 2019;Mokrik et al, 2021;Samalavičius, 2022;Mokrik and Samalavičius, 2022). This, of course, has an influence on results of geochemical ages.…”

Late Pleistocene and Holocene groundwater flow history in the Baltic Artesian Basin: a synthesis of numerical models and hydrogeochemical data

“…The results of the studies on the origin, geochemical evolution and age of groundwater in the aquifer systems of the northern BAB have been published in recent decades in a number of papers and dissertations (Mokrik 1997;Vaikmäe et al 2001aVaikmäe et al , 2001bVaikmäe et al , 2008Karro et al 2004;Marandi 2007;Raidla et al 2009Raidla et al , 2012Raidla et al , 2014Raidla et al , 2016Raidla et al , 2019aRaidla et al , 2019bRaidla 2010;Pärn et al 2016Pärn et al , 2018Pärn et al , 2019Gerber et al 2017;Suursoo et al 2017;Pärn 2018;Vallner et al 2020). However, several recent studies on the palaeowaters in the BAB have employed other meth odologies besides geochemistry and 14 C dating, which have been traditionally used for that purpose.…”

Interpretation of the anomalous groundwater chemistry and 234U/238U activity ratio disequilibrium in the northern part of the baltic region

“…LGM groundwater background parameters of the Tallinn intake are δ 18 O -22.5‰, δ 13 C -16‰ and TDS 0.35-04 g/L. The highest observed AR (26) in boreholes are distant from paleo valleys. Groundwater extraction caused background values to change, toward freshening, a drop of AR (down to 3.0), enrichment of δ 18 O‰ and δ 13 C‰ content to -12.0‰ and -12.4‰, respectively.…”

“…The latest investigation of the wellfield intake of the Viimsi Peninsula revealed that groundwater chemistry changes (chloride content increased) over time in the Cm-V aquifer. The authors' opinion is that potential seawater intrusion could occur in the future [26].…”

“…Estonian researchers proposed a three end-member mixing model to explain the origin of the Estonian Cm-V and O-Cm groundwater as a consequence of glacial meltwater, crystalline basement brackish water mixing and modern groundwater recharge [24][25][26][27][28][29]. Another point of view suggests that the cryogenesis could explain the isotope-geochemistry anomaly because EH groundwater was strongly affected by permafrost [17,18].…”