Valle Raidla

We review our current understanding of groundwater flow history in the northern part of the Baltic Artesian Basin (BAB) from the end of the Late Pleistocene to current conditions based on the hydrogeological studies carried out in 2012–2020 by the Department of Geology, Tallinn University of Technology and its partners. Hydrogeochemical data and various numerical models are combined in order to understand the link between glaciations and groundwater flow. The results of our earlier research and published literature on groundwater flow history in the BAB are also taken into account. The reconstruction of groundwater flow history is based on the database of the isotopic, chemical and dissolved gas composition of groundwater. The database contains data on 1155 groundwater samples collected during 1974–2017. We find that groundwater in the BAB is controlled by the mixing of three distinct water masses: interglacial/modern meteoric water (δ¹⁸O ≈ –11‰), glacial meltwater (δ¹⁸O ≤ –18‰) and an older syngenetic end-member (δ¹⁸O ≥–4.5‰). The numerical modelling has suggested that the preservation of meltwater in the northern part of the BAB is controlled by confining layers and the proximity to the outcrop areas of aquifers. Aquifers containing groundwater of glacial origin are in a transient state with respect to modern topographically-driven groundwater flow conditions. The most important topics for future research that can address gaps in our current knowledge are also reviewed.

It is proved that a transient 3D distribution of ¹⁸O concentration in a regional-scale heterogeneous multi-layered aquifer system can be numerically simulated by codes MODFLOW-2005 and MT3DMS as a boundary problem. An optimum method of the transition of the observed negative δ¹⁸O values to respective positive units of the absolute ¹⁸O concentration needed for simulations has been substantiated. The practical applicability of the elaborated method has been verified by the reconstruction and interpretation of the geohydrological history of the Estonian Artesian Basin during the Late Pleistocene and Holocene. The adequacy of regional hydrodynamic calculations proceeded by eight consecutive modelling scenarios has been verified by a good correlation between the measured and simulated ¹⁸O values. The set of functionally interconnected groundwater flow and ¹⁸O transport models forms an integral hydrogeological model of the Estonian Artesian Basin for the last 22 ka. The paper contributes to a wider application of ¹⁸O concentration as a conservative tracer in the investigation of the complex problem of groundwater flow and transport in real-world conditions.

The study focuses on chemical composition of stream and subsurface water in the catchments of two small arctic alpine lakes in the Kilpisjärvi area (northwest Finland). Differences and changes in chemical components of both water types are followed in order to detect spatial variability and impact of environmental factors. To achieve this, ion compositions of subsurface water and streams were measured at 12 sites in the catchments of Lakes Saana and Saanalampi during four years (2008–2010, and again in 2017). In the Lake Saanalampi catchment, the salinity of stream water (7.0 to 12.7 μS·cm⁻¹) corresponded to that of snow. In the catchment of Lake Saana, however, the conductivity in stream water was much higher (40 to 220 μS·cm⁻¹), connected mainly to the increase of SO₄²⁻ and less with Mg²⁺ and Ca²⁺ contents, especially in the western part of the Saana catchment. These results demonstrate that arctic conditions do not preclude intense chemical weathering where conditions are favourable. Although chemical composition of the soil fluid does not match the geochemical signal from the local soil, rock composition, especially the presence of pyrite, is the main controller of chemical weathering rates of the rocks on the area. This supports earlier views that the character of precipitation mostly controls water chemistry of local lakes in the Kilpisjärvi area.

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.