Rein Vaikmäe

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.

A fundamental knowledge of processes that control groundwater composition is required for informed management of water quality. The Voronka groundwater body in northeastern Estonia represents a good example of a complicated, overexploited groundwater system where conceptual understanding of baseline quality and governing hydrogeochemical processes can support sustainable aquifer management. A conceptual understanding or conceptual model is a simplified representation or a working understanding of the real hydrogeological system and its processes. The baseline chemical composition of the Voronka ground­water body was formed during the last glaciations, when glacial meltwater intruded into water-bearing rocks. Two main processes that can change Voronka groundwater body quality at the present day are: (1) seawater intrusion and (2) water exchange between buried valleys and formation’s groundwater. Future monitoring and management should focus on changes in the natural composition of groundwater caused by abstraction. The HCO₃^–/Cl^– value is the best parameter to describe the fluctuations in natural back­ground chemistry in the Voronka groundwater body and to assess significant trends induced by abstraction. In case of the discovered trends, a suite of isotope methods, especially ¹⁴C, ³H, δ²H, δ¹⁸O and δ¹³C, can be used to detect whether the intrusion of seawater or exchange of water with buried valleys is taking place.

The chemical composition of groundwater was studied and plausible reasons for its peculiarities were evaluated on the basis of long-term groundwater monitoring data. Fluoride and boron anomaly was distinguished in the Silurian–Ordovician aquifer system in western Estonia. The chemical type of groundwater controls the dissolution of fluorides and boron in water. Their concentrations are highest in deep groundwater having long residence time in the host rocks. The dissolution of carbonate rocks and clayey K-bentonite beds are the sources of F in groundwater. Barium anomaly in the Cambrian–Vendian aquifer system occurs in northeastern Estonia, where the intrusion of saline groundwater from the weathered crystalline basement contributes to the formation of Ba-rich groundwater. The overexploitation of fresh groundwater has resulted in the increase in groundwater salinity, however, detectable intrusion of seawater into the Cambrian–Vendian aquifer system is ruled out by the stable isotope signature.