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The study investigates the mechanism of glacial meltwater recharge under the Fennosciandian Ice Sheet during the last glacial maximum (LGM) and its impact on regional groundwater flow in the northern Baltic Artesian Basin (BAB) in Estonia and Latvia. The current hypothesis is that a flow reversal occurred in the BAB due to subglacial recharge during the LGM. This hypothesis is supported by an extensive dataset of geochemical and isotopic measurements in the groundwater of northern Estonia, exhibiting significant depletion in δ18O with respect to modern precipitation. To verify the consistency of this hypothesis and better understand groundwater flow dynamics during the LGM period, a numerical model is developed for this area. Two cross-sectional models have been created across the northern BAB, in which groundwater flow and the transport of δ18O have been simulated from the beginning of the LGM to present-day. Several simulations were performed with different subglacial boundary conditions, to investigate the uncertainty related to subglacial recharge of meltwater during the LGM and the subsequent flow reversal in the northern BAB. Several simulations provide a satisfying fit between computed and observed values of δ18O, which means that the hypothesis of subglacial recharge of meltwater is consistent with δ18O distribution. The numerical model suggests that preservation of meltwater in northern Estonia is controlled by confining layers and the proximity to the outcrop area of aquifers, located in the Gulf of Finland. The results also suggest that glacial meltwater has been preserved under the Baltic Sea in the Gulf of Riga.

The specific impact of glacial processes on groundwater flow and solute transport under ice-sheets was determined by means of numerical simulations. Groundwater flow and the transport of δ18O, TDS, and groundwater age were simulated in a generic sedimentary basin during a single glacial event followed by a postglacial period. Results show that simulating subglacial recharge with a fixed flux boundary condition is relevant only for small fluxes, which could be the case under partially wet-based ice-sheets. Glacial loading decreases overpressures, which appear only in thick and low hydraulic diffusivity layers. If subglacial recharge is low, glacial loading can lead to underpressures after the retreat of the ice-sheet. Isostasy reduces considerably the infiltration of meltwater and the groundwater flow rates. Below permafrost, groundwater flow is reduced under the ice-sheet but is enhanced beyond the ice-sheet front. Accounting for salinity-dependent density reduces the infiltration of meltwater at depth. This study shows that each glacial process is potentially relevant in models of subglacial groundwater flow and solute transport. It provides a good basis for building and interpreting such models in the future.

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.

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.

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.

Monitoring of the confined Cambrian-Vendian aquifer system utilised for industrial water supply at Kopli Peninsula in Tallinn over 24 years reveals remarkable changes in chemical composition of groundwater. A relatively fast 1.5 to 3.0-fold increase in TDS and in concentrations of major ions in abstracted groundwater is the consequence of heavy pumping. The main sources of dissolved load in Cambrian-Vendian groundwater are the leaching of host rock and the other geochemical processes that occur in the saturated zone. Underlying crystalline basement, which comprises saline groundwater in its upper weathered and fissured portion, and which is hydraulically connected with the overlying Cambrian-Vendian aquifer system, is the second important source of ions. The fractured basement and its clayey weathering crust host the Ca-Cl type groundwater, which is characterised by high TDS values (2-20 g/L). Intensive water abstraction accelerates the exchange of groundwaters and increases the area of influence of pumping. Chemical and isotopic studies of groundwater indicate an increasing contribution of old brackish water from the crystalline basement and rule out the potential implication of an intrusion of seawater into aquifer.[PUBLICATION ABSTRACT]

Analyses for \\(^{81}\\)Kr and noble gases on groundwater from the deepest aquifer system of the Baltic Artesian Basin (BAB) were performed to determine groundwater ages and uncover the flow dynamics of the system on a timescale of several hundred thousand years. We find that the system is controlled by mixing of three distinct water masses: Interglacial or recent meteoric water \\((\\delta^{18}\\text{O} \\approx -10.4\\unicode{x2030})\\) with a poorly evolved chemical and noble gas signature, glacial meltwater \\((\\delta^{18}\\text{O} \\leq -18\\unicode{x2030})\\) with elevated noble gas concentrations, and an old, high-salinity brine component \\((\\delta^{18}\\text{O} \\geq -4.5\\unicode{x2030}, \\geq 90 \\text{g Cl}^{-}/\\text{L})\\) with strongly depleted atmospheric noble gas concentrations. The \\(^{81}\\)Kr measurements are interpreted within this mixing framework to estimate the age of the end-members. Deconvoluted \\(^{81}\\)Kr ages range from 300 ka to 1.3 Ma for interglacial or recent meteoric water and glacial meltwater. For the brine component, ages exceed the dating range of the ATTA 3 instrument of 1.3 Ma. The radiogenic noble gas components \\(^{4}\\)He* and \\(^{40}\\)Ar* are less conclusive but also support an age of > 1 Ma for the brine. Based on the chemical and noble gas concentrations and the dating results, we conclude that the brine originates from evaporated seawater that has been modified by later water-rock interaction. As the obtained tracer ages cover several glacial cycles, we discuss the impact of the glacial cycles on flow patterns in the studied aquifer system.

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.

It is proved that a transient 3D distribution of [.sup.18]O 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 [delta][.sup.18]O values to respective positive units of the absolute [.sup.18]O 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 [.sup.18]O values. The set of functionally interconnected groundwater flow and [.sup.18]O 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 [.sup.18]O concentration as a conservative tracer in the investigation of the complex problem of groundwater flow and transport in real-world conditions.

Ice cores from the relatively low-lying ice caps in Svalbard have not been widely exploited in climatic studies owing to uncertainties about the effect of meltwater percolation. However, results from two new Svalbard ice cores, at Lomonosovfonna and Austfonna, have shown that with careful site selection, high-resolution sampling and multiple chemical analyses it is possible to recover ice cores from which part of the annual signals are preserved, despite the considerable meltwater percolation. The new Svalbard ice cores are positioned in different parts of Svalbard and cover the past 800 years. In this paper we focus on the last 400 years. The δ18 O signals from the cores are qualitatively similar over most of the twentieth century, suggesting that they record the same atmospheric signal. Prior to AD 1920, the Austfonna ice core exhibits more negative δ18 O values than Lomonosovfonna, although there are intermittent decadal-scale periods throughout the record with similar values. We suggest that the differences reflect the effect of the inversion layer during the winter. The pattern in the δ18 O records is similar to the Longyearbyen air-temperature record, but on an annual level the correlation is low. The Austfonna record correlates well with the temperature record from the more distant and southwesterly located Jan Mayen. A comparison of the ice-core and sea-ice records from this period suggests that sea-ice extent and Austfonna δ18 O are related over the past 400 years. This may reflect the position of the storm tracks and their direct influence on the relatively low-altitude Austfonna. Lomonosovfonna may be less sensitive to such changes and primarily record free atmospheric changes instead of variations in sea-ice extent, the latter is probably a result of its higher elevation.