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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.

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.