Using stable isotopes to characterize the conditions of groundwater formation on the eastern slope of the Baltic Shield (NW Russia)

Abstract Repeated continental glaciations and marine transgressions had a strong influence on the geochemical evolution of groundwater systems of the eastern slope of the Baltic Shield (Arkhangelsk region, Russia). The spectrum from modern fresh waters to very old brines is distributed from the surface to a depth of 1 km in Quaternary and Vendian (Ediacaran) sediments. The purpose of this study was to i) assess the isotopic composition of various types of groundwater, ii) assess the degree of participation of glacial melt and meteoric waters in the processes of dilution of seawater and brines according to isotope-geochemical data, and iii) employ the use of stable isotopes to clarify previously performed groundwater dating. It has been established that the salty water in the upper part of the Vendian aquifer near the modern sea coast has maximum enrichment by heavy isotopes δ18O = −7.7‰ and δ2H = −98.9‰, on average. It is a mixture of 19–51% glacial meltwater and 49–81% seawater. The age of the marine component is 115–130 ka, and the age of the glacial water is similar to that of the Middle Pleistocene. The relict brackish water in the middle part of the Vendian aquifer has the most depleted isotope composition: δ18O = −14.1‰ and δ2H = −106.7‰, on average. It is a mixture of ~3% brine, 8–19% glacial and 78–89% interglacial water, and was formed in the range of 35.3–22.3 ka (Late Pleistocene). The maximum brine content (10–17%) is characteristic of salt water in the lower part of the Vendian aquifer. The isotope composition of the fresh water is close to the weighted annual composition of modern precipitations δ18O = −13.2‰ and δ2H = −98.9‰. Fresh water is a mixture of about 2% seawater, up to 16% glacial water, and 84–100% Holocene water. The age of this water is in the range from the present day to 11.8 ± 1.6 ka. The maximum depth of penetration of melt glacial waters of the Middle Pleistocene is assumed to be 600 m.