Features of Groundwater Formation of the Main Aquifers of St. Petersburg and Suburbs Based on Data of Chemical and Isotope Composition

Abstract

The main aquifers used for water supply in St. Petersburg and Leningrad Oblast contain waters with total dissolved solids (TDS) and chemical composition that vary considerably with the distance from modern recharge areas, as well as depending on the composition of the host rocks. Waters of the Quaternary, Devonian, Ordovician, Cambrian-Ordovician and Vendian aquifers were sampled and analyzed. With low mineralization, a bicarbonate calcium-magnesium composition predominates in waters of low TDS, while with its increase over 400– 600 mg/dm3, chlorides dominate in the anion composition, and sodium — in cation composition. In some cases, low-mineralized water contains significant amounts of iron and can be considered mineral-medicinal. A regional feature is the local excess (sometimes significant) of the activity of natural radionuclides over the intervention levels. To identify formation conditions, data on the isotopic composition (deuterium and oxygen content-18) of groundwater were used. The greatest variety of isotopic compositions is observed in the waters of the Vendian aquifer. On the Karelian Isthmus, it contains modern water (which is also indicated by tritium). When moving to the south, in the direction of increasing TDS, the isotopic composition of water in the Vendian aquifer gets lighter, up to the appearance of waters with obvious signs of cryogenic metamorphosis. Light isotopic compositions are explained by the formation of groundwater in the cold climatic period, which ended in this area at least 12,000 years ago. Changes in the isotopic composition and chemistry of the waters of the Vendian aquifer are simultaneous with an increase in their age at a distance from the recharge area. Hence, in natural conditions, there is no areal recharge of the aquifer outside the area of its discharge to the land surface or under the permeable Quaternary deposits. In terms of water management, this means that its resources in the area where TDS exceeds 0.7–1.0 g/ dm3 should be calculated only as lateral inflow.