Interpretation of the anomalous groundwater chemistry and 234U/238U activity ratio disequilibrium in the northern part of the baltic region

Abstract

The anomalous isotope-hydrogeochemistry phenomena in the groundwater of Estonian Cambrian-Vendian (Ediacaran) and Ordovician-Cambrian aquifer systems were formed in the Late-Middle Pleistocene. In the periglacial environment, in northern and northwestern Estonia, these aquifer systems with fracture porose crystalline basement are connected to hydraulically joint unit characterized by high radioactivity groundwater. A significant alteration of groundwater occurred by series of isotope and chemistry facies fractionation. In this study, uranium isotopes activity ratio (234U/238U), 4He content, isotope-hydrogeochemistry and adjusted 14C ages are coupled for a new prospect of the estimation of northern Baltic Basin groundwater evolution. Analyzing radiocarbon and 4He groundwater residence time results and uranium isotope activity ratio distribution suggests a prolonged periglacial environment in which groundwater evolved. Stable isotope ratios of δ18O and δ2H correlation and hydrochemical composition changes support the cryogenic origin of groundwater. Pleistocene glaciations cyclically affect groundwater in multiple ways: permafrost isotope-geochemistry partitioning; periodically changing reversed flow directions of recharge and discharge areas; oscillations of the sea, river system, and periglacial lakes level, surface and sub-permafrost water mixing via taliks and fractured basement rocks. These processes lead to forming the sequence of isotope-hydrogeochemistry types and specific zoning; in general, two separate groundwater fractions – brackish in the lower part and freshened above. An extensive groundwater exploitation on the northern coast sites influenced a sharp dysfunction in the groundwater body, destabilizing the natural equilibrium state formed in the Holocene and Pleistocene.