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Analyses for $^{81}$Kr and noble gases on groundwater from the deepestaquifer system of the Baltic Artesian Basin (BAB) were performed to determinegroundwater ages and uncover the flow dynamics of the system on a timescale ofseveral hundred thousand years. We find that the system is controlled by mixingof three distinct water masses: Interglacial or recent meteoric water$(\delta^{18}\text{O} \approx -10.4\unicode{x2030})$ with a poorly evolvedchemical 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 gasconcentrations. The $^{81}$Kr measurements are interpreted within this mixingframework to estimate the age of the end-members. Deconvoluted $^{81}$Kr agesrange from 300 ka to 1.3 Ma for interglacial or recent meteoric water andglacial meltwater. For the brine component, ages exceed the dating range of theATTA 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, weconclude that the brine originates from evaporated seawater that has beenmodified by later water-rock interaction. As the obtained tracer ages coverseveral glacial cycles, we discuss the impact of the glacial cycles on flowpatterns in the studied aquifer system.

Using 81Kr and Noble Gases to Characterize and Date Groundwater and Brines in the Baltic Artesian Basin on the One-Million-Year Timescale