Using 81Kr and noble gases to characterize and date groundwater and brines in the Baltic Artesian Basin on the one-million-year timescale

Abstract

Analyses for 81 Kr and noble gases on groundwater from the deepest aquifer system of the Baltic Artesian Basin (BAB) were performed to determine groundwater ages and uncover the flow dynamics of the system on a timescale of several hundred thousand years. We find that the system is controlled by mixing of three distinct water masses: Interglacial or recent meteoric water (δ 18 O ≈ −10.4‰) with a poorly evolved chemical and noble gas signature, glacial meltwater (δ 18 O ≤ −18 ‰) with elevated noble gas concentrations, and an old, high-salinity brine component (δ 18 O ≥ −4.5‰, ≥ 90 g Cl − /L) with strongly depleted atmospheric noble gas concentrations. The 81 Kr measurements are interpreted within this mixing framework to estimate the age of the endmembers. Deconvoluted 81 Kr ages range from 300 ka to 1.3 Ma for interglacial or recent meteoric water and glacial meltwater. For the brine component, ages exceed the dating range of the ATTA-3 instrument of 1.3 Ma. The radiogenic noble gas components 4 He* and 40 Ar* are less conclusive butalso support an age of > 1 Ma for the brine. Based on the chemical and noble gas concentrations and the dating results, we conclude that the brine originates from evaporated seawater that has been modified by later water-rock interaction. As the obtained tracer ages cover several glacial cycles, we discuss the impact of the glacial cycles on flow patterns in the studied aquifer system. Virbulis et al., 2013), the latter of which estimated the hydraulic age of groundwater in the CAS to be on the order of several hundreds of ka to 1 Ma. In the light of such long residence times, it is crucial to consider the effect of repeated glacial cycles on the long-term evolution of groundwater composition and flow. Sampling the deeper parts of the CAS on a regional scale for chemistry, noble gases, and multiple dating tracers ( 81 Kr, 85 Kr, 39 Ar, 14 C, 4 He, 40 Ar) allows us to elucidate the evolution of the brine, mixing proportions of the different groundwater components, and the flow dynamics over the last 1 Ma.

The Demographics of Water: A Review of Water Ages in the Critical Zone

“…The most recent development in this field is the introduction of an analytical method from atomic physics (Atom Trap Trace Analysis), which greatly facilitates the use of the noble gas radioisotopes 39 Ar, 81 Kr, and 85 Kr (Lu et al, 2014). This new method of groundwater dating is now increasingly being applied, making successful use of the advantageous properties of the noble gas radioisotopes (e.g., Aggarwal et al, 2015;Gerber et al, 2017;Matsumoto et al, 2018;Ritterbusch et al, 2014;Yechieli et al, 2019).…”

39Ar dating with small samples provides new key constraints on ocean ventilation

“…The general concept of ATTA has first been demonstrated for the rare isotopes 85 Kr (half-life of 10.76 years) and 81 Kr (half-life of 229 000 years) [13,14] and is applied for dating groundwater [15][16][17] and ice [18]. While the first 39 Ar detection by this approach was reported in a proof of concept experiment in 2011 [19], the first explicit demonstration for dating groundwater samples was achieved in 2014 [20].…”

A Review of the Hydrochemistry of a Deep Sedimentary Aquifer and Its Consequences for Geothermal Operation: Klaipeda, Lithuania

“…The particle size in the water is typically 4-40 μm, approximately the same size as the porosity distribution in the reservoir (see above [11]). Micron-scale filter bags and cartridges are installed immediately prior to the heat pump and Table 3: Chemical composition of groundwater from Klaipeda Geothermal Demonstration Plant, showing analysis on a sample collected 5/10/2012 [17], three samples collected and analysed by GFZ-Potsdam in March-July 2016 from well 2P, and a sample collected on 5/4/17, from bulk water storage, and analysed by Glasgow University/SUERC. The final column shows standard sea water as reported by [18,19].…”

“…Additional analyses are available in GTN reports [16]. [17]. As some of the TIC may be carbonic acid, the figure of 84 mg/L may be somewhat overestimated.…”