Présentations Colloques

Oral Presentation
8.04
Session 8.04: Isotopic and residence time tracers
Gerber Christoph
81Kr Dating of Groundwater in the Baltic Artesian Basin on the One-Million-Year Timescale
With the recent improvements in analytical techniques, 81Kr dating is becoming a viable and powerful alternative to tracers such as 36Cl and 4He for estimating groundwater ages and flow patterns on time scales of hundred thousands of years. Here we present a case study of the multi-layered sedimentary Baltic Artesian Basin (BAB), which is a source of drinking water for Estonia, but also considered for storage of CO2 or radioactive waste. We focus on the Cambrian Aquifer System (CAS), which covers the whole BAB and is sandwiched between the bedrock and a thick aquitard.**Samples from seven deep wells (up to 1800 m) were analyzed for a suite of dating tracers (85Kr, 39Ar, 81Kr, 4Herad, and 40Arrad), noble gas concentrations, chemistry, and stable isotopes of the water. The results are consistent with a mixing of three water types, as proposed in earlier studies- (i) fresh meteoric water, (ii) glacial meltwater, and (iii) a brine end member. These end members are expected to also have different residence times, which need to be deconvoluted.**We find that the three end members also have very distinct noble gas concentrations varying over one order of magnitude. As a consequence of this, end-member proportions calculated based on chemistry and stable isotopes are different from end-member proportions of Kr, which are relevant for interpreting the 81Kr measurements. For the brine component, 81Kr ages exceed the dating range of the ATTA 3 instrument of 1.3 Ma. Ages of > 1 Ma for the brine component are also supported by 4Herad and 40Arrad. In turn, 81Kr ages of the fresh meteoric and glacial meltwater components range from 300 ka to 1.3 Ma. The spatial pattern of these ages implies that in the past, flow directions in the CAS must have been different from those inferred from today’s hydraulic heads. A conceptual model shows that it is plausible that the flow direction reversed during the advances of the Scandinavian Ice Sheet.
Switzerland