Dating of glacial palaeogroundwater in the Ordovician-Cambrian aquifer system, northern Baltic Artesian Basin

The Ordovician-Cambrian aquifer system in the northern Baltic Artesian Basin contains glacial palaeogroundwater that originates from the Scandinavian Ice Sheet that covered the study area in the Pleistocene. Previously, no absolute dating of this palaeogroundwater has been attempted. In this multi-tracer study, we use H-3, C-14, He-4 and stable isotopes of water to constrain the age distribution of groundwater. We apply the geochemical modelling approach developed by van der Kemp et al. (2000) and Blaser et al. (2010) to calculate the theoretical composition of recharge waters in three hypothetical conditions: modern, glacial and interstadial for( 14)C model age calculations. In the second phase of the geochemical modelling, the calculated recharge water compositions are used to calculate the C-14 model ages using a series of inverse models developed with NETPATH. The calculated C-14 model ages show that the groundwater in the aquifer system originates from three different climatic periods: (1) the post-glacial period; (2) the Late Glacial Maximum (LGM) and (3) the pre-LGM period. A larger pre-LGM component seems to be present in the southern and north-eastern parts of the aquifer system where the radiogenic He-4 concentrations are higher (from similar to 3.0.10(-5) to 5.5.10(-4) cc.g(-1)) and the stable isotopic composition of water is heavier (delta O-18 from - 13.5 parts per thousand to -17.3 parts per thousand). Glacial palaeogroundwater from the north-western part of the aquifer system is younger and has C-14 model ages that coincide with the end of the LGM period. It is also characterized by lower radiogenic( 4)He concentrations (similar to 2.0.10(-5) cc.g(-1)) and lighter stable isotopic composition (delta O-18 from -17.7 to - 22.4 parts per thousand). Relations between radiogenic He-4 and C-14 model ages and between radiogenic He-4 and Cl(- )concentration show that groundwater in the aquifer system does not have a single well-defined age. Rather, the groundwater age distribution has been influenced by mixing between waters originating from end-members with strongly differing ages. Overall the results suggest, that in the shallower northern part of the aquifer system, significant changes in groundwater composition can be brought about by glacial meltwater intrusion during a single glaciation. However, multiple cycles of glacial advance and retreat are needed to transport glacial meltwater to the deeper parts of the aquifer system.