Pleistocene groundwater – evidence from stable and radiogenic isotopes

Robert van Geldern1, Sigrid Kowol2, Johannes A.C. Barth1
1 GeoZentrum Nordbayern, Universität Erlangen-Nürnberg
2 Erlanger Stadtwerke AG

P 6.8 in Grundwasser-, Boden- und Oberflächenwasser-Interaktionen

The metropol region Nuremberg and its vicinity in Northern Bavaria is an area that has been used intensively for industry and agri­culture for several decades. Whereas the upper aquifers are influenced from this anthropo­genic use to various degrees the deeper aqui­fers in the region should contain poten­tially pristine water that is not influenced by modern me­teoric recharge. In order to secure and main­tain high-quality drinking water reserves it is essential to investigate and under­stand ground­water re­charge and hydro­geologi­cal con­ditions.

In this study water samples from a confined, deeper aquifer (‘Benkersandstein’; Keuper) were ana­lysed for hydrogen and oxygen stable isotope composition (δ2H and δ18O), tritium (3H), and radiocarbon (14C). To investigate po­tential groundwater recharge by modern pre­cipitation a local meteoric water line (LWML) was calculated from the stable iso­tope preci­pitation data analysed over several years at the GeoZentrum Nordbayern.

Tritium content was below limit of detection (LOD) in all samples and indicates that sampled ground­water did not interact with surface water during the last ~50 years. Except for one sample, 14C-analyses revealed no percentage modern carbon (pMC) above LOD and indicate a water age older than ~26.000 years BP. A radiocarbon age >21.000 years BP is calculated after applying corrections after Pearson and Hanshaw (1970) for the single well containing low pMC concentrations.

The oxygen and hydrogen stable isotope data from all wells are virtually identical within analytical errors and show a calculated mean value of –10.6(±0.1)‰ and –74(±0.4)‰. These values are depleted in 18O and 2H relative to the mean value of the local modern recharge water (Fig. 1). This clearly indicates that the groundwater in the aquifer is not recharged by modern precipitation.

Evidence from stable and radiogenic isotope data show that the analysed groundwater was not part of the hydrological water cycle in the more recent human history. The water is older than 20.000 years and most likely originates from the isotopic depleted melt water from the last Pleistocene ice age and might therefore provide a resource of pristine water for the future.

Figure 1: Oxygen and hydrogen stable isotope data from deep aquifer compared to LMWL and mean value of modern recharge water.
Figure 1: Oxygen and hydrogen stable isotope data from deep aquifer compared to LMWL and mean value of modern recharge water.

Pearson, F.J. and Hanshaw, B.B. (1970): Sources of dissolved carbonate species in groundwater and their effects on carbon-14 dating; in Isotope Hydrology 1970; IAEA Symposium 129, p. 271-286.

Letzte Änderung 01.11.2013