Background and objectives
Groundwater age or the residence time of groundwater refers to the period of time that the water has spent in the water cycle from its infiltration into the saturated zone to sampling. Knowledge of groundwater age generally improves the understanding of the dynamics of groundwater systems. For example, groundwater age is an important parameter for evaluating groundwater recharge and flow processes, for spatially differentiating between mobile and stagnant groundwater and for estimating the vulnerability of aquifers. In Germany and worldwide, the recording of groundwater ages is incomplete and, in many cases, concentrates on aquifers close to the surface that are used for water supply.
The aim of the project is to make groundwater age data available in an efficient manner and to improve the methodology for determining, evaluating and using groundwater ages. Newly acquired findings and data are to be used in particular for the exploration of deep aquifers (focus on Germany and Africa) and for the future exploration of potential repository sites (section 22 of the German Site Selection Act “StandAG”). They can also be useful for modelling groundwater age distribution, calibrating regional flow models and, if necessary, investigating the distribution of pollutants (e.g. nitrate).
Pursuant to § 22 of the German Site Selection Act, groundwater age is defined as an exclusion criterion. This means that, if “the presence of young groundwaters is detected in [...] rock zones which can potentially serve as effective enclosure, [...]” then these must be excluded as a location for a geologic repository for high-level radioactive waste. Dating groundwater in these rock zones in Germany is therefore important. Tritium (3H) and carbon-14 (14C) are specified in the German Site Selection Act as indicative tracers that can be used to identify young groundwater.
Project activities
Data collection
A core element of the project is to compile existing age dating of groundwater in the Federal Republic of Germany and to store it in projects using the relational database AQUA and to make it available in a well-documented and secure form for statistical and spatial analyses. The age dating data include tritium (3H), tritium-helium (3H/3He), 14C and, to a much lesser extent, sulphur hexafluoride (SF6) and chlorofluorocarbons (CFCs) as well as noble gas isotopes (e.g. 4He, 39Ar, 85Kr, 81Kr). In addition to the age data, accompanying data such as coordinates, sampling depth, type of measuring point and sampling date are recorded digitally, where available. To a limited extent, re-sampling for hydrochemistry and age dating is planned to close gaps in the database and improve the development of methods. This particularly concerns old groundwater at greater depths and in selected areas.
Regional evaluation of selected groundwater systems
The available data is used for a regional evaluation of selected groundwater systems (scale: large basins or hydrogeological units). The investigations focus on the northern German lowlands and areas that fall within the Buntsandstein Group in southern Germany.
- In the North German Plain, a unique large data set of groundwater samples for tritium and helium isotopes from the last 20 years was analysed in collaboration with the University of Bremen and the University of Oldenburg, among others (Desens et al. 2023). The various natural and technical influences on the tritium-helium age, such as filter depth and length of the sampled wells, groundwater recharge rate and climatic effects, were investigated. Up to a sampling depth of ~40 m below ground level, the mean tritium-helium age increases almost linearly with depth and reaches a maximum of 40 years. Below this depth, the proportion of older, tritium-free water increases. The regional distribution for shallow wells shows that the tritium-helium age increases from west to east, which is related to the decreasing groundwater recharge due to the more continental climate in the east.
- The current focus is on characterising groundwater in the overburden of crystalline rocks in southern Germany, in particular the Lower to Middle Triassic Buntsandstein Group. We have added 6080 new stations in Baden-Württemberg, Bavaria, and Hesse to the database. This includes 14 830 data sets, among them 1174 tritium and 228 14C values from the Permian and Triassic aquifers.
Literature
- DESENS, A., HOUBEN, G., SÜLTENFUß, J., POST, V. & MASSMANN, G. (2023): Distribution of tritium-helium groundwater ages in a large Cenozoic sedimentary basin (North German Plain). – Hydrogeology Journal, 31: 621–640. DOI: 10.1007/s10040-023-02600-1
- NEUKUM, C., SEIBERT, S., POST, V., KÖNIGER, P., BÄUMLE, R., DESENS, A. & HOUBEN, G. (2020): Standortauswahl – Ausschlusskriterium Grundwasseralter. – Abschlussbericht. 127 pp.; BGR, Hannover.
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