Hydrological Model Studies and Natural Isotope Data as Indication for Groundwater Flow in Deep Sedimentary Basins

Summary

The study of the flow system in deep aquifers is relevant to the safety assessment studies of geological disposal of radioactive waste. Because, deep aquifers are the first porous medium in which migration of radionuclides can occur, it is essential to establish whether the direction of this flow permits migration toward shallow fresh-water aquifers and finally to the biosphere.

Although natural isotopes provide indications about the age and origin of groundwater, interpretation of the findings is difficult. A hypothesis about the flow system can be reached on the basis of an interpretation of natural isotopes taking into account the paleogeographic situation. To reduce the uncertainty associated with the interpretation of natural isotopes, we made an inventory of all available oxygen-18 and carbon-14 data concerning The Netherlands, which we divided into two sets, i.e., data originating from areas outside the area of Zechstein salt deposition and data from salt- influenced aquifers. The first series of ¹⁸O data was combined with ¹⁴C data which yielded a curve of ¹⁴C-age vs. ¹⁸O. The ¹⁸O data of the second series was corrected with help of this graph which enabled the calculation of the contribution of dissolved rock-salt.

Another approach to obtain support for such a hypothesis is the construction of a model. Groundwater flow lines can be calculated and travel times, starting from surface infiltration, estimated. These models can show the likelihood of the flow system assumed for the hypothesis. It is concluded that the best results can be obtained by combining both approaches -isotope hydrology and groundwater modelling- to reduce the uncertainty about the deep groundwater system.