The Use of Some Ion-Exchange Sorbing Tracer Cations in In-situ Experiments in High Saline Groundwaters
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THE USE OF SOME ION-EXCHANGE SORBING TRACER CATIONS IN INSITU EXPERIMENTS IN HIGH SALINE GROUNDWATERS J. BYEGARD, G. SKARNEMARK AND M. SKALBERG Department of Nuclear Chemistry, Chalmers University of Technology, S-412 96 G6teborg, Sweden.
ABSTRACT The possibility to use alkali metals and alkaline earth metals as slightly sorbing tracers in in-situsorption experiments in high saline groundwaters has been investigated. The cation exchange characteristics of granite and some fracture minerals (chlorite and calcite) have been studied using the proposed cations as tracers. The results show low Kd's for Na, Ca and Sr (-0.1 ml/g), while the sorption is higher for the more electropositive cations (Rb, Cs and Ba). A higher contribution of irreversible sorption can also be observed for the latter group of cations. For calcite the sorption of all the tracers, except Ca, is lower compared to the corresponding sorption to granite and chlorite. Differences in selectivity coefficients and cation exchange capacity are obtained when using different size fractions of crushed granite. The difference is even more pronounced when comparing crushed granite to intact granite.
INTRODUCTION The retardation process in fractured rocks of radionuclides released from a nuclear waste repository is believed to consist of sorption on the fracture walls combined with diffusion into the rock matrix [1]. In-situ experiments have shown that the flow in fractured rock is very channelled [2] and it is therefore believed that only part of the fracture wall surface (the flow wetted surface) will come into contact with the released radionuclides. An important process however, which is believed to enhance the sorption of the radionuclides
is the diffusion of the radionuclides into minor fractures of the surrounding rock [3]. In order to perform a correct safety assessment, it is important to quantify the flow wetted surface and the matrix diffusivity in in-situ experiments. Furthermore, it is necessary to verify the sorption and diffusion data obtained in laboratory experiments for field experiments. A series of experiments are therefore proposed to be performed at the Swedish Asp6 Hard Rock Laboratory [4]. The rock at Asp6 consists mainly of diorite and granite which is in contact with a saline groundwater with a relatively high concentration of Ca [5]. A possible way to obtain information about the in-situ sorption capacity is to perform flow experiments using slightly sorbing tracers with known sorption behaviour. By comparing the breakthrough times of the slightly sorbing tracers to the breakthrough time of a non-sorbing tracer, it may be possible to quantify the interaction between the groundwater and the rock matrix. Adjusting the hold up time of the tracers makes it possible to observe the effect of the matrix diffusion. Alkali metals and alkaline earth metals are proposed to be used as tracers in this type of experiment [6]. The sorption mechanism for these cations is supposed to be ion exchange, which is considered as a kinetically fast and rev
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