Technical Report NTB 90-39

Grimsel Test SiteThe Grimsel Migration Experiment:A hydrogeochemical equilibrium test

Within the migration experiment at the Grimsel Test Site (GTS) various breakthrough tests, including non-sorbing as well as reactive tracers, are performed in a single fracture. During a special experiment, the water naturally flowing in the migration fracture (MI­water), was replaced by a groundwater of a somewhat different chemical composition (EM-water), which was pumped from borehole BOEM 85.012 located 200 m south of the migration site. The dipole flow field for this breakthrough experiment was established between borehole BOMI 86.004 (injection well) and borehole BOMI 87.006 (extraction well). The initial objective of this hydrogeochemical equilibrium test was to observe how the system responded to the injection of a "foreign" water and to determine the time required to reach new steady state conditions. A total of 22 water samples was taken from the extraction well for subsequent laboratory analyses. This test was part of the necessary preparation work required for future tracer migration experiments in which it might be necessary to inject a water with a different chemistry if a constant supply of natural migration water in the quantities required can not be guaranteed over the anticipated experimental time scales.

The results of the hydrogeochemical equilibration test showed clearly that anions such as F- and CI- reached steady state concentrations in the extracted water within ~ 50 hours whereas the major cations, Na+ and Ca2+ required ~ 150 hours. The three cations, Sr2+, K+ and Mg2+ showed evidence of being retarded more strongly. Sr2+ did not reach steady state until after ~ 250 hours and K+ and Mg2+ appeared to be still increasing slowly when the experiment was terminated after ~ 312 hours. The conclusion from this first part of the work was that EM-water should not be used as a source of injection water in any future migration experiment with sorbing tracers.

These data were further analysed and interpreted assuming that the protomylonite fracture material behaves as a weak cation exchange medium giving rise to composition changes in the extracted water in addition to those resulting from simple mixing processes.

From the concentration-time data for the extracted water, curves representing the "interaction" component of the "foreign" water with the fracture material could be deduced. These results, together with selectivity coefficients for the major ions with respect to Ca2+, and some simple assumptions concerning the fracture geometry, enabled a cation exchange capacity for the fracture material to be estimated. Using the estimated CEC value, in situ Kd values for Na+ and Sr2+ were predicted. The sorption characteristics of Na+ and Sr2+ are of direct relevance to the Nagra/PSI programme at the GTS since field migration experiments with radioisotopes of both of these tracers are being carried out during 1990/1991.