Evaluation of some tracer tests in the granitic rock at Finnsjoen

Tracer tests in a granite rock have been analysed. A nonsorbing tracer -iodide- was use to investigate the hydralic properties of the pathway. This gives the mean residence time(s) and spreading. The latter may be caused by channeling, hydrodynamic dispersion and interaction with the matrix. Four different models emphasizing different combinations of the various mechanisms were used. It was found that even the nonsorbing tracer interact considerably with the matrix. Recent laboratory data wich show a high porosity of fissure coating materials indicate that the cause for this interaction may be diffusion of the tracer into the pore water of the matrix. The residence time(s) and dispersion data obtained from the iodide test were used to predict the behaviour of the sorbing tracer strontium. Independent laboratory data for strontium sorption and diffusion in granite were used in the predictions. The predicted data were in very poor agreement with the field data. The field data were then used to fit the model parameters. The resulting combination of sorption and diffusion data wich gives a good fit to the field data was off by a few orders of magnitude. An examination of some recent sorption and diffusion data show at at the high concentration, wich was used in the field test, the sorption isotherm is nonlinear and the sorption capacity considerably smaller than that used in the original predictions. The new data would account for the lower retardation. As, however, the new sorption data have not been performed on the actual fissure coating material in the flowpath and no diffusion measurements at very high concentrations have yet been performed the conclusions are only tentative. (Forf)