LABORATORY AND IN-SITU INVESTIGATION OF THE ISOTOPIC RATIO BR-79 BR-81 AS A TRACER OF WATER MIGRATION IN CRYSTALLINE ROCKS/

The natural isotope Br-79 in the bromide form was used as a hydrologic al tracer of groundwater movement. Laboratory-scale infiltration exper iments using cylinders cored from granite and repacked columns of crus hed granite showed that the change of the natural isotopic ratio Br-79 /Br-81 with time can be used to monitor the tracer breakthrough. Compa rison was made with other non-sorbing tracers, such as tritiated water , chloride and iodide. The difference in travel time of halides was su ggested to be related to the solute size. A Br-79(-) migration experim ent in crystalline rock was carried out between two boreholes connecte d by a low-permeability fracture. This showed the suitability of Br-79 (-) as a groundwater tracer also for field tests.