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 hydrological tracer of groundwater movement. Laboratory-scale infiltration experiments using cylinders cored from granite and repacked columns of crushed granite showed that the change of the natural isotopic ratio Br-79/Br-81 with time can be used to monitor the tracer breakthrough. Comparison was made with other non-sorbing tracers, such as tritiated water, chloride and iodide. The difference in travel time of halides was suggested to be related to the solute size. A Br-79(-) migration experiment in crystalline rock was carried out between two boreholes connected by a low-permeability fracture. This showed the suitability of Br-79(-) as a groundwater tracer also for field tests.