Isotope hydrological study of mean transit times and related hydrogeologica- l conditions in Pyrenean experimental basins (Vallcebre, Catalonia)

Measurements of tritium and oxygen-18 concentrations in water samples from streams at low discharge, wells, and permanent springs are used to provide insight into hydrogeological conditions, storage properties, runoff formation, and flow partitioning of Pyrenean experimental basins 100 km north of Barcelona. Transit times as derived from tritium range between 8-11.5 years for wells, 8.5-13.0 years for streams, to 10.5-13.5 years for springs. These results are twice the usual values which suggests low permeability of the bedrock formations. Minimum groundwater reserves can be determined from low discharge and mean transit times, and groundwate- r plays an important role in flood generation. Tritium is still found to be a useful tracer provided that transit times are high enough, and geological information is available.