Analysis of fracture-induced water mixing by natural tracers at a granitic pluton (El Berrocal, Spain)

We use hydrochemical data and natural isotopic tracers to study the groundwater flow regime, at the regional scales of a granitic pluton. The tracers involved in this study are tritium, deuterium and ¹⁸O. Renewal times inferred by tritium content considering an exponential mixing flow model, allowed us to distinguish between the local discharge zones (shallow waters) from the regional ones, with renovation times above 1000 years. The exponential model was validated using stable isotope data. Most of the springs appear associated to highly conductive fractures in the middle-slope of the batholith. They display conspicuous differences in stable isotopes composition, tritium content as well as major and minor chemical elements. The contrast in hydraulic conductivity of the rock mass and the transmissivity of the fractures induces an important mixing of groundwater of different ages and origins. Hence, the “mixing model” which relies on geologic and hydraulic evidences explains hydrochemical and isotopic anomalies.