Dissolved Ba as discriminator between two adjacent karst catchments that are both subject to allogenic recharge (Sohodol Valley, Vâlcan Mountains, Romania)

In a fluviokarst region, three rarely used natural tracers, SiO2, Na and Ba, were considered for tracking the allogenic, silicate-derived water contribution to cave streams and to final karst outflows. The concerned allogenic recharge originates in watersheds that consist of metamorphic formations intruded by magmatic rocks, for which available whole rock chemistry data indicate rather uniform contents of SiO2 and Na but contrasting (up to one order of magnitude) contents of Ba. All three considered natural tracers proved to behave conservatively along karst flow-paths and indicated binary mixing between allogenic and autogenic inputs. However, only the dissolved Ba concentrations enabled chemical distinction between two adjacent karst catchments: one with allogenic inputs presumably derived mainly from the weathering of Ba-rich rocks (essentially granites), and the other with allogenic recharge originating mostly from the weathering of Ba-poor formations. In contrast, if only the SiO2 and Na concentrations of the sampled waters had been considered, it would have been virtually impossible to establish whether the two adjacent karst catchments were distinct - or not - from each other. When considering each of the two karst catchments separately, the concentrations distribution of each of the three natural tracers, SiO2, Na and Ba, consistently indicated that between a swallet and a connected cave stream, then further between cave streams and final karst outflows, the allogenic water relative contribution gradually diminished to the benefit of autogenic water.