Dissolved Ba as discriminator between two karst catchments that are both
subject to allogenic recharge (Sohodol valley, Vâlcan Mountains,
Romania)
Abstract
In a fluviokarst region, three seldom used natural tracers, SiO
2, 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 SiO 2 and Na, but contrasting (up to
one order of magnitude) contents of Ba. All three considered natural
tracers proved to behave, along karst flowpaths, conservatively, and
indicated binary mixing between allogenic and autogenic inputs. However,
only the dissolved Ba concentrations enabled chemical distinction to be
made between two separate karst catchments: one having allogenic inputs
presumably derived mainly from the weathering of Ba-rich rocks
(essentially granites), while the other had allogenic recharge
originating mostly in the weathering of Ba-poor formations. In contrast,
if only the sampled waters SiO 2 and Na concentrations
had been considered, it would have been virtually impossible to
establish if the two 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, SiO
2, 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. A two-component
mixing model indicates that the allogenic water fraction in the major
karst outflows remained, nevertheless, always important: about 70 -
80%, with a larger percentage being recorded during a low water stage.