Natural tracers (silica, barium, rubidium and uranium) provide insights into the karst hydrology of Grijii Valley (Vâlcan Mountains, Romania)

Abstract

When underground flows in a karst region are derived not only from autogenic, but also from allogenic recharge, natural chemical tracers can be used for distinguishing between these two contributing inputs. And since allogenic recharge prevalently originates in weathering of silicate rocks, the most appropriate natural tracers are those which are more abundant in such formations than in carbonate ones. Moreover, the tracers must not interact with the limestone or dolomite rocks hosting the underground flow-paths. The karst setting addressed by the present study includes a sinking stream, a cave intercepting the sunken streamflow, and three karst springs, two perennial, and one temporary. The objective of the present study was to assess, for the concerned setting, the capability of four natural tracers, SiO2, Ba, Rb and U, of tracking the specific contributions of allogenic and autogenic inputs along underground karst flow-paths. Those chemical constituents were noticed to comply with the specified requirements for natural tracers, as their maximum concentrations were recorded in the swallet flow, and gradually declined toward the cave stream, then further to the temporary spring. The latter was thus proven to act as an overflow of a currently inaccessible underground course, which is supplied by the cave stream, but whose final outlet is not yet identified. Alternatively, the perennial springs resulted to be proxies of two chemically distinct autogenic inputs: one supplementing the groundwater flow between the swallet and the cave stream, and the other contributing to the flow between the cave stream and the temporary karst spring. It is worth mentioning that so far, the four indicated natural chemical tracers have been only sporadically utilized in karst aquifers with allogenic recharge.

Keywords

• natural tracers
• allogenic recharge
• silicate rocks
• autogenic recharge proxy
• vâlcan mountains