Characterizing fluxes of trace metals related to dissolved and suspended matter during a storm event: application to a karst aquifer using trace metals and rare earth elements as provenance indicators

The origin of suspended matter and colloids in karst aquifers during storm events is not well understood in spite of their potentially important role in the transport of poorly soluble inorganic, organic, and microbiological pollutants. This work aims to characterize the fluxes of trace metals related to dissolved and suspended matter during a storm event at the spring of a karst aquifer in the Jura Mountains in France that is mainly recharged by diffuse infiltration. Various natural tracers, including rare earth elements (REE), were used to identify the origin of the contributing dissolved and suspended fractions. The storm event was characterized by the temporally delayed arrival of two different fractions of suspended particulate matter (SPM). A first SPM peak corresponded to autochthonous conduit sediment mobilized by a piston effect at the beginning of the event. A second SPM peak, related to the arrival of allochthonous soil particulates, was characterized by elevated turbidity and high concentrations of K, Al, Fe, and Mn. In the dissolved fraction, this second SPM peak was accompanied by various poorly soluble trace elements, which were interpreted by the presence of nanoparticles and colloids (NPCs). The REE distribution patterns demonstrated that both the NPCs and the SPM were derived from soil particles whatever the storm stage. This suggests that the SPM of the first stage was reworked cave sediment from previous storms composed of residual clays and soil particles, which excludes authigenic precipitates (such as Fe-Mn oxyhydroxides and speleothems) as a significant source for autochthonous sediments.