Geochemical flood deconvolution in a Mediterranean catchment (Hérault, France) by Sr isotopes, major and trace elements

This study, based on a chemical and isotopic approach (major, trace elements and strontium isotopes), focuses on the geochemical hydrograph separation during flood events in a Mediterranean watershed (Hérault, Southern France). Major element contents present a typical decreasing logarithmic curve when plotted against river discharge. The concentrations measured during floods are, however, always higher than the theoretical dilution curve and can be related to (1) rainfall input in this sea-side basin, especially for Cl, and (2) water-rock interactions, which release elements in solution especially from the carbonate compartment of the watershed. Ca and HCO₃ are not correlated with the river discharge, their content in the dissolved phase being controlled by calcite and aragonite mineral phases. Sr is a proxy of Ca regarding its physico-chemical properties, but it has been demonstrated that Sr is not controlled by these mineral phases and it presents a logarithmic variation with discharge. It is also very well correlated with Cl, which is a conservative element par excellence. Sr and its isotopic ratio can thus be used to constrain the geochemical behaviour of the dissolved phase during flood events. The main tributaries, characterised under various hydrological conditions, defined very well constrained end-members as the result of the diversity of the lithologies drained. Monitoring at the outlet of the watershed during four flood events showed that (1) during low flow periods, the signature at the outlet results from a binary mixing between the two main water sources in the basin; (2) each flood event studied presents a different temporal geochemical evolution (counter- or clockwise evolution in a ⁸⁷Sr/⁸⁶Sr vs. reciprocal Sr-content diagram) reflecting the predominance of different water sources during the flood, i.e., the tributaries versus the karst aquifers depending on rainfall intensity, duration and location; (3) the sequence of water origins during the flood seems also be controlled by the initial state of the soil and epikarst-karst continuum, inducing a more or less rapid and pronounced response after rainfall events.