Base flow and stormwater net fluxes of carbon and trace metals to the Mediterranean sea by an urbanized small river

Base flow and storm flow events from a small, urbanized Mediterranean river located in the South of France were studied to evaluate net fluxes from the continent to the coastal sea water. Considered variables were: pH, conductivity, redox potential, temperature, dissolved O2, SPM (Suspended Particulate Matter), Cl−, NO3−, SO42−, Na+, K+, Mg2+, Ca2+, DOC (Dissolved Organic Carbon), Cu, Zn, Cd and Pb in the dissolved fraction and POC (Particulate Organic Carbon), Cr, Mn, Fe, Ni, Cu, Zn, Cd and Pb in the particulate fraction. If rainfall intensity and antecedent rainfall history were sufficient to explain many observed variations, the patterns of particles and solutes transport greatly varied from one storm event to another. SPM, POC, particulate Cu, Zn, Cd and Pb had similar behavior characterized by an immediate increase at the beginning of the storm flow and the highest values at the first high discharge peak. Among dissolved species, Cl−, SO42−, Na+, Ca2+ and Mg2+ had a behavior very similar one to the other. They exhibited high concentrations and enrichment factors at the beginning of the storm flow, due to fast leaching of highly labile species. Their concentrations decreased during posterior discharge peaks but positive enrichment factors indicated permanent sources for these ions. DOC, K+ and NO3− had different behaviors which indicated sources positively correlated with rainfall intensity. A 3D-fluorescence study showed that the humified organic matter contribution to DOC increased during storm flow. Relationships between DOC and dissolved Cu and Pb indicated differences in organic-matter binding properties between dry and humid periods. Storm flow were responsible for more than 90% of the annual output to the sea of SPM, POC and particulate Cu, Zn, Cd and Pb and more than 70% for dissolved Pb, Cd, NO3− and DOC. For the other dissolved species, outputs were balanced between base flow and storm flow. Contrary to what was observed in large rivers, organic carbon was mainly transported in the POC fraction. The average specific fluxes of Cu and Pb to the sea were 3.8 and 3.4 kg km−2 y−1, respectively, of the same order of magnitude than specific fluxes of other North-Mediterranean rivers, but outputs were more intense during shorter durations. The extrapolation of the Eygoutier River data to the Mediterranean non-desert coastline showed that the order of magnitude of Cu and Pb annually brought to the sea by the whole of small anthropized coastal rivers can be similar to the annual input by the Rhône or the Po River.

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► Base flow and flash floods were studied in detail for dissolved and particulate species.
► Annual fluxes are mainly due to flood events for most species.
► Particles and solutes transport patterns greatly vary from one flood to another.
► Metal-binding properties of transported organic matter exhibit seasonality.
► Urbanized Mediterranean coast would transfer Cu and Pb net fluxes similar to large rivers as Rhône.