Contribution of small coastal rivers to copper export to the Gulf of Lions

Soils in the Gulf of Lions region (NW Mediterranean) show some of the highest copper (Cu) levels in Europe (Ballabio et al, 2018). The episodic and erosive nature of rainfall in the Mediterranean area (González-Hidalgo et al, 2007), historical intensive agricultural practices using Cu to fight the downy mildew and its relatively high solubility make the study of its transfer and consequences along the land-sea continuum a major issue. The main objective is to quantify the Cu fluxes from coastal rivers discharging in the Gulf of Lions and characterise their temporal variability highlighting the importance of floods on the transfer processes of matter and associated contaminants in the Mediterranean region.

Since 2006, the SNO MOOSE, a multi-platform and multi-site observation network designed to monitor the evolution of the Mediterranean basin in a context of global change, has been carrying out. Therefore, on a monthly monitoring basis, trace metal concentrations in suspended particulate matter (SPM) have been estimated in the five main small coastal rivers of the Gulf of Lions as well as the Rhone River.

These observations coupled with a sediment flux model (Sadaoui et al, 2016) allow the estimation of elemental fluxes. Small rivers have the highest average Cu content in suspended matter (80.3 µg/g) and in the soils of the catchment areas (79.7 µg/g) (approximatively a factor of 2 compared with the Rhone). Mean annual estimation of Cu fluxes are about 316T/year with an interannual variability of 36%. The Rhone River is by far the major contributor to the fluvial exports of particulate copper to the Gulf of Lion. However, although small coastal rivers account only for 6% of SPM inputs, their contribution to particulate Cu fluxes averages 9.7%.

Interannual variability of fluxes is controlled by the occurrence of episodic flash floods on coastal rivers typical of the functioning of Mediterranean watercourses (Roussiez et al, 2011, 2012).

The SPM transport originating from surface soil erosion and associated copper mainly takes place during these brief events for small coastal rivers (annual average of 66% of total Cu export against 19% for the Rhone). According to the number of events occurring each year their relative contribution of Cu fluxes compared to the Rhone is highly variable (from 2 % in 2012 up to 41% in 2011). Thus, the influence of coastal rivers on the global Cu budget to the Gulf of Lions is not negligible.

Moreover, copper transferred in rivers from the erosion of wine-growing soils is mainly in extractable form, which is more hazardous for the environment because it is both mobile and potentially assimilable by organisms.