Extreme hydrological regimes of a small urban river: impact on trace element partitioning, enrichment and fluxes

The input of trace elements from a small urban river (Las River, Toulon, France) located on the northern Mediterranean coast was studied during both base flow and flood events. A 2-year monitoring period of water flow and suspended particulate matter (SPM) showed a typical Mediterranean hydrological regime: a strong increase in water flow and SPM during short flood periods. During the flood event, an up to 2-fold increase in dissolved trace element ( D TM ) concentrations and particulate trace element content in SPM ( P TM ) was observed compared to the baseline discharge. The enrichment factor of elements in the SPM ranges from low or moderate for Co, Ni and Cr (1.0–4.7) to extremely high for Cd (157). However, the enrichment factors decrease from base flow to flood, indicating a dilution effect with a large yield of weathering particles with higher particle size. The most significant total trace element loading occurred during flood, ranging from 78% for As and Ni to 91% for Pb, while P TM loading during flood ranged from 35% for As to 77% for Pb. The specific dissolved fluxes during the flood are significantly higher for Pb, Cu and Zn than in the surrounding rivers, indicating specificity in the catchment (lithology). This study shows the importance of monitoring the transport of pollutants through small urban rivers and their potential impact on the coastal region, especially when they enter small and closed bays, as a receiving pool.