Temporal variability of metallic properties during flood events in the Deba River urban catchment (Basque Country, Northern Spain) after the introduction of sewage treatment systems

The aim of this study was to assess the particulate metal pollution status and the evolution of particulate metal contamination after the implementation of sewage treatment systems in the Deba River catchment during flood events. Concentrations of suspended particulate matter (SPM), dissolved (DOC) and particulate (POC) organic carbon and metals (Fe, Mn, Zn, Ni, Cu, Cr and Pb) were monitored in the urban watershed over four hydrological years. Geoaccumulation index was used to determine the pollution degree and a number of statistical analyses were performed to elucidate temporal changes. 14 flood events were analyzed from October 2011 to September 2015. The content of metals (Fe, Mn, Zn, Ni, Cr, Cu and Pb) in suspended solids showed a high degree of temporal variability which depended on number of factors being the hydrodynamic process the main one controlling the behaviour of particulate metals. Zn, Ni Cr and Cu appear to be mainly related to anthropogenic sources, whereas Pb appears to be lithogenic in character (like Fe) possibly related to an ageing process. The decrease of metal pollution degree and the homogenisation of organic and metal sources could be a consequence of the wastewater treatment, despite it seems likely that this will be a long-term process.