Micropollutants removal and storage efficiencies in urban stormwater constructed wetland

Stormwaters is identified as a major source of pollution in waterbodies. Particularly, heavy metals (HMs) and Polycyclic Aromatic Hydrocarbons (PAHs) in stormwater are highly toxic compounds for living organisms. To limit the impact of these micropollutants on hydrosystems quality, stormwater constructed wetlands (SCWs) have been built worldwide. This study aims to i) assess the efficiency of a SCW that combines a sedimentation pond followed by a vertical flow sand filter in urban area (Strasbourg 67, France) and ii) determine micropollutants storage in water and soils during dry periods. Stormwater quality was analysed during 13 sampling sessions and the SCW storage ability during dry period was highlighted. The rainfall events sampled are characterized by very high variability: dry periods lasted from 5 h to 10 d, rain durations varied from 15 min to 22 h and the return periods were between 2 and 4 wk. and 3-6 mo. The inflow stormwater included a high amount of Zn and a variety of PAHs. Cu, Zn and some PAHs concentrations are impacted by hydrological characteristics. During a rain event, the filter catches the majority of both dissolved and particulate micropollutants and the mobilization of particulate micropollution by incoming flow decreases pond removal efficiency. The treatment removal efficiency varied from 50% (naphthalene) to 100% (particulate Zn). Four HMs (Co, Cu, Pb, Zn) were found in the pond and seven (Cd, Cr, Co, Cu, Ni, Pb, Zn) in the filter during a dry period at high concentrations compared to their occurrence in rainfall. A release of HMs from the filter sand to the interstitial water is highlighted. In water and the soil matrix, PAHs occurrence was consistent with their water solubility, logKow and logKoc.