Mobility of trace metals associated with urban particles exposed to natural waters of various salinities from the Gironde Estuary, France

Background, aim and scope  Urban systems are hot spots of environmental pollution caused by manifold anthropogenic activities generating traffic-related, industrial and domestic emissions. Besides air, soil and groundwater pollution, pollution of surface water systems is of major concern because they are often (ab)used to export waste of various consistence out of urban areas and become contaminated on varying scales. The Gironde Estuary (southwest France) is affected by various anthropogenic contaminations derived from historic polymetallic pollution mainly due to former mining and ore-treatment and, additionally, from agriculture and urban areas. Although detailed knowledge is available on the impact of mining and anthropogenic activities on the water quality of the Gironde Estuary, almost nothing is known on the urban impact, even though the Garonne Branch which is one tributary of the Gironde system crosses the large urban agglomeration of Bordeaux. The present work links urban geochemistry and estuary research and aims at evaluating the mobility of potentially toxic trace elements (Cd, Cu, Zn, V, Co, Mo, Pb) associated with urban particles under estuarine conditions owing to the particles' role as potential vectors transporting urban pollutants into the estuary. For this, environmentally available fractions of trace elements in representative urban particles (urban dust, road sediment, riverbank sediment, construction materials) from the city of Bordeaux were extracted by natural estuarine waters of varying salinities and compared to commonly applied HNO₃ extractions. Materials and methods  For the assessment of the urban particles' contribution to the pollution of the Gironde/Garonne system, various particle types were sampled in Bordeaux: road sediments, urban bulk deposition, construction materials (concrete, asphalt, tile and gravel) and flood sediments. Potentially environmental available fractions of Cd, Cu, Zn, V, Co, Mo and Pb were extracted by means of concentrated HNO₃, estuarine freshwaters and waters of two different salinities (S = 15 and S = 31). Analysis of trace elements was carried out by means of quadrupole inductively coupled plasma-mass spectrometry. Furthermore, single particles from road sediments were characterised with scanning electron microscopy (SEM). Results  SEM analysis clearly showed that some particles contained fairly high concentrations of potentially toxic trace elements. Extractions of materials investigated by varying acidities and salinities documented that the potentially bioavailable fractions extracted by concentrated HNO₃ may cover wide concentration ranges. Natural estuarine waters of various salinities (S = 0.5; S = 15; S = 31) extracted high proportions of Co, Ni, Cu, Zn and Cd from urban particles, especially for high-salinity water (S = 31). Extractions with freshwater revealed the lowest concentrations of desorbed trace elements. Particulate Mo, Pb and V showed similar or lower mobility in saline water compared with freshwater, depending on the sample type. Discussion  Trace element mobility in estuarine waters varied according to the type of urban particles and depended on salinity for Co, Ni, Cu, Zn and Cd. This is of high importance for towns located directly at the coast or for cities like Bordeaux, where water courses crossing the agglomerations are connected to saline water masses. Since trace elements desorbed from particles in saline waters may become highly bioavailable, they bear a potential risk for organisms. Comprehensive studies on the behaviour of urban particles in estuarine waters and the related potential environmental impact are still missing. Conclusions  Saline waters mobilise relatively high amounts of Co, Ni, Cu, Zn and Cd from urban particles suggesting considerable metal fluxes from riverine urban systems into coastal waters. Although estimates of trace metal inputs by urban bulk deposition (urban dust) and other types of urban particles are preliminary for Bordeaux and may bear important uncertainties due to several assumptions and extrapolation to the annual timescale, the orders of magnitude are probably realistic. Thus, these fluxes are not negligible and need (1) further and improved observation and (2) to be taken into account in both mass budgets at the estuary scale and emission control strategies. Recommendations and perspectives  New approaches combining geochemical and mineralogical characterisation of single urban particle types help identify their role in metal emission into the environment and develop potential limitation strategies (e.g. the ban of priority pollutants in tyres, etc.). Therefore, prioritisation of urban particle sources in terms of fluxes, reactivity of associated pollutants and feasibility of emission reduction is strongly recommended. Coastal cities should integrate extractions of urban particles with saline water into their environmental monitoring programs owing to the fact that saline conditions might cause efficient desorption of potentially toxic trace elements. In continental cities, winter salting is likely to induce intense mobilisation of metals from road sediments that may then reach the aquatic environment, instead of being retained in runoff decantation reservoirs followed by subsequent disposal/treatment with road sediments. However, also particles from continental cities reach coastal waters via rivers and have to be assessed with respect to trace metal desorption under various salinities. There is a strong need for the quantification of fluxes and for the identification of carrier phases and reactivity of metals exported from urban areas to aquatic systems.