Flood inputs in a Mediterranean coastal zone impacted by a large urban area: Dynamic and fate of trace metals
Introduction
Estuaries constitute a transition zone between continent and ocean, and represent a major source of material to the coastal zone. The presence of trace elements in rivers relates to the regional geochemistry and inputs from various anthropogenic sources, especially where population density is high. These anthropogenic inputs are mainly due to leaching of impervious urban surfaces (roads, carparks, roofs) and soil surfaces by runoff, and to sewage overflow from sanitary sewer system during high rainfall events; locally treated wastewaters (TWW) discharge and industrial effluents can also be of importance (Bay et al., 2003, Bothner et al., 2002, Gonzalez et al., 1999, Matthai et al., 2002, Nicolau et al., 2012, Oursel et al., 2013, Wei et al., 2010). When discharged to the aquatic environment, trace elements can raise toxic levels and cause adverse effects to organisms or even human health (Gupta et al., 2009).
In freshwater/seawater mixing zones such as estuarine systems, the biogeochemistry of trace metals is controlled by a complex interplay of hydrodynamic factors, industrial and municipal wastewater discharges and biogeochemical processes. Differences of physico-chemical conditions between river and seawater can cause a redistribution of metals between solution and suspended particulate matter (SPM). The geochemical reactivity of trace metals in estuaries is commonly ascribed to changes in metal adsorption–desorption equilibrium and flocculation processes along the salinity gradient (e.g. Boyle et al., 1977, Zwolsman et al., 1997, Thill et al., 2001). Mobilisation of dissolved metals from solid phase is indeed the subject of considerable interest and numerous field data confirm the significance of metal release from river SPM along the salinity gradient (Elbaz-Poulichet et al., 1996, Kraepiel et al., 1997, Waeles et al., 2005, Audry et al., 2007). The morphological characteristics of the river watershed as climate, catchment area hydrology, geology and land use, and the physico-chemical characteristics of the water column as mixing time of waters or metal speciation can be decisive (Nicolau et al., 2012).
The Mediterranean environment is known for its high-intensity rains, promoting high rates of terrestrial runoff that can occur after long dry periods which allow contaminant accumulation in the watershed surfaces. The spatial and seasonal variability of rainfall follows a complex pattern, with wide and unpredictable rainfall fluctuations from 1 year to another (Martínez-Casasnovas et al., 2002, Nicolau et al., 2012). Numerous studies focused on Rhône River, the main river of the western Mediterranean basin and the largest contributor with regard to water and SPM discharge to the Mediterranean Sea (Ollivier et al., 2011, Radakovitch et al., 2008, Raimbault and Durrieu de Madron, 2003). Small coastal rivers, despite their low flows, can also have a high ecological impact because they rapidly bring to the sea the contamination occurring in the coastal area, especially where population density is high, however only few studies dealt with small Mediterranean rivers impacted by urbanisation (Dassenakis et al., 1997, Elbaz-Poulichet, 2005, López-Flores et al., 2003, Nicolau et al., 2012, Oursel et al., 2013). In Marseille agglomeration (1.7 million inhabitants), Huveaune and Jarret Rivers constitute a typical example of such rivers. Their watersheds are representative of modern anthropisation in most of the Mediterranean coastal strip, mixing urban, semi-urban, small industrial and some preserved natural areas. Their waters are mixed together and with city wastewater treatment plant (WWTP) effluents before their outlet to the sea (Fig. 1) at the Calanque of Cortiou, situated in the French National Park of “Calanques”. Studies on the dynamics of elements in transitional coastal zone (e.g. estuaries) are well documented (Elbaz-Poulichet et al., 1996, Ollivier et al., 2011, Shiller, 1997, Waeles et al., 2008), but a system such as Marseille is quite specific. Oursel et al. (2013) have demonstrated that the WWTP effluents represent the main source of inorganic contaminants brought to the coastal zone during baseflow periods. During flood events, the relative contributions of rivers vs. WWTP effluents are likely different. Indeed, studying experimentally the fate of Marseille watershed particles and associated pollutants when discharged to the coastal zone, Oursel et al. (2014) have observed that characteristics of particles from flood events were dominated by that of river waters, except during WWTP by-pass.
In this context, the aims of the present study were (1) to quantify and characterise carbon and trace metals dynamics and inputs to the sea during flood events in comparison to baseflow periods and (2) to understand the mechanisms governing elements behaviour and fate in the mixing zone during these contrasted conditions.
Section snippets
Study site
Huveaune River extends over 48.4 km long and runs through a watershed with an area of 523 km2 which consists of karstic formation (60%) and detrital sediments. Land-use in the downstream part of the watershed is urban and industrial. Jarret River extends over 21 km with a 102 km2 watershed mainly urban and industrial. For the period 1961–2009, the mean annual precipitation in Marseille was 544 mm and the mean number of days with total rainfall of 1 mm or more was 59 (Infoclimat, 2014).
In baseflow
Dynamic of elements in rivers during flood event
Table 1 gives the average of the measured parameters in Huveaune and Jarret Rivers, at the outlet and in the marine end-member during the flood, compared to those observed during floods in Eygoutier River, a small coastal river of the same geographic area (Nicolau et al., 2012) and to those observed in floods of Rhône River (discharge over 1500 m3 s− 1), the main river of the French Mediterranean area (Ollivier et al., 2011).
Fig. 2A represents the discharge in rivers and the rainfall at Marseille
Concluding remarks
During rain periods, the dissolved and particulate trace metal temporal dynamics in rivers were flood, river and element dependent. The studied system was highly dynamic and rapidly answered from the beginning of rain, which is typical of small Mediterranean rivers, especially when the water catchment is urbanised, requiring an adapted monitoring strategy. Metals contamination was mainly brought by the particulate fraction that originated from the leaching of impervious surfaces and soil
Acknowledgements
The authors would like to thank all the persons who participated at sampling campaigns, for their help in samples preparation. The authors would also like to thank “Météo-France” (Y. Bidet), “MIO” (C. Yohia), “SERAM” (A. Queau, F. Jonot), and “DEA” of Marseille City (Z. Djelalli, S. Barde) for access to the study site, material installation, flow and rainfall values, meteorological data at the Marseille observatory station (“GIRAC-PACA” project). The authors would also like to thank D.H. Dang
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