Fonctionnement d’un bassin de rétention d’eaux pluviales en milieu urbain

Au cours du projet OPUR et de ma thèse, le fonctionnement d’un autre système aquatique a été étudié. Il s’agit du lac du Héron, bassin de rétention d’eau pluviale qui se jette dans la Marque. Afin d’évaluer l’impact de ce système sur l’état de la rivière, plusieurs campagnes de mesures ont été réalisées sur l’année 2014 visant à caractériser le contenu en contaminants et en sels nutritifs du lac, à évaluer sa capacité épuratoire et à estimer l’impact de son rejet sur la rivière. De plus, les données haute-fréquence, fournie par la bouée de l’AEAP en 2015 ont permis de mieux appréhender la dynamique de la production primaire, qui est une problématique récurrente pour la MEL (Métropole Européenne de Lille), gestionnaire de ce bassin d’orage. Le chapitre IV compile les données provenant des eaux et des sédiments de surface du lac du Héron, un bassin de rétention d’eaux pluviales, situé à Villeneuve d’Ascq. La première partie de ce chapitre est sous forme d’article, en préparation, qui sera prochainement soumis dans une revue à comité de lecture de rang A (Environmental Science : Processes and Impacts). Il traite du fonctionnement du lac du Héron et de l’estimation de l’impact de son déversement sur La Marque Rivière. La seconde partie de ce chapitre se base sur le suivi haute-fréquence obtenu grâce à la bouée de l’AEAP. Les données enregistrées entre mars et décembre 2015 sont celles relatives aux paramètres physico-chimiques et aux pigments photosynthétiques.

Capacité épuratoire du lac du Héron

Stormwater ponds became an alternative management during the last decades in order to avoid flooding and to contain rainwaters and runoff waters in urban areas where impervious land covers have increased. The second goal of this kind of basin is to treat this waters which can be contaminated with nitrogen, phosphorus, metals and organic micro-pollutants by using natural methods as settlement for example, to minimise the impact of these inputs to the natural receiving aquatic system. This study aims to better understand the behavior of a wet stormwater pond, the Fonctionnement d’un bassin de rétention d’eaux pluviales en milieu urbain Chapitre IV – Fonctionnement d’un bassin de rétention d’eaux pluviales en milieu urbain 106 Heron Lake (33 ha) located in the city of Villeneuve d’Ascq in northern France through different indicators (trace metals, PAH, PCB, caffeine and carbamazepine, nutrients and pathogens). Our results from monthly monitoring and spatial screening on water and sediment samplings highlights (i) the presence of wastewater inputs into the Heron Lake; (ii) a globally efficient removal rate from the entrance to the outlet of the basin; (iii) highly contaminated sediments at the entrance channel with metals, PAHs and PCB which need to be dredged as soon as possible; (iv) eutrophication of this pond because of still high nutrients contents in waters and sediments, leading to the development of an invasive macrophyte, the Elodea Nuttallii; and (v) a negligible impact, at this monitoring scale, of the discharge from the lake to the natural watercourse, the Marque River. Keywords: stormwater pond, water runoff, waste water, nutrient, micro-pollutants, efficiency removal, pressure and impact. IV.1.a. Introduction The large increase of impervious land cover in urban area during the last decades led to the construction of many stormwater ponds. Firstly constructed in order to prevent flooding and/or to drain wetlands, their functions were broadened to the treatment of runoff stormwater to protect the receiving natural ecosystems from pollution. Treatments are generally based on settlement processes (Marsalek et al., 2005) since runoff pollution is mainly bound to particles (Gaspéri et al., 2009; Rule et al., 2006; Pitt et al., 2004; Gromaire-Mertz et al., 1999). Bacterial degradation processes could also occur in such constructed wetlands (Tixier et al., 2011). Runoff water quality depends on the local situation (e.g. runoff on roofs or pavement, road traffic, agricultural practices) and could be highly contaminated by Suspended Particles Matter7 (SPM; Athayde et al., 1983), trace metals (Huber et al., 2016; Robert-Sainte et al., 2009; Davis et al., 2001; Gromaire-Mertz et al., 1999), Polycyclic Aromatic Hydrocarbons (PAH) (Krein and Schorer, 2000), pesticides (Richards et al., 2016) or bacteria (Paule-Mercado et al., 2016; Paule et al., 2015). In addition to these anthropogenic pressures, stormwater basins could also receive untreated wastewater due to the management of sewage overflows during storm events or leakages into the sewer systems. Furthermore, runoffs in urban areas contain less nitrogen and phosphorus than wastewater treatment plant (WWTP) effluents (Graças-Silva, 2014). Consequently, untreated wastewater inputs in stormwater ponds could lead to the increase of Chapitre IV – Fonctionnement d’un bassin de rétention d’eaux pluviales en milieu urbain 107 nutrients contents. In this case, an additional challenge for the management of stormwater ponds is to control the enrichment in nutrients and as a result to limit the eutrophication of this aquatic system. Indeed, this process could disturb the quality of the aquatic system and lead to the development of invasive species (e.g. macrophytes) which could have several impacts on the management of the risk of flooding (e.g. clogging pumps of the water pond). Another challenge when the pond is also used for leisure activities is to limit blooms of cyanobacteria which could release toxins in the waters. To integrate the different goals, two main types of basins exist (Tixier et al., 2011; Marsalek et al., 2005): (i) ponds which are partly or completely empty between two storm events and (ii) wet ponds which are permanently kept at a certain level of water storage and allow the existence of aquatic habitats. The dimensions are also an important factor considering the capacity to integrate high storm events, the residence time of the water, and for example the depth to avoid the transport and/or the release of suspended materials in the entire area, in case of important events. This study is dedicated to the behavior of a large constructed wet stormwater pond (the Heron Lake in Northern France). The aims of this publication are: (i) to assess the contamination of this basin through the analysis of targeted substances (nutrients, trace metals and organic micropollutants: PAHs, PCB, caffeine and carbamazepine); (ii) to evaluate the treatment capacity of such pollutants; and (iii) to quantify the impact of the treated water on the quality of the receiving river (The Marque River). IV.1.b. Material and methods IV.1.b.i. Study area The Heron Lake is a stormwater management facility of 33 ha constructed in the 70’s (Figure 29). Its volume is estimated at 634 000 m3 , with an average and maximal depths of 1.5 m and 2.5 m, respectively. It receives the runoff water of the city of Villeneuve d’Ascq (64 000 inhabitants) pretreated by 5 successive smaller ponds but also direct inputs of runoff and sewage waters. Overall, these 6 lakes drain off water runoff from 1 440 ha of impervious areas (mainly pavement, streets, carparks and highways) and to a lesser extent, untreated urban waste waters. As reported by the Lille European Metropolis services (MEL), the runoff coefficient is estimated at 0.47 in this area. Otherwise the water level in the Heron Lake is controlled by three automatic Chapitre IV – Fonctionnement d’un bassin de rétention d’eaux pluviales en milieu urbain 108 pumps (each of them has a capacity of 750 L s-1 ). This system is located at the outlet and ensures the discharge of the water into a natural watercourse, The Marque River (average water flow of 0.8 m3 s -1 in 2014-2015; Ivanovsky et al., 2016). This discharge is representing an annual average of 7 % of the river flow. For example, in 2014, a total average discharge of 196 161 m3 to the river has been recorded. Finally, the residence time of the water within the Heron Lake is estimated around 2 to 3 months (MEL, 2016). In addition to the water management purpose, these lakes have been converted to a recreational area, open to the public with several activities (walking and running, fishing and bird sanctuary). In 2013, sailing stopped due to the presence of high density of macrophytes (principally Elodea Nuttallii) within the lake. Additionally, the senescence of the macrophytes at the middle of summer induces visual and odor nuisances and causes recurrently the clogging of the pumps.

Sampling, pre-treatment of the samples and in-situ measurements

Surface waters at the entrance (station 2, Figure 29) and at the outlet (station 12, Figure 29) of the lake were sampled monthly during one year (from February 2014 to February 2015) from a small boat. Then, the year 2015 will be notified only to February within the whole paper to differentiate those two months. Additionally, a spatial screening has been performed in January 2015 with 11 samples of surface waters and surface sediments (0-2 cm) in order to get a spatial view of the pollutant distribution within the entire lake (stations 2-12, Figure 29). An additional site (station 1, Figure 29) has been sampled, located in the channel entrance of the lake where a large sedimentation of SPM occurs. Chapitre IV – Fonctionnement d’un bassin de rétention d’eaux pluviales en milieu urbain 109 Figure 29. Location of the study area and sampling sites for the monthly monitoring (stations 2 and 12) and for the spatial screening in January 2015 (stations 1 to 12). The white arrow represents the main water flow through the lake. For trace metals analysis, surface waters were sampled using perfluoroalkoxy bottles previously washed with nitric acid (10 % v/v, Fisher Chemical suprapure, 65 %) and rinsed thoroughly with ultrapure water (Milli-Q gradient, Millipore, ρ = 18.2 MΩ cm). Water samples were filtered on site (0.45 µm, cellulose acetate, Sartorius), acidified with ultra-pure nitric acid (2 % v/v, Fisher Chemical Optima grade, 67-69 %) and stored in the dark at 4°C prior to analysis. Aliquots of filtrated samples were stored in glass tubes, previously pyrolized at 450°C during 24 h for DOC analysis. For organic micro-pollutants, sampling bottles in amber glass were washed using successive batch mixtures: detergent of a basic solution (Decon90®, 5 % v/v), HCl (10 % v/v, Merck suprapure) and finally, soaking and rinsing with ultrapure water. Water samples were filtered in the lab at 0.7 µm using GF/F Whatman filters, within 24 h after the sampling campaign. Surface sediments were sampled using a manual corer equipped with a Perspex tube (length of 35 cm and internal diameter of 7.5 cm). Only the 2 first centimeters of the sediment core were taken into account for this study. Back to the laboratory, the sediments were dried under a laminar flow Chapitre IV – Fonctionnement d’un bassin de rétention d’eaux pluviales en milieu urbain 110 hood. Afterwards, the samples were gently crushed and sieved to keep the fraction lower than 63 µm. The average rainfall between the data recorded at the closest station (i.e. < 2 km from the lake) by the MEL and at the meteorological station of the city of Lesquin was used as the daily pluviometry. The data relative to the water outflow of the lake were also provided by the MEL.