Délimitation des zones de protection autour de la retenue du barrage Hachef (Maroc) par télédétection et SIG
A. El Garouani and A. Merzouk
La présente étude concerne le barrage Hachef (Barrage nommé 9 Avril 1947), mis en service en 1995, situé dans le Rif Occidental Marocain à environ 30 km au sud de la ville de Tanger et 15 km à l’est de la ville d’Asilah. La retenue du barrage d’une capacité utile de 270 millions de m3 reçoit un apport solide de 609 000 tonnes/an à cause de l’érosion des versants favorisée par le déboisement. Des foyers de pollution sont également identifiés dans les régions limitrophes du barrage. Dans le but de préserver et protéger cette ressource destinée à l’alimentation en eau potable de la population des régions voisines, il s’est avéré nécessaire de mettre en place des périmètres de protection. Ceci entraîne également une plus grande rentabilité des investissements et infrastructures réalisés. Dans ce travail, une solution informatisée pour la délimitation des périmètres de protection a été conçue et réalisée. Ainsi, un modèle d’analyse spatiale par le SIG basé sur l’évaluation de la vulnérabilité à la contamination de la ressource a été élaboré. Toutes les données disponibles, aussi bien celles issues de la télédétection que celles du terrain, ont été modélisées et intégrées au sein d’une base de données géographique gérée par un SIG. Dans cette expertise, les différents aspects de la problématique concernant la sauvegarde de la ressource en eau ont été cernés et les besoins des gestionnaires ont été recensés. L’utilisation de cette méthode a permis de délimiter les trois zones de protection autour de la retenue du barrage (zones de protection immédiate, rapprochée et éloignée).
Due to the degradation of the quality of Moroccan water resources, it is necessary to set up mechanisms and actions to ensure their conservation and protection. The creation of protection zones is one of the more effective means for establishing this protection. The study region involved the Hachef dam, operating since 1995 and located in the Moroccan Occidental Rif, approximately 30 km south of Tangiers City and 15 km east of Asilah City. The watershed area is 220 km2 with an annual average precipitation of 900 mm. The useful capacity of the dam is 270 million m3. By storing 90 million m3/year in normal years, this dam supplies a water supplement to the existing resources, covering drinking water requirements and irrigation for the region located between Tangiers and Asilah beyond the year 2015.
In order to classify the importance of protection from pollution, the watershed was divided into three zones (zones I, II and III), as defined by their distance from the resource to be protected. In the immediate protection zone (zone I), it is necessary to assure protection of the dam against water pollution. The dimension of this zone should allow for appropriate intervention in the event of an accident and also allow for planting of a forest or a zone of protective vegetation. In the next protection zone (zone II), one should assure protection against pollution and other anthropogenic stressors that could threaten the harnessed waters. In zone III, located furthest away from the resource to be protected, the protection of waters should focus on major pollutant problems, in particular those cases difficult to eliminate such as chemical or radioactive contamination. The demarcation of these zones requires several disciplines and different expertises.
In order to facilitate the management, storage and analysis of necessary data for the demarcation of protection zones, we used a Geographic Information System (GIS), in which remote sensing allowed the description of the natural environment (land use, crop types, infrastructure, hydrographic network, etc.). The following cartographic approach represents the analysis of the spectral signatures of the main types of land use by a supervised classification based on the minimum distance method. The study of data in the context of a dam allowed us to identify the information flows, as well as to analyse and collect data in the form of information layers including both cartographic and alphanumeric data (Table 1).
In the demarcation of the protection zones around the reservoir, we conducted adjustments and multi-criteria analyses while taking into account a number of parameters (Figure 3). The manipulation of the system was assured by structured tools that offer to the user all the possibilities of GIS management, from data entry to the output of results (Figure 2). The examination of the study region by remote sensing data (Spot HRV image of 17-08-96 and Landsat TM image of 28-03-96), allowed the discrimination of the distribution of the different land use classes (Figure 4). The interpretation of aerial photos and observations allowed us to establish the ground truth against which the image processing results were compared. The adopted GIS contained software to plan the various tasks required for the demarcation of the protection zones demarcation.
After analysis of all the potential factors to be considered for the demarcation of the protection zones (geologic, hydrogeologic, climatic, anthropogenic factors), we noted that geologic and hydrogeologic factors make a negligible contribution to the pollution of the reservoir. In contrast, morphology, degradation of vegetation, the hydrographic network and agriculture are important factors for determining the protection zones. Integrating the previously mentioned factors allowed the demarcation of three protection zones, as presented in Figure 5.
This work has taken advantage of GIS implementation to define protection zones for a potable water supply (dam). This research has also defined different necessary stages for the conception of a prototype that integrates the remote sensing data into a GIS for the delimitation of these zones. The implementation of these zones represents a possibility for effectively protecting water resources and extending their potential lifetime. This action also generated a better profitability for the investments and the infrastructure.
Butylétains dans les eaux du fjord du Saguenay (Canada) : menace pour l’écosystème d’un milieu semi-fermé ?
Liza Viglino and Émilien Pelletier
Dans le cadre de travaux sur le comportement des composés des butylétains dans les milieux côtiers froids, des échantillons d’eau, de matière particulaire en suspension et de seston (phyto- et zooplancton) ont été prélevés à huit (8) stations le long du fjord du Saguenay (Canada) et dans la baie des Ha! Ha! en mai 2001. Les concentrations en butylétains totaux (MBT + DBT + TBT) étaient significativement plus élevées en surface (26 à 206 ng Sn L-1) que dans les échantillons de fond (7 à 30 ng Sn L-1). Les niveaux trouvés à l’embouchure du fjord étaient deux fois plus élevés que ceux observés dans son axe principal et cinq fois plus élevés que ceux dans la baie des Ha! Ha! dont les concentrations variaient de 40 à 55 ng Sn L-1 avec les plus élevées à proximité de Port‑Alfred. Le tributylétain (TBT) est toujours le composé minoritaire, que ce soit dans les eaux de surface (de 1 à 5 %) ou dans la couche d’eau profonde (5 à 24 %). Le dibutylétain (DBT) domine dans les deux masses d’eau avec des proportions d’environ 85 % pour la surface et de 34 à 90 % au fond. Le monobutylétain (MBT) est présent dans toute la colonne d’eau avec des pourcentages fluctuant de 4 à 15 % dans les eaux de surface et de 2 à 46 % dans les eaux profondes. Les butylétains sont également présents dans tous les échantillons de seston. En surface, les concentrations des métabolites (DBT + MBT) sont plus élevées (25 à 59 ng Sn g-1) que celles du TBT (10 et 20 ng Sn g-1). Dans la couche de fond, le TBT est majoritaire dans le seston avec des concentrations similaires entre les stations d’environ 30 ng Sn g-1. Les facteurs de bioconcentration obtenus à partir des données du seston confirment que les niveaux de TBT dans l’eau sont suffisants pour induire une bioaccumulation par étape au sein de la chaîne alimentaire. Enfin, les concentrations en TBT dans la colonne d’eau semblent bien au-dessus du niveau susceptible de perturber l’écosystème en causant des effets chroniques sur la reproduction de plusieurs organismes ou en affaiblissant leurs systèmes immunitaires.
In order to understand the fate of butyltin compounds in cold coastal ecosystems, samples of water, suspended particulate matter and seston (phyto- and zoo-plankton) were taken in the Saguenay Fjord, a deep and narrow glacial valley filled with seawater from the St. Lawrence Estuary (Canada) and used as a navigation channel to reach an upstream industrial area. On one hand, this study was designed to evaluate the butyltin contamination level of waters of the Fjord. On the other hand, the samples were used to better understand the behaviour of tributyltin (TBT) and its breakdown products (dibutyltin (DBT), monobutyltin (MBT)) in the water column and its associated ecosystem. In addition, this study enabled us to establish if concentrations found in the Fjord may represent a toxic threat for the biota.
The water sampling was carried out at eight stations and various depths along the Saguenay Fjord and in the Baie des Ha! Ha! in May 2001. During this same expedition, samples of seston were also collected at 5 and 70 m depth for the same stations. Butyltins were extracted according to a well defined protocol and quantified by gas chromatography - mass spectrometry (GC-MS) on an Ion Trap GC/MS operated in single ion monitoring (SIM) mode. In the water column, concentrations of total butyltins (MBT + DBT + TBT) were significantly higher in surface samples (26 to 206 ng Sn L-1) than in deep samples (7 to 30 ng Sn L-1). The levels found at the mouth of the Fjord were two times higher than those observed along its main axis and five times higher than those in the Baie des Ha! Ha! where concentrations varied from 40 to 55 ng Sn L-1 with the highest values observed near Port Alfred. Tributyltin (TBT) was always a minor component in water surface (from 1 to 5%) as well as in the deep water layer (5 to 24%). Dibutyltin (DBT) dominated in the two water masses with proportions of approximately 85% for surface and from 34 to 90% at the bottom. Monobutyltin (MBT) was present in all waters, contributions fluctuating from 4 to 15% in surface water and from 2 to 46% in deep water. Butyltin compounds were also present in all seston samples. In the surface layer, concentrations of metabolites (DBT + MBT) were higher (25 to 59 ng Sn g-1) than TBT itself (10 to 20 ng Sn g-1). However, TBT dominated in the deep samples (70 m depth) with similar concentrations among stations of approximately 30 ng Sn g‑1.
This study presented the first evidence of the widespread butyltin contamination of Saguenay Fjord with levels typically reported for contaminated coastal areas. The shipping traffic was considered as the main source of continuous butyltin inputs, essentially TBT, but industrial activities located along the St. Lawrence Estuary were also suspected of generating the release of significant quantities of DBT, and thus contributing to the total reservoir of butyltins. The particular oceanographic conditions of the Saguenay Fjord (tidal cycles, strong stratification of water column) were responsible for the variation of concentrations among stations but also among depths. In addition, results observed in this study (high bioconcentration factors) confirmed the affinity of TBT for particulate matter compared to its metabolite, DBT. The chemical properties of TBT (low solubility, high water-sediment and water-octanol partition coefficients) are such that the processes of sorption and bioaccumulation in organisms are favoured. Bioconcentration factors calculated from seston data confirmed that the levels of TBT in water were sufficient to induce a stepwise bioaccumulation throughout the food chain. Consequently, suspended particular matter and organisms were the two compartments responsible for the TBT elimination from the water column to sediment. On the other hand, because of its higher solubility and a lower water-sediment partition coefficient, DBT was the major compound in the dissolved phase and its behaviour was influenced by hydrographic conditions. Lastly, TBT concentrations in the water column seemed well above the level likely to disturb the ecosystem by causing chronic effects on the reproduction of several organisms or by disturbing their immune systems.
Adsorption des matières organiques des eaux usées urbaines sur la bentonite modifiée par Fe(III), Al(III) et Cu(II)
Z. Meçabih, S. Kacimi and B. Bouchikhi
Une argile naturelle, la bentonite de Maghnia (ouest d’Algérie), a été purifiée et caractérisée par calcination, pH‑métrie, IR, XRD et SEM. Les résultats obtenus ont montré que l’argile a une conductance de 66,4 µS, une capacité d’échange cationique (CEC) de 0,91 méq/g et contient 12 % de matières organiques. Les caractérisations par IR, XRD et SEM ont montré que la bentonite de Maghnia (B) est composée de quartz comme impureté majeure, d’illite (7 %) et principalement de montmorillonite.
Cette argile a été saturée au sodium (Na+) et associée au fer (III), à l’aluminium (III) et au cuivre (II). Ces derniers sont insérés, par couple (Al-Cu, Fe-Cu et Fe-Al, 50 ‑ 50 % en atomes), dans l’espace inter-feuillets de la montmorillonite sodée. Le rapport massique métaux/B est égal à 0,0625.
Les échantillons obtenus sont appliqués pour fixer les matières organiques (MO) des eaux usées, très chargées, de la ville de Sidi Bel-Abbès (ouest d’Algérie). Ces matières organiques représentent 60 % des matières en suspension (MES).
L’adsorption des MO a été suivie par photométrie à 470 nm et à température ambiante (20 °C). Cette adsorption a été ajustée au modèle de Freundlich dans le cas de la bentonite et au modèle d’Elovich dans le cas de la bentonite modifiée. Sur l’argile seule l’adsorption des MO est en monocouche, alors qu’elle est en multicouche sur tous les trois autres systèmes. Les taux de fixation des matières organiques sont égaux à 67,1; 76,0; 82,6 et 87,7 % respectivement sur B, Al‑Cu/B, Fe‑Cu/B et Fe‑Al/B. Ces systèmes ont des capacités maximales d’adsorption (qm), respectives, de 570; 860; 890 et 1 010 mg MO/g. Les systèmes au fer sont les plus performants, en particulier Fe‑Al/B.
Clay functions as a recyclable surfactant support for the adsorption and subsequent combustion of organic pollutants. The fact that this technology is coupled with general water treatment processes is in itself advantageous. The adsorption of organic matter (OM) from the urban wastewater of Sidi Bel-Abbes City (western Algeria) onto modified clays was investigated in this study. Bentonite from Maghnia (western Algeria) was used in a purified form. The natural clays were washed several times with distilled and deionised water and were completely dispersed in water. After 17 hours at rest, the dispersion was centrifuged for one hour at 2400 rpm. The size of the clay particles obtained was < 2 µm.
These clay particles were dispersed in water and heated at 75°C in the presence of a solution composed of the sodium salts of bicarbonate (1 M), citrate (0.3 M), and chloride (2 M). The purpose of this operation was to eliminate inorganic and organic compounds, aluminium found in the inter-layer spaces and various free cations. Carbonates were removed by treatment with HCl (0.5 M) and chloride was eliminated after several washings. The organic matter was eliminated completely by treatment with H2O2 (30% v/v) at 70°C. The purified clay was dried at 110 °C, and then saturated with sodium (Na+). To ensure complete transformation into the sodium form, all samples were washed several times with a NaOH solution (1 M).
The exchanged clay was examined by X-Ray Diffraction (XRD), infrared (IR) and scanning electron microscopy (SEM). The results obtained demonstrated that the clay had a conductance of 66.4 µS, a cation exchange capacity of 0.9 meq/g (CEC) and contained 12.2 % organic matter. These techniques also show that the purified bentonite of Maghnia (B) was composed essentially of Na-montmorillonite and some illite (~ 7%).
The clay was associated with iron (III), aluminium (III) and copper (II). These metals were inserted in couples (Al‑Cu, Fe‑Cu and Fe‑Al; 50/50% on an atomic basis) in the interlayer space of the montmorillonite. The mass ratio of metals/bentonite was 0.0625. The obtained structures were based on the intercalation of metals into the clay, thus ensuring uniform porosity with a high interlayer pore volume.
These systems (metals/bentonite) were applied to fix the organic matter (OM) present in urban wastewater from the city of Sidi Bel-Abbes (western Algeria). The samples of wastewater were taken from the effluent of the wastewater treatment station located on the Mekerra River at the exit of the city. Wastewater was sampled at 1:00 pm on four dates (June 1998, September 1998, April 1999 and December 1999) and preserved according to standard protocols (AFNOR). Water quality parameters of this effluent were indicative of pollution: suspended solids (675 ‑ 940 mg/L), Chemical Oxygen Demand (COD 340 ‑ 1000 mg O2/L), Biological Oxygen Demand (DBO5 190 ‑ 520 mg O2/L), Total Kjeldahl Nitrogen (79.8 ‑ 82.6 mg N/L) and total phosphates (20 ‑ 47 mg P /L). The treatments were carried out on diluted wastewater; the dilution factors were 3.16, 1.92, 3.72 and 4.53 respectively for the four dates cited above. These dilutions were used so as to obtain an initial organic matter concentration 126 mg OM/L. For the four sampling periods the average dilution factor was 3.33. At ambient temperature, a mass (10 ‑ 30 mg) of bentonite alone (B) or modified (Al-Cu/B, Fe-Cu/B and Fe‑Al/B) was added to 50 mL of the polluted and diluted water.
The adsorption of organic matter was followed by spectrophotometry at 470 nm, using batch equilibration, under various conditions established by controlling the following parameters: adsorbate/adsorbent ratio, contact time, pH and temperature. This adsorption was adjusted to the Freundlich model in the case of bentonite alone and to the Elovich model in the case of the modified bentonite. Indeed, the isotherm studies showed that the adsorption of organic matter was best described by the Freundlich isotherm equation (with a correlation coefficient R = 0.98 and an adsorption coefficient KF = 70.91 L•g‑1) for bentonite alone and the Elovich isotherm equation for the modified clay (with R > 0.96 and KE = 2•10‑3, 15•10‑3 and 11•10‑3 L•g‑1 respectively for Al-Cu/B, Fe-Cu/B and Fe-Al/B). The adsorption was in monolayers on bentonite and in multilayer form on the other systems. The degree of OM removal was 67.1, 76.0, 82.6 and 87.7% respectively for bentonite (B), Al-Cu/B, Fe-Cu/B and Fe-Al/B. The maximal adsorption capacities (qm) for these systems were: 570, 860, 890 and 1010 mg OM•g‑1, respectively for the four sorbents. The qm values indicate the degree of surface covering of the solid phase (θ); this coverage was 0.80, 0.72 and 0.86 respectively for Al-Cu/B, Fe- Cu/B and Fe-Al/B.
The systems containing iron were powerful, particularly Fe-Al/B; this system was able to completely remove the organic matter found in the wastewater. The result was confirmed by the abatement of the COD. This parameter decreased from 537 to 55 mg/L in a trial where one litre of wastewater was treated with 0.67 g of bentonite and 42 mg of Fe-Al/B (50/50 atomic ratio).
Salah Jellali and Olivier Razakarisoa
Deux expériences ont été réalisées sur un site expérimental contrôlé de dimensions décamétriques reconstituant un aquifère alluvial. L’originalité de ce travail est basée sur le fait que cette plate-forme expérimentale permet de rendre compte du rôle de la frange capillaire dans les phénomènes de transfert, ce qui est difficilement accessible sur des systèmes réduits de laboratoire ou dans les investigations sur site réel. L’objectif principal est l’évaluation quantitative des mécanismes de transfert de Composés Organiques Volatils (COV) depuis la zone non saturée vers la nappe dans le cas d’une source de pollution localisée en zone non saturée. Le cas du transport du trichloroéthylène (TCE) a été abordé où une analyse comparative du transfert du TCE depuis la zone non saturée vers la nappe via la frange capillaire est présentée en étudiant les deux mécanismes : dispersion et dissolution. Dans la première expérience, la dispersion passive de la pollution par les vapeurs depuis la zone non saturée vers la nappe via la frange capillaire est étudiée. Dans la seconde expérience, l’impact sur la pollution de la nappe du lessivage des vapeurs par une pluie contrôlée est quantifié. Les résultats montrent que la dispersion passive des vapeurs peut causer une pollution significative de l’eau de la nappe, et ce, malgré la lenteur du processus de diffusion dans la partie inférieure de la frange capillaire suffisamment saturée en eau. Le lessivage des vapeurs par la pluie provoque une pollution de nappe plus importante et plus étendue. La quantification des flux de pollution partant de la zone non saturée vers la nappe a été réalisée dans la première expérience en se servant de la méthode de JOHNSON et PANKOW (1992), et du code de calcul (Hydrus) dans la seconde expérience. Les résultats expérimentaux et analytiques mettent en évidence, d’une part, le rôle d’écran joué par la frange capillaire contre le transfert de la pollution vers la nappe, et d’autre part, l’augmentation significative du degré et de l’étendue de la pollution de la nappe en cas de lessivage des vapeurs par les eaux de pluie.
Two large-scale experiments were conducted on a controlled artificial aquifer referred to as SCERES (Site Contrôlé Expérimental de Recherche pour la Réhabilitation des Eaux et des Sols: 25 x 12 x 3 m). The experimental tool SCERES was completely buried in the subsurface in order to get stable temperature conditions in the aquifer. The hydraulic gradient, flow rate, visualization of the water table and water sampling were managed and monitored in two technical pits located at the upstream and downstream ends of the SCERES basin. It was packed with a main layer of uniform quartz sand and a 0.5 m-deep drainage layer at the bottom of the basin having hydraulic conductivities of 8 x 10‑4 and 6 x 10‑3 m/s respectively. The quartz sand had a mean grain diameter of 0.45 mm, a total porosity of 0.4 and a uniformity coefficient of 2.1; its longitudinal dispersivity was determined in laboratory column experiments to be approximately 1 mm. The aquifer is composed of a 1 m-thick saturated zone and a 2 m-thick unsaturated zone, making it possible to monitor the propagation of vapours in this zone. The hydraulic gradient of the groundwater was fixed at 0.003 m/m, which corresponded to a flow rate of 0.5 m3/h and an average velocity of approximately 0.4 m/day. The thickness of the capillary fringe was estimated to be approximately 0.25 m as deduced from water profile measurements by exploration with a TDR (Time Domain Reflectometry) probe. In the capillary fringe, water saturation at depths of 1.85 and 1.95 m was approximately 57% and more than 90%, respectively.
The contaminant chosen for these experiments was trichloroethylene (TCE), because it is among the most frequently detected volatile organic compounds (VOCs) in subsurface environments. TCE as the pure phase was injected 0.35 m beneath the soil surface of SCERES with an experimental design that maintained a uniform infiltration of TCE in the vadose zone for 15 min. The chosen injection device was built from a stainless steel tank (0.58 m diameter and 0.15 m height), on which 31 screened brass rods were fixed at the bottom and separated by a distance of 0.1 m. Each rod contained four injection holes of 0.2 mm diameter in the lower extremity (JELLALI, 2000; JELLALI et al., 2001). In order to prevent TCE volatilization from the device during injection, the TCE volume in the injection tank was covered with a 0.02 m thick water layer.
The resulting vapour and aqueous phases were monitored along with temperature and moisture content in order to investigate the mass transfer of VOC from the unsaturated zone to groundwater. The originality of this research was based on the fact that the controlled SCERES site allowed the study of transfer phenomena in the capillary fringe, which is difficult to reach in a reduced laboratory physical model or in a real contaminated site. This characteristic offers an exceptional opportunity for data acquisition in controlled conditions between laboratory and site scales. This aspect is of importance due to the fact that in this capillary zone, water content varies with depth and this situation causes changes in flow rate affecting the intensity of the pollution flux. This study aimed to quantify, in an experimental way, the pollution flux from the unsaturated zone towards the groundwater.
In the first experiment, carried out in the summer (July-September), the infiltrated TCE volume was 5 L. In the second experiment, carried out in autumn (October-December), the TCE volume was 3 L in order to increase the distance between the pollution source and the water-table. These volumes were selected on the basis of previous knowledge of TCE residual saturations determined in laboratory column tests (JELLALI, 2000) and in order to obtain a pollution source limited to the unsaturated zone. The first experiment was conducted without water infiltration to study the dispersion of TCE vapours across the capillary fringe, while the second experiment was carried out with a limited rain infiltration in order to investigate the effect of vapour leaching on groundwater pollution. The transport of TCE was monitored in the vadose zone, the capillary fringe and the groundwater where a comparative analysis of two mechanisms of TCE transfer from the unsaturated zone to groundwater via the capillary fringe was carried out: dispersion and dissolution. In both cases, the coupling of measurements of pollutant concentrations in the unsaturated zone, the capillary fringe and the groundwater of SCERES allowed us to take into account the mechanisms intervening near the source area, on a scale close to that of a real pollution problem. The concentration of dissolved TCE was analyzed by a gas chromatograph equipped with a flame ionization detector (GC-FID) after liquid-liquid extraction with hexane. The online quantitative analysis of TCE vapours was performed by a multigas monitor equipped with a photoacoustic infrared detector.
For the first experiment, the TCE mass fluxes from the vadose zone to groundwater were quantified using the method of JOHNSON and PANKOW (1992) where we applied the analytical solution of GRATHWOHL (1998) and incorporated parameters due to the capillary fringe. For the second experiment, we used the numerical code HYDRUS that allowed the simulation of one-dimensional flow in the unsaturated zone. In this example, the TCE mass fluxes leaving the unsaturated zone to the groundwater due to rain infiltration were obtained from the knowledge of the infiltration flow rate and the measured concentrations at the top of the capillary fringe.
The observed results indicate that the hydrodynamic dispersion of TCE vapours within the capillary fringe (vertical dispersion) can cause significant groundwater pollution despite the slowness of the aqueous diffusion in the lower region of the highly water saturated capillary fringe. Vapour leaching due to controlled water infiltration causes more significant groundwater pollution in degree and extent than vertical dispersion. The experimental and analytical results demonstrate, on the one hand, the role of the capillary fringe as a barrier against pollution transfer to groundwater when the only mechanism is hydrodynamic dispersion, and on the other hand, significant enhancement of groundwater contamination due to the capture and leaching of vapours from the vadose zone by infiltrating water.
Algorithme Génétique (AG) pour le choix optimal des stations d’appoint de chlore sur les réseaux d’eau potable
Issam Nouiri and Féthi Lebdi
La chloration sur les réseaux de distribution d’eau potable constitue une tâche délicate. Elle assure la protection contre la reviviscence microbienne et contre la contamination du réseau. Les réactions du chlore avec la matière organique du système entraînent la formation des sous‑produits chlorés, indésirables pour la santé humaine. Ainsi, le maintien du taux de chlore libre à des valeurs admissibles, sur tout le réseau et à tout moment, constitue un objectif principal des gestionnaires des réseaux.
Lors de la chloration à partir des sources d’eau, les processus de réaction-transport créent sur les réseaux, caractérisés par des temps de séjours importants, de mauvaises distributions des taux de chlore libre. Les stations d’appoints sur les réseaux constituent une alternative efficace. La détermination de leur nombre et le choix des emplacements optimums constituent les deux difficultés auxquelles sont confrontés les gestionnaires. Le présent travail utilise un algorithme génétique (AG) pour la détermination du nombre et des emplacements optimums des stations d’appoint de chlore sur les réseaux. Deux objectifs ont été fixés : (1) l’amélioration de l’homogénéité spatio-temporelle de la chloration et (2) la minimisation du nombre de stations d’appoint. L’application du modèle développé sur un réseau test a permis d’identifier les emplacements des deux stations d’appoint de chlore. La solution optimale a considérablement amélioré l’homogénéité et a assuré, pour 98 % des noeuds, des taux de chlore libre dans l’intervalle admissible (0,1-0,5) mg/L.
The chlorination of drinking water networks represents a delicate task. It ensures protection against microbial regrowth and network contamination. However, chlorine also reacts with organic matter in the system and leads to the formation of chlorinated by-products, which are undesirable for the human health. Thus, one of the main objectives of the water network manager is to maintain acceptable levels of free residual chlorine, at all network nodes and at all times. When water sources are chlorinated, reaction-transport processes create an unbalanced distribution of free chlorine concentrations, especially in networks characterized by long water residence times. Booster stations in networks constitute an efficient alternative to improve the spatial and temporal chlorine distribution. Their number and their optimum locations are two challenges facing network managers. In this respect, this paper suggests the use of a genetic algorithm (GA) to determine the number and the optimum locations of chlorine booster stations in networks. The two main objectives of this study were: (1) the improvement of the spatio-temporal homogeneity of chlorination and (2) the reduction of the number of booster stations.
A solution to this optimisation problem is an arrangement of ns booster stations in n consumption nodes of the water network. To resolve this problem, we linked the toolkit of the hydrodynamic computer program EPANET, which uses a one-dimensional reaction-transport model, to the proposed genetic algorithm (GA). In this application, we assume that free chlorine reactions through distribution networks are first-order. First, the EPANET computer program simulates temporal and spatial chlorine spreading in the network for each solution. Next, the GA calculates the sum square deviation E(NN, T), for the average chlorine concentration required for adequate sanitation (0.1‑0.5) mg/L, which corresponds to an optimal free chlorine concentration of 0.3 mg/L in the network. The number of booster stations is also defined for each solution studied. The optimal solution must minimize the sum square deviation and the number of booster stations used. Therefore, solution j can be evaluated by its fitness representing the weighted sum of the homogeneity function Fh(j) and the function of booster stations number Fsmin(j). According to the fitness of the solution, genetic operators (tournament selection, two points crossover and mutation) associated with an elitist evolution strategy (ES), combine individuals and create new populations. This iterative process explores the solution space and improves the maximum population fitness until stagnation, to achieve the optimal individual.
The test network configuration maintained for modelling is formed by a tank and 20 km of pipes. The stretched-out shape of the network imposed long water residence times and usually created an unbalanced distribution of free chlorine concentrations at consumption nodes. In order to evaluate chlorination, three control nodes were chosen: N145 (first consumption node), N168 (middle of the network) and N206 (extremity of the network). For this network example, the best initial chlorination homogeneity was reached with a constant free chlorine concentration equal to 0.5 mg/L, usually imposed by the manager at the exit of the water tank. This initial management approach created in the network a sum square deviation of 65.9 mg2/L2, as well as an unbalanced spatial chlorination distribution with 27% of the consumption nodes having free chlorine concentrations less than the minimum required value for adequate sanitation (0.1 mg/L). The simulation of higher free chlorine concentrations at the exit of the water tank (0.8 mg/L) led to concentrations above the maximum value required for adequate human health protection (0.5 mg/L) at 26% of the consumption nodes. Also, 10% of the nodes had concentrations lower than the minimum concentration required for human health. For all constant chlorination scheduling studied (0.4; 0.5; 0.6; 0.7 and 0.8 mg/L), an important fraction of consumption nodes remained with free chlorine concentrations outside the desired concentration range. Therefore, this management approach is not suitable for networks with long residence times.
Before its application, the GA required the definition of the crossover and the mutation probability (respectively 0.9 and 0.05), the size of the population (100) and the maximum number of generations (300). In addition, in the present application, chlorination homogeneity and the reduction of the booster stations number (p1 = p2 = 0.5) were given the same importance. According to boundary conditions, we imposed a free chlorine concentration of 0.5 mg/L at both the exit of the water tank and at the booster stations. The application of the GA, developed in this study, ensures the evolution of the initial population until stagnation of its maximum fitness. The resulting optimal solution involved the creation of two booster stations at nodes N179 and N196. The simulation of this chlorination management improves the spatio-temporal homogeneity of free chlorine concentrations in consumption nodes. It reduced the sum square deviation from 65.9 to 38.8 mg2/L2. Finally, 98% of the consumption nodes had free chlorine concentrations (0.1-0.5 mg/L) that were required in the context of human health.
This model represents a first step in the optimization of chlorination homogeneity in networks characterized by long water residence times. The use of this tool requires network hydraulic modelling, the simulation of the free chlorine behaviour, the definition of the chlorine reaction coefficients and the GA parameters. The optimization of the chlorine concentration scheduling (in water sources and in booster stations) represents an additional step in the present work.
Tolérance et accumulation du cuivre et du chrome chez deux espèces de lentilles d’eau : Lemna minor L. et Lemna gibba L.
Mohammed Ater, Nadia Aït Ali and Hicham Kasmi
Certains procédés d’épuration extensive des eaux usées utilisent les lentilles d’eau. Les rejets des eaux usées sont de plus en plus sujets à des pollutions de natures diverses, notamment les métaux lourds. Dans ce travail, on procède à une évaluation comparée de la tolérance et de l’accumulation de deux métaux lourds, le cuivre et le chrome, par deux espèces de lentilles d’eau Lemna minor et Lemna gibba. Bien que sous climat de type méditerranéen, L. gibba soit plus utilisée que L. minor, les données disponibles dans la littérature concernent plutôt L. minor. L’évaluation des paramètres toxicologiques montre chez L. gibba une tolérance nettement supérieure aux effets toxiques des deux métaux expérimentés. Le chrome est moins toxique que le cuivre et s’accumule à des taux supérieurs à ceux du cuivre dans les tissus des plantes. Pour les deux métaux, l’accumulation est plus importante chez L. minor. Cependant, on pense que L. gibba serait plus indiquée dans des applications en phytoremédiation que L. minor, vu sa tolérance et sa productivité en biomasse. Les potentialités de ces deux espèces pour des applications de ce type se justifient par des taux d’accumulation fort élevés, spécialement pour le chrome où ils dépassent largement les 1000 µg g‑1 de poids sec. En effet, les concentrations des plantes en Cr obtenues dans ce travail atteignent 2140 µg g‑1 chez L. minor et 1710 µg g‑1 chez L. gibba. Ces performances montrent un potentiel fort intéressant en comparaison à d’autres macrophytes comme la jacinthe d’eau par exemple.
Natural wastewater treatment technologies are common practice in many regions of the world. Although these technologies are normally meant for domestic wastewater treatment, they can have broader applications such as the treatment of water contaminated by heavy metals.
Among various existing technologies, systems that use duckweed are exploited for wastewater treatment in various regions of the world. In Morocco, this type of system has been tested on an experimental scale and a pilot scale. The most common duckweeds species are Lemna minor L. and Lemna gibba L. L. gibba is used in Mediterranean climates more often than L. minor, but literature data on the tolerance and accumulation of heavy metals are mostly available for L. minor.
In the current study, we compared the tolerance and accumulation of two heavy metals, copper (Cu) and chromium (Cr), for these two duckweed species. Thus, this study compares the potential of these two duckweed species to be used as a method of decontaminanting Cu‑ and Cr‑contaminated water. The experimental design used hydroponic crops of the two species. Forty fronds of each species were sown in plastic pots containing 100 mL of White nutritive solution with a pH adjusted to 6.8. As experimental treatments, we used exposures corresponding to the following concentrations: 0.5; 1; 2; 3; 4; 5 mg Cu/L and 3; 5; 10; 20; 30; 35 mg Cr/L. Each treatment was repeated five times. In order to compensate for water loss by evapotranspiration, we added 15 mL of the corresponding solution to each treatment daily.
Generally, the results obtained confirm that Cu is more toxic than Cr and that the two species of duckweed have different tolerance levels; L. minor is more sensitive than L. gibba for both metals. For Cu, we observed a highly significant inhibition of growth in response to the gradient of Cu concentrations used, the growth of L. minor being more affected than that of L. gibba. For example, the time required for L. minor colonies to double was more than four days for 1 mg Cu/L, whereas for L. gibba this was observed only at 3 mg Cu/L. The toxicological parameters reflect this observation as L. minor had both a lower NOEC (No Observed Effect Concentration) and a lower IC50 (50% Inhibition Concentration) than those reported for L. gibba. Comparatively, these results show a higher tolerance of Cu contamination for L. gibba.
For Cr, L. minor was more sensitive than L. gibba, but in much less marked way than for Cu. Indeed, the time required to double the colony size and the NOEC value were similar for the two species, whereas the IC50 of L. minor was lower than that of L. gibba. The comparison of the rate of inhibition confirms a slightly greater sensitivity of L. minor to Cr exposure.
The Cu and Cr concentrations in the biomass increased with the concentration of metal; the highest contents were observed in the treatments with the highest concentrations used: 5 mg Cu/L and 10 mg Cr/L. However, the BCF (Bioconcentration Factor) decreased with the concentration of the treatment; the highest values were observed for the treatments with the lowest concentrations (1 mg Cu/l and 3 mg Cr/L). With respect to possible applications in phytoremediation, the potential use of these two species would be more efficient for low levels of contamination.
The two species accumulate the two metals at different rates; the accumulation of Cr is greater than that of Cu and L. minor shows higher rates of accumulation. Copper concentrations of approximately 800 µg/g were obtained in L. minor exposed to 5 mg Cu/L. For Cr, concentrations were approximately 2140 µg/g obtained in L. minor exposed to 10 mg Cr/L. For L. gibba, the maximum concentrations obtained were 745 µg/g and 1710 µg/g respectively for the treatments of 5 mg Cu/L and 10 mg Cr/L. Similarly, the BCF estimated at L. minor was higher than that of L. gibba.
Compared with other macrophyte species, the duckweeds show a very interesting potential for metal accumulation. Indeed, for Cu, accumulation was definitely higher than that for other species such as Eichhornia crassipes and Polygonum hydropiperoides. The BCF obtained was higher than those observed for species of Typha and Spartina. These results confirm that L. minor and L. gibba could be good Cu accumulating species compared to other macrophytes. For Cr, other species of macrophytes accumulate this metal to a greater or lesser extent: E. crassipes (slower rate than that of the duckweeds), Nymphea alba (similar rate to that of the duckweeds) or Azolla pinnata (higher rate than the duckweeds). The Cr contents accumulated by the two duckweed species would justify their classification as hyperaccumulator species.
With respect to tolerance, accumulation potential and biomass productivity, L. gibba shows potential as a species that could be used in phytoremediation and in particular the rhizofiltration of wastewater contaminated by Cu and Cr.
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