Valérie Villeneuve, Stéphane Légaré, Jean Painchaud and Warwick Vincent
La concentration en oxygène dissous en milieu fluvial varie selon un cycle diurne (24 h) qu’il est essentiel de considérer dans l’évaluation de l’état d’oxygénation d’un cours d’eau. En principe, seules des mesures en continu recueillies au cours de cycles de 24 h permettent d’évaluer correctement l’état d’oxygénation d’une rivière, ce que, en pratique, les contraintes logistiques et budgétaires ne permettent pas de réaliser. Le présent article vise à faire la synthèse des connaissances sur les facteurs de contrôle et la modélisation des variations diurnes de la concentration en oxygène dissous en rivière. Parmi les facteurs biologiques et physico-chimiques, les activités autotrophe et hétérotrophe sont les facteurs dominants responsables des variations diurnes de l’oxygène dissous. Certains modèles de qualité de l’eau permettent de modéliser la teneur en oxygène et, dans certains cas, la variation diurne. Toutefois, ces modèles sont souvent complexes, d’utilisation ardue et impliquent la mesure directe sur des cycles de 24 h des variables qui régissent la concentration en oxygène dans le milieu. Une démarche est proposée pour l’élaboration d’un modèle et a été appliquée dans une étude de cas réalisée dans la rivière Saint-Charles (Québec, Canada). Un modèle simple (une fonction sinusoïdale) dont les paramètres ont été corrélés à la température et à la concentration moyenne en nitrate a permis de générer des valeurs simulées d’oxygène dissous très proches des valeurs observées in situ. Un modèle alternatif utilisant des valeurs ponctuelles de température et de concentration de nitrate a donné des résultats équivalents. L’approche proposée constitue donc une alternative simple et pratique à la mesure en continu de l’oxygène et permet une évaluation plus réaliste de l’état réel d’oxygénation d’une rivière que la prise de mesures ponctuelles.
In rivers, dissolved oxygen concentrations typically show diel variations with maximum values during daytime and minimum values at night. The diel cycle must be taken into account when assessing the state of oxygenation of a watercourse. However, in water quality monitoring programs, dissolved oxygen concentrations are usually obtained from single measurements taken during daytime. The resulting data do not represent the real overall oxygen levels of a watercourse and thus can lead to erroneous conclusions regarding the oxygen status of a river. Continuous data collected over 24‑hour cycles are required for an accurate oxygen status assessment, but in practice, logistic and budget constraints do not allow such samplings. Modelling can be a convenient alternative to direct measurements. However, the water quality models that take into account the diel cycle of oxygen are generally complex to run. The purpose of this study was to review the information relating to the dynamics and the modelling of the diel variations in dissolved oxygen in rivers and to apply a simple model in a case study involving dissolved oxygen, nutrients, temperature and chlorophyll a data collected over 24‑hour cycles in the St. Charles River near Quebec City (Canada).
Photosynthesis by algae, both benthic and planktonic, as well as by macrophytes, is an important and sometimes dominant factor in the oxygen budget of a river. Sediments and heterotrophic activity by bacteria, particularly in rivers receiving important loads of wastewaters, can be important sinks for oxygen. Temperature determines the solubility of oxygen, thereby directly influencing oxygen concentrations. Diel variations in oxygen thus reflect diel variations in temperature. Temperature also has an effect on biological processes such as respiration and photosynthesis. Reaeration varies not only with temperature, but also with the type of flow (laminar vs. turbulent) and current velocity. In rivers with important slopes and current velocities, reaeration can be sufficient to make up for oxygen losses due to high heterotrophic activity. Nevertheless, light is the first causative factor for the diel variations in oxygen, determining both autotrophic activity and water temperature. However, suspended matter in the water column reduces light penetration. Higher levels of suspended matter result in lower levels of photosynthesis and oxygen production. The sudden or large influx of runoff waters after heavy rain or snowmelt can also have an important impact on the oxygen budget of a river. Finally, chemical factors can have an influence on the diel variations in oxygen: nutrient inputs, in particular, can stimulate the rate of photosynthesis and oxygen production. Overall, oxygen dynamics are determined by the relative importance of biological, physical and chemical factors, which vary in time and space. However, biological processes often dominate over the other causative factors affecting diel variations in oxygen. In temperate climates, biological processes have a controlling function only during warmer months, with temperature and flow being the dominant controlling factors during colder months.
Water quality models that take into account diel variations in oxygen are often designed to assess primary production and respiration. These models are based on either ODUM’s (1956) concept of oxygen curves or on direct and continuous measurements of dissolved oxygen. Periodic functions or Fourier series are also used to simulate diel variations in oxygen. The widely used USEPA QUAL2e model predicts diel variations in oxygen from different measurements including light intensity and from computation of the rates of photosynthesis and respiration. Several other modelling approaches use various combinations of indirect methods to predict the variations in oxygen, based on light intensity, algal biomass, primary production, and reaeration. Specific models are sometimes necessary due to particular regional characteristics or environmental issues.
In general, water quality models designed for water management purposes are complex and require the measurement of a large number of parameters, which necessitates elaborate and costly logistics. Estimating the parameters controlling the oxygen budget in a river thus ends up being more time and labour consuming than the direct measurement of diel variations in oxygen. A simpler model leading to the estimation of the concentrations and the amplitude of the variations of dissolved oxygen was developed and applied to the St. Charles River.
The St. Charles River flows from the Laurentians north of Quebec City to the St. Lawrence River. The vast upper watershed is mostly forested, but the lower part is heavily urbanized. Two stations were located in the upper watershed and one in the section of the river within Quebec City. Water quality was excellent at the upstream stations and poor at the downstream station, due to wastewater inputs and low flow rates. Sampling was carried out in the months of July and August of 1996 and 1997. Physico-chemical and light measurements were made every two hours for periods of 24 hours. Nutrient and chlorophyll a samples were collected every four hours. Small diel variations in oxygen (amplitude: 1.48 mg/L) were observed at the upstream station, while much larger ones (amplitude: 4.23 mg/L) were measured at the downstream station. Modelling of the diel variations in oxygen was carried out using a sine function. Oxygen concentrations over 24 hours were successfully predicted from average concentration of dissolved oxygen, amplitude, and phase of the cycle. Average oxygen concentrations and amplitude can be derived from physico-chemical and/or biological variables easily measured in standard water quality monitoring programs. Average oxygen concentrations showed a very strong correlation with temperature (r2 = 0.91) and amplitude of oxygen level variations was strongly correlated with average nitrate concentration (r2 = 0.58). These relations were used in the sine function and resulted in significantly correlated modelled and measured oxygen concentrations for six of the seven cycles. Overall correlation between modelled and observed values was high (r2 = 0.77). Modelled values obtained with single measurements of temperature (taken at 2:30 P.M.) and nitrate concentration (which shows no diel variation) were also highly correlated with observed data (r2 = 0.75). Absolute and relative bias as well as root-mean-square error also showed the validity and the equivalence of the two approaches.
This study shows that simple models based on available water quality data may generate realistic oxygen values over 24‑hour cycles. These models would be a valuable diagnostic and decision making tool for the management of water quality in rivers.
Utilisation de bactériophages pour contrôler les populations de Aeromonas salmonicida résistantes aux antibiotiques
Sandra Imbeault, Serge Parent, Jean-François Blais, Michel Lagacé and Carl Uhland
La furonculose, causée par la bactérie Aeromonas salmonicida, représente une des principales causes de mortalité chez les salmonidés d’élevage. L’antibiothérapie constitue l’approche la plus largement répandue pour contrer les effets néfastes de cette maladie. Cependant, le développement de bactéries résistantes aux antibiotiques représente un problème de plus en plus préoccupant. La présente recherche a visé à explorer une nouvelle option pour lutter contre la furonculose, soit la possibilité d’utiliser des bactériophages comme moyen de prévention pour contrôler les populations de A. salmonicida. La sensibilité de 19 souches de A. salmonicida, résistantes à aucun, un, deux ou trois antibiotiques, a été évaluée vis‑à‑vis de 12 bactériophages. Les résultats ont montré que les souches de A. salmonicida résistantes aux antibiotiques utilisés dans l’industrie piscicole canadienne sont aussi sensibles à de nombreux bactériophages, tout comme des souches sensibles aux antibiotiques. Il serait donc possible d’envisager un traitement préventif à base de bactériophages pour lutter contre la furonculose chez les salmonidés d’élevage.
Aquaculture represents an increasingly important source of food fish worldwide. The aquaculture industry currently produces between 25 and 30% of all seafood for human consumption. In Canada, salmonids (salmon, rainbow trout, arctic char and brook trout) account for the majority of food fish production. Furonculosis involving the bacterium Aeromonas salmonicida is one of the most important infections observed in salmonid farms. An A. salmonicida infection results either in morbidity and mortality with few clinical signs, or in weakened fish with skin ulcers that make them unmarketable for human consumption. The A. salmonicida bacterium uses a number of mechanisms to counteract the natural barrier of the immune system. Bacterial growth is encouraged by an increase in the ambient temperature and in the concentration of organic matter in the water.
During recent years, a relationship between therapeutic failures and the development of bacterial resistance to antibiotics has been reported in salmonid farms. This problem is complicated by the fact that only four antibiotics are authorized for the aquaculture industry in Canada. One consequence of this increasing resistance is a renewed interest in alternative therapies and prevention. Bacteriophages (bacterial viruses) may represent one such alternative. In recent decades, interest in bacteriophages as antibacterial agents has been growing in the Americas and in Asia. Some researchers have tried to exploit the potential of bacteriophages to reduce bacterial populations in infections affecting humans, while others have tried to identify uses in veterinary medicine.
The overall objective of this research was to explore a new treatment against furonculosis infection based on the use of bacteriophages to inhibit growth of A. salmonicida cells. In this study, we looked at 19 strains of A. salmonicida, resistant to zero, one, two or three antibiotics, and evaluated their sensitivity to 12 bacteriophages. The results showed that the antibiotic-resistant strains were sensitive to as many bacteriophages as were the bacterial strains sensitive to antibiotics. They also showed that all the A. salmonicida strains were sensitive to several bacteriophages and, conversely, that several bacteriophages were effective against all the A. salmonicida strains. It may thus be possible to consider a preventive treatment using bacteriophages to fight against furonculosis in salmonid farms.
Analyse géostatistique des répartitions horizontales printanières de la biomasse zooplanctonique et des variables physico-chimiques dans un petit lac
Nadine Angeli, Daniel Gerdeaux and Jean Guillard
À partir d’un échantillonnage intensif du lac de Nantua, nous avons déterminé à quelles échelles les structures spatiales de la biocénose et de son environnement sont le plus fortement exprimées à l’aide de techniques géostatistiques. Ces résultats obtenus au printemps portent sur des descripteurs globaux (biomasse totale, moyennes déduites des profils verticaux enregistrés par sonde multiparamètre). Les modèles de variogrammes ajustés aux données montrent qu’une forte part de la variance totale s’exprimait à grande échelle pour le zooplancton total, à très petite échelle, au contraire, pour les descripteurs reflétant l’activité métabolique lacustre. La liaison des différentes variables explorées avec la température variait en outre fortement avec l’augmentation de la distance inter-stations, confortant l’hypothèse que des facteurs structurants distincts se succédaient sur la plage d’échelles considérée. Cette étude fondée sur des variables globales souligne la difficulté d’extrapoler les liaisons entre variables ou processus à d’autres échelles que celle(s) observée(s).
On the basis of a dense sampling survey, and for a given site and a given season (Lake Nantua; mid-spring), we assessed on which scales the spatial patterns of the biocenosis and its environment were most strongly expressed. These preliminary results focused on global descriptors (total biomass per surface unit, means computed from vertical multiprobe recordings). The theoretical variograms fitted to the data showed that most of the total variance was expressed on a broad scale for total zooplankton biomass. In contrast, the variance of descriptors reflecting lake metabolic processes was expressed on very short scales. For most of the explored descriptors, their relations with temperature also varied dramatically on a spatial scale, supporting the hypothesis that distinct structural factors prevail at different scales. The practical implications of these multiscale characteristics are discussed.
Edgar Herrera, Taha B.M.J. Ouarda and Bernard Bobée
La littérature scientifique de la dernière décennie contient un grand nombre de travaux qui détaillent le développement des méthodes de « désagrégation » (downscaling) de l’échelle globale à l’échelle hydrologique pour tenter d’estimer les impacts du changement global sur la disponibilité et la distribution des ressources en eau. Cet article présente une revue et une synthèse des méthodologies de « désagrégation climatique » présentées dans la littérature afin de simuler les réponses régionales au changement global du climat. L’accent est mis sur les avancées récentes et sur les problèmes reliés à l’application pratique des modèles dans les études d’impact. L’article présente également une discussion des avantages et limites des différentes approches, ainsi que quelques suggestions pour l’étude future des impacts du changement global sur les ressources en eau.
During the last decade, a large volume of literature has been published on the development of “downscaling” methods from the global to the hydrological scale in order to estimate the impact of global climate change on the availability and distribution of water resources. The present paper proposes a comprehensive review and a synthesis of climatic downscaling methodologies presented in the literature in order to simulate the regional response to global climate change. The paper focuses mainly on recent advances in the field, and on the practical problems that may arise from the application of the various models in impact studies. The paper presents also a discussion of the advantages and limitations of the various methods. The paper concluded with some suggestions for future work dealing with the impacts of global change on water resources.
Exploration des séries chronologiques d’analyse de la qualité des eaux de surface dans le bassin de la Tafna (Algérie)
A.E.B. Tidjani, D. Yebdri, J.C. Roth and Z. Derriche
La qualité des eaux de surface est sujette à de nombreuses fluctuations et par conséquence les gestionnaires sont souvent confrontés à des pics de pollution problématiques. À cet effet, nous présentons dans cette étude les résultats d’une exploration des séries d’analyses de la qualité des eaux de surface dans le bassin de la Tafna (Algérie). Nous avons retenu trois descripteurs de la qualité des eaux; la matière en suspension (MES), la conductivité et la demande chimique en oxygène (DCO). On dispose pour cette étude de séries de prélèvements instantanés d’une fréquence au moins mensuelle, couvrant une période de sept ans (avril 1995-mars 2002). Les stations de prélèvements choisies se situent sur les principaux cours d’eau du bassin de la Tafna (Mouillah, Isser et Tafna) de manière à couvrir l’ensemble du bassin versant.
Afin d’aboutir à des conclusions exploitables, nous avons commencé, dans une première étape, par une vérification de la cohérence des résultats d’analyse, suivie d’une analyse conjointe de tous les paramètres pour explorer la structure globale des données et appréhender les principaux facteurs qui contrôlent l’évolution de la qualité physico-chimique des eaux.
Nous proposons dans cette étude deux modèles mathématiques prévisionnels : le premier est un modèle de simulation du comportement de la qualité des eaux basé sur les relations concentration-débit. Le second est une approche prédictive qui permet de mettre en évidence les saisonnalités et les tendances d’évolution des paramètres étudiés.
A river is a complex system in interaction with the whole of its catchment area. River water acquires its physicochemical composition under the combined action of several factors: weathering of rocks, decomposition of terrestrial organic matter, diffuse and point-source pollution, and atmospheric precipitation.
The large catchment area of Tafna (7,245 km2 surface area), located at the extreme northwest sector of Algeria and extending into Moroccan territory (23%), is subdivided into four sub-basins. However, the downstream portion of the basin, which represents the sub-basin of maritime Tafna, was not retained in this study; its principal river Tafna is often dry because several hydraulic constructions have been installed in the upstream part of the basin. The three sub-basins concerned then with this study are the upper Tafna, the Mouillah and the Isser.
In the basin of Tafna, surface water quality is being continually degraded. In addition to the phenomenon of erosion, which has been estimated to be 400 t/km2/year in the Mouillah sub-basin and 1,000 t/km2/year in the Isser and upper Tafna sub-basins, domestic and industrial wastes increasingly threaten the quality of the surface water.
The basin of Tafna has a surface water quality network composed of 41 sampling stations. Three stations have been selected for this study. They are distributed in the three sub-basins studied, and are located upstream of dams, so as to cover the whole of the basin: Station ST160202 in the Mouillah sub-basin, ST160426 in the upper Tafna sub-basin and ST160614 in the Isser sub-basin. This work presents the results of an exploration of a series of instantaneous samples collected with at least monthly frequency over a period of seven years (April 1995-March 2002). The descriptors used in this study are the total concentrations: suspended matter concentration (SS), conductivity, which expresses the dissolved matter concentration, and the organic matter concentration as estimated with the chemical oxygen demand (COD).
To be able to interpret the influence of the characteristics of the catchment area on the physicochemical quality of surface water, we started by describing the principal characteristics of the sub-basins studied. Given the great variability of the results of the analysis, we considered it to be useful and necessary to evaluate the coherence of the measurements with ion balance and statistical analysis (Box and Whisker plots). A joint analysis of all parameters was then applied to explore the total structure of the data in order to understand the principal factors that control the evolution of the physicochemical water quality.
This study proposes two models. The first model is a simulation of the behaviour of surface water quality based on concentration-flow relations. The second is a predictive approach that makes it possible to highlight the seasonal effects and the temporal trends of the studied parameters.
Based on this modelling, it appears that the concentration variance explained by the concentration-flow models varies between 50% and 72%, which means that the reconstitution of the concentrations is only approximate. To improve the reliability of these models, we propose several solutions.
As for the forecasting models, a comparison of the values calculated to those measured shows that the results for conductivity and the COD are close to the measured values, with average relative variations of the order of 15%, but the forecasts of SS are considered to be less precise because they produce greater average relative variations. This is explained by the fact that the suspended matter concentrations do not follow a seasonal trend but are strongly related to flow variations.
Finally, it should be noted that the reliability of the established models remains very sensitive to the size of the measurement series one has. Better information contained in longer series of analyses would undoubtedly make it possible to reduce this uncertainty.
The methodology has been suggested on a purely exploratory basis and was limited to the three descriptors of the surface water quality: SS, conductivity and COD. The work will be extended by applying it to other parameters, which should make it possible to establish a complete model for the behaviour of surface water quality in the catchment area of the Tafna.
Emna Ellouze-Gargouri and Zoubeida Kebaili Bargaoui
Une méthodologie a été proposée pour la prédétermination des débits à partir de la pluviométrie. Il s’agit d’associer, en entrée, les simulations Monte Carlo de hyétogrammes de pluie nette avec un modèle d’hydrogramme unitaire à base de géomorphologie pour obtenir des hydrogrammes simulés en sortie. À la fin des simulations, l’analyse statistique des sorties permet d’apprécier l’étendue de la variabilité de la réponse du bassin aux événements pluvieux et de caractériser les débits et les temps de pointe. Un total de 44 simulations ont été menées pour chacun des 15 événements observés pour un petit bassin versant situé en Tunisie centrale. L’analyse des hydrogrammes générés a montré une assez faible dispersion des débits de pointe, d’une simulation à l’autre, pour un événement donné, et a mis en évidence la dissymétrie des distributions des débits et temps de pointe. L’exploitation des résultats de l’ensemble des simulations permet de dégager des relations empiriques caractérisant le comportement du bassin selon les débits de pointe, les temps de pointe, les temps de base et les volumes écoulés.
The predetermination of peak discharges and flood volumes of ungauged basins is an important aspect of the management of surface waters, protection against floods, water supply, etc. In this study, a method is proposed for the predetermination of discharges from rainfall data. The method associates effective rainfall obtained from Monte Carlo Simulations (MCS) with a unit hydrograph based on geomorphology. The unit hydrograph (UH) based on geomorphology is selected knowing that the parameters can be obtained from topographic charts, soil charts and ground occupation charts, as well as from soil data. The UH used was produced from the Nash cascade model in which the scale and shape parameters were taken from the literature. These parameters depend on the hydrographical network, the Horton ratios and the average peak flow velocity, which is assumed to be constant throughout the network and with respect to time. The average peak flow velocity can be expressed as a function of 1) geomorphologic parameters such as the total surface area of the basin, the slope of the highest order stream, the Manning-Strickler coefficient, the width of the channel, the kinematic wave parameter of the highest order stream and the length of the main channel, and 2) the effective rainfall intensity and duration.
With respect to effective rainfall intensities, the idea is to consider the effective rainfall as a vector of the parameters of the hydrological model, and then to use the MCS method to generate the corresponding components. The proposed simulation framework includes: 1) the specification of the data for which the geomorphologic parameters and the time increments are fixed for all simulations, whereas the duration of the total rainfall and the effective rainfall volume vary from one event to another, and constitute constraints determining whether or not simulations should be rejected, 2) the random drawing of effective rainfall intensities and durations, 3) the computation of resulting hydrographs and 4) the analysis of the simulated hydrographs, where the hydrographs are first simulated for each event and then simulated in their entirety to highlight indicators to characterize outputs.
In order to statistically interpret the simulated hydrographs, the generated peak discharges were classified for each event, and their 25th, 50th and 75th percentiles were analyzed. The same treatment was applied to the simulated times to attain peak values. The use of the 25th and 75th percentiles makes it possible to evaluate the extent of the 50% interval of the simulated discharges, whereas the median and the mode make it possible to position values representative of the distribution of the generated discharges. The hydrographs are assumed have the same “recurrence” as their peak discharge. Hydrograph generation by the MCS method is a two step process: 1) the generation of effective rainfall intensities based on the assumption that the total volume is observed, and 2) the convolution of the unit hydrograph resulting from each interval of effective rainfall.
The study site, Saddine1, is a small catchment with a surface area of 384 hectares. It is located adjacent to Makthar in Tunisia (northern latitude 35°48’06’’ and longitude 9°04’ 09’’) in a mountainous zone. The catchment is controlled by a small headwater dam and was monitored from 1992 to 1999. Observed over periods of five minutes, the maximum rainfall intensity was 324 mm/h and the minimal intensity was 10 mm/h. The maximum total rainfall recorded for an event was of 106 mm. The longest duration for an event was of approximately 5 hours (299 min) and shortest was 12 minutes. A great disparity in the volumes was also noted: the maximum volume observed was 67,200 m3 whereas the minimum was 1,275 m3. The peak discharges of the recorded hydrographs were very variable with a minimum/maximum ratio of about 1/1370. Indeed the maximum discharge observed was 85.6 m3/s, and the minimum discharge only 0.062 m3/s. The time to attain peak flows for the rainfall events varied from 10 to 120 minutes. The effective rainfall intensities were calculated using the infiltration index method, ϕ, which remains a method still largely used in spite of its rudimentary character. The effective rainfalls estimated for each event varied from 0.3 mm with 17.5 mm.
Before using the MCS, the model was calibrated. The results of the calibration analysis showed that the calculated hydrographs were reasonable comparable to the observed hydrographs. In addition to the shape, the peak discharge and the peak time reconstitutions were satisfactory. A total of 44 simulations were carried out for each of the 15 events observed, of which 13 allowed for the identification of the distributions of effective rainfall intensities and durations. The remaining two events were used for the validation of the approach. The analysis of the generated hydrographs showed a rather weak dispersion of the peak output from one simulation to another, for a given event. Moreover, the discharges and times to attain peak discharge resulting from the generated hydrographs followed a dissymmetrical distribution. The observed values of the peak discharges and times to attain peak discharge represent realisations of output simulations with different probabilities of occurrence. In order to capitalize on the model, relationships between simulated peak discharges, times to peak discharge, base times and volumes were constructed.
Feten Horriche and Mustapha Besbes
L’observation régulière du niveau des eaux souterraines a débuté en Tunisie il y a plus de soixante ans. Depuis, le réseau piézométrique national, composé aujourd’hui de près de 3 700 points de surveillance (puits, forages et piézomètres), permet l’observation de 148 systèmes aquifères et comporte des séries de mesures parfois longues d’une cinquantaine d’années. Pour caractériser le réseau piézométrique national, deux types d’indicateurs sont définis : a) des indicateurs portant sur la qualité intrinsèque des réseaux existants; b) des indicateurs décrivant le besoin d’informations complémentaires et la nécessité de consolidation des réseaux. Les indicateurs de qualité sont liés à la densité spatiale, la durée de surveillance, le fonctionnement, la couverture du réseau par des piézomètres, le contrôle de l’exploitation et l’évolution temporelle du réseau. Les indicateurs de consolidation des réseaux sont relatifs à la variabilité piézométrique, au taux d’exploitation ou de surexploitation, aux ressources exploitables et aux superficies des systèmes aquifères. Se fondant sur ces indicateurs, la classification des systèmes aquifères surveillés met en évidence les nappes disposant de réseaux de meilleure qualité et celles nécessitant une consolidation de leur réseau. La synthèse globale des différents indicateurs a permis de dégager les systèmes aquifères nécessitant des actions prioritaires en matière de rationalisation des réseaux piézométriques. Les résultats obtenus au terme de cette analyse concordent bien avec la situation actuelle observée sur le terrain.
The regular observation of groundwater levels began in Tunisia more than sixty years ago. The national groundwater level network consists presently of almost 3,700 monitoring points (wells, boreholes and piezometers), monitoring 148 aquifers with some long-term series of about fifty years. To characterize the national piezometric network, two types of indicators were defined: a) indicators connected with the intrinsic quality of the existing networks, and b) indicators describing the need for additional information for network consolidation. Quality indicators relate to space density, monitoring duration, functioning, spatial coverage of the network by piezometers, exploitation control and temporal evolution of the network. Consolidation indicators relate to piezometric variation, exploitation or overexploitation rates, resources and aquifer area. Based on these indicators, the classification highlights aquifers having the best quality networks and those requiring a network consolidation. The global synthesis highlights aquifers requiring priority actions in terms of rationalization of the piezometric network. The results obtained at the end of this analysis agree well with the present situation as observed in the field.
Développement d’une nouvelle méthode de régionalisation basée sur le concept de « régime des débits naturels » : la méthode éco-géographique
Ali A. Assani, Simon Tardif, Safia Benseghir, Alain Chalifour and Mesfioui Mhamed
Nous proposons une nouvelle méthode de régionalisation des débits fondée sur le concept de « régime des débits naturels » introduit en écologie aquatique : l’approche éco-géographique. Elle se distingue de deux approches de régionalisation existantes (approches hydrologique et écologique) sur les trois points suivants : le choix des variables hydrologiques, l’échelle d’analyse et la finalité de la régionalisation. En ce qui concerne le choix des variables hydrologiques, la nouvelle méthode est fondée sur le choix des caractéristiques des débits et non sur les variables hydrologiques. Ces caractéristiques des débits sont définies au moyen de l’analyse en composantes principales appliquée sur les variables hydrologiques. Contrairement aux autres approches, l’approche éco-géographique tient compte de toutes les caractéristiques des débits dans la régionalisation conformément au concept de « régime des débits naturels ». Quant à l’échelle d’analyse, à l’instar de l’approche écologique, la nouvelle méthode s’applique aussi à toutes les échelles d’analyse (annuelle, mensuelle et journalière) mais en les considérant séparément afin de tenir compte de toutes les caractéristiques de débits dans la régionalisation. Enfin, la finalité de la nouvelle méthode est de pouvoir déterminer les facteurs de variabilité spatiale des caractéristiques de débits (et non des variables hydrologiques) au moyen de l’analyse canonique des corrélations, notamment afin d’assurer une gestion durable des ressources hydriques dans un contexte de changement de l’environnement. Nous avons appliqué cette nouvelle méthode aux débits moyens annuels au Québec.
Flow regionalization has been the subject of numerous hydrologic studies. However, despite the development of regionalization methods, there are still differences in the approaches used amongst hydrologists on the one hand, and between hydrologists and experts in other fields (aquatic ecology and physical geography) on the other hand. Those differences relate to five aspects of the regionalization process: the choice of hydrologic variables, station grouping methods to produce homogeneous hydrologic regions, the choice of appropriate statistical laws to estimate quantiles for non-gauged or partially-gauged sites, the scale of flow analysis, and the ultimate purpose of the regionalization exercise. Depending on the choice of hydrologic variables, the scale of analysis and their ultimate purpose, regionalization studies may thus be divided according to two distinct approaches: the hydrologic approach and the ecologic approach.
The ultimate purpose of the hydrologic approach is to estimate flows at non-gauged or partially-gauged sites. For this reason, it has been primarily concerned with methods that allow the grouping of stations into homogeneous hydrologic regions and with the choice of statistical laws to estimate quantiles for non-gauged or partially-gauged sites. However, despite its undeniable interest from a practical point of view, this approach does not address the concerns of ecologists and geographers for three reasons: 1) the choice of hydrologic variables used for regionalization is not based on a scientific concept (this choice is arbitrary, and the variables selected do not constrain all the flow characteristics); 2) the ultimate purpose of the regionalization exercise is limited to estimating flows and is of limited interest to geographers and ecologists; 3) regionalization is performed at a daily scale, without taking into account other scales.
To make up for these limitations, ecologists have recently proposed regionalization based on the “natural flow regime” concept (the ecologic approach), which allows all fundamental flow characteristics (magnitude, frequency, duration, timing of occurrence and variability) to be taken into account. The rationale for considering all flow characteristics is that each characteristic has an effect on the behaviour of river ecosystems. Hence, regionalization based on the ecologic approach relies on a large number of hydrologic variables that define the fundamental flow characteristics. Rather than being arbitrary, the choice of variable is based on this new paradigm. Regionalization using the ecologic approach considers all time scales, and its ultimate purpose is to account for differences in the structure and biological composition of aquatic ecosystems.
However, one of the limitations of studies based on this approach is that, no matter how numerous they are, the variables used for regionalization do not constrain all flow characteristics, as required by the natural flow regime concept, so that application of this concept is incomplete. In addition, simultaneous analysis of all time scales does not allow consideration of all flow characteristics. To overcome these limitations, we propose a new regionalization approach based on the natural flow regime concept, an “ecogeographic” approach that differs from the ecologic approach in three ways. First, the proposed method is based on the use of flow characteristics rather than hydrologic variables. The reason for this is that there are an infinite number of hydrologic variables to define the five fundamental characteristics, making it impossible to account for all of them in the regionalization process. In contrast, since the number of fundamental flow characteristics is limited, they can all be taken into account, consistent with the “natural flow regime” requirements. Second, the ultimate purpose of the proposed regionalization method is to identify the physiographic and climatic factors that explain the spatial variability of these fundamental characteristics. To achieve this goal, it is necessary to analyze the different time scales (daily, monthly, annual) separately given the fact that it is impossible to constrain the effect of these various physiographic and climatic factors at all time scales. Indeed, some factors may show an effect at some time scales and not at others. This ultimate purpose addresses the concerns of geographers interested in explaining the spatial variability of such phenomena, among other things. Finally, separate analysis of the various time scales makes it possible to define all flow characteristics linked to a given time scale. As such, application of the “natural flow regime” concept to regionalization is complete.
Application of the ecogeographical method involves four separate steps: 1) the definition of the flow characteristics for the hydrologic series of interest; 2) the determination of minor and major characteristics using principal component analysis, where a “major” flow characteristic is defined as one which meets the following criterion: TVE ≥ (100% / N), where N is the total number of characteristics that define the analyzed hydrologic series and TVE is the total variance explained; 3) the grouping of stations in homogeneous hydrologic regions based on factorial scores. Homogeneous hydrologic regions are divided in two types based on the presence or absence of stations: effective homogeneous regions contain stations whereas fictive homogenous regions do not; 4) the determination of the factors that affect the spatial variability of flow characteristics. This is achieved using canonical correlation analysis, an approach that we have applied to average annual flows in Quebec watersheds.
Étude préliminaire du traitement d’effluents contenant de l’encre de seiche par centrifugation et procédés à membranes
Imen Aloulou, Khaled Walha, Raja Ben Amar, Francis Quemeneur and Pascal Jaouen
Il s’agit d’une étude préliminaire sur le traitement d’effluents de conditionnement de la seiche avant congélation en vue de réduire la charge polluante des rejets et de valoriser l’encre qu’ils contiennent. Deux types de procédés ont été mis en oeuvre : d’une part, la centrifugation, qui permet de fractionner la suspension d’encre de seiche entre un culot noir à DCO (Demande Chimique en Oxygène) élevée et un surnageant limpide et, d’autre part, l’ultrafiltration (UF) et la microfiltration (MF). Les flux de perméat obtenus par les deux procédés à membranes sont du même ordre de grandeur (25 à 30 L·h‑1·m‑2 sous 1,5 bar). La rétention moyenne en DCO est de 65 % et la rétention en COT (Carbone Organique Total) et azote protéique (NTK) de plus de 95 %. Cependant le colmatage irréversible de la membrane de MF conduit à préférer l’UF, plus facilement régénérable.
Industries that condition fish products have to cope with the problem of processing their usually protein-rich wastewaters. An example of such an industry that discards a large amount of wastewater is the CALEMBO Company (Sfax-Tunisia), which uses 50 m3 per metric ton a day to condition cuttlefish for freezing. In order to conserve water, high-salinity bore water is sometimes used. This high salinity water is responsible for the difficulties encountered during the biological treatment of wastewaters and the recovery of valuable by-products. In this respect, membrane processes, used in the treatment and exploitation of effluents from industries that process sea products, are very attractive. The first membrane filtration trials on sea-product effluents date back to the 1980’s, but they did not result in major developments. Legislative pressures and the increasing costs of water and effluent-processing, as well as the improvement of membrane efficiencies, have made membrane treatment processes much more interesting for wastewater treatment processes. The GEPEA Laboratory at Nantes University has carried out research on membrane technologies to clean up polluted process waters, enhance substances such as soluble fish proteins, and to recover substances responsible for the flavour of bivalves and shellfish.
This paper presents preliminary research on the treatment and exploitation of water used in cuttlefish conditioning. Treatment processes used include centrifugation, microfiltration and ultrafiltration. Centrifugation is used to determine the distribution of the effluent between the black residue and the clear supernatant, whereas membrane processing is used to reduce wastewater pollution and concentrate pigments.
The effluent studied was reconstituted from pure cuttlefish-ink samples taken directly from the animal and salt waters of the same salinity as the bore water used by the CALEMBO Company (Table 1). The samples were reconstituted in ratios of 1 to 50 for centrifugation and 1 to 100 for membrane filtration. Centrifugation trials were carried out using a KR 22i type JOUAN centrifuge, whereas ultrafiltration and microfiltration trials were carried out using the laboratory apparatus represented in figure 1. The main characteristics of membranes used are indicated in table 2. Operating conditions were determined according to the capacities of the feed pump: transmembrane pressure Ptm = 1.5 bar, circulation velocity U = 1.5 m·s‑1 and temperature T = 25°C. The parameters measured on initial feed solutions and the fractions obtained were COD (Chemical Oxygen Demand), TOC (Total Organic Carbon) and nitrogen content (NTK). Filtration trials were carried out according to two different procedures, either with constant feed composition to determine the best operating conditions, or with increasing effluent concentration together with monitoring of the Volumetric Reduction Factor (VRF).
Centrifugation of the cuttlefish-ink suspension produced two phases: a very dense black residue and relatively clear supernatant. The volumetric distribution and the COD and TOC contents of the different fractions are presented in table 3. The supernatant represented about 75% of crude effluent volume. Organic matter was concentrated in the residue and consisted primarily of suspended particles.
At a constant concentration, the ultrafiltration (UF) and microfiltration (MF) processes behaved differently. A rapid drop in flux in the first minutes followed by stabilization at 30 L·h‑1·m‑2 after 30 min was observed for the MF process, whereas a rapid stabilization at approximately 25 L·h‑1·m‑2 was observed for the UF process. The drop in flux at the beginning of MF process may be due to the partial fouling of the membrane pores by melanin particles ranging in sizes from 55 to 160 nm, which are of the same order of magnitude as the membrane pores of 100 nm. On the other hand, the small decrease in flux in the case of ultrafiltration resulted essentially from the formation of a polarization layer and possible interactions between the membrane material and the solution.
Batch-concentration trials were carried out for 5 and 4 h using UF and MF respectively, the operating time being dictated by the dead volume of the equipment (0.75 L). The permeate flux variation as a function of the volumetric reduction factor (VRF) is illustrated in figure 3. The MF flux was slightly higher despite the higher initial concentration of organic substances. For a VRF of 2.64 (final concentration), J = 2.8 L·h‑1·m‑2 for MF and 15.2 L·h‑1·m‑2 for UF. Despite the significantly different permeabilities of the MR and UF membranes to pure water (2690 against 34 L·h‑1·m‑2·bar‑1), their very similar J values are a consequence of the internal pore fouling of the MF membranes.
Analyses performed on the initial feed samples, and on the different fractions of ink suspensions obtained by MF and UF following concentration, are presented in table 5. Retention ratios for UF were very slightly higher than those found for MF, about 65% for COD, 98% for TOC and 95% for NTK. From the point of view of pollution remediation, and considering permeate COD values, the efficiency of the membrane technique does not seem sufficient.
Following ultrafiltration, membrane regeneration was possible by simply rinsing the membrane with water. On the other hand, the same procedure proved inefficient for the microfiltration (PVDF) membrane. The black pigment remained stuck to the membrane surface and most likely inside the pores as well. Furthermore, chemical regeneration (NaOH 0.1 M, 20 min, 25°C) was not enough to recover the membrane’s initial permeability.
To conclude, the ultrafiltration process is better adapted to the treatment of cuttlefish washing wastewater. However, considering the level of residual COD in the ultrafiltration permeate, more efficient post-treatment techniques must be developed.
Jean-Pierre Larue and Alain Giret
L’analyse des données hydrométriques des cours d’eau du bassin de la Maine permet de déterminer la durée et la gravité des étiages durant les années de sécheresse. Les grandes différences dans l’espace mais aussi dans le temps s’expliquent plus par la diversité géologique que par les variations climatiques. Les aquifères jouent un rôle efficace dans le soutien des étiages. Hormis les aménagements réalisés pour régulariser l’écoulement, les autres activités humaines tendent à accentuer les étiages, surtout du fait de l’augmentation des prélèvements d’eau et de la diminution de l’infiltration liée à l’utilisation du sol.
Droughts in France during the years 1976, 1989‑1992, 1996‑1998 and 2003 had a marked effect on its hydrology. Analysis of the hydrometric data collected by DIREN Pays de la Loire made it possible to assess the duration and the importance of low water flows during these years in the Maine watershed. We used low flow limits (half of the minimum monthly discharge) to measure the duration of low flow events. A low flow duration frequency model, hydrological profiles and drought coefficients were used to characterize the low water levels.
Low flows in 1976 were the most pronounced, but did not last the longest. Maps of rainfall, flow and discharge coefficients for the Maine basin show high spatial and temporal fluctuations. These differences can be accounted for more precisely by geological diversity than by climatic changes. All watersheds with an oceanic climate suffered from pluviometric deficits and ground water played an efficient part in sustaining low water levels. Three types of stream response could be distinguished. First, the drought affects secondary streams flowing on the clay soil of the Armorican basement (Vaige, Oudon, Chéran, Uzure), but also in the Paris basin, such as the Rhonne River, where structure is responsible for the capture of a part of the ground water by other rivers. Second, the Sarthe, Huisne, Braye, Veuve and Narais rivers, which have appreciable ground water reserves, did not dry up and where low flows were not important. Third, between these two groups, the other rivers had more or less pronounced low flows, according to morphostructural conditions and also water consumption patterns.
In oceanic regions, low flows mainly take place during the summer when pluviometric deficits and strong evapotranspiration occur. Cumulative rainfall and discharge values show that low water flows occur at the end of the descending stages of the hydrograph. Seasonal rainfall distributions indicate that low flows are strongly favoured by pluviometric deficits occurring during cold seasons. From 1989 to 1992, four consecutive years with pluviometric deficits and especially with insufficient rains during the winter, an increase in the duration of low flow events and droughts was observed. In regions with sedimentary geology, low waters occurred when rainfall during the cold season was insufficient to recharge the aquifers that control base flow. In non-sedimentary basement regions, low flows are more pronounced and lengthy because of the lack of groundwater inputs. Therefore, the schistose areas of the Armorican Massif are particularly susceptible to drying up. Moreover, smaller watersheds appear to be more sensitive to drying up than are larger ones. Note too that apart from work that has been carried out specifically to regularize river flows, human activities tend to accentuate low flows, especially because of increases in irrigation pumping and decreases in water infiltration linked to changes in land use.
The study of low flows is a useful method for understanding watershed hydrology and for the management of water resources. The recorded spatial differences suggest that measures that must be taken to reduce the occurrence of low flows are different for the basement and sedimentary regions. For the former, it is necessary to build water reserves, whereas for the latter, the monitoring of water table levels and the implementation of controls should be sufficient for predicting and managing drought periods.
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