Volume 9, Number 2, 1996

FR:

Les modèles de simulation hydrologiques sont reconnus comme des outils mathématiques très performants mais généralement d'application difficile, principalement à cause du nombre élevé des paramètres requis. À l'inverse, les outils actuels de gestion sont généralement développés à l'aide d'approches empiriques limitant leur application. De plus, leurs paramètres ne sont pas des grandeurs mesurables et doivent être ajustés pour chaque situation. Une approche est proposée pour le développement d'outils semi empiriques de gestion. Elle consiste à simuler un grand nombre de scénarios en utilisant un modèle complexe de simulation puis à rationaliser l'information obtenue pour développer un nouveau modèle semi empirique. L'exemple illustrant cette approche concerne l'évaluation des flux d'eau ruisselée à la surface des champs, lessivée vers la nappe souterraine et drainée par les drains agricoles spécifiquement pour le contexte du Québec. À partir des résultats de simulation de 4500 scénarios, une simplification.rationalisation a permis de réduire à 120 le nombre de scénarios de référence à l'aide desquels peuvent être évalués tous les scénarios possibles par de simples interpolations linéaires. Une application de l'algorithme résultant sur un site du Québec a montré la bonne concordance entre les résultats calculés et mesurés. À la fois l'ordre de grandeur du ruissellement et du drainage et leur grandeur relative sont bien évalués.

EN:

Considering the complexity of the water cycle in soil systems, models are used more than ever in parallel with field investigations to assist in the decision making process (KHAKURAL et ROBERT, 1993). Most available models are either too complicated (many non-measurable parameters) or too simple (empirical or site-specific) to be used as management tools. Such tools should conform to known theory and should be structured to enable efficient analysis of field situations with minimal requirements for parameters (CARSEL et al 1984). However, if the mechanistic models are very performing tools with regards to their representation of the processes and for the accuracy and reliability of their results, they are criticized for their complexity and for the large number of parameters they require. For this reason, their potential application as management tools cannot be recommended especially in preliminary investigations when the methodology has to be straight forward and rapidly implemented. On the other hand, existing management tools are often developed using an empirical approach for a specific context which considerably limits their transferability to different situations. Moreover, their empirical parameters often cannot be measured for the new situations, and must be adjusted for each new application.

A new approach conciliating the qualities of both kinds of tools was elaborated for the development of management tools. This approach consists in using mechanistic models for simulating a set of possible situations and in rationalizing the information obtained by simulation through regression analyses or other methods. An example of this methodology is presented in this paper with the development of the hydrological part (runoff, leaching and drainage) of a management tool dedicated to the evaluation of nutrient losses related to manure applications. Developed for the Quebec conditions, 4500 theoretical situations were considered corresponding to ten climates, nine soil textures, 25 crops and two slope values. Independently, agricultural management practices and drainage were taken into account.

For the mechanistic simulation of the water budget in the 4500 theoretical situations, the hydrologic module of the mechanistic-stochastic model AgriFlux was used (BANTON et al. 1993b). Because of the important field variability of most parameters, the stochastic AgriFlux model incorporates the variability resulting from field heterogeneity, measurement errors and intrinsic uncertainty related to parameter definition. The soil profile is divided in plot scale homogeneous horizons (or compartments) and a daily time step is used in the calculations. The water budget module in AgriFlux is named HydriFlux and simulates all the water-related processes (precipitations, snowmelt, infiltration, runoff, water uptake by plants, evaporation, percolation and drainage) using characteristic water contents and unsaturated hydraulic conductivity.

In the example presented, the simulation results obtained by running HydriFlux have shown that the soil water fluxes (runoff and percolation) vary as linear functions of both the annual rain volume (the most important characteristic of the climate) and the logarithm of the saturated hydraulic conductivity (the most important characteristic of the soil type). A reduction of the number of crops could also be achieved by taking into account the water needs and the water uptake curves of the crops. This rationalization-simplification reduced the number of theoretical simulations to be stored in the management tool to 120 (2 climates x 3 textures x 10 crops x 2 slopes). These represent only 2.7% of the initial situations simulated by the mechanistic HydriFlux model. The different water fluxes are stored in the management tool as tables in which direct interpolations are performed to calculate the fluxes corresponding to all the potential intermediary situations. Such developed management tool presents good qualities at the same time for its calculation speed, for its easy parameterization, for the reliability of its evaluation (through the evaluation of the mechanistic model) and for its high transferability and applicability to various situations. The calculations are rapidly done and their programming can be very easily made by using a spreadsheet software.

An application of this evaluation method has been done on an experimental site located in Quebec (ENRIGHT et MADRAMOOTOO, 1994), the only one for which both the runoff and the drainage have been measured during many years (1989 to 1991, April to December). The application on two fields (1.84 et 4.63 ha) has shown a good concordance between the calculated and measured results, as well for the magnitude of the fluxes than for the relative importance of these fluxes. Moreover, this application has shown that the variability of the measured values is higher than the calculated ones, attesting of the great influence of the variations in climatic, soil, crop and management conditions on the water budget. However, the good evaluation of the fluxes (for relative and absolute values) confirms the reliability of the proposed approach and of the simplification.

EN:

A study of the removal of dissolved organic carbon (DOC) and biodegradable dissolved organic carbon (BDOC) has been performed in a water treatment plant where several biological steps are involved. The influence of the temperature of the raw water has been studied. The behavior of slow sand filtration (SSF) differs from that of granular activated carbon (GAC) filtration: in the case of slow sand filtration, optimum efficiency is obtained at a temperature above or equal to 15°C, whereas optimum efficiency for GAC is obtained starting from a temperature of 10°C. Slow sand filtration seems to be more efficient in BDOC removal where high temperatures are present. For low temperatures, efficiency of the two methods is comparable. For medium temperatures (10°C), GAC seems to be more efficient.

FR:

Le suivi du carbone organique dissous (COD) et de sa fraction biodégradable (CODB) a été effectué à différentes températures comprises entre 5 à 15°C sur une chaîne de traitement d'eau de consommation comportant plusieurs étapes biologiques. L'influence de la température de l'eau brute est particulièrement importante pour les étapes biologiques telles que la filtration lente sur sable (FLS) et la filtration sur charbon actif en grains (CAG).

L'efficacité de la filtration lente est très bonne à 15°C avec une élimination de 76 % du CODB. À cette température, néanmoins, la concentration en CODB de l'eau issue des filtres lents est supérieure à celle observée à des températures inférieures. L'activité biologique des filtres lents produit du CODB. À la température de 5°C, l'élimination du CODB sur les filtres lents n'est que de 20 %. Il semble que l'optimum de température pour une efficacité optimale soit de 10°C.

Les résultats obtenus sur le charbon actif en grains sont très différents: à 8°C, l'élimination du CODB est de 35 %, alors que nous avons observé un maximum d'élimination de 55 % de CODB sur les filtres à charbon à une température de 9°C. L'efficacité de la filtration lente sur sable pour l'élimination du CODB semble supérieure à celle des filtres à CAG à des températures supérieures à 10°C. Pendant l'étape d'ozonation, une augmentation du carbone organique biodégradable a été observée. Cette hausse est fonction de la température et de la dose d'ozone appliquée. Quatre fractions du carbone organique biodégradable de l'eau ozonée ont été différenciées: le CODB libéré par l'activité biologique des filtres lents, le CODB particulaire redissous, le CODB attribuable à l'attaque du COD réfractaire par l'ozone et le bruit de fond du CODB libéré des filtres lents, constant quelle que soit la température.

Une augmentation du CODB libéré par l'activité biologique des filtres lents a été notée, ainsi qu'une augmentation du CODB particulaire redissous avec la hausse de la température. Par contre, une diminution du CODB provenant de l'attaque du COD réfractaire a été observée avec l'augmentation de la température.

EN:

The purpose of this study is to select a process control parameter for monitoring microbial regrowth in a network and to develop a more accurate and relevant quality control of supply water. Two parameters were examined as potential process control parameters: the water residence time in the network and the concentration of biodegradable organic matter. Residence time calculations were carried out and validated by tracer studies in a branched network and then in a simply looped network. The measurement of the natural dissolved organic carbon (DOC) consumption in the network was preferred to the determination of any in vitro biodegradation. The measurement of consumption requires the determination of DOC in treated water and in supply water. It is simpler and less expensive than other biodegradable organic matter determinations. A model for colony counts as a function of the residence time was developed in order to demonstrate that this parameter can be used for process controlling. This model was very well adjusted to data collected in a network in winter, spring and summer. This process control parameter was then used in order to locate and estimate the quantity of water whose colony counts exceed the European directive guide level.

Accurate correlation measurements between colony counts and DOC consumed in the network were carried out in three distinct systems. No significant correlations were measured. For these three networks, biodegradable organic matter measurements based on DOC determinations were demonstrated to be unreliable process control parameters for monitoring bacterial regrowth.

FR:

De nombreux paramètres sont mesurés aux usines de production d'eau potable pour contrôler l'efficacité du traitement : pH, turbidité, concentration en désinfectant... Cependant, l'eau traitée n'est pas un produit fini; sa qualité, notamment microbiologique, peut évoluer en cours de distribution. La livraison d'eau aux consommateurs peut ainsi être considérée comme l'étape finale du processus de production d'eau potable. La complexité du contrôle de la qualité de l'eau en cours de distribution repose sur l'identification des points d'échantillonnage et du nombre de prélèvements. L'objectif de cette étude est de sélectionner un paramètre de contrôle de la recroissance microbienne pour développer un contrôle de qualité plus rigoureux et précis de l'eau de distribution. Deux paramètres ont été étudiés en tant que paramètres potentiels de contrôle : le temps de séjour de l'eau dans le réseau et la matière organique biodégradable. S'il est possible de démontrer que ces paramètres ont une influence significative sur la qualitê microbiologique, ils pourront être utiles à l'identification des points d'échantillonnage et à la détermination des nombres minimaux de prélèvements. Le temps de séjour a été déterminé dans un réseau en antennes, puis dans un réseau légèrement maillé. Ce paramètre a été calculé à partir de mesures de débit et de volume de réservoirs. Les conditions de mélange parfait dans les réservoirs et d'écoulement piston dans les canalisations ont été supposées. La détermination du temps de séjour a ensuite été validée par des traçages au chlorure de sodium. La mesure du carbone organique dissous (COD) naturellement consommé dans le réseau a été préférée, pour sa simplicité et son plus faible coût, aux analyses reposant sur la biodégradation de la matière organique in vitro. Ces déterminations reposent sur la différence entre les teneurs en COD de l'eau traitée et de l'eau en cours de distribution. Un modèle prédictif du nombre de micro-organismes déterminé sur gélose et fonction du temps de séjour a été développé. La fonction logistique, souvent appliquée à la croissance des micro-organismes dans des réacteurs de laboratoire, a été choisie comme modèle. Deux paramètres microbiologiques ont été considérés : le nombre de micro-organismes déterminé à 20ºC après trois jours (N3D) et quinze jours (N15D) d'incubation. La fonction logistique a été ajustée, à un niveau de signification inférieur à 0.05, aux données de N3D et N15D collectées en hiver, au printemps et en été dans un réseau faiblement maillé. L'ajustement du modèle à différentes saisons et différentes parties d'un réseau a permis de mettre en évidence les facteurs influençant la recroissance microbienne et par conséquence les coefficients du modèle : la saison, l'origine de l'eau (eau de surface, eau souterraine) et le type de conduite. Le modèle de N3D a permis de localiser et d'estimer la quantité d'eau de distribution dont le paramètre N3D est supérieur au niveau guide européen. Ce modèle a aussi été appliqué à la localisation de postes de rechloration sur le réseau.

L'utilisation de la mesure de la matière organique biodégradable en tant que paramètre de contrôle de process a été évaluée à partir de la mesure de la corrélation partielle entre le COD consommé dans le réseau et N3D et N15D. La corrélation partielle permet, dans ce contexte, de mesurer uniquement le lien entre le COD consommé dans le réseau et le nombre de micro-organismes en supprimant les interactions avec d'autres paramètres tels que le temps de séjour. Les résultats ont montré que pour les trois réseaux étudiés aucune corrélation partielle significative n'a été observée entre ces deux paramètres. Le COD consommé dans le réseau ne peut pas être utilisé efficacement comme paramètre de contrôle.

FR:

L'approche expérimentale choisie pour cette étude a eu pour objet de mesurer en conditions batch, les cinétiques d'élimination de la demande chimique en oxygène soluble d'un effluent synthétique mis en contact avec des boues activées d'origine différente.

Les essais conduits en laboratoire ont été réalisés en faisant varier le rapport So/Xo (mg de DCO initiale par mg de matières volatiles initiales) telles que les concentrations en So et en Xo correspondent aux concentrations en DCO soluble (DCOs) et en matières volatiles (MV) rencontrées sur les stations d'épuration.

Les essais ont été effectués sous aération continue à 20 °C en mettant en contact l'effluent synthétique et de la boue activée prélevée depuis 24 h et stockée à 4 °C dans l'attente de l'essai. De ce fait, la valeur de So mesurée au début de l'essai représente la concentration en DCO amenée par l'effluent synthétique (Seff) et celle amenée par l'inoculum de boue activée (Sb) représentant selon les essais de 5 à 70 % de la DCO de l'essai.

Les profils de cinétique d'élimination de la DCO soluble obtenus pour différentes conditions d'essai s'ajustent, selon les valeurs de So/Xo (So/Xo variant de 0,15 à 2,17 ) à une fonction du premier ordre par rapport au substrat ou à une fonction sigmoïde. Le type de fonction cinétique d'élimination est également contrôlé par la proportion de la DCO amenée par l'inoculum.

EN:

The conventional activated sludge process used for wastewater treatment removes from 80 to 95% of the total organic matter. However, a quantity of "not well identified" (particular, colloidal and soluble) organic matter is always present in the treated effluent. Reducing this residual (and improving the treatment efficiency) requires knowledge of the origin of that organic matter and especially to determine the fraction originating from the influent and the fraction generated by the biomass.

This research has been conducted in batch conditions and studies the soluble COD (CODs) removal kinetics of a synthetic effluent (casein + starch + acetate + mineral salts), in contact with different activated sludge originating from six different wastewater treatment plants (loads varying from 0.06 to 1.14 kg BOD[inf]5/kg VSS. d).

Experiments have been conducted with different So/Xo values (ratio between CODs initial concentration and VSS initial concentration) in order that these values correspond to the CODs and VSS values found in the plants.

In accordance with GRAU et al. (1975), CECH and CHUDOBA (1983), PITTER and CHUDOBA (1990), CHUDOBA et al (1992), the So/Xo ratio is a fundamental parameter governing the kinetics reactions.

Experiments have been conducted under continuous aeration at 20°C where the synthetic wastewater (500 ml) is in contact with activated sludge (200 ml) collected 24 h before and stored at 4°C until the batch is started. In this manner, the initial So is due to the CODs of the synthetic effluent (So eff=197 mg/l) and to the CODs originating from the sludge (5 to 70% of the initial CODs). The initial VSS concentration (Xo) is between 0.6 and 2.5 g/l.

Kinetics of CODs removal are simulated by two functions: the first order function, where the initial rate is the maximal, and the sigmoidal function where the maximal rate is reached after a lag time (3 to 8 h).

Concerning the first order functions, the degradation rate is faster when the ratio is low (So/Xo lower than 0.44). This is not the case for the sigmoidal functions. In the results of this study, the residual of COD is always lower when the degradation kinetic follows the exponential model.

Our experiments show that the type of degradation kinetics (first order or sigmoidal) is not only controlled by the So/Xo parameter but also by the proportion of CODs brought by the sludge and that parameter can play a determinant role.

When the proportion of CODs brought by the sludge is very large (between 41% to 46%) the degradation reactions follow the sigmoidal type. These results can possibly be explained by the low biodegradability of the polymers or molecules originating from the inoculum which has been stored during 24 h, or by the low activity of the biomass after 24 h of storage on the biodegradation of the soluble organic matter

FR:

L'étude des écoulements souterrains et du drainage est généralement réalisée par l'établissement des cartes et des profils piézométriques (en effectuant des campagnes de relevés piézométriques dans les forages et les piézomètres captant la nappe). On essaye ici de confirmer les résultats obtenus à partir de l'interprétation de la piézométrie par le traitement statistique multidimensionnel des données hydrogéochimiques des sources, et plus précisément l'analyse factorielle discriminante, et de caractériser l'importance et la dominance de l'un des paramètres responsables de la minéralisation des eaux karstiques : les phénomènes climatiques (saisons), et les caractéristiques du système (lithologie, structure du drainage, gradients).

L'application à la nappe des calcaires du Bathonien-Bajocien du plateau de Besançon (Jura externe, France) a permis de démontrer que la variabilité chimique des eaux aux exutoires karstiques est conditionnée plus par les caractéristiques hydrogéologiques de l'aquifère (unités hydrogéologiques) que par les évènements hydroclimatiques (effet "campagne" ou période d'étude), et de resituer les forages par rapport aux sources testées ; l'association d'un groupe de forages avec certaines sources est à relier avec des circulations rapides en zone libre, ce cas est celui du regroupement de la source du Trébignon et les puits de Chailluz (situés dans la zone libre de l'aquifère) qui présentent le même comportement hydrochimiques. Alors que d'autres forages sont groupés avec le fonctionnement de la nappe en zone captive, ce cas correspond à celui de l'affectation des puits de Thise (implantés dans la partie captive de l'aquifère) à la source de Roche- lez-Beaupré. Ces résultats corroborent ceux obtenus par l'étude piézométrique.

EN:

The plateau of Besançon (Eastern France) spreads between the large valley of the Ognon to the north and the enclosed valley of the Doubs to the south. Stratigraphically, the principal formations belong to the middle Jurassic era and the region is marked by a brittle tectonic characterized by NE-SW faults. Bajocian and Bathonian limestones constitute a karstic aquifer (200 m in thickness), which is free for much of its extent but becomes confined under Oxfordian marls towards the Doubs valley. The hydrogeological and structural boundaries of the drainage basin are complex, as are its hydrodynamic functioning and the hydrochemical responses observed at the outlets. The reservoir is drained principally by the Mouillère, Trébignon and Roche-lez-Beaupré springs, these outlets being situated mainly in the Doubs valley.

The region is largely covered by dense vegetation, essentially forests, which constitute a groundwater recharge zone, and by the conglomeration of Besançon. One of the most karstified regions in France, the site includes two groups of wells that supply the town of Besançon with drinking water: the first group is situated on the unconfined zone of the aquifer (Chailluz), and the second is located on the confined part of the karstic aquifer (Thise). The wells are equipped with piezometers to monitor water levels continuously.

The rainfall distribution in this karstic region is homogeneous. During the 1991- 1992 hydrological year, the observed precipitation was 1144 mm at Besançon, with an evapotranspiration constituting about 50 % of the measured precipitation. On the Besançon plateau, the water mineralization of the different karstic springs is related to the nature of the underlying geological media (lithology, importance of karstification, etc.), to the water residence time and to seasonal variables (characteristics of the rainfall episode, hydroclimatic events, etc.). The water chemistry is relatively stable during dry intervals, but exhibits high fluctuations during flood periods. Outlets were sampled every 7 to 10 days from January 1991 to August 1992, in order to follow variations in water physico-chemistry. The sampling frequencies covered diverse water-level situations in the aquifer (low-water period, high-water period, drying-up period, etc.), and allowed the application of some multidimensional statistical tools.

The piezometric profile and lithologic correlations between the boreholes of Chailluz and Thise show that the highest hydraulic gradient is calculated between the Chailluz boreholes and the Trébignon spring. The hydraulic gradient is low in the confined part of the aquifer (between the Thise boreholes and the spring of Roche-lez- Beaupré). Two zones can be identified:

1. a free zone distinguished by flow and drainage relationships between the Chailluz boreholes and the Trébignon spring;

2. a confined zone linking the Thise boreholes and the Roche-lez-Beaupré spring, which corresponds to a common water circulation.

To confirm the results obtained from the piezometric study, we have used multidimensional statistical techniques to process the hydrochemical data from the karstic springs.

Factorial discriminant analysis is a multivariate method that allows the testing of hypotheses. It belongs to the field of decisional statistics, and is based on the comparison of multidimensional intragroup variances to intergroup variance. These methods can show the presence of certain relationships between a qualitative explained criterion and a group of quantitative explanatory characters, and they allow one to describe these latter relationships. Cross discriminant analysis can be used to evaluate the pertinence of a given hypothesis and to identify the dominant factor(s). The introduction of a qualitative variable within a population allows the division of this population into different groups, with each individual assigned to one group. Discrimination of the groups consists of maximizing the variance between their centres of gravity (a principal components analysis is performed on the matrix of the centres of gravity of the defined groups a priori); one can then clarify the properties that distinguish the different groups. If the individual is close to the centre of gravity of its group, it is "well classified". In the case where the distance to the centre of gravity of its group is superior to that to the centre of gravity of another group, the individual is "poorly classified" and it will be reassigned to this other group.

Another advantage of these methods is the use of supplementary individuals, which are not involved in the definition of the groups. Supplementary individuals are reassigned to the groups to which they are statistically linked. In karstic hydrogeology, cross factorial discriminant analysis of a population enables one to validate the classification criteria based on the analysis of flows and concentrations. These units of classification deal with the hydrokinematic phases (seasonal variable) or with the intrinsic systems (geographic variable). Therefore, in our case we have crossed two qualitative factors, "system" (Trébignon, Roche-lez-Beaupré) and "season" (spring, summer, autumn, winter) by undertaking two different analyses on the same series of data.

The application of these techniques to the limestone aquifer of the Bathonian- Bajocian shows that intrinsic factors predominate in determining the mineralization of the outlet water. On the other hand, this method enables one to associate borehole chemistry to tested springs: the association of a group of boreholes with particular springs is explained by the rapid circulation and drainage in the unconfined zone of the karstic media. This is the case of the association of the Chailluz boreholes (situated in the unconfined zone of the aquifer) with the Trébignon spring, which behave hydrochemically in the same manner. Other boreholes are grouped with the confined aquifer; this case corresponds to the assignment of the Thise boreholes (located in the confined zone of the aquifer) to the Roche-lez-Beaupré spring. These results corroborate those obtained from the piezometric study.

Factorial discriminant analysis allowed the verification of the appropriation of the predefined groups to the discriminant factors: hydrogeologic character (lithology, structure of the drainage, gradients, etc.) and hydroclimatic phenomena (season, hydrometeorological episodes, etc.). The choice of the property groups based on the geographic and geologic criteria proved pertinent.

FR:

Cet article rassemble les résultats de l'étude en laboratoire des conditions de formation des ions bromate (BrO3-) lors de l'ozonation des ions bromure. Les expériences ont été réalisées en réacteur fermé, en milieu tamponné à la température ambiante, (ª23 °C) et en présence de matière organique. La concentration initiale en ions bromure a été fixée à 200 mg/L et le taux d'ozone appliqué à 5 mg/L. La matrice étudiée a été préparée à partir de différentes fractions de la matière organique extraites d'eaux naturelles (acides fulviques et acides hydrophiles) et d'une fraction synthétique (tripeptide). Les fractions ont été étudiées individuellement ou en mélange dans des proportions compatibles avec la matière organique naturelle. L'étude a porté sur l'impact de la matière organique, du pH, de l'azote ammoniacal, de l'alcalinité et du peroxyde d'hydrogène sur la formation des ions bromate. L'analyse de BrO3- a été effectuée par chromatographie ionique après 24 heures de temps de réaction (ozone résiduel négligeable) ; la limite de détection de la mesure est de 2 mg/L.

EN:

The objective of our work was to evaluate the importance of various parameters (natural organic matter, pH, ammonia, bicarbonate, hydrogen peroxide) affecting the formation of bromate during the ozonation of natural waters containing bromide.

Bench scale experiments were carried out on synthetic solutions prepared in phosphate buffer (10-² - 10-³ M). Bromate concentrations were determined by ion chromatography after the complete consumption of the added ozone (24 h contact time); the ion chromatograph was equipped with a suppressed conductivity detection system (DIONEX) with a detection limit of ca. 2 µg BrO3-/L.

The impact of organic matter was evaluated with model organic compounds: fulvic acid and hydrophilic acids (FA and HyA) isolated from a river water with XAD8 and XAD4 resins, and a tripeptide (Tyr-Gly-Gly). These different organic compounds were studied individually or as a mixture (60% FA, 30% HyA, 10% tripeptide).

All solutions were spiked with 200 mg/L of bromide and the applied ozone dose was 5 mg O3/L; the DOC content varied from 0 to 5 mg C/L.

For given experimental conditions, bromate formation varies depending on the origin and nature of the organic matrix (natural organic matter or tripeptide), probably due to their respective ozone consumption.

For a particular ozone dose, an increase in DOC results in decreasing bromate production. In fact, ozone consumption by the natural organic matter reduces the available ozone and somewhat inhibits the efficiency of bromate formation reactions.

More bromate is formed with increasing pH, for the relative proportion of hypobromite increases in relation to hypobromous acid.

In the presence of ammonia and natural organic matter, bromate formation is lowered because ammonia diverts part of the HOBr to form organobrominated compounds, and thus participates indirectly in ozone consumption.

Bicarbonate addition favors molecular ozone type reactions by scavenging free radicals. Ozone is stabilized in the medium and thus more bromate is formed than in the absence of any scavenger. The reaction of OH radicals leading to bromate formation needs the presence of BrO-. Thus the use of hydrogen peroxide with an excess of ozone, or ozonation prior to hydrogen peroxide addition, may lead to the production of high bromate concentrations.

This experiments shows that the higher the available ozone, the higher the bromate production. The use of ozone for disinfection purposes involves maintaining a residual ozone concentration during a due time in order to achieve the required C. t for bacterias and viruses inactivation. This increasing C. t may result in high bromate formation.

FR:

Deux campagnes de mesures effectuées sur le lac du Bourget en période de faible stratification (avril et décembre 1994) ont mis en évidence des oscillations de la thermocline de période comprise entre deux et trois jours. Ces oscillations atteignent 40 m d'amplitude pour une profondeur maximale de 145 m. Elles ont probablement un impact sur les processus biologiques et physico-chimiques qui gouvernent l'évolution de la qualité des eaux du lac.

L'analyse des données brutes des températures révèle une corrélation étroite entre la génération des ondes internes et les événements de vents importants. Il apparaît en particulier que seuls les vents violents (< 8 m/s) affectent la stratification thermique de manière significative.

Deux approches sont utilisées pour caractériser ces ondes :

- une approche par traitement du signal qui donne accès aux périodes d'oscillations prédominantes ainsi qu'à la répartition de l'énergie dans la colonne d'eau en fonction de la fréquence.

- une approche par modélisation mathématique au cours de laquelle les résultats obtenus par tjjois techniques distinctes utilisant plusieurs degrés de représentation de la bathymétrie du lac sont comparés. Ces modèles permettent de calculer les périodes d'oscillations ainsi que les déplacements de l'interface de densité et les vitesses dans chaque couche. A partir des valeurs des amplitudes d'oscillation obtenues expérimentalement, des vitesses maximales de l'ordre de 7 cm/s dans l'épîlîmnion et 3 cm/s dans l'hypolimnion ont pu être estimées pour les deux épisodes considérés.

On montre que les modèles mathématiques et l'analyse spectrale corroborent les observations.

EN:

There is a great concern about the understanding of water mass movements in lakes as they play a crucial role in the way nutrients and pollutants are trans-ported. This work brings new insights to the study of internal waves as it compares field data to various mathematical approaches.

During the months of April and December 1994, a thermistor chain was deployed in Lake Bourget, France (length 18 km, width 3 km, maximum depth 145 m) to record temperature every 10 min, over nine unevenly spaced depths, from 10 to 51m. The time series of measurements provide a detailed picture of the characteristics and dynamics of internal waves. Records are discussed in view of the wind data observed at a meteorological station located at the south-end of the lake. According to the intensity of the wind forcing, the thermal structure is altered in different ways. When the winds are weak, the thermo-cline tilts and sets up a hydrostatic pressure gradient which balances the wind stress (TURNER, 1973). When the wind stops, the density interface oscillates until buoyancy is strong enough to balance the baroclinic pressure field.

Strong winds, however, enhance large amplitude nonlinear waves which may break (Kelvin-Helmoltz instabilities), and therefore give rise to vertical mixing in the hypolimnion. During the recording periods wind stress in general is low, but occasional bursts of energy generate internal waves (fig. 2 and 3). In this paper, we focus on linear internal waves as records display a dominant response of the first longitudinal mode. The internal seiche continues to oscillate with decreasing amplitude after the wind has ceased.

Two approaches have been implemented in order to characterize the internal waves. One consiste of signal treatment through spectral analysis and the second one involves mathematical modeling.

Spectral analysis discloses responses of the first mode with periods of about 80h and 40h, respectively, for the April and December fleld survey (fig. 4 and 5). Further analysis of the April spectra shows that winds generate highly non-linear waves with high energy levels in a large band located in the first 30m. This band results from the mergence of two peaks of high energy at 80 and 40h respectively which probably correspond to the fundamental and second harmonie of a nonlinear wave. Then, as the wind stops, internal seiche of the first mode develops in the layer located between 30 and 50m indicating a deepening of the thermocline.

Those fluctuations and their energy spectra are compared with the prédictions of three methods which are based on linear théories and consequently are not valid when the magnitude of oscillations is too high. The full phenomenon of wind-forced motion in a lake is not treated here. However, the analysis of postforcing phase is undertaken to charaterize free internai waves. One method is the Merian formula, which considers the lake as a two-layer system of constant properties and assumes the lake as a rectangular box. Another is a modified version of the Defant procédure (MORTIMER, 1979) which again assumes two layers but solves the momentum and mass équations with a varying cross section. The third method is the two layered variable depth model (TVDM) deve-loped by Schwab (HORN et aL, 1986), fitted to the basin topography and inclu-ding the free surface displacement It is expressed here through a one dimensional version directed along the main axis of the lake (i.e. the lateral variations of depth are not considered).

The models display pattems of thermocline displacements (illustrated in fig. 7 and 8) which, in periodicity, are closely similar to those observed. Moreover, they give estimates of the maximum velocity induced by the seiche. Values of the order of 7 cm/s and 3 cm/s are found in the epilimnion and hypolimnion respectively. Finally, the influence exerted by the morphometry on the wave shape and associated field velocity is emphasized. In particular, the difference in the maximum speed calculated in the hypolimnion probably stems from the lateral contraction of the lake (and thus increasing speed) near Aix-les-Bains which is not taken into account in the TVD Model. The validity of the models implemented here is thrown back into question when the magnitude of the oscillations is sufficiently high to steepen the thermocline and in this particular case, a nonlinear theory (Korteweg-de Vries Equation) would be appropriate.

The importance of a better knowledge of internal seiches goes beyond the field of physics. Through their influence on mixing and dispersal, those motions profoundly affect the chemical and biological economies of many lakes. Internal waves are responsible for periodic vertical displacement of the resuspended biomass, and consequently for variation in the light intensity to which algal cells are exposed. Furthermore, associated bottom currents can enhance dissolution and remobilization of nutrients by transporting the products of bacterial decomposition away from the sediment-water interface into the water column.