FR :

Cette réflexion sur la modélisation en hydrologie, bien que se référant souvent à ce domaine précis de l'hydrologie, se veut d'ordre plus général et propose une classification très réductrice des modèles en deux genres: ceux établis à partir de données d'observation des processus étudiés, et ceux pour lesquels il n'existe aucune observation du phénomène à modéliser, à l'échelle étudiée.

A partir de cette classification et des règles bien connues de la tragédie classique (unité de lieu, de temps, d'action), une pratique de la modélisation est proposée et des pistes de recherche sont dégagées.

On conclut en rappelant qu'il doit exister aussi, dans la communauté scientifique, en sus de recherches en modélisation à caractère « utilitaire », d'autres travaux portant sur des modèles qui ne servent à rien.

Ces quelques réflexions, à caractère quelques peu polémique, ont pour objet d'initier si possible une discussion; d'où la rubrique « tribune libre » où elles paraissent.

EN :

This discussion article addresses the issue of the nature of models used in hydrolory. Although its emphasis is on contaminant transport in groundwater, I believe it is relevant to most areas of hydrologic modelling. It proposes a minimalist classification of models into two categories: models built on data from observations of the processes involved and those for which there are no observation data on any of these processes, at the scale of interest.

The argument is that the former should (or rather, ought to, since the question seems to attract little interest) obey serious working constraints, well-known from classical tragedy:

- unity of place,

- unity of time,

- unity of action.

The meaning of these rules, in terms of model calibration, validation and extrapolation, is analysed. They impose very strong limitations on the applicability of such models.

As to the models in the latter category which, in my opinion, are the more interesting and useful ones, several suggestions are made for their development and application.

1. MODELLING OBSERVABLE OR OBSERVED PHENOMENA

Observable phenomena such as nitrate or pesticide pollution are there to be measured and obserrved, although this might in practice involve considerable effort. Modelling is then used to forecast future behaviour of these pollutants.

The archetypal model for observable phenomena is that of the « black box ». If one can provide the box with one or several inputs and outputs and place something numerical inside, it will produce results. The modeller's task is to introduce a serviceable « engine » into the box, if possible. However, the least demanding of approaches is protrably the neural network method. Here, an engine is not even necessary: the series of observed inputs and outputs is given to the network which itself carries out a « weighting » of the input data resulting in the given output. The « engine » in the black box is created by the data, whereas in more familiar black boxes, the modeller decides on a form of relationship between the input and the output (e.g. a convolution equation, a groundwater model equation), and only tries to fit a limited set of parameters describing the black box. This phase of the fitting is called, in neural network terms, the « learning process » - it is modelling reduced to its bare essentials. Almost no physical understanding of the system is needed and the range of decisions left to the modeller is almost zero (e.g. number of neurons, of neural layers, forward or backward learning, etc.). This type of model looks simple but it must be remembered that most other models currently used in hydrology on series of observed data are actually quite similar. They may be called deterministic or even stochastic, conceptual or distributed but the basic principle of all these « fitted » models is the same: the « engine » in the box is created by the data set and its exact nature is irrelevant.

Validation of models fitted by the learning process

This important questions is at present being debated by modellers. For a blackbox model (as defined above), valdiation consists in testing it with a set of parameters that has not been used during the learning process trut for which the output is knovn and then, comparing the real and calculated outputs. A number of criteria has been proposed for this comparison and many conflicting opinions aired as to the possibility of validating a theory, and the model which is its expression, from observations. A counter- argument holds that a model is not an expression of a theory but of universally accepted principles, e.g. mass conservation, and of experimental laws, e.g. Darcy's law. Moreover, although a model may be invalidated at some point, it represents the « least unsatisfactory » way of trying to forecast the future, and each successful validation attempt increases the confidence in the model. Therefore, it would seem a good idea to separate the learning data into two groups: one for fitting and the other for validating.

Unity of spaceThis means that the model only applies to the domain on which it has been fitted through the learning process. There are three examples of how this rule is infringed: (i) Extrapolation in space. The set of parameters derived at site A, said to be « representative » of the medium, are used at site B. To my knowledge, no evidence exists at present of the merits of this method nor has it been reliably validated by experience. Quite the opposite, except in rare cases ofvery uniform media. (ii) Transposing in space through a formal link between the parameters obtained through the learning process for site A and the corresponding geometrical and geological data for site B. A new set of parameters is derived characterizing the latter. This could prove a fruitful line ofresearch but it has not as yet, received much attention . (iii) Method of « relay element », which applies specifically to transport in porous media. As movement of strongly retarded solutes cannot be observed, a laboratory experiment is done with the solute in question and another weakly retarded « relay element ». The relay element is then used alone in the field, at the transport distance of interest The difference in their retardation, measured at the laboratory scale, is then extrapolated to the real medium in order to make the prediction. This approach is certainly preferatrle to a simple extrapolation from the laboratory to reality of a retardation coefficient related to a perfect tracer, since it uses information of the retardation over the entire transport distance of interest. However, it is totally dependent on the similarity of retardation mechanisms affecting the two elements and on the « representativeness » of the tested sample.

Unity of action

This rule is simple and no exceptions should be tolerated. If the modelled action were to change, the model fitted by the learning process is dead. The learning process is based on a given medium, driven by given mechanisms. It does not identify any general intrinsic characteristics.

Unity of time

If the system is modified by time, the model is no longer relevant. The changes may be seasonal, long-term or due to inherent nonstationary conditions. For example, nitrate transport depends on the type of winter soil cover, ploughing techniques, etc., and the representative parameter sets will change in consequence.

These constraints severely limit the use of black-box models.

2. MODELLING OF UNOBSERVABLE PHENOMENA

There are several reasons why certain phenomena cannot be observed. Consider nuclear waste disposal in deep repositories: if this becomes a source of pollution, it will happen untold years hence - the pollution phenomenon is therefore unobservable today, predictive impact studies generally make use of this type of modelling.

Here, the important question is: what parameters to introduce ? The decision must be based on the ability to describe the system and its behaviour without the benefit of observation. At present the general tendency seems to be:

(i) to identify the real geometry of the system,

(ii) to thoroughly analyse and represent the physics of the underlying driving mechanisms,

(iii) to analyse scenarios.

(i) Identifrication of the real geometry. Since it is impossible to blindly fit global coefficients by the learning process, the medium must be observed and described, starting with the geometry. This is an entirely new discipline in groundwater hydraulics. Several methods are being developed, genetic models (e.g. alluvium sedimentation in streams, diagenetic processes in pores), stochastic facies models (e.g. Boolean techniques, Indicator simulation techniques, Truncated Gaussian techniques) which produce a very detailed description of the geometry and properties of the medium. In general, an infinite number of possible « realizations » of the real medium is generated and the variatlility between them indicates the urrcertainties on the medium under consideration. These realizations can be conditioned by available measurements of medium properties, e.g. at boreholes. These models usually function with a very small discretization (on the order of decimeters to meters) and produce descriptions which are coherent with what can be observed in the field at the sample scale. Then, a change of scale, as exact as possible, has to be made in order to provide a description of the medium that can be handled by the model. This is a difficult process on which much more work needs to be done.

(ii) Anatysis and representation of the underlying physical processes. In the black-box models ihere is a tendency to globalize the processes. Here, the tendency is reversed: the individual, elementary mechanisms must be examined and their strength and kinetics studied and measured. This involves lengthy and expensive investigations but it is the only way to obtain physically significant parameters. However, this method results in extremely complicated models and although it seems to be the only rigorous one, it is, as yet impossible to judge its success.

(iii) Scenario analysis. When the model is ready, it is used in forecastng. This is done through the development of scenarios which take into account all possible factors of change and evolution, natural and man-made. This by itself is a very demanding task, which requires considerable effort both from the technical and the sociological viewpoint, considering all inter - and retroactions between the physical and the decision-making worlds.

The concluding passage suggests that somewhere in the scientific community, over and above the modelling work guided by « useful » aims, work must also continue on models that serve no particular purpose. consider the lattice gas models, which are fascinating tools, although it is impossible, at this stage, to know if they will ever play a significant role in any facet of hydrology.

FR :

La présence de concentrations élevées en métaux lourds retrouvés dans les boues de stations d'épuration est un facteur important limitant ainsi leurs différents modes de dispositions (rejet en mer, enfouissement, incinération, ou même recyclage comme fertilisant agricole ou forestier) par crainte de dégâts considérables qui pourraient être causés à l'environnement. L'objectif de ce travail consistait à mettre au point un procédé qui permettrait conjointement la stabilisation des boues municipales et l'enlèvement des métaux lourds qui y sont associés. L'approche poursuivie dans cette étude était de vérifier si ce procédé microbien, conçu pour enlever les métaux lourds associés aux boues municipales, pourrait éventuellement remplacer les procédés conventionnels de stabilisation des boues municipales, ce qui réduirait considérablement les temps et coûts de traitement. Ce procédé microbien consiste à utiliser des souches de thiobacilles qui oxydent le soufre élémentaire, produisant ainsi de l'acide sulfurique, et par la même occasion une forte baisse de pH (1.5). Les résultats obtenus en cultures discontinues, en bioréacteurs, avec des boues primaires provenant de la station d'épuration de la Communauté Urbaine de Québec (C.U.Q.- Est) montrent que cette chute de pH entraîne une solubilisation importante des métaux (Cr: 56 %, Cu: 97 %, Fe: 30 à 40 %, Pb: 69 %, Zn: 98 % ), du phosphore (52 %), ainsi qu'une réduction appréciable des matières volatiles en suspension (40 à 50 % ), et ce, après seulement 7 jours de traitement. Le soufre nécessaire à la biolixiviation- stabilisation est introduit, dans le cas de notre étude, sous forme de granules ou de blocs. Ce choix de la forme de soufre influe beaucoup sur la qualité de la boue produite, ainsi que sur son pouvoir acidophile après neutralisation. Le soufre en blocs s'avère plus efficace et aussi préférable au soufre en granules quant au pouvoir acidophile, après neutralisation, de la boue produite.

EN :

Given the potential geochemical mobility and recognizd toxicity of heavy metals, their presence at high concentrations in sewage sludges imposes serious limitations on various sludge disposal practices (ocean disposal, landfill, incineration, or use as a fertilizer in agriculture or forestry), The objectve of this work was to develop a process that would permit the simultaneous stabilization of sewage sludges and the removal of heavy metals associated with them. The approach followed was to verify if this microbial leaching procedure could eventually replace conventional sludge stabilization processes and hence considerably reduce the time and cost of treatment. The microbial process consists of using thiobacillus strains which, in the presence of air, oxidize elemental sulfur to sulfuric acid, thus reducing the pH to very acidic levels (pH 1.5). This biological oxidation of elemental sulfur is brought about by two groups of sulfur-oxidizing bacteria, the less-acidophilic and the acidophilic thiobacilli. The initial acid production and pH reduction is due to the less-acidophilic bactena (Thiobacillus thioparus) which lower the pH to about 4.0. This is followed by the growth of acidophilic bacteria (Thiobacillus thiooxidans) and fruther pH reduction.

Batch culture experiments were carried out in 30 L and 8 L reactors with primary sludges obtained from the Quebec urban community's wastewater treatment centre. Elemental sulfur and inoculum were added at the beginning of each experiment The inoculum was prepared by adding 1 % tyndalized sulfur powder to fresh secondary sludge and incubating for 8 days (final pH 1.5 to 2.0). A small portion (5 %) of this acidified sludge was then used as an inoculum for another batch of fresh sludge and this process was repeated several times until an acclimatized inoculum was obtained which could oxidize elemental sulfur rapidly, without an appreciable lag phase. The elemental sulfur necessary as substrate for the simultaneous bioleaching and sludge stabilization was introduced in the form of granules (2.4 to 4 mm diameter) or blocks (25 mm diameter).

Sludge pH and ORP were measured at 24 hour intervals and all other measurements were carried out at 48 hour intervals. The results demonstrate that the addition of elemental sulfur and inoculum resulted in a considerable lowering of the sludge pH during the incubation period. Such pH lowering was not observed in cultures to which sulfur and inoculum were not added. This lowering of pH (2.1) was related to the quantity of substrate (sulfur) and inoculum present in the medium. An increase in the medium ORP (from -50 to about 500 mV) was also observed. Acidification of the medium along with the elevated ORP levels resulted in the solubilization of metals initially present in the sludge. This pH reduction, after only 7 days of treatment, effected an important metal solubilization (Cr: 56 %, Cu: 97 %, Be: 30-40 %, Pb: 69 %, Zn: 98 %), as well as an appreciable reduction in phosphorus (52 %) and in the volatile suspended solids concentration (40-50 %). In addition, the sludge which initially had a highly repulsive odour was rendered odourless.

In a previous study we had showm that for an optimum rate of acidification of the sludge a minimum concentration of elemental sulfur (2 g/L) was necessary, even though only 40 % of this sulfur was oxidized. In the present experiment the physical form of the sulfur was shown to influence both the quality of sludge produced and its acid-generating capacity after neutralization. Sulfur in the form of blocks was more efficient than granules in that the elemental sulfur could be readily separated from the sludge at the end of the treatment, yielding a sludge after neutralization that had a low acid-generating capacity. In the case of granules, the unused sulfur was broken down into a fine powder during the course of the bioleaching experiment and could not be separated from the leached sludge - even after neutralization, the leached sludge exhibited a high acid-generating capacity, which would limit its use as a fertilizer on agricultural land.

FR :

Des essais en erlenmeyers de 0,5 L ont été faits afin d'évaluer les possibilités d'application d'un procédé de solubilisation biologique aux sédiments. Ce procédé utilise la capacité des thiobacilles de lixivier les métaux en milieu acide. Au cours des essais, l'influence, sur la solubilisation des métaux, de la quantité de substrat (FeSO4 × 7H2O), du pourcentage d'inoculum et de la teneur en solides totaux a été suivie. La concentration de substrat n'a pas d'effet sur l'enlèvement du Cd, du Zn, du Mn et du Cu. Les taux d'enlèvement moyens pour le Cd, Zn et Mn, sont, après 46 heures, de 70 à 90 %, 90 à 100 % et 64 à 72 % pour des concentrations de substrat variant de 0 à 30 % FeSO4 × 7H2O (poids sec/poids sec). A 29 et 46 heures, des coefficients de détermination (r²) de 0,46 et 0,59 entre le pH et la concentration de FeSO4 × 7H2O, et de 0,92 et 0,55 entre le POR et la concentration de FeSO4 × 7H2O ont été trouvés. A plus forte teneur en FeSO4 × 7H2O, le POR est moins élevé, surtout au début des essais. La présence des ions ferreux en plus grande quantité expliquerait les plus faibles valeurs du potentiel-rédox.

A 20 % d'inoculum, la solubilisation des métaux est optimale avec une solubili- sation de 86 % pour le Cd et 73 % pour le Cu et le Mn. La forte corrélation entre le POR, le pH et le pourcentage d'inoculum (r² à 29, 46 et 70 heures de 0,94, 0,94 et 0,63 pour le POR et le pourcentage d'inoculum et de 0,80, 0,69 et 0,60 pour le pH et le pourcentage d'inoculum), a permis d'établir une corrélation entre le POR, le pH et la solubilisation du Cu. Les seuils trouvés permettant une solubilisation du Cu supérieure à 10 % sont un pH de 3,75 et un POR de 260 mV.

L'augmentation des solides totaux (ST) semble limiter le développement des thiobacilles et diminue les rendements de solubilisation du Cd (à 21 heures) et du Zn (à 45 heures). Ainsi, pour le Cd, les pourcentages de solubilisation passent de 71 % à 24 % pour 3 et 15 % ST, alors que dans le cas du Zn, ils diminuent de 94 % à 81 % pour 3 et 15 % ST. La solubilisation du Mn serait, pour sa part, plutôt de type chimique.

EN :

In the province of Québec, the St-Lawrence Seaway as well as the harbour areas undergo frequent dredging. Each year, more than 10⁶ cubic metres of sediments must be dredged from the st-Lawrence river. The dredged sedirnents are generally dispersed in the water column. However, heavy metal contamination of these sediments may preclude open-water disposal and necessitate their confinement.

The research work presented in this article applies to dredged sediments that are too contaminated for open-water disposal. In order to permit their safe disposal, it is necessary to have a method which effectively removes metals from these sediments. One of the effective methods of metal removal from contaminated sediments could be a microbial solubilization process, which was developed at INRS-Eau for sewage sludge decontamination and which has been tested on a pilot scale during the past year. In order to appty such a process to decontaminate sediments, it was necessary to study the influence of various factors affecting the microbial leaching process. This research verified the possibility of using a modified microbial leaching process to decontaminate sediments and also studied the influence of the enerry substrate, the microbial inoculum and the sediment total solids concentration on such a modified process.

The experimental methodology consisted of mixing the contaminated sediment in a 500 ml Erlenmeyer flask for a period of 96 hours in the presence of thiobacillus inoculum and ferrous sulfate (FeSO₄ • 7H₂O) as the enerry substrate. Before each experiment sediments were adjusted to the desired total solids concentration with deionized water and were acidified with sulfuric acid to obtain the starting pH conditions.

The solubilization of Cd, Cu, Zn and Mn was monitored at regular intervals in order to evaluate the influence of the various factors studied. Subsamples were removed, the sediments were digested and the metals were analyzed by inductively coupled plasma atomic emission spectrophotometry (ICP-AES). The elements analyzed were Al, Ca, Cd, Cr, Cu, Fe, Mg, Mn, Ni, Pb,Zn as well as S, P and C. All the digestions and analyses performed made according to the APHA guidelines. The total solids, both volatile and inert, were also determined according to APHA method no.209F (APHA, 1985).

Regarding the influence of the ferrous sulfate concentration on the application of the microbial leaching process, the removal rates of Cd, Zn and Mn did not vary more than 12 %. For all experiments, the final pH value approached 2,5. The oxidation-reduction potential (ORP) varied for all experiments from 462 to 560 mV. An energy source could therefore be present in the sediments, which would support the growth of thiobacillus bacteria.

During the experiments conducted to study the influence of inoculum concentration on the microbial leaching process, it was found that the pH decreased from 3.5 to 2.4 and the ORP increased from 284 to 593 mV when the inoculum addition increased from 0 to 30 % (v/v). This tendency was also observed during the removal of Cd and above all, Cu. Between 0 and 20 % (v/v) inoculum concentration, Cd removal increased from 64 to 86 % whereas Cu removal increased from 20 to73 %. However, these two metals did not show any appreciable increase in solubilization when the inoculum was increased from 20 to 30 % (v/v). In the case of Mn, it was not possible to determine the solubilization pattern since the inoculum concentration did not influence the solubilization of this element, Between 0 and 30 % inoculum, the Mn removal varied from 56 to 73 %. Hence the microbial influence on Mn solubilization may be negligible.

The study on the influence of total solids content on the microbial leaching process showed that the total solids concentration had an influence on pH and ORP. The final pH increased from 2.43 to 3.32 and ORP decreasedfrom 492 to 321 mV when the total solids content was varied from 3 to 15 % . The percent solubilization of Cd after 21 h was 7l %, 69 %, 43 %, 44 % and, 24 % at 3, 5, 7, 10 and 15 % total solids respectively, On the other hand, Zn solubilization after 45 h was 94 %, 90 %, 84 %, 97 % and 8l % for the same concentrations of total solids. An increase in total solids content decreased the solubilization efficiency of Cd (at t=2l h) and, to a lesser extent, Zn (att=48h). However,in the case of Mn, the maximum solubilization was achieved right at the beginning and varied little with time. Its solubilization varied between 61 and 70 % for 3 % total solids, between 67 and, 79 % for 5 % total dolids, between 52 and,62 % for 7 % totaf solids, between 57 and, 75 % for l0 % total solids and tretween 40 and 63 % for 15 % total solids. In general, for the sediments studied here, total solids content has little influence on Mn solubilization.

In conclusion, use of a microbial leaching process to remove metals from contaminated sediments seems to be less complex than for sewage sludges, since the maximum removal of metals is achieved in less than 48 h in a bench-scale experiment. The average metal solubilization was above 80 % for Cd, 73 % for Cu, 90 % for Zn and 65 % for Mn. The development of thiobacillus does not seem to be dependent upon the presence of an added energy substrate. Only a minimal quantity of substrate, say 5 % ferrous sulfate (dry wt./dry wt.), is required in order to assure optimal conditions for thiobacillus growth. It is probable that with time, without ferrous sulfate, the microbial conditions would deteriorate due to exhaustion ofthe enerry substrate. A 20 % microbial inoculum seems to be enough to assure an efficient metal solubilization process. Except for Mn, the increase in total solids concentration had a negative influence on the metal solubilization. It is hence preferable to work with a total solids concentration of less than 5 %. Nevertheless, it should be noted that this percentage is more than that used for sewage sludge decontaminaton (around 2 % total solids).

FR :

La question des précipitations acides est devenue une des principales préoccupations environnementales de ce siècle. Les dommages engendrés touchent l'ensemble des composantes atmosphérique, aquatique et terrestre de notre écosystème. Dans cet article, l'intérêt est tourné vers le transport des substances polluantes en faisant intervenir le vecteur des précipitations acides: le vent. Il existe une croyance populaire, largement répandue dans l'est du Canada, consistant à admettre que les vents d'est ou du nord sont associés à des précipitations faiblement chargées en éléments acides, contrairement aux vents du sud-ouest qui transporteraient les charges plus contaminées des industries situées dans le Midwest nord-américain. Une confirmation expérimentale de la réalité du phénomène est présentée ici.

Une analyse factorielle des correspondances permet de mettre en évidence des relations entre la direction des vents et la composition chimique des précipitations. Des analyses de variance permettent ensuite de montrer la signification de l'effet de la direction des vents sur les concentrations de sulfates et de nitrates, en plus de mettre en évidence un effet saisonnier significatif pour ces deux variables. Les concentrations de nitrates et de sulfates associées aux vents de l'ouest sont respec- tivement de 0,33 mg/l-¹ et 1,73 mg/l-¹ comparativement à 0,24 mg/l-¹ et 1,48 mg/l-¹ pour les vents provenant de l'est. En ce qui concerne l'effet saisonnier, les concentrations moyennes de nitrates sont plus élevées durant les mois de janvier et de mars alors que les concentrations de sulfates sont plus elevées durant les mois d'été.

EN :

The problem of acidic precipitation has become an important environmental concern; related damages can affect atmospheric, terrestrial and aquatic components of our ecosystem. This paper focuses on wind-driven atmospheric transport of contaminants, notably niirates and sulfates. Conventional wisdom in eastern Canada holds that winds originating from the east bring precipitation less loaded with acidic components than winds originating from the southwest; the latter carry contarninants generated by industries located in the American Midwest.

An experimental confirmation of this phenomenon is presented here. SPERBER (1987) showed that hourly series of piecipitation content and wind direction, measured at a reception site are adequate to represent the lagrangian history of precipitating systems (New York City region). Following this result, we suppose that the northesastern continental atmospheric system is homogeneous enough so that winds measured at our reception site (Québec City region) are representative of the whole system. Thus, our experiment is performed in eulerian coordinates.

The data bank used in the statistic alana|yses contains 10 time series: the weekly concentrations of 9 compounds found in the precipitation (H, Ca, Cl, K, Mg, Na, NO₃, NH₄ and SO₄) gathered at the local Montmorency site and a s:ries ofweekly prevailing winds measured at the nearby Québec City airport. The time series contain 312 observations covering a full six year period (December 1981 to December 1987). As the original data bank of corcentrations is episodic, i.e. an obsercvation is available for each day with a significant precipitation event, volumes and loadings are used to derive the average weekly values of concentrations. In contrast, hourly series of direction (projected in 36 directions) and velocity of the prevailing winds are used to build, via a vectorial addition, a weekly series of wind direitions projected on a 12 point wind rose where directions corespond to the nind origin and not its destination.

Classic statistical methods are used to deal with this data bank. Principal component analysis studies relationships between series of concentrations in the precipitations, while correspondence analysis highrights the relationships betwlen tile series of precipitation content and the series ofwind direction. The final statistical method, analysis of variance, is used to test the signilïcance of relationships higtrlighted by the correspondence analysis.

The principal component analysis shows that all variables were positively correlated with the first component which reflects the fact that a higtrly loaded precipitation event will show high concentrations for each ofthe nine variables. The second component discriminates two groups of variables: one includes NO₃, NH₄ and SO₄, the other Ca, Mg and Cl. The acidity variable, H, is nearer to the acid ion group (NO₃, NH₄ and SO₄) than to the other ion group (Ca, Mg and Cl).

The correspondence analysis shows that high concentrations of acidic compounds (NO₃, NH₄ and SO₄) are highly rerated to winds from the W and WSW directions; conversely lower concentrations of the same compounds are associated with winds from the E and ENE directions. The elements Ca, Mg and Cl, reputed to be of oceanic origin, show high functional relationships between high concentrations and E, ENE winds and also between low concentrations and W, WSW winds. These results support the popular belief initially presented that acid precipitation is largely associated with winds from the southwest.

The analyses of variance show that precipitation concentrations (NO3 and SO4) are significantly different according to the wind directions. Nitrate and sulfate concentrations associated with winds originating from the west are respectively 0.33-mg.l^-l and 1.73 mg.l^-l compared, to 0.24 mg.l^-l and 1.48 mg.l^-l for winds originating from the east. The analyses ofvariance also indicate a significant seasonal effect where mean monthly concentrations in nitrates are hilhest for winter months and early spring whereas sulfate concentrations are highest for the summer months.

FR :

Les méthodes de calcul des crues de projet pour des grands barrages sont de type statistique ou de type déterministe (PMP/PMF). Ce dernier type de méthodes est principalement utilisé dans le monde anglo-saxon (États-Unis, Australie), mais peut offrir une alternative intéressante aux méthodes statistiques, qui présentent des lacunes pouvant être importantes.

Une application des méthodes PMP/PMF sur des bassins suisses a mis en évidence l'intérêt de ces méthodes, mais aussi leurs faiblesses. Une adaptation des outils déterministes est nécessaire pour prendre en compte les particularités des bassins de montagne. Il s'agit principalement de mieux tenir compte des propriétés physiques des bassins versants et des phénomènes de non linéarité.

Dans ce but, un concept de modélisation utilisant la description géomorphologique du bassin versant a été utilisé et amélioré.

Le premier modèle utilisé est l'hydrogramme unitaire géomorphologique, qui existe depuis 1979. Le deuxième modèle utilise le concept de cascade de réservoirs hydrologiques à vidange non linéaire, dont les éléments sont issus de la description géomorphologique du bassin. C'est là une modélisation nouvelle qui est particulièrement adaptée au contexte des crues extrêmes dans les Alpes. Un travail de calibration et validation du modèle a montré que ce modèle a un bon potentiel d'utilisation. La principale difficulté revient à identifier la signification physique des paramètres du modèle, qui pourrait s'appuyer sur les résultats d'un autre type de modélisation couplant la géomorphologie du bassin aux equations de l'onde cinématique. Ce modèle est en cours de test à l'IATE.

EN :

Design floods for large dams are computed either with statistical or deterministic methods (Probable maximum precipitation, PMP/probable maximum flood, PMF). The latter presents an interesting alternative to statistical rnethods, whose drawbacks may be significant, especially when hydrometeorological data are deficient.

PMP/PMF methods are based on assumption of the existence of an upper limit of the hydrometeorological processes. The prevalent method for PMP estimation is the maximization - transposition technique. Simpler methods, such as the statististically based technique, also exist. However, the most accurate method may well be the use of a meteorological model, but this aspect is still in the research domain.

PMF is derived from a PMP using a simplified loss function and a transfer function for which parameters are maximized. The prevalent transfer function is the unit hydrograph model.

Application of traditional PMP/PMF method on alpine catchments showed important weaknesses due to the transfer function. Alpine catchments are characterized by steep slopes, thin soil cover, poor vegetation, and large floods are due to heavy, short thunderstorms. Consequently, hydrological response is very sensitive to the topography of the catchment. Nonlinearity must be taken into account in many cases. However, experience showed that a model cannot take into account a detailed description of the catchment.

In order to take into account the characteristcs of alpine catchments while staying as simple as possible, we have used the geomorphologic description of the catchment as a modeling basis. This representation is based on the Strahler ordering scheme, and defines all possible path types that a surface runoff dropret may follow to reach the outlet. This description has been used since 1979 in the geomorphologic unit hydrograph (GUH) formulation.

We developed a geomorphorogic nonlinear cascade (GNC) in order to take into account nonlinear processes. The GNC model uses the geomorphologic description of the catchment to define the reservoirs of hydrological cascades. A cascade represents a path type; the first reservoir ofa cascade is an overland element and subsequent reservoirs are channels with increasing Strahler's order. Outflow from a reservoir is the inflow of the downstream reservoir. Outflows from all cascades are combined to produce the global catchment response. The two equations used in the GNC model are a global continuity equation: I - O = dV/dt, and a nonlinear outflow equation: O = k V^x, where I is an input term, that is precipitation for the first reservoir, and outflow from the upstream element for channel reservoirs, V is the volume of water stored in a reservoir at time t, k is a consfant, and x an exponent. The parameter k varies with each reservoir as a function of surface area (for overland erements) or rength (for channel elements). The exponent x should be between 1 and 2.

The two models, GUH and GNC, have been calibrated with an automatic optimization procedure, and tested on the Vogelbach catchment. This catchment is located in the swiss alps (Arptar, canton of schwytz); it is a third order catchment, and its area is 1,55 km². Both models gave good results, although the GUH model had a tendency to smooth the dischage.

Inorder to improve the physical meaning of the moder parameters, we are testing a third model linking the geomorphologic description with the kinematic wave equations. Results are promising, but are not shorm in this paper.

The hydrologic modeling based on the geomorphologic description of the catchment seems to be a good compromise between lumped modeb and detailed distributed models, which are difficult to apply.

FR :

L'équation cinétique qui permet de calculer l'oxydation d'un micropolluant dans les réacteurs d'ozonation s'écrit:

-(d[P]/dt)=(K_D[O₃]_L+K_ID[OH^∘])[P]

Kd et Kid: constantes de vitesse de l'ozone et des radicaux hydroxyles sur le micropolluant P.

Dans la première partie, l'approche théorique de la concentration en radicaux hydroxyles a montré que [OH·] est proportionnel à la concentration en ozone ([OH·] = k'[03]). On a donc:

(d[P]/dt)=K_G[O₃]_L[P] with K_G=K_D+K_IDK'

Dans un réacteur parfaitement agité, les concentrations en ozone et en micropolluant sont constantes et l'élimination s'écrit:

([P]/[Po])=(1/1+K_G[O₃]_L τ) with τ=(V/Q)

Dans un réacteur piston, les concentrations varient tout au long de la colonne et il est habituel de modéliser un tel réacteur comme un grand nombre de R.P.A. en série de volume DeltaV et de hauteur DeltaH (Dans notre approche DeltaH = 0,01 m).

Dans les deux cas, la simulation de l'élimination du micropolluant est basée sur la connaissance de la valeur de kG et de la concentration en ozone dans l'eau [03]L

[03]L est obtenue de la résolution des bilans massiques dans un volume V ou ~V.

ozone à l'entrée + ozone transféré = ozone à la sortie + ozone consommé

L'ozone transféré utilise pour son calcul des relations semi-empiriques donnant la constante de Henry et la valeur du kLa.

L'ozone consommé est déduit de la relation établie dans la partie 1:

(d[O₃]_L/dt)=w[O₃]_L

Les résultats de la simulation sont comparés aux résultats expérimentaux obtenus avec un pesticide organo-phosphoré, le parathion. Les paramètres variables sont le temps de contact (300 - 600 s), le pH (6,7 - 8,2) et le taux de traitement (1 à 5 g/m³).

Une valeur de kG comprise entre 500 et 600 M-¹s-¹ donne une bonne corrélation entre les valeurs expérimentales et calculées.

Cependant, on peut noter quelques différences, en particulier dans la partie basse de la colonne, ce qui montre la nécessité de prendre en compte pour des calculs plus précis l'hydrodynamique du réacteur.

L'emploi du programme de simulation permet de tracer deux abaques qui montrent l'influence pour n'importe quel micropolluant des facteurs kGteta et w.

EN :

Micropollutant (P) oxidation in an ideal ozonation reactor uses the kinetic équation:

(d[P]/dt)=(K_D[O₃]_L+K_ID[OH^∘])[P]

k_D and k_ID : kinetic rate constant of ozone and hydroxy radicals on the micro -pollutant P.

In part 1, the theoritical équation shows that [OH°] is proportional to the ozone concentration ([OH°] = k'[O₃]) and thus the following equation is obtained :

(d[P]/dt)=K_G[O₃]_L[P] with K_G=K_D+K_IDK'

In a completely stirred tank reactor, ozone concentration in liquid phase is constant and pesticide elimination is given by the equation :

([P]/[Po])=(1/1+K_G[O₃]_L τ) with τ=(V/Q)

In a plug flow reactor, ozone concentration in liquid phase varies along the column. To modelize them, we use the model of completely stirred tank reactors in series where the unit volume is ∆V. In our calculations, this volume is obtained by S (reactor cross section area) and ∆h equal to 0,01 m. In this volume ∆V, ozone and micropoliutant concentrations are considerad as a constant.

Simulation calculations are based on the knowledge of global kinetic constant k_G and ozone concentration.

The value of the ozone concentration is obtain from mass balances on the oxklant (on a ∆V or V volume reactor) :

ozone inlet + transferred ozone = consumed ozone + ozone outlet

The quantifies of transferred ozone are calculated from the Henry law and a semi empirical k_La equation.

The quantity of consumed ozone is calculated from the equation in part I

(d[O₃]_L/dt)=w[O₃]_L

Experimental results are obtained with parathion, an organo-phosphorus pesticide on a bubble column pilot plant:

Parameters are contact time (300-600 s), pH (6,7-8-2) and ozone treatment rate (1 to 5 g/m3).

A k_G value of 500 or 600 M^-l s^-l shows a good correlation between predicted and simulated pesticide concentrations.

However, there are noticable differences, especially at the bottom of the column. This shows the necessity to take into account the hydrodynamic properties of the reactor during next works.

The use of the simulation program lets to calculate the elimination of pesticide versus the two main parameters : the factor k_Gteta and the value of w.