Volume 6, numéro 1, 1993

FR :

Un site expérimental représentatif du milieu calcaire fissuré a été étudié dans le Jurassique supérieur de la vallée de la Loue. Un forage carotté de trente mètres a été réalisé et étudié par différentes techniques de laboratoire (porosité, perméabilité, vitesse ultrasonique, quantification de la fissuration) et de terrain (essais hydrauliques par injection entre obturateurs, diagraphies soniques, densité). Le massif rocheux concerné a été exploré en surface à partir de la sismique réfraction.

Les principaux résultats obtenus permettent de relier les paramètres hydrauliques à la fissuration du massif calcaire. On constate une mauvaise relation entre les mesures de laboratoire et de terrain par suite d'un effet d'échelle, les investigations étant faites sur des volumes variant de quelques centimètres cubes à plusieurs mètres cubes. En effet, la matrice homogène et isotrope est perturbée par des fissures (fermées ou ouvertes). Le milieu continu représenté par la matrice et observé au laboratoire s'avère discontinu sur le terrain. L'indice le obtenu à partir de la diagraphie sonique paraît intéressant, mais doit être utilisé avec prudence et après correction dans les zones à argilosité reconnue, en effet dans ces zones on constate que pour des valeurs supérieures à 10 de l'indice lc brut, aucun écoulement n'a été mis en évidence par essai hydraulique. Un abaque de correction C (lc) a donc été établi qui tient compte de la teneur en argile et qui permet de proposer un indice corrige le -C (lc) plus fiable.

EN :

An experimental site in fissured calcareous media has been analysed in the upper jurassic of the Loue valley. A tore of some 30 metres has been drilled and analysed by different laboratory techniques (porosity, permeahility, ultrasonic speed, quantification ol the fissuration) and in the field (hydraulic injection tests between shutters, sonic and density diagraphics data). The rock volumes have been explored at the surface by the refraction sesmic method.

ln the tore the fissures are distributed according to four families :

- subvertical stylolitic fractures, the horizontal pealo; of which denote the maximum compressive direction of the original tectonic stresses,

- mineralized crevices with calcitic filling, closed fissures,

- opened fractures which induced the hydraulical quality of limestones.

The total porosity of the limestones is characterized by homogeneous values (front 2 to 6 %) and three differents familles of porosity o and the average diameter d_m (0.00310 0.27µm) of the accessible apertures in the matrix :

- family 1 : 0.003 µm < d_m < 0.018 µm, 1.05 % < ø < 2.14 % in relation with intergranular spacing in the cementic phase. The clay in the basal part of the tore indures a great reduction of this kind of porosity,

- family 2 : 0.044 µm < d_m < 0.11 µm, 0.49 % < ø < 4.39 % in liaison with intergranular spacing between the carbonatic figurative elements (biociasts, Iithoclasts, oolithes and microorganisms),

- family 3 : 0.14 µm < d_m < 0.27 µm, 0.28 % < ø < 3.13 % in relation with the increasing of the sizes of the figurative elements.

A good agreement between permeability k and porosity ø is observed (k=3.4 10^-5 - ø^4-96 in darcy units).

Three diagraphic investigations has been carried out with gamma-gamma measurements (bulk density), with gamma-ray measurements (clay content) and with acoustic compensated measurements (sonic waves speed). The bulk density log is uniform (2.5 to 2.7 g/cm³), the gamma-ray log is increasing down the hole (21 to 29 m) whereas the clay content rises.

The acoustic log records the total wave signal chat can be traduced by two computed logs :

- speed propagation of the S waves (Vs speed) and P waves (Vp speed) with ihe ratio Vp/Vs,

- Poisson coefficients and Young modulus.

The principal results suggest the existence of a liaison between hydraulic parameters and the fissuration of the calcareous formation where three levels of fracturation are observed (from 11 to 12 m, from 16 to 18 m and from 19 to 20m).

The permeability values of the limestone matrix cores ara relatively homogeneous (1 to 19 10^-5 darcy) with a decrease in the 23 to 27 m section of the bore-hole where a clayed cernent is detected. The hydraulic conductivity of the fractured zone, as determined by means of hydraulic injection tests between shutters leads to values of 1.10^-6 to 8.10^-6 m/s (# 0.1 to 0.8 darcy). Five zones have been choses for the hydraulic tests in liaison with different parts of the bore hole. The high values ot permeability k are in accordance with the high le index values of the open fractured zones.

A tirst local calibration function is proposed :

lc=7.13 Log k + 51

A pour relation connects the laboratory and field results because the elementary volume is increasing from cubic centimeters to some cubic matera. The homogeneity and the isotropy of matrix is disturbed hy the fissures (shut or open).

The matrix medium is continuous on the laboratory scate and discontinuous in the field. The ultra-sonic investigation confirms the liaison between the high values of the sonic P waves speed and the principal regional direction ot tectonical stress. The total acoustic recording yields the knowledge of the S and P waves speeds of in correlation with subvertical fractures when the speed value variations are numerus.

Three fracture zones are distinguished between 11 to 12 m, 17 to 18 m and 19 to 20 m. The lc index obtained from the sonic log must be used cautiously aller correction in the true clayed zones. In these zones the values of the le index are above 10 without waler flowing in the bore hale alter hydraulic testing. We propose the use of a corrective chart C (lc) to correct for the clay effect and a new corrected index lc - C (lc) is more reliable.

An adjusted statistical graphical chart is proposed with the following expression : C (lc) = ß • X^α with α= 1.84 and ß = 0.088 (valid only for limestones) and X : lite clay index obtained from the gamma-ray measurements. The le index values histogram localizes the permeable zones of limestones with hydreulically active open fractures.

Measurement of the speed variation of P sesmic waves on the bottom of the quarry by the means of the refraction method leads to a good agreement between the maximum speed anisotropy direction and the major fracture direction (N 120°-N 140°) which is perpendicular to the minimum anisotropy sesmic speed direction (N 40° -N 50°). The open fractures direction measured on the core is adjoining about N 135°-N 145° when the direction of maximum anisotropy in the quarry is near N 140°.

FR :

On analyse par événement les périodes sèches ou longs événements secs se produisant au cours d'une saison humide en pays semi-aride. Une période sèche se définit comme une série de jours avec pluies quotidiennes inférieures à un seuil donné. Le cas d'espèce de la région de Dodoma en Tanzanie, où l'on observe surtout des précipitations de type convectif illustre la méthodologie. Une analyse conventionnelle des périodes sèches, qui ne fournit pas de relation entre la fréquence et la durée de ces périodes, semble cependant indiquer que les périodes sèches se produisent de façon aléatoire pendant la saison pluvieuse qui est elle-même de longueur aléatoire. L'analyse par événement comprend le nombre d'événements par saison, quisuit approximativement une loi de Poisson, la durée des événements secs, qui est supposée suivre une loi binomiale négative et la durée des événements pluvieux qui est supposée suivre une loi géométrique. On utilise la loi de Pearson III pour estimer les événements secs saisonniers de durées maxima et on compare les résultats obtenus avec ceux de l'analyse par événement, les écarts observés étant expliqués par les différences de conception existant entre ces deux méthodes. La distribution spatiale des événements secs est également analysée, on trouve qu'une forte majorité des événements secs est simultanée sur au moins deux stations, et que près de la moitié des événements secs de durée modérée est simultanée sur trois stations. L'analyse par événement permet de calibrer les modèles de précipitation avec peu de données et de procéder à la génération d'événements synthétiques par simulation.

EN :

One form of drought is the interruption of the rainy season by a sa called dry spell. Dry spell can be defined as a sequence of dry days including days with less than a threshold value of rainfall.

A dry spell, defined a on daily scale, becomes untraceable by statistics using longer than one-day-long equidistant time intervals. If the daily discretization of the rainy season is te be avoided, an intermediate technique is needed.

Event-based analysis of the rainfall and dry spell provides This approach. The method is demonstrated with data from the Dodoma Region, situated in the semiarid highlands of Tanzania. The climate is characterized by one rainy tesson from the and of November until the end of April. The occurrence of rainfall is erratic.

The average seasonal precipitation is about 600 mm with variations between 450 and over 800 mm. Rainy seasons are separated by an almosl 7 month long dry season.

During the rainy season convective type storms prevail. Single storms lasi a few hours, but their occurrence is clearly grouped within the timespan of a few days, separated by the dry spells which are usually much longer.

Conventional statistics of dry spells are summarized in tables 1 and 2 using 1.0 mm daily precipitation as the threshold.

It is shown that dry spells occur randomly during the rainy season. For the event-based analysis dry spell is detined as a dry event. Dry events are considered as a sequence of dry days separated by rainfall events from each other. Thus the rainy season is detined as a series of rainfall and subsequent dry events. Rainfall events are defined as the uninterupted sequence of rainy days, when at least on one day more than a threshold amount of rainfall has been observed- Rainy days with less Man the threshold depth of precipitation are accounted for the rainfall event if they occur in an uninterrupted sequence. Only isolated subthreshold rainfall will be discarded, and considered as part of a dry event (fig. 2). ln this analysis the threshold value of 5.0 mm/day was seiected.

The comparison of tables 1 and 4 shows that the length of the mean maximum dry spell doubles by replacing the 1.0 mm/day threshold by 5.0 mm/day. The sequence of rainfall and dry events is characterized by D_{n, m}, duration of the mth rainfall of the nth rainy season, and by the inter-event time Z_{n, m} (duration of the dry event) between the end at the preceding and the start of the succeeding rainfall event.

In case of convective type storms the series of the subsequent events (either dry or rainfall) could be considered independent, thus their number/season should follow the Poisson distribution.

In case of independence of subsequent events, the waiting lime for a new event must follow the exponential pdf.

By measuring the waiting time in days the discrets equivalent of the exponential pdf can be used.

Since the sequences of convective type storms de not contain purely independent events, the waiting time t follows instead the discrete counterpart of the 2 parameter gamma pdf, the negative binomial pdf. This modified Poisson-modal, Poisson pdf for the number of rainfall events and the negative binomial pdf for the length of the inter-event lime has been applied to describe the rainy season. Table 3 summanzes the parameters r and p for different rein gauges.

By focusing on the dry spell event, the duration of the rainfall events D_{n, m} will in fact be identified as inter-event time. This change of rotes fils the original Poisson model better. Since rainfall events are shorter, their duration follows the geometrical pdf, as theoretically required.

The Poisson pdf seems to fit slightly better the number of dry events than the rainfall ones (fig. 5). If has to be pointed out that the event-based definition of the rainy season dues not exactly fit the theoretical precondition, i. e. to have a certain fixed period. Rainy seasons have variable longths, as they are a stochastic fonction of the events themselves. For "modal fitting" the consideration of the core of the rainy season, from January to April would be a better choice. However it would truncate the physical phenomenon wilh the potentiel omission of extrema long dry events. Therefore, in spite of mediocre fitting, the Poisson modal will be used ln analysis.

Tables 4 and 5 summarize the statistical characteristics of the dry events for the selected rain gouges for bath the whole, and for the core of the rainy season.

Dry events accurring in the core of the rainy season were identified as those ending within the timespan of January -April.

The mean lengths of the longest dry spells in the core are less than the corresponding value for the whole season. However, at two stations, Kondoa and Gwandi, in more han 70 % of the season the longest dry spell did occur during the core. This coincidence was only 40 % for the Farkwa rain gauge.

For planning purposes, the longest dry spells associated with the varions statistical recurrence periods are derived on the basis of the fitted Pearson III type probability distribution functions (fig. 6, table 6).

The event-based analysis, relying on the expected number ot events/season and the negative binomial pdf for the length of the dry events, can also be used to approximate the distribution of the extreme long dry spells. Contrary to the Pearson III distribution fitted to the seasonal extreme values, the negative binomial pdf f (n) determines the probability that a random dry event would last n days.

Consequently, the exceedence probability P_e (N), that an extreme long dry event would occur at least once within a given statistical recurrence period of T years must be equal to the reciprocal value of the product λ•T, where λ denotes the expected number of dry eventslyear (season). λ•T specifies the expected number of "trials" needed to observe at least once the extreme duration of N days associated with the return period of T years. The length of this extreme dry spell can then be obtained from the cumulative negative binomial pdf (table 6). The deviations observed for low number of "trials" between the event-based and the extreme seasonal value approach are due to the conceptual difference.

Table 7 displays the simultaneous occurrence of dry events at several rain gauges. By using Farkwa as the reference station, table 7 does not account for dry events that might have occurred simultaneously at Gwandi and Kandoa without having been recorded at Farkwa.

Except for the very short (1 or 2 days) and the very long dry events (over 30 days), the overwhelming majority of the dry events occurred at least at two stations simultaneously. Furthermore, excluding the 1, 2 or more than 30 day - long events, more than 63 % of the dry events have been observed at all three stations.

Two (or three) dry events were only classified as simultaneous if more than hall of the duration of the reference event at Fartwa was overlapped by an uninterrupted dry event at the other station (s).

Event-based analysis, even if it is carried out on the bases of a few years of observation, can rely on large number of data points (table 3). While the expected number of events/season is still derived from very few data, this estimate is more reliable than the approximative expected length of the longest seasonal dry spell, since the variability of the former is usually less than that of the latter, for the same data sets (table 8).

FR :

L'objectif de cette étude est de comprendre le fonctionnement épuratoire de l'écosystème particulier que constitue le lagunage à haut rendement (LHR) afin de déterminer les principaux facteurs responsables de l'élimination de l'azote et du phosphore.

Sur un bassin de 48 m², alimenté eu eaux usées domestiques préalablement traitées pendant une semaine dans un bassin primaire ont été suivies selon un rythme hebdomadaire les formes carbonées, azotées et phosphorées dis-soutes et particulaires, les variables caractéristiques de l'activité photosynthétiques (chlorophylle a, pH et 0₂) et les données climatiques (rayonnement solaire et température). Une analyse en composantes principales réalisée sur l'ensemble des résultats a montré, d'une part l'opposition des variables climatiques et photosynthétiques aux formes minérales de l'azote et du phosphore (N-NH₄ et P-PO₄) et d'autre part l'influence de la charge organique sur le fonctionnement du système.

L'évolution des formes azotées et phosphorées présente un effet saisonnier marqué. En hiver ou lors de surcharge organique importante, une augmentation du temps de séjour peut améliorer les rendements épuratoires. L'évolution de la matière organique dissoute est indépendante du cycle saisonnier. Au cours des deux années du suivi on observe une adaptation progressive de l'écosystème à la dégradation de la matière organique.

Dans le lagunage à haut rendement l'élimination de l'azote et du phosphore est liée principalement à l'activité algale, qui entraîne une assimilation biologique et une élévation du pH, responsables des phénomènes chimiques de volatilisation de l'azote ammoniacal et de précipitation de phosphate de calcium.

EN :

High rait algal ponds (HRAP) for wastewater treatment have been the locus of a lot of attention since their creation by Professor W.J. OSWALD in the early 1960' s. These aquatic ecosystems are hypereutrophic because of massive nutrient introduction with the wastewater influent. This kind of pond is very different from oxidation ponds because al short residence times (2 to 12 days), shallow depths (0.30 to 0.60 m) and constant mechanical mixing which improves algal growth.

This study has been carried out in a small town in the department of Herault (France). The objective is to establish the efficiencies and mechanisms for nitrogen and phosphorus remval from secondary domestic wastewater by treatment by HRAP. A high rate algal pond of 48 m² surface area and 35 cm depth, constantly mixed with paddle wheels was studied (fig. 1). Residence limes changed from 2 te 12 days with solar radiations and influent loading. The samples were collected once a week from the in- and outflow at 3 p.m. - Dissolved and particulate COD, varions nitrogen and phosphorus compounds, suspended solide, chlorophyll-a, pH, dissolved oxygen, temperature and irradiation were determined.

The results of principal component analysis show a negative correlation between (1) the climatic and photosynthetic parameters and (2) inorganic nitrogen and phosphorus compounds, as well as the influence of organic loading on the HRAP efficiency.

The chronological clustering analysis was performed on the data taking into account the discontinuities of effluent autrient contents (fig. 4). During the first year, from February to September 1988, good climatic conditions were shown to favour photosynthetic activity and consequently a good ammonia and orthophosphate removal. Front September 1988 to February 1989, the effluent was characterized by high ammonia and orthophosphate levels because of unfavourable climatic conditions and low photosynthetic activity. During the second year, the objective was to improve removal efficiencies ; the residence time was changed with solar radiations and influent loading. The results of the chronological clustering analysis of ammonia and orthophosphate levels showed only one sequence (February 1989-January1990) because the seasonal variations of nutrient removal were attenuated. The optimal residence time was found to be 4 days in summer and 12 days in winter.

The evolution of dissolved chemical oxygen demand is independent of seasonal conditions but decreases with the maturity of the system (< 50 mg . l^-1).

The mass balance of nitrogen and phosphorus compounds has been established (fig. 5). In high rate algal pond influent, the main nitrogen form is ammonia. In the effluent the main nitrogen form is suspended organic nitrogen. The oxidized forms (NO₂- et NO₃) present very low levels ; nitrification is a minor process in the system. Ammonia is reduced by both assimilation and stripping ; suspended organic nitrogen in the effluent is an indicator of assimilation by algal biomass and loss of nitrogen is an indicator of ammonia stripping. With regard to directive 91/271 EEC concerning domestic wastewater treatment, partial algal separation is necessary in order to achieve 15 mg • l^-1 total nitrogen, which is the limit for zones sensitive to eutrophication.

The residual phosphate levels are controlled by pH, which is responsible for instantaneous calcium phosphate precipitation. As the dissolved calcium concentrations are high in the calcareous region near the site (Ca = 125 mg • l^-1), the pH of the water will be the main contraint on phosphate elimination in the HRAP. Theoretical calculations indicate that the precipitate is probably an amorphous tricalcium phosphate in pseudoequilibrium (fig.6). Phosphate concentrations are much higher titan the values expected for calcium hydroxyapatite in equilibrium (MOUTIN et al., 1992). The limit of 2 mg • l^-1 for phosphores can only be attained when pH reaches or exceeds 9.

Nitrogen and phosphorus removal is correlated mainly with phytoplanktonic activity, which controls biological nutrients assimilation and pH levels. High pH values are responsable for ammonia stripping and calcium phosphate precipitation.

FR :

L'adsorption en phase liquide sur charbon actif est un sujet très travaillé sur le plan expérimental et de plus en plus dans le domaine de la modélisation. Les tentatives de description des courbes de percée ou de fuite des filtres montrant la saturation du matériau adsorbant remontent aux travaux de BOHART et ADAMS en 1920. D'autres équations avec d'autres approximations ont été proposées par la suite (THOMAS 1944 ; DOLE et KLOLZ 1946), HUTCHINS (1973) ; plus récemment, WOLBORSKA (1989) ou CLARK (1987) ont proposé d'autres modèles. Nous avons essayé de faire le point sur ces différents modèles, de montrer leurs origines communes, souvent à partir des équations de BOHART et ADAMS, les approximations apportées limitant leur domaine d'application, les grandeurs qu'ils permettent de déterminer : capacité maximum d'adsorption, constante cinétique d'adsorption, vitesse de déplacement du front d'adsorption. De tous ces modèles, un seul (CLARK 1987) permet une bonne représentation des courbes de percée. Nous en avons proposé une linéarisaüon qui facilite la détermination des paramètres nécessaires au calcul des courbes de fuite. Tous ces modèles ont été testés sur les résultats expérimentaux obtenus pour l'adsorption d'un tensioactif anionique : le décylsulfonate de sodium et ceci sur cinq petites colonnes de hauteurs différentes de charbon actif. Le modèle de CLARK a également été appliqué à des résultats obtenus au laboratoire (El HANI, 1987) sur l'adsorption et la dégradation biologique des acides humiques sur un filtre de charbon de 1m de haut, sur une période beaucoup plus longue (1 mois) et avec des lavages du filtre. Ce modèle permet de calculer la part qui n'est pas simplement de l'adsorption rapide (dégradation biologique et adsorption lente).

EN :

Low concentrations of organic contaminants are not easily removed by conventional treatment methods, but activated carbon bas a good affinity for various organics and is used in batch or column reactors.

Much has been written concerning the prediction of the performance of powdered activated carbon (PAC) ; adsorptive capacity and equilibrium isotherms determined in « batch » reactor are proposed to simulate the performance of PAC for single or bisolute systems (DUSART et al. 1990, SMITH 1991). Some investigators have attempted to simulate column performance with mathematical models and the aim of this work is to present the principal models and verify how the different models are applied to break-through curves ; parameters which can be evaluated by the different equations will also be compared.

As early as 1920 BOHART and ADAMS presented differential equations which govern the dynamics of the adsorption of vapours and gases on fixed beds and the final result, applied to the liquid-solid phase, yields the kinetic adsorption rate (k) and the maximum adsorption capacity (No) (eq. 3). By transposition to the liquid phase, we have calculated the concentration distribution in the bed (eq. 5) by using the kinetic constant k and the maximum adsorption capacity No obtained by equation 4; it was noted that only the low concentration range of the break-through curve can be used. Some approximations from DOLE and KLOTZ (1946) lead to the « Bed Depth/Service-Time (BDST) equation 7 proposed by HUTCHINS (1973) ; the service time of a column tb has a linear relationship with the bed depth Z (fig.3). The activated carbon efficiency No can be estimated and the adsorption rate constant calculated from the slope and the y-intercept.

Recently, WOLBORSKA (1989) proposed a rectilinear equation InC/Co = At + B (eq. 10) for the initial segment of the break-through curve. The form of this equation is similar to equation (4) obtained tram the BOHART-ADAMS hypothesis. The mass transfer coefficient, ßa, the maximum adsorption capacity and the migration velocity v (eq.9) of the concentration fronts can be calculated from the constants A and B.

The model developed by CLARK (1987) is based on the use of e mass-transfer concept in combination with the Freundlich isotherm (fig.4). The originality of this modal, in comparison to the others, consists in the existence of the equilibrium concentration Ce and the driving force equilibrium « C-Ce ». The general equation is equation (14). Two parameters A and r are determined by regression equations ; we proposed a simple method to calculate A and r by a linearization of the preceding equation (eq. 14). This is equation (16) In [(Co/C)^n-1 -1] = In A -rt.

Sodium decanesulfonate at a concentration of 20 mg ·l^-1 was used as influent and activated powdered carton (200 ≤ ø ≤ 315 µm) as the fixed bed adsorbent layer to illustrate the comparison between the different models. The linear flow rates were 3.0 m . h^-1 and the five columns tested were 3.1 ; 4.0 ; 7.5 ; 10.2 ; 12.5 cm high with a 1.45 cm² cross section. The Freundlich isotherm equation (fig. 4) obtained in a batch system for an equilibrium time (t = 24 h for this activated carbon) gives a « n value » equal to 2.38.

Figure 2 presents the experimental break-through curves obtained for the different bed heights ; by using equations (4 or 10) in the system (In C/Co, t) they are represented on the same figure by the dotted line. The agreement is only for the low values of C in the break-through curves.

The coefficients A and B (table 1) are determined from the straight lines obtained with the low break-curve concentrations (fig. 1). The kinetic coefficient Sa, and the maximum capacity adsorption No are shown in table 1. The No value is similar to those obtained from the other equations. The migration velocity of the concentration fronts (r = 0,133 cm · h^-1) is in good agreement with the experimental value (0,128 cm · h^-1).

The linearized Clark equation (16) gives a good representation of experimental results (fig. 6) alter the determination of A and r parameters (fig. 5 and table 2). With the use of the two parameters, the break-through curves have been recalculated (fig.6) and compared to experimental results. Their is good agreement. The A parameter is related to the depth Z of the adsorbant : A = Be^z ·. B value can be determined with the different columns (fig.7).

The Clark model can be applied to filers which have a biological activity ; the results obtained in the laboratory by EL HANI (1987) for the adsorption of humic acids (10 mg · l^-1) on a 1m granular activated carton bed were analyzed by the Clark equation (fig. 8). The initial concentrations of humic acids are never obtained in the effluent because of biological degradation and/or slow adsorption in mesopores. From the difference in the area of the two curves, it is possible to calculate the supplementary biological degradation. For 95 cm of activated carbon in the column and after 800 h, the biological degradation represents 55 % of the total elimination. The percentage is constant alter 35 cm depth of the activated carton in agreement with the electon microscopy study that showed that the flora was only present in the 10 first centimeters.

The use of this model is facilitated by our linearization and the case of particular phenomena : biological degradation or desorption. in the case of successive muld adsorbates fixation (REYDEMANEUF et al. to be published) can be studied and compared to the only adsorption phenomena.In conclusion, nome of the tested models lead to different parameters by using low break-through curve concentrations or others with the whole range of experimental points, but only one (CLARK) gives a good description of the break-through curves in our actual knowledge.

FR :

Parmi les différentes théories généralement considérées pour expliquer la formation et l'entretien des dépressions piézométriques régionales : surexploitation ; drainage profond ; tectonique subsidente ; réajustement du niveau marin ; reprise évapotranspiratoire, seule la dernière semble pouvoir s'appliquer à l'ensemble des cas décrits en zone sahélienne. Les études hydrogéologiques en cours dans cette région, et notamment les résultats des isotopes de l'environnement, mettent en effet en évidence le rôle prépondérent des transferts verticaux sur les écoulements latéraux.Le schéma conceptuel présenté est basé sur les hypothèses suivantes: considérant une nappe libre alimentée par ses bordures, en milieu à faible perméabilité ; (1) il existe, en tout point de la nappe, un déficit entre l'infiltration saisonnière et la reprise évapotranspiratoire ; (2) infiltration, évapotranspiration et déficit décroissent de façon exponentielle avec la profondeur de la nappe ; (3) la piézométrie actuelle est le résultat d'une évolution commencée depuis la fin d'une période humide correspondant au remplissage maximun de la nappe (dernier pluvial Holocène pour les grandes dépressions sahéliennes).Dans un premier temps, la validité de ce schéma est testée à l'aide d'un modèle analytique simple prenant en compte les caractéristiques climatiques (évolution temporelle du déficit hydrique), les propriétés hydrodynamiques (perméabilité, coefficient d'emmagasinement) et le « pouvoir évaporatoire » du sol (« facteur de forme » k). Ce dernier, expression principalement de la texture du terrain, apparaît logiquement comme le paramètre le plus sensible pour le modèle.Un modèle transitoire unidimensionnel est ensuite présenté afin de simuler la baisse piézométrique entre deux limites à potentiel constant. La surface piézométrique obtenue semble en accord avec les données observées sur le terrain.

EN :

The discovery of the large piezometric depressions in the Sahelian zone dates back to the 1950s, when the early hydrogeological studies took place in West Africa (DEGALLIER,1954 ; ARCHAMBAULT, 1960). Since then, numerous examples have been described throughout the region (aquifers of Trarza in Mauritania ; Ferlo in Senegal ; Gondo, Nara, Azaouad in Mali ; Kadzell in Niger ; Yaere in North Cameroon...), raising what was, certainly, the major enigma in Sahelian hydrogeology (fig. 1).

So far, none of the different theories put forward to explain this phenomenon has met with general agreement : (1) Overexploitation cannot be involved due to the insignificant pumping in comparison to the reserves. (2) Geological subsidence due to neotectonic causes would never be active enough in this stable part of the African shield. (3) Drainage to a deeper aquifer is impossible to - consider in this particularly flat area. (4) Changes in sea water level may explain the piezometric evolution of the coastal aquifers (DIENG, 1987) but cannot be considered for the whole Sahelian zone. (5) Evaporation and transpiration losses were often considered as insufficient to generate deep piezometric depressions. However, only this last hypothesis seems to fit the whole of the hydrogeologic and climatic Sahelian conditions and will therefore be considered as the governing factor in this paper.

Evidence of a vertical water deficit

The major role of evaporation and transpiration processes in the formation of the piezometric depressions is deduced from various data : (1) the geographical distribution of the « depressed aquifers », which is in accordance with the regional isohyets, suggesting that climatic factors are compulsory conditions ; (2) the piezometric measurements (figs.2, 3), which show a deficit between direct aquifer recharge by precipitation and evapotranspiration losses during the dry season ; (3) the environmental isotopic results, which demonstrate that vertical water movements prevail over lateral transfers (fig.4). Furthermore, evaporation effects and plant uptake may still be noticeable at relatively great depth : the presence of root activities of Acacia was, for instance, recently proven at a depth of 35m in North Senegal (DUPUIS et DREYFUS, 1989).

According to theoretical and experimental results from different approaches (isotopic interpretation of unsaturated soil profiles ; calculation of unsaturated soil suction profiles ; piezometric measurements and mathematical simulations) it can be assumed that steady-state exfiltration from an unconfined aquifer decreases exponentially with water table depth (ARANYOSSY, 1991).

Conceptual scheme

The conceptual model presented here (fig. 5) is based on the following assumptions : (1) there is, everywhere, a water deficit (D) between the seasonal infiltration and the evapotranspiration losses; (2) Infiltration, evapotranspiration and water deficit follow an exponential decrease with respect to the water table depth ; (3) the present piezometric profile is the result of an evolution that started at the end of a past period corresponding to a total replenishment of the aquifer (last pluvial Holocene period for the Sahelian region). Furthermore, it is considered that the climatic evolution towards arid conditions is accompanied by a parallel increase of the water Deficit (for simplification, D is assumed to be a linear function of time).

Considering a low permeability, unconfined aquifer system, the lateral water movement coming from the imposed constant head recharge zones on the boundaries (lake, river or sea) is not important enough to counterbalance evaporation losses. The water table therefore progressively drops, decreasing the evaporation losses, up to a quasi-steady state where, finally, lateral transfers just compensate the vertical water deficit (ARANYOSSY, 1988).

Mathematical simulation

In a first step, a simple analytical model is introduced to check the validity of this hypothesis. Parameters included depend on climatic conditions (evolution of the water deficit D) ; on hydrodynamic characteristics (permeability, storativity) and on the soil aptitude for evaporation (k factor). It follows that : (1) the water table decreases very rapidly during the first millenium and then slows down to a quasi stationary state ; (2) the maximum calculated water table depths correspond to those observed in the field ; (3) the most sensitive parameter is the « k factor » - the value of which depends mainly on the soil texture (fig. 6).

A transient numerical model is finally presented to depict, in one dimension, the evolution of an unconfined aquifer between two constant head boundaries. Simulation on an 8 000 year time scale generates a piezometric profile (fig.7) in conformity with the configuration observed in the field.

In conclusionThe formation of the Sahelian piezometric depressions results from the conjunction of several compulsory factors : (1) climatic conditions : evolution from a humid to an arid climate accompanied by a vertical water deficit ;(2) geological conditions : low permeability sedimentary formations ;(3) geographical conditions : constant recharge zones on the boundaries of the aquifer.

FR :

Dans cette étude, des mesures électrochimiques ont été utilisées pour caractériser l'efficacité inhibitrice de produits dérivés de phosphonates associés à des sels de zinc, employés pour le traitement des eaux des circuits de refroidissement. L'influence de la concentration de cette formulation ainsi que l'effet du pH du milieu ont été étudiés.

Les courbes courant-tension stationnaires et les diagrammes d'impédance électrochimique ont été obtenus avec des électrodes tournantes en acier au carbone dans une solution de chlorure de sodium à 200 mg l^-1. Ce milieu a été choisi car sa faible conductivité électrique est proche de celle rencontrée dans les eaux naturelles.

La méthode stationnaire (relevé des courbes courant-tension) a permis de déterminer la vitesse de corrosion en l'absence et en présence de l'inhibiteur et par conséquent, le taux de protection. Ainsi, le composé présente une très bonne efficacité dès les faibles concentrations (50 mg · l^-1). Entre 50 et 200 mg · l^-1, l'efficacité inhibitrice augmente de 95 à 98 %. Pour la concentration de 100 mg · l^-1 il est efficace dans un large domaine de pH (de 5,5 à 9). Cette efficacité apparaît légèrement supérieure à pH = 7 et à pH = 8.

Les valeurs des résistances de polarisation mesurées à partir des diagrammes d'impédance confirment les résultats obtenus à partir des courbes stationnaires.

En outre, l'efficacité inhibitrice du composé a été comparée à celle de produits déjà testés au Laboratoire pour des utilisations identiques. Le chlorure de zinc, le monofluorophosphate de zinc et l'association d'une amine grasse et d'acide phosphonique présentent des efficacités moindres que le mélange testé ici à base de produits dérivés de phosphonates associés aux sels de zinc.

EN :

In this study, electrochemical measurements were carried out to characterize the inhibitive efficiency of the association of phosphonate-derived products with zinc salts, employed for water treatment in cooling circuits.

The influence of parameters such as the concentration of the formulation and the effect of the solutions pH on the protection of the metal were studied.

Steady-state current-voltage curves and electrochemical impedance diagrams have been obtained for a carbon steel rotating disc in a 200 mg · l^-1 NaCl solution. The choice of this medium is based upon the following criteria :

(i) its low electrical conductivity is close to that encountered in natural saline waters ;

(ii) its corrosivity is fairly high ;

(iii) it Is formed by an easily reproducible baseline solution.

The inhibitive efficiency has been determined tram the plotting of the cathodic potentiostatic curves. It is important to note that in low conductivity media, the current-voltage curves are distorted by a non-negligible uncompensated ohmic drop (I.R_e), R_e being the electrolyte resistance between the working and the reference electrodes. The interpretation of these curves (i.e. corrosion rate evaluation, Tafel slopes determination) is only possible if they have been corrected according to the following relationship.

E_apparent = E_true + R_eI

In this study, the electrolytic resistance value has been determined from electrochemical Impedance measurements : the high frequency limit of the impedance gives R_e.The ohmic drop is then automatically compensated by the potentiostat to obtain the corrected steady-state curve.

The inhibitive efficiency is evaluated by the relationship :

E_i = (i_corr⁰-i_corr / i_corr⁰) x 100

in which i^o_corr and i_corr represent the current densities without and with inhibitor, respectively.

In addition to the determination of the current densities obtained from the steady-state current-voltage curves, the polarization resistance, R_p, has been measured from the impedance diagram : the low frequency limit of the impedance gives the sum R_e + R_p ; R_p is equal to the slope of the steady-state current voltage curve.

In this study, only the evolution of this value as a function of concentration has been examined.

It has been shown from the steady-state technique that the compound presents a very good efficiency for low concentrations (50 mg · l^-1). Between 50 and 200 mg · l^-1, the inhibitive efficiency increases from 95 to 98 %. For 100 mg · l^-1 it is effective over a wide pH range (5.5 to 9) with a better protection for pH = 7 and pH = 8.

The measurements of the polarization resistance corroborate these results.

In a second part, the inhibitive efficiency of the compound has been compared to different inhibitors used for similar uses and tested in the laboratory. Thus, zinc chloride, zinc fluorophosphate and the association of fatty amine and phosphonic acid present a lower efficiency than the product tested here.