Sensibilité à la pollution métallique de deux grands lacs africains (Tanganyika et Malawi)

Les lacs Tanganyika et Malawi sont, de par leur volume, les deux plus grands lacs africains. Ces réservoirs semblent pour l'instant épargnés par la pollution en éléments en trace. Il est toutefois crucial, en raison de leurs caractéristiques hydrologiques, de poser la question du temps de réponse de ces systèmes à une pollution chronique potentielle véhiculée par les affluents. Cet article simule ainsi cette réponse dans la fraction dissoute suite à l'introduction pendant 50 ans de polluant par tous les affluents. Cette démarche s'appuie sur un modèle hydrologique intégrant les trois compartiments des colonnes d'eau (épi-, méta- et hypolimnion) et sur la prise en compte de la réactivité des éléments dissous dans ces compartiments par l'intermédiaire du taux de rétention élémentaire. Ainsi quatre types d'éléments sont considérés, (i) le type Cl, non réactif, (ii) le type Si, réactif-nutritif, (iii) le type Mn et (iv) le type V tous deux réactifs sensibles aux conditions d'oxydo-réduction. La réactivité de l'élément, l'efficacité du mélange vertical ainsi que la position de l'oxycline dans la colonne d'eau conditionnent l'amplitude et la cinétique de réponse des systèmes ainsi que le temps de retour à la situation initiale après l'arrêt des apports polluants. Ces caractéristiques propres à l'élément et au lac influent sur le risque potentiel encouru par l'écosystème et l'homme. Ainsi la pollution affecte principalement les eaux de surface (types Cl et V), les réseaux trophiques (type Si), les eaux profondes (types Si et Mn) et le compartiment sédimentaire (types Mn et V).Lakes Tanganyika and Malawi are the largest African lakes as measured by volume. They constitute essential water and protein resources for the surrounding populations. These aquatic systems have become stressed due to high human population density, growth and associated activities. While eutrophication was apparent locally and organic pollutants were detected in fish and water, concentrations of several dissolved trace elements of potential concern corresponded to uncontaminated systems. However, due to their hydrological features, it was important to characterise the lake response time to chronic contamination loaded by the tributaries. This paper presents two simulations of this response, in the dissolved fraction, following 50 years of pollutant input by the tributaries. The first simulation corresponded to an annual pollutant input that was the same for both lakes, resulting in mean river input concentrations of 5.0 U L-1 and 3.7 U L 1 (where U is a weight or molar unit), respectively, for lakes Tanganyika and the Malawi. The second simulation corresponded to an annual input proportional to the lake volume, with mean river input concentrations of 5.0 U L 1 and 1.5 U L 1, respectively, for lakes Tanganyika and the Malawi. The polluted input was loaded by the dissolved fraction with the exception of Mn-type elements, which were carried by the particulate fraction. This approach was based on an annual hydrological model of three water column compartments (epi-, meta- and hypolimnion) of these meromictic lakes. In addition, the reactivity of dissolved elements in the water column was taken into consideration. The reactivity was characterised by the elemental retention rate that quantifies dissolved-particulate interactions linked to biological and physico-chemical processes. The reactivity of trace elements was assessed through their concentration distribution profile in the water column. Four element types were considered: the non-reactive elements characterised by homogenous concentrations in the water column (Cl-like); the micronutrient-type elements (Si-like) characterised by a strong positive concentration gradient below the thermocline; redox-sensitive elements (Mn-like) characterised by a strong positive concentration gradient below the oxycline and other redox-sensitive elements (V-like) characterised by a strong negative concentration gradient below the oxycline. Trace elements (F, Al, Fe, Mn, V, Ba, Sr, Mo, Cr, Ni, Co, Cu and Pb) in both lakes were associated with these element types but they did not necessarily belong to the same type in both lakes. Other elemental types likely occurred (e.g., carbonate type and Fe types) but they were not clearly identified. After 50 years, surface concentrations ranged from 0 to 1.15 U L 1 in Lake Tanganyika and from 0 to 2.40 U L 1 in Lake Malawi. The difference between the lakes was linked to the greater volume of Lake Tanganyika, mainly in its hypolimnion, and to the longer vertical water exchange time in Lake Tanganyika. For Cl-type elements the concentration response decreased for both lakes from the epi- to the hypolimnion with similar kinetics for the epi- and metalimnion and a delay for the hypolimnion. For Si-type elements the response decreased in Lake Malawi from the hypo- to the epilimnion and for Lake Tanganyika the maximal concentration was calculated in the metalimnion. The concentration range was higher in Lake Malawi than in Lake Tanganyika. For the Mn-type elements, the maximum concentration was calculated in the hypolimnion with a higher response in Lake Malawi. The metalimnetic water concentration of Lake Tanganyika increased slightly and epilimnetic and metalimnetic waters of Lake Malawi did not react. For V-type elements the epilimnetic waters were more sensitive to the increase, with a higher response for Lake Malawi. In Lake Malawi concentrations also increased in the metalimnion. Concentrations in the hypolimnetic zone of both lakes and metalimnetic zone in Lake Tanganyika remained zero. Depending on the element type and on the lake, the time required to return to initial conditions, when contaminant inputs stopped, varied from 30 to 7 300 years. In the epilimnetic zone of both lakes the intensity of reaction and the pollution persistence were higher for Cl-type elements. For Si-type elements, mainly in Lake Malawi, the vertical input from deep waters was sufficient to sustain productivity even after the input of pollutants was stopped. For these elements the dissolved contamination was mainly stored in deep waters. For Mn-type elements the contamination was also stored in deep waters with a relatively slow net transfer to the sedimentary compartment. V-type pollutants were transferred from the dissolved to the particulate phase in deep waters leading to a relatively rapid net transfer to the sediment. Once the pollutant was in the system and until its evacuation to the outlet or to sediment, the risk for the ecosystem and for the population was associated with its presence in the dissolved phase of the surface water. The risk was then higher for Cl- and V-type elements as well as for the Si-type elements that were introduced into the web food. For the Si- and Mn-type elements that were mainly stored in deep waters, the associated risk was linked to a breaking of the thermo-haline stratification or to a reinforcement of vertical mixing. For the V-type elements and also for the sedimentary fraction of the Mn-type elements, the risk was also associated with possible remobilization from the sediments due to physico-chemical changes at the water-sediment interface.Element reactivity, efficiency of the vertical mixing and the depth of the oxycline control the importance and the kinetic response. They also controlled the time to attain initial conditions once contaminant inputs were stopped. These features, relative to the element and to the lake, were key parameters in the assessment of the potential risk for both the ecosystem and people that rely on these lakes. Even if the elemental typology was the same for both lakes, elements can be considered a different type from one lake to another. Contamination from the same pollutant would then have different consequences, for instance regarding the associated risk. Computed hydrochemical budgets were simple but realistic, illustrating the behaviour of elements in the water column. Computation of this budget requires the knowledge of global water column fluxes, which have to be improved mainly for Lake Tanganyika. The element's reactivity was mainly linked to liquid-solid reactions. It would be interesting in future studies to characterise particulate phases and their reactivity and to introduce such processes in hydro-geochemical models. Computations of chronic contamination response indicate that for both lakes, due to the inertia of the hydrochemical system, the lack of lake water contamination does not imply a systematic lack of pollution in the tributaries. Once pollution is detected, it will be persistent. A global watershed monitoring program should be organised in the near future. Monitored parameters should be relevant to metallic and organic pollutants, as well as eutrophication

Hydrochimie des lacs dans la région de Trinidad (Amazonie bolivienne) : influence d'un fleuve andin : le Rio Mamoré

La caractérisation physico-chimique des cours d'eau et des lacs de la région de Trinidad (plaine du Mamoré central) a permis de distinguer deux grands types d'eau. Les eaux d'origine andine (le Rio Mamoré et les lacs qu'il alimente) qui sont relativement bien minéralisées (> 100 mg.l-1), de type bicarbonaté calcique et avec de fortes teneurs relatives en calcium, magnésium et sulfates. Les eaux de la plaine (Rio Mocovi et les lacs peu profonds éloignés du Rio Mamoré) qui sont de type variable, toujours faiblement minéralisées (< 50 mg.l-1), et avec de fortes teneurs relatives en fer et en potassium. L'évolution de l'hydrochimie de ces différents milieux, plus ou moins proches, étudiée au cours de deux cycles hydrologiques, montre une forte influence du Rio Mamoré en période de hautes eaux (de janvier à avril) sur les milieux lacustres adjacents, liée à l'existence de zones d'inondation. (Résumé d'auteur

L'analyse discriminante, un puissant moyen de validation des hypothèses hydrogéologiques

L'étude des tableaux de données hydrochimiques acquises au cours de campagnes synchrones (« instantanés ») ou de suivis diachroniques à pas régulier (hebdomadaire, quotidien, horaire) s'opère généralement en résumant l'information par des méthodes statistiques. Ces méthodes descriptives, qui négligent nécessairement une partie de l'information initiale, permettent l'interprétation de la structure du tableau de données en termes de fonctionnement hydrocinématique (BAKALOWICZ, 1979, 1982, MUDRY et BLAVOUX, 1988, ROLET et SEGUIN, 1986 8 et b).Ces méthodes, fondées sur l'analyse d'une seule population statistique (bi ou multidimensionnelle) impliquent un mode de raisonnement déductif.Leur application, ainsi que l'examen du tableau des données brutes (ou de ses représentations graphiques), peut mettre en évidence des sous-groupes fondés sur des critères hydrogéologiques. La réalité de ces sous-groupes peut être testée à l'aide de méthodes statistiques basées sur l'analyse de la variance. Certaines méthodes utilisent le rapport des variances qu'elles comparent à la distribution de Snedecor (analyse de la variance à une ou deux voies), d'autres comparent des variances multidimensionnelles intraclasses à la variance interclasses, c'est le cas de l'analyse discriminante. Les sous-groupes constituent une variable qualitative dont la pertinence peut être démontrée par la calcul. L'analyse discriminante apparaît donc comme un outil décisionnel. Le présent article présente brièvement la méthode du point de vue statistique et montre deux exemples d'application à des sources karstiques.Le premier exemple traite de l'appartenance chimique d'une phase de basses eaux à la petite crue qui la précède et non à un tarissement au sens hydrocinématique. L'analyse discriminante permet d'affirmer qu'une recharge peu perceptible sur l'hydrogramme de la source amène une évolution chimique irréversible de l'eau de la réserve, responsable des phénomènes d'hystérésis observés sur les courbes concentration-débit. Ce cas est celui de l'aquifère de la Fontaine de Vaucluse (Sud-Est de la France) pendant un suivi quotidien d'étiage.Le second exemple permet de rattacher, par son comportement physico-chimique hebdomadaire, une émergence karstique à une autre et non à une troisième. Ce cas est celui du karst de la Rochefoucauld (Charente), avec les sources du Bouillant, de la Font de Lussac et de la Lèche. Les sources du Bouillant et de la Font de Lussac ont un comportement physico-chimique semblable, alors que la Lèche réagit de manière totalement indépendante. Elle constitue un système globalement distinct du point de vue hydrocinématique, ce qui permet de minimiser les relations mises en évidence par traçage artificiel entre les deux systèmes.The study of the hydrochemical data tables obtained during synchronous sampling (weekly, daily or hourly) is generally carried out by resuming the information by statistical methods. These methods, that disregard part of the initial information, allow to explain the structure of the data table in terms of hydrokinematics (BAKALOWICZ, 1979, 1982; MUDRY et BLAVOUX, 1986; ROLET et SEGUIN, 1986 a et b).These descriptive methods, based on the analysis of a single (bi or multivariate) statistical population, imply deductive reasoning. Their application, as well as the study of the untreated data table (or of its scattergrams), can show the presence of sub-groups based on hydrogeological criteria. The existence of such sub-groups can be tested by statistical methods based on variance analysis. Several methods use the variance ratio and compare it to Snedecor's distribution (single or double path variance analysis), others compare multidimensional intragroup variances to intergroup variance. This is the case with discriminant analysis.This paper describes the method from a point of view of statistics and presents two examples of application to karst springs.The first study deals with the chemical relationship of a low-water period with the preceding period of small floods and not to a hydrokinematical water drying up. The discriminant analysis allows to say Chat a recharge less visible on the hydrograph of the spring induces a non reversible chemical evolution of the reserve water. This is the case of the Fontaine de Vaucluse karst spring (Southeastern France) during a daily low water sampling.The second study allows to relate a karst spring to another one, thanks to its weekly physico-chemical behaviour, and not to a third one : this is the case of the La Rochefoucauld karst system (Charente, Western France) with the Bouillant, Font de Lussac and Lèche springs. The Bouillant and Font de Lussac springs behave in the same way hydrochemically, whereas the Lèche works independently. It is, hydrokinematically, a separate karst system, even if there is a certain relationship between all of them

Apports des outils chiniques et isotopiques à l'identification des origines de la salinisation des eaux : Cas de la nappe de La Chaouia côtière (Maroc)

L'étude hydrochimique des eaux de la nappe libre de la Chaouia côtière montre une teneur excessive en sels dans ces eaux, spécialement des chlorures (jusqu'à 3 g.l-1).Cette salinité élevée peut, à terme, nuire gravement à l'économie de la région qui se consacre essentiellement à la culture maraîchère sous irrigation.Des mesures isotopiques (18O/16O, 2H/1H) couplées aux éléments chimiques, notamment Br-/Cl-, excluent l'hypothèse d'une intrusion marine généralisée. L'existence d'une paléosalinité (SHIVANNA andal.,1993) n'est pas soutenue par les mesures des teneurs en isotopes radioactifs (3H, 14C). Il en est de même pour le lessivage de dépôts chlorurés préexistants dans les sédiments. L'apport essentiel en chlorures provient de l'altération de la roche mère, des fertilisants et des embruns marins lessivés par suite de l'irrigation et entraînés vers la nappe par percolation. Le recyclage de l'eau souterraine depuis au moins trente ans, par l'irrigation, n'a fait qu'augmenter cette salinité.Toutefois en bordure de l'océan quelques puits semblent montrer la présence d'eau marine, en effet les débits d'exhaure des puits sont à la limite d'exploitation de la nappe et toute augmentation de débits dans ces puits engendrera fatalement une invasion marine généralisée.In West of Morocco, the coastal plain Chaouia is located between Casablanca and Azemmour (south-west of Casablanca) over a distance of 65km (Figure 1). This plain aquifer spreads over 1100 km2 in semi-arid climatic conditions with about 370 mm.year-1 of mean precipitation and 17°C in mean air temperature.Irrigated agricultural farming is the main economic resource of the region. The only source of water is provided by highly mineralized groundwaters, which are harmful for the rural population and agricultural irrigation. The chloride content represents the main contribution of the salinity of ground waters. Most of the previous authors and authorities have attributed the high mineralization to seawater intrusion (YOUNSI, 1994). The objective of the present study is to determine the cause of salinity, considering three hypothesis: seawater intrusion, leaching of salts and evaporation of water. The environmental isotopes (2H,18O, 3H, 14C) have been used with hydrogeology together with major and minor ion chemistry to identify the source of salinity and to estimate the residence time of ground water. Moreover an environmental isotope study was carried out in order to identify the origin of the salinization (MERLIVAT et al., 1970; COTECCHIA et al., 1974; GASPARINI, 1989; CABRAL et al., 1991; GOMEZ-MARTOS et al., 1993; HASHASH et al., 1995). Both isotopic studies and Br-/Cl- ratios have been used to identify the salinity origin. The potential sources for this salinization are: seawater encroachment as a consequence of intense exploitation of the aquifer, dissolution of the halite from the several diapiric structures intruded in the aquifers. The isotopic waters sampling were carried out in August 1993 on several wells (Figure 2), in order to study the variations of the oxygen-18, deuterium, tritium and carbon-14 isotopes. Table 1 shows the results of the isotope and chemical analyses carried out. As oxygen-18 data for local precipitation are not available in Morocco, the oxygen-18 and deuterium mean contents in precipitation were taken from stations of the International Atomic Energy Agency (IAEA) network near the studied area (Gibraltar, Faro, Ponta Delgada) and/or deduced from published isotopic composition of shallow groundwaters in Morocco (KABBAJ et al., 1978; LOUVA and BICHARA, 1990; BOUCHAOU et al., 1995); the mean values for the precipitation are -4.0 ± 0.5‰ and -23.0 ± 2 ‰ (vs. V-SMOW) respectively. The oxygen-18 and deuterium mean contents in ground waters are -3.7 ± 0.3 ‰ and -20.1 ± 1.8 ‰ (vs.V-SMOW) respectively. The relationship between the oxygen-18 and deuterium contents of some groundwater samples indicates a good fitting along the world meteoric line (CRAIG, 1961) (Figure 3), suggesting that the ground waters are mainly of meteoric origin. There is no indication of possible mixing with seawater because there is no indication of a relation between the oxygen-18 content of the wells and their distance from the sea (Figure 4). On the other hand, no isotopic enrichment is noted during the infiltration of water from precipitation and/or irrigation.The diagram d18O-Cl- (Figure 5) shows that all the data do not lie along a theoretical mixing line with seawater and/or an evaporation line. Every 18O data point is very close to the isotopic values of precipitation [-4.0± 0.5 ‰ (vs. V-SMOW)] but with a large scatter in Cl- content. In this zone, an increase in the salt concentration is not accompanied by an isotopic effect. The sources of the chloride must be found among the products of rock alteration, fertilizers and marine airborne salts. The recycling of salt water by irrigation increases the salinity of soil and ground waters. Halite is characterized by a very low Br-/Cl- ratio(0.183 x 10-3). Water in some wells presents Br-/Cl- ratios similar to those of halite, but most of the groundwater samples have a greater ratio. These results argue against the hypothesis of dissolution of halite in the aquifer formation. But the Br-/Cl- ratio alone is not sufficient for detecting a possible seawater intrusion.During August 1994, 20 drilled wells were sampled over the study area for determining tritium concentrations. Carbon-14 activities were measured only on some samples. The relatively high tritium content in the majority of the sampled waters [between 2 and 9 tritium units (TU)] indicates a recent groundwater recharge (Table 1). The use of natural radioactive isotopes (3H, 14C) in the ground waters gives an estimate of mean residence time of 50 ± 20 years (mixing model). Some waters show a mean residence time greater than 100 years because they are isolated from the general circulation and limited to local spots. The recent 14C activity of the groundwater (_100 pcm) allows us to reject the hypothesis of a mixing with paleoconnate waters as demonstrated elsewhere (SHIVANNA et al., 1993).In the case of the coastal Chaouia (semi-arid environment), the use of both chemical tracers (Cl-, Br-) and isotopes (18O, 2H,3H, 14C) has shown that rainfall constitutes the essential origin of ground water in the area. The high salinity caused by seawater intrusion is probably limited in space to some coastal wells; neither evaporation of water before and during percolation, nor dissolution of evaporitic deposits (halite), has been found. The past marine transgression salt origin has also been discarded. The main source of the salinity is the washout of the rock alteration salts, fertilizers, marine airborne salts, and the unusable salts rejected by plants in the soil. Thus the increase of salinity is well explained by the recycling of salty ground waters by irrigation during at least thirty years

Variabilité de l’érosion actuelle et holocène : le cas des marbres de Grenville en Outaouais québécois

Cet article souligne la difficulté de comparer les taux d'érosion mesurés dans les marbres de Grenville selon différentes méthodes. La variabilité des taux d'érosion a été mise en évidence par l'étude des nodules en relief, des plaquettes de calcaire et par l'hydrochimie. La mesure moyenne du déchaussement postglaciaire des nodules emprisonnés dans les marbres se situe à 5 mm/1000 ans pour une période qui couvre l'Holocène (58 nodules, 1429 mesures). L'étude hydrochimique a fourni une vitesse moyenne d'érosion de 21,4 mm/1000 ans. Le taux médian de 17,2 mm/1000 ans se rapproche plus de la vitesse réelle d'érosion chimique actuelle, car les mesures ont surtout été prises en été et au printemps. Parmi les nombreux facteurs qui expliquent l'écart entre les taux d'érosion fournis par les deux méthodes, la différence de milieu étudié est primordiale. Elle est mise en évidence par la méthode ubiquiste de l'étude des plaquettes qui a fourni des taux moyens d'érosion presque huit fois plus importants dans les ruisseaux que dans les sols. Dans ces derniers, le résultat obtenu après une expérience de 18 mois a été de 12,4 mm/1000 ans alors qu'il a été de 94,3 mm/1000 ans pour les plaquettes immergées dans diverses eaux courantes. Dans les eaux calmes, le résultat a été de 41,7 mm/1000 ans.This paper demonstrates the difficulties of comparing the variability of erosion rates within Grenville marbles by the use of experimental limestone tablets, the measurement of deterration height of insoluble nodules, and hydrochemical analyses. The mean value of postglacial deterration is 5 mm/1000 years (58 sites, 1429 measurements). Hydrochemical monitoring yielded a mean erosion rate of 21,4 mm/1000 years. The median of 17,2 mm/1000 years likely reflects the actual rate of chemical erosion, as most monitoring took place during spring and summer. There are numerous factors that can account for differences in erosion rates obtained from these two methods, the most predominant one being the site selected. This is evident when the results obtained from experiments using marble tablets are considered. The tablets, when suspended in streams, yielded mean rates of erosion which proved to be almost eight times higher than those measured in soils. Following 18 months of monitoring, the mean rates obtained for tablets in soils and various stream currents were 12,4 and 94,3 mm/ 1000 years respectively. In calm waters, a mean rate of 41,7 mm/1000 years was calculated.Dieser Aufsatz hebt hervor, wie schwierig es ist, die im Marmor von Grenville mit verschiedenen Methoden gemessenen Erosionsraten zu vergleichen. Die Variabilitàt der Erosionsraten konnte durch das Studium der hervorstehenden Knollen, der Kalkplâttchen und durch hydrochemische Analysen deutlich gezeigt werden. Der durchschnittliche Wert der post-glazialen Freilegung der in den Marmor eingeschlossenen Knollen betrâgt 5 mm/1000 Jahre fur die Zeit des Holozân (58 Knollen, 1429 Messungen). Die hydrochemische Analyse ergab eine durchschnittliche Erosionsgeschwindigkeit von 21,4 mm/1000 Jahre. Die mittlere Rate von 17,2 mm/1000 Jahre kommt der realen gegenwârtigen chemischen Erosionsgeschwindigkeit naher, denn die Messungen wurden vor allem im Sommer und Frùhling durchgefùhrt. Unter den zahlreichen Faktoren, welche die Abweichung zwischen den durch die zwei Methoden erbrachten Erosionsraten erklà-ren, ist der Unterschied des studierten Milieus entscheidend. Er wird deutlich dargelegt durch die allgegenwàrtige Méthode der Analyse der Plattchen, welche fast acht mal hôhere durchschnittliche Erosionsraten in den FluBchen als in den Bôden erbrachte. In den letzteren hat man nach einer 18 monatigen Studie ein Ergebnis von 12,4 mm/1000 Jahre erhalten, wogegen es 94,3 mm/1000 Jahre fur die in verschiedenen flieBenden Wassern eingetauchten Plattchen betrug. In stehenden Gewassern betrug das Ergebnis 41,7 mm/1000 Jahre