Effect of the environment on the hydrochemical characteristics of an alluvial aquifer following an exceptional multiyear drought (Mediterranean Seashore,Hérault,France): Part II—Climatology and agronomy

The hydrochemical evolution of an alluvial groundwater located along the Mediterranean seashore is analyzed with respect to its agricultural and climatic environment, including degree of saturation of the soil; types of cultures, chemical fertilizers and phytosanitary treatments; selective irrigations of the cultivated areas; precipitations. It is shown that the environmental effect on the groundwater's vulnerability depends on two groups of factors:

1. Transfers from surface to groundwater of nitrates, chlorides, sulfates, and potassium, which are governed by: (a) the climatic conditions before the agricultural activities begin, (b) the kinds of agricultural activities and their distribution in time with respect to a given climatic context. Phenomena of retention of and/or rapid diffusion are related to the sedimentary heterogeneities of the reservoir and to the differences of temperatures between irrigation waters and precipitations.
2. Cationic exchanges related to the presence of clays of Montmorillonite-Kaolinite type.

     

An extension to the DRASTIC model to assess groundwater vulnerability to pollution: application to the Haouz aquifer of Marrakech (Morocco)

An extension to the DRASTIC model is proposed in order to assess aquifer vulnerability to pollution. In contrast to the DRASTIC model, which considers the unsaturated and saturated zones together and computes a global intrinsic vulnerability index, the suggested approach discriminates between the aquifer vertical vulnerability (a concept related to the pollutant percolation) and the groundwater susceptibility (a concept that depends on the behaviour and uses of the groundwater). This approach is applied to the Haouz aquifer (Morocco) that supplies water to the Marrakech area. This aquifer is widely overexploited and there is evidence that the groundwater quality is threatened by various sources of pollution. Evaluation of the vertical vulnerability indicates that the aquifer mainly presents a moderate-to-weak vertical vulnerability. The zones potentially most favourable to pollutant percolation are mainly located in Central Haouz, along or near the surface wadis. The aquifer susceptibility is high in places located near the N’Fis, Baaja and Issil wadis. Everywhere else, low-to-moderate susceptibility is observed. This new approach therefore enables areas of vertical vulnerability and areas of susceptibility to be delineated separately. As a result, it constitutes a valuable decision-making tool for optimising the management of aquifer water resources and land-use planning.     

Hydrochemical characterization of complex volcanic aquifers in a continental rifted zone: the Middle Awash basin, Ethiopia

The Middle Awash basin is an arid region in Ethiopia where surface waters are scarce and local communities are dependent on groundwater resources for water supply. The complex hydrogeological system of this basin has been conceptualized. Multivariate statistical analysis of hydrochemical variables and water isotopes were used to study the rock?Cwater interaction, geochemical reaction processes and the hydrological link between aquifers. Groundwaters from aquifers of the high-rainfall plateau bounding the rift are slightly mineralized, as well as depleted in ??¹⁸O and ??D, and contain ³H above 0.8?TU. This suggests a low degree of rock?Cwater interaction and that groundwater is under recharge from heavy rain that falls on surrounding highlands. On the other hand, groundwaters from aquifers of the rift floor are highly mineralized and show slight enrichment in ??¹⁸O and ??D with positive oxygen shift, but contain ³H below 0.8?TU. The positive oxygen shift in rift floor groundwaters may be caused by the isotopic exchange of oxygen between groundwater and aquifer materials during rock?Cwater interaction, whereas the low ³H content could be due to the decay of tritium along relatively long flow paths. The approach utilized in this study may be applicable to understanding hydrogeochemical processes in other complex volcanic terrains.     

Assessment of hydraulic properties of sedimentary and volcanic aquifer systems under arid conditions in the Republic of Djibouti (Horn of Africa)

The Republic of Djibouti (23,000 km²; 500,000 inhabitants), located within the Horn of Africa, undergoes an arid climate with an average annual rainfall less than 150 mm. Water resources are provided up to 98% by groundwater. Two types of aquifers are encountered: volcanic and sedimentary aquifers. This paper focuses on the assessment of their hydraulic properties, which is necessary for future tasks regarding the management of these aquifers. To this end, a data base consisting of all available pumping test data obtained since the 1960s was compiled. Pumping tests have been interpreted to determine transmissivity. Solely for volcanic aquifers, transmissivity also has been estimated through an empirical relationship using specific capacity corrected for turbulent well losses. The transmissivity of each type of aquifer can span up to four orders of magnitude, pointing out their strong heterogeneity. For the various volcanic rocks, the younger the rock, the higher the transmissivity. The transmissivity of volcanic rocks has therefore decreased in the course of geological time. At present, a much better understanding of the hydraulic properties of these complex aquifers has been obtained, which should enable optimal management of their groundwater resources through the use of numerical modeling.

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Resumen La República de Yibuti (23.000 km², 500.000 habitantes) está situada en el Cuerno de África, donde se han formado diversas unidades volcánicas—basaltos y riolitas—y rocas sedimentarias desde la expansión de los continentes acaecida al inicio de la deriva continental (hace 30 millones de años). La precipitación media anual es inferior a 150 mm. Las rocas volcánicas y sedimentarias, con dimensión inferior a 2.000 km², constituyen acuíferos locales. Los basaltos estratificados forman un acuífero regional que se extiende a más de 9.000 km². La multitud de datos existente ha sido almacenada desde los años 1960 en una base de datos. La transmisividad de estos acuíferos ha sido determinada mediante datos de ensayos de bombeo y una relación empírica que usa la capacidad específica corregida con las pérdidas turbulentas en el pozo. La transmisividad de estos acuíferos se comporta como una variable lognormal, hecho importante para los trabajos previstos de modelación con métodos estadísticos. La transmisividad de cada acuífero puede variar hasta en cuatro órdenes de magnitud, manifestando su gran heterogeneidad. Para los materiales volcánicos, la transmisividad es mayor cuanto más joven es la roca. La permeabilidad de las rocas volcánicas ha evolucionado por tanto con el tiempo geológico. Actualmente, se posee un mayor conocimiento sobre las propiedades hidráulicas de estos acuíferos complejos, de manera que se puede hacer una gestión óptima de sus recursos hídricos subterráneos con la utilización de modelos numéricos.

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Résumé La République de Djibouti (23,000 km²; 500,000 habitants), située dans la Corne de lAfrique, subit un climat aride avec une pluviométrie moyenne annuelle inférieure à 150 mm. Les ressources en eau sont fournies à plus de 98% par les eaux souterraines contenues dans des aquifères sédimentaires ou volcaniques. Cet article a pour objectif lévaluation des propriétés hydrauliques de ces aquifères, étape indispensable pour entreprendre par la suite des études en vue de la gestion de ces aquifères. Une base rassemblant les données dessais par pompage disponibles depuis les années Soixante a dabord été établie. Les essais par pompage ont été interprétés pour déduire la transmissivité. Concernant les aquifères volcaniques, la transmissivité a également été estimée à laide dune relation empirique reliant la transmissivité et le débit spécifique corrigé des pertes de charge anormales. La transmissivité pour chaque aquifère couvre jusquà quatre ordres de grandeur, montrant la forte hétérogénéité de ces milieux. Pour les roches volcaniques, on observe que la transmissivité est dautant meilleure que la roche est plus jeune. Leur transmissivité a ainsi diminué durant les temps géologiques. La compréhension des propriétés hydrauliques de ces aquifères complexes est à présent bien meilleure, ce qui permet denvisager une gestion optimale de leurs ressources à laide de modèles numériques.

     

Survey of a coastal karstic aquifer by analysis of the effect of the sea-tide: Example of the Kras of Slovenia,Yugoslavia

A hydrodynamic study of a coastal karstic aquifer is attempted by analyzing the effect of sea-tide on its water table Under certain conditions, this approach, which involves quite easy data processing, may yield the hydrodynamic parameters of the aquifers, diffusivity, in particular. The aquifer involved here is the Kras of Slovenia, Yugoslavia, which is bounded on one side by the Adriatic Sea (Bay of Trst)Kras is the Slovenian word from which the wordkarst originates The Kras aquifer is located in a thick carbonate sequence (Cretaceous) with an anticline structure. This sequence outcrops and is affected by opened vertical fractures. The geological setting could suggest a free groundwater with negligible piezometric variations with respect to the thickness of the carbonate sequence The study was carried out by the identification of observed data (fluctuations of the water table) and computed data under various assumptions (confined, semi-confined, free groundwater). The best fit is obtained when considering a groundwater with semi-confined behavior, which, of course, does not agree with the geological setting of the Kras However, this behavior can be justified by the fact that an important karstification occurred below the existing sea-level during the Quaternary and Tertiary marine recessions, creating diffusive zones in lower parts of the saturated karst it should be noted that similar situations must prevail in all the peri-Mediterranean karsts as they too were affected by the Quaternary and Tertiary recessions     

Comportement d'une source karstique soumise à une sécheresse prolongée : la source Bittit (Maroc)

The behaviour of a karstic spring (Bittit spring, Middle Atlas, Morocco) subjected to a prolonged drought (>20 yr), is analysed using spring hydrographs analysis methods and correlation and spectral methods. Two series of discharges, corresponding to a normal rainfall period and a drought period, are analysed. The results show that the karstic system is inertial, with very significant storage. However, in spite of the great regulating capacity of the storage, the long drought period significantly reduced the spring discharges, due to less diversified inputs, and to the karstic system storage depletion. To cite this article: F. Amraoui et al., C. R. Geoscience 336 (2004).