Constraints on alluvial clay mineral assemblages in semiarid regions. The Souss Wadi Basin (Morocco, Northwestern Africa)

Illite, chlorite and kaolinite, resulting directly from substratum weathering, dominate the clay assemblages recorded in altered rocks and soils of the Souss Wadi watershed. Nevertheless, the transformation and neoformation of diverse clay minerals, which is mainly controlled by changing climatic, lithological and topographic conditions, enables one to establish three distinctive source areas. High altitude, largely watered zones where chlorite is transformed into vermiculite; middle altitude drier areas where more pronounced seasonal contrast induces the transformation of chlorite into chlorite-smectite mixed layer; finally, semiarid low altitude zones with gentle slopes where the weathering of carbonate-dominated substrata, evaporation and chemical concentration result in smectite and palygorskite neoformation. These minerals, which result from transformation and neoformation, provide suitable environmental indicators and local to zonal tracers of sediment contribution. Nevertheless, the original clay mineral assemblages undergo a significant homogenisation due to the destruction of most of the transformed and neoformed minerals during fluvial transport. As a consequence, inherited clay minerals (illite, chlorite and kaolinite) usually dominate the assemblages recorded in the downstream alluvial sediments. The clay assemblages recorded in different tributary wadi alluvial deposits reflect major sediment contribution from the High-Atlas and the older alluvial plain deposits affected by intensive erosion. On the other side, Anti-Atlas contribution is lesser as a consequence of its wide tableland topography and the widespread outcropping of karstified carbonate rocks, which enhance water infiltration and preclude sediment supply

Apports des traçages artificiel et naturel à la connaissance des circulations et des zones de recharge en région semi-aride: cas de l'Atlas de Beni Mellal (Maroc)

Pour mieux comprendre les modalités d'alimentation des sources de l'Atlas, des campagnes de coloration et d'analyses isotopiques ont été effectuées sur l'ensemble des sources. La disparité des circulations, la variabilité des vitesses et des taux de restitution, révèlent un aquifère complexe ou à plusieurs entrées. La majorité des écoulements convergent vers la source de Ain Asserdoune qui constitue une zone d'abaissement d'axe de plis. L'étude isotopique montre la variation des teneurs en 180 avec l'altitude des zones de recharge des différents exutoires et avec la saison. Les gradients définis autorisent la détermination de l'altitude moyenne des impluviums des systèmes jusqu'alors mal connus, en particulier celui de l'exutoire principal du massif, Ain Asserdoune.The chalky massif of Beni-Mellal, which supplies many springs, spreads over a 365 km2 area of 500 to 2400 meters elevation (average altitude, 1200 m). Rainfall in this sector varies between 400 mm and 700 mm/year. All springs emerge along the major coastal fault giving the Liassic massif overthrushing the Cenozoic lands of piemont. The most important is Ain Asserdoune spring with an average discharge of about 1.1 m3/s. The mainly chalky and dolomitic formations are very fractured according to preferential directions (N150-180 and N50-70) and show some karstification features at the surface. Some tracing experiments with fluorescein allowed the delimitation of the boundaries of the intake area of Ain Asserdoune Basin and the determination of the groundwater flow velocity. Two types of flow were identified with fast (100 to 231 m/h) and slow circulation (< 100 m/h). Most of circulations converge towards the spring of Ain Asserdoune (fig. 1) with an important intake area. This convergence results from an axis lowering. The variability of the circulation axes, the velocity and the rate of restitution suggest either a complex aquifer or an aquifer with several inputs.Two water samplings (the first for the Beni Mellal Atlas) were carriet out in March and in September 1993 on the several springs, in order to study the variations of the oxygen-18 isotope. The oxygen-18 contents range between - 5.5 per thousand and - 7.7 per thousand (table 2). The most depleted waters directly exit from the Liassic aquifer or emerge at high altitudes; those which are richer in oxygen-18 emerge at the level of foothills, either from the Tertiary or from Liassic scales. The relationship between the oxygen-18 and the H-2 content of some samples indicates that the points are aligned along the world meteoric line (Craig, 1961). Even if the waters of Dir (piemont) springs are slighly evaporated because they partly originate in seeping Liassic waters, oxygen-18 content is probably mainly controlled by an altitude effect. Oxygen-18 data for local precipitations are not available; however, a local relationship between oxygen-18 content and mean recharge altitude was etablished for eleven (11) springs with small recharge areas whose mean altitude may be estimated from topography and geology. Oxygen 18 ratio is very well correlated to recharge average elevation for spring samples ( [delta]per thousand (O - 18)=- 3 - 0,0026 x altitude, r=0,96, fig. 3), when it is not for autumn samples. This gradient is very close to the regional relationship proposed by Marce (1975) and Kabbaj et al. (1978) (slope: 3 per thousand in oxygen-18 per 1,000 m ). This difference is attributed to the seasonal variations in the origin of water that feeds the springs. In the high water season, the whole basin area contributes to the recharge. In contrast, during low water period, water mainly originates from altitudes higher than the mean altitude of the basin, probably with a longer transit time. The seasonal variations are particularly marked for the Dir (piemont) springs (with a lower flux and intake area range of 800 to 1,200 m). They are weaker for the Lias springs and even non-existent for the main spring of the massif, Ain Asserdoune. In this latter case, the smoothing (- 7.6 per thousand and - 7.5 per thousand) of seasonal variation (which is also observed for temperature: 15.5 to 16°C ) gives evidence of important water reserves. A mean recharge altitude higher than 1,700 m may be proposed for Ain Asseroune. Based on this analysis, some emergences which in the part were assigned to the same hydrologic system can now be discriminated. On the other hand, with a similar value of oxygen-18 ratio, rather distant springs can be considered as belonging to the same hydrogeological Atlasic unit. So the relationship which was quantified (slope: 2.6 per thousand in oxygen-18 per 1,000 m) is usable for the whole Atlasic range on account of regionaly similar weather. The methodology selected in this work corroborates the results obtained by hydrometric analysis concerning the very important groundwater reserves in the Beni Mellal aquifer

Characterization of groundwater in the Souss upstream basin: Hydrochemical and environmental isotopes approaches

The hydrochemistry of major ions and environmental isotope compositions (18O, 2H) of water samples have been used to identify the chemical characteristics and the origin of groundwater in the Souss upstream basin. The total dissolved solids (TDS) did not exceed 1000 mg.L-1 with an average of 635 mg.L-1. Two chemical water types were observed: A calcium and magnesium bicarbonate type (Ca2+-Mg2+-HCO3-), forming the dominant water type generally observed in the plain along the Souss river and towards the piedmont of the Anti-Atlas Mountains; and a calcium sulphate type (Ca2+-SO42-), from the northwest of the study area, linked to the presence of phosphatic gypseous marls and limestone marls (Upper Cretaceous). The calculations of saturation indexes versus the main mineral phases have been carried out, using the PHREEQC program. The groundwater is saturated and slightly oversaturated with respect to carbonate minerals and under saturated with respect to evaporite minerals; surface waters show an oversaturation with respect to carbonate mineral phases, mainly dolomite. The groundwater composition is largely controlled by the dissolution of carbonate rocks known in this part of the basin. Stable isotope contents of groundwaters ranged from -7.96 to -6.26&permil; for &delta;18O and from -49.47 to -39.28&permil; for &delta;D. The hydrogen (&delta;D) and oxygen (&delta;18O) isotope signatures indicate a low evaporation of precipitations during infiltration and that the aquifer is highly influenced by the contribution of recharge water recharge from the High Atlas Mountains.Key words: Groundwater, hydrochemistry, stable isotopes, arid climate, Souss, Morocco

Détermination des modalités de recharge des aquifères par le biais des traceurs chimiques et isotopiques. Cas de la zone de contact Haut Atlas occidental – Plaine du Souss (SW du Maroc)

Determination of the origin of recharge of the unconfined aquifer in the right side of the Souss wadi between Agadir and Taroudant (South-western of Morocco) was based on the use of hydrochemical and isotopic analysis of groundwater, surface water and springs of the contact zone between the High-Atlas Chain and the Souss plain.The correspondence in the space evolution of the various chemical elements of evaporitic origin (SO42-, Cl-, Sr2+) in groundwater, piedmont springs, and surface water reveals the existence of recharge water from the adjacent High-Atlas Chain.The various recharge modes of the different aquifers (High Atlas and Souss plain) determined by isotopic analysis, shows that the source of groundwater for the unconfined Souss aquifer seems to be composite between a direct infiltration on the High-Atlas tributaries and a remote recharge from the bordering High Atlas aquifers.La determinación del origen de los aportes de agua de la capa freática de la ribera derecha del rio Souss entre Agadir y Taroudant (Suroeste de Marruecos) se ha basado en la hidroquímica y el análisis isotópico de las aguas subterráneas, aguas superficiales y manantiales de la zona de contacto entre el Alto Atlas y la llanura de Souss.La correspondencia en la evolución espacial de los diferentes elementos químicos de origen evaporítico (SO42-, Cl-, Sr2+) en las aguas subterráneas, manantiales de pie de monte y aguas superficiales, revela la existencia de una recarga de agua procedente de la cadena del Alto Atlas. El análisis de los modos de recarga de los diferentes acuíferos (Alto Atlas y llanura de Souss) determinado por análisis isotópico, demuestra que la alimentación de la capa freática de Souss a partir del Alto Atlas parece ser mixta, compuesta por una infiltración directa de los afluentes del Alto Atlas y una alimentación lejana desde los acuiferos que limitan con el borde del Alto Atlas

Qualite des eaux souterraines et risque de pollution en milieu semi-aride. Cas de la cuvette de Cheria (NE Algerien)

Water shortage has become a key problem for all countries and particularly for those in development. In fact, the increase of the populations and the development of urbanism, industries and cultivated land lead to a degradation of the quality of the groundwater and a very significant reduction of the reserves that constitute at times the only source of water for human consumption. After the analisis of the situation of the land, the present work comes in an important moment to establish the characteristics of the complex aquifer of the plateau of Chéria in Algeria and the risks of contamination of the groundwater. This study, which relies in the recent causes of contamination takes into account several factors. We can mention the contamination by blackwaters (waste), the impact of livestock and the climatic conditions. The exploitation of the groundwater from carbonated aquifers, is today the only source that responds to the needs of water for the industrial, human, and agricultural comsumption in the region. In recent years there has been an increase in agricultural and agricultural-related activity. The first results show the existence of a lowering of the piezometric level, as well as important concentrations in nitrates due to anthropic activity which are leading to a deterioration of the quality of drinking water in Chéria.La escasez de agua se ha convertido en un problema crucial para toda la Sociedad y en particular para los paises en vias de desarrollo. En efecto, el crecimiento de la población y el urbanístico, de la industria y de los cultivos pueden conducir a una degradación de la calidad de las aguas subterráneas y una significativa reducción de las reservas que, a menudo, constituyen la única fuente de agua de consumo humano. En este trabajo se evalúan las características del complejo acuífero de la cuenca de Chéria (Argelia) basado en un inventario de las fuentes de contaminación teniendo en cuenta diversos factores, como son la contaminación por aguas residuales, el impacto debido al ganado y las condiciones climáticas. Esto es necesario ya que la explotación de las aguas subterráneas a partir de acuíferos carbonatados es actualmente la única fuente de agua de consumo humano, industrial y agrícola de la región, donde se ha producido un fuerte incremento de consumo en los últimos años. Los resultados de este estudio demuestran la existencia de un descenso del nivel piezométrico y de concentraciones elevadas de nitratos debido a la actividad antrópica, con valores de 80 a 120 mg/l

Use of environmental isotopes to assess the sustainability of intensively exploited aquifer systems (2012‐2015)

Intensive exploitation of groundwater over longer period has led, in many important aquifers, to marked lowering of water tables, increasing exploitation costs, and often, to a progressive deterioration of water quality. Concentrated pumping may also alter flow patterns permanently with the risk of migration of pollutants into aquifers from the surrounding aquifers or surface water bodies due to lack of physical protection to prevent them. Isotope hydrology tools have proven to be very useful in assessing groundwater hydrology, addressing aspects related to recharge processes, delineation of flow patterns, water quality issues and interactions with other water bodies; this unique information can be further used to evaluate long term aquifer sustainability. The objective of the Coordinated Research Project F33019 is to develop and review approaches and methodologies, mostly based on the combined use of conventional hydrogeological techniques and environmental isotopes, to assess the response of groundwater systems to intensive exploitation and groundwater availability. Access to new dating tools and approaches for groundwater dating covering different time scales offers the possibility to evaluate changes in groundwater dynamics and flow patterns, providing key data to predict the evolution of aquifers and their sustainability as major sources of water. The CRP aims to assess the performance of these new tools and approaches and the possible adoption of these methods by water management experts

Disentangling the complexity of groundwater dependent social-ecological systems

Groundwater resources are part of larger social-ecological systems. In this chapter, we review the various dimensions of these complex systems in order to uncover the diversity of elements at stake in the evolution of an aquifer and the loci for possible actions to control its dynamics. Two case studies illustrate how the state of an aquifer is embedded in a web of biophysical and sociopolitical processes. We propose here a holistic view through an IGM-scape that describes the various possible pathways of evolution for a groundwater related social-ecological system. Then we describe the elements of this IGM-scape starting with physical entities and processes, including relations with surface water and quality issues. Interactions with society bring an additional layer of considerations, including decisions on groundwater abstraction, land use changes and even energy related choices. Finally we point out the policy levers for groundwater management and their possible consequences for an aquifer, taking into account the complexity of pathways opened by these levers

Constraints on alluvial clay mineral assemblages in semiarid regions. The Souss Wadi Basin (Morocco, Northwestern Africa)

Illite, chlorite and kaolinite, resulting directly from substratum weathering, dominate the clay assemblages recorded in altered rocks and soils of the Souss Wadi watershed. Nevertheless, the transformation and neoformation of diverse clay minerals, which is mainly controlled by changing climatic, lithological and topographic conditions, enables one to establish three distinctive source areas. High altitude, largely watered zones where chlorite is transformed into vermiculite; middle altitude drier areas where more pronounced seasonal contrast induces the transformation of chlorite into chlorite-smectite mixed layer; finally, semiarid low altitude zones with gentle slopes where the weathering of carbonate-dominated substrata, evaporation and chemical concentration result in smectite and palygorskite neoformation. These minerals, which result from transformation and neoformation, provide suitable environmental indicators and local to zonal tracers of sediment contribution. Nevertheless, the original clay mineral assemblages undergo a significant homogenisation due to the destruction of most of the transformed and neoformed minerals during fluvial transport. As a consequence, inherited clay minerals (illite, chlorite and kaolinite) usually dominate the assemblages recorded in the downstream alluvial sediments. The clay assemblages recorded in different tributary wadi alluvial deposits reflect major sediment contribution from the High-Atlas and the older alluvial plain deposits affected by intensive erosion. On the other side, Anti-Atlas contribution is lesser as a consequence of its wide tableland topography and the widespread outcropping of karstified carbonate rocks, which enhance water infiltration and preclude sediment supply