Structural pattern of the Saïss basin and Tabular Middle Atlas in northern Morocco: hydrological implications

International audienceThe plain of Saïss is a fertile area of great agricultural production with major economic interests. Therefore, the improved knowledge about the water supply is imperative within a context of recurrent droughts and overexploitation of the groundwater. This plain is located in the Meknes-Fes basin and between two deformed domains: the Rif and Middle Atlas. The aquifers are fed by water coming from the Tabular Middle Atlas, for which the pathways are poorly constrained. This study provides new data to determine the water pathways based on a structural map produced from a novel analysis of SPOT images and a digital elevation model. This structural map reveals two fracture sets trending NE-SW and NW-SE. The first set is well known and corresponds to a main trend that controlled the tectonic and stratigraphic evolution of the study area. On the other hand, the NW-SE set was poorly described until now: it is both diffuse and widespread on the Tabular Middle Atlas. A comparison between the regional water flow trend, drainage pattern and structural map shows that the NW-SE fractures control the water flow from the Tabular Middle Atlas to the Saïss plain. A hydrological model is discussed where the water flow is confined onto Liassic carbonates and driven by NW-SE fractures. This study explains how a detailed structural mapping shows hydrology constraints

Implication of Guigo and L'Hajeb Causses in the renewal and circulations of Saïs basin groundwaters (Middle-Atlas Causses, Morocco).

International audienceIn a context of overexploitation of the karst system of the Middle Atlas Causses feeding the Saïs basin and the current climatic variations, the study of circulations and of renewal of waters of this system in the Fes-Meknes area becomes essential for the population, in order to maintain a sufficient quality of waters with a good management.By coupling hydrochemical and isotopic analysis methods (delta-D, delta O-18, Radon-222), saturation indices obtained from PHREEQC code and principal component analysis (PCA) of ten springs and three wells, a first conceptual model of groundwater flows of this karst system was obtained . These waters are mainly renewed by the rainfall of L'Hajeb Causse and secondarly by the rainfall of Guigo Causse containing several springs. Hydrochemistry and saturation indexes allowed us to highlight two types of waters: a main contribution of Liasic origin and two low contributions of Triassic origin at the southern extremities (SE, SW) of the basin. We pointed out the existence of five local recharge zones of different altitudes (900 to 1500 m asl.) including the two main mixing zones to the south (SE, SW). Radon-222 showed areas of rapid exchange (upwelling time less than two weeks) between waters of Liasic aquifer and the ones of Triassic origin with high radon activity.The use of PCA on hydrochemical data, allowed us to refine the kind of waters, their transit times and highlighted the existence of several mixing zones between the Triassic aquitard and the Liasic aquifers in more or less faulted structures for the two causses. Our results allow us to obtain a first conceptual model of groundwater circulations between the two causses and the Saïs basin.Previous campaigns of electrical resistivity tomography coupled with electromagnetic measurements (EM34) revealed lateral and vertical variations of electrical conductivity changing with the depth along the North-South axis, and a preferential drain perpendicularly to the causses (EGU 2009). We can note that groundwater flows are conditioned by two major fracture networks, NE-SW and NW-SE directions (Bentayeb & Leclerc 1977, Amraoui 2005). A karstic complex water circulation is especially developed locally at the border of the two hydrogeological units (Qarqori 2015)

Geophysical constraints on the groundwater circulation and its renewal at the main Bittit spring, supplying Meknes and its region (Sais Basin): Characterization & conceptual model.

International audiencePrevious hydrochemical and isotopic studies of the tabular Middle-Atlas karst showed that the Sais basin is mainly fed at its northern border by the carbonated Causses of the fractured Lias, in particular by El Hajeb causse to the east and the Guigo causse west (Miche et al, 2018).In a context of over-exploitation of the water karst system accentuated by the current climatic variations, with recurrent droughts and sparse strong thunderstorms, the karstic spring of Bittit, main water source of Meknes and its region, is strongly solicited. Since we have obtained a conceptual model using hydrochemical and isotopic data, Bittit is mainly involved by the causse of El Hajeb (1500m) and partially by the causse of Imouzzer (1340m). It is necessary to localize the depth of mixing zones and the direction of preferential flows around Bittit spring, for a better understanding of aquifers renewal. The coupling of electromagnetic sounding (EM34) and electrical resistivity tomography (ERT) show a tightening of conductivity zones according to the SE-NW direction. It means preferential flows at depth quasi-perpendicularly to the causses, contrary to what was supposed.This was confirmed by depth estimates of potential sources (drainage area in the porous structure) by using electrical multi-scale wavelet tomography of spontaneous potential (SP) around Bittit. A global conceptual model based on hydrochemical, isotopic and geophysical data is proposed

Geochemical and isotopical analyzes of groundwater in a karst system - the case study of Fez-Meknès basin (Morocco).

Plenary Flash Oral Presentation and poster.International audienceThe karst Middle Atlas Causses reservoir mainly composed of Liasic dolomitic limestone, overlyingTriassic clays, evaporates and basalts, is the main drinking water supply in Fez-Meknès region (SaïsBasin). Because of the increase in population, the number of drilling for irrigation of agricultural landand some longer drought periods in recent years, the issue of sustainable groundwater managementarises. Recent analyzes have shown a decline in its chemical quality and punctually some turbidityproblems. In order to enhance understanding of this hydrosystem and the interactions between thedifferent main springs of Liasic and Triassic origin, and their connections to the major spring (Bittit), anintegrated hydrogeochemical study was conducted coupled to isotopic analyzes of delta O-18, deltaDand Radon-222. Four surveys were performed in fall and spring seasons (2009 -2011). Watergeochemistry shows two kinds of water- a major Liasic origin along the transition between theCausses and the Saïs basin, and a major Triassic origin for El Mir, Sbaa, and Maarouf springs locatedat the southern border of Saïs basin extremities. Deuterium excess highlights a local recharge whiledelta O-18 versus delta D characterize two aqui fers with different evaporations. Radon -222,underlined the existence of areas of rapid exchange between waters of Liasic aquifer characterized bylow Radon activity (3500 Bq m3) and waters in contact with Triassic aquitard characterized by a highRadon activity (> 15000 Bq m3). The transit time is less than two weeks to surface. The watersrenewal is only conducted by major rainfalls. Four recharge zones of different altitudes have beenfound and two main mixing zones (limits of Saïs Basin)

Geophysical constraints on the groundwater circulation and its renewal at the main Bittit spring, supplying Meknes and its region (Sais Basin): Characterization & conceptual model.

International audiencePrevious hydrochemical and isotopic studies of the tabular Middle-Atlas karst showed that the Sais basin is mainly fed at its northern border by the carbonated Causses of the fractured Lias, in particular by El Hajeb causse to the east and the Guigo causse west (Miche et al, 2018).In a context of over-exploitation of the water karst system accentuated by the current climatic variations, with recurrent droughts and sparse strong thunderstorms, the karstic spring of Bittit, main water source of Meknes and its region, is strongly solicited. Since we have obtained a conceptual model using hydrochemical and isotopic data, Bittit is mainly involved by the causse of El Hajeb (1500m) and partially by the causse of Imouzzer (1340m). It is necessary to localize the depth of mixing zones and the direction of preferential flows around Bittit spring, for a better understanding of aquifers renewal. The coupling of electromagnetic sounding (EM34) and electrical resistivity tomography (ERT) show a tightening of conductivity zones according to the SE-NW direction. It means preferential flows at depth quasi-perpendicularly to the causses, contrary to what was supposed.This was confirmed by depth estimates of potential sources (drainage area in the porous structure) by using electrical multi-scale wavelet tomography of spontaneous potential (SP) around Bittit. A global conceptual model based on hydrochemical, isotopic and geophysical data is proposed

Geochemical and isotopical analyzes of groundwater in a karst system - the case study of Fez-Meknès basin (Morocco).

Plenary Flash Oral Presentation and poster.International audienceThe karst Middle Atlas Causses reservoir mainly composed of Liasic dolomitic limestone, overlyingTriassic clays, evaporates and basalts, is the main drinking water supply in Fez-Meknès region (SaïsBasin). Because of the increase in population, the number of drilling for irrigation of agricultural landand some longer drought periods in recent years, the issue of sustainable groundwater managementarises. Recent analyzes have shown a decline in its chemical quality and punctually some turbidityproblems. In order to enhance understanding of this hydrosystem and the interactions between thedifferent main springs of Liasic and Triassic origin, and their connections to the major spring (Bittit), anintegrated hydrogeochemical study was conducted coupled to isotopic analyzes of delta O-18, deltaDand Radon-222. Four surveys were performed in fall and spring seasons (2009 -2011). Watergeochemistry shows two kinds of water- a major Liasic origin along the transition between theCausses and the Saïs basin, and a major Triassic origin for El Mir, Sbaa, and Maarouf springs locatedat the southern border of Saïs basin extremities. Deuterium excess highlights a local recharge whiledelta O-18 versus delta D characterize two aqui fers with different evaporations. Radon -222,underlined the existence of areas of rapid exchange between waters of Liasic aquifer characterized bylow Radon activity (3500 Bq m3) and waters in contact with Triassic aquitard characterized by a highRadon activity (> 15000 Bq m3). The transit time is less than two weeks to surface. The watersrenewal is only conducted by major rainfalls. Four recharge zones of different altitudes have beenfound and two main mixing zones (limits of Saïs Basin)

Implication of Guigo and L'Hajeb Causses in the renewal and circulations of Saïs basin groundwaters (Middle-Atlas Causses, Morocco).

International audienceIn a context of overexploitation of the karst system of the Middle Atlas Causses feeding the Saïs basin and the current climatic variations, the study of circulations and of renewal of waters of this system in the Fes-Meknes area becomes essential for the population, in order to maintain a sufficient quality of waters with a good management.By coupling hydrochemical and isotopic analysis methods (delta-D, delta O-18, Radon-222), saturation indices obtained from PHREEQC code and principal component analysis (PCA) of ten springs and three wells, a first conceptual model of groundwater flows of this karst system was obtained . These waters are mainly renewed by the rainfall of L'Hajeb Causse and secondarly by the rainfall of Guigo Causse containing several springs. Hydrochemistry and saturation indexes allowed us to highlight two types of waters: a main contribution of Liasic origin and two low contributions of Triassic origin at the southern extremities (SE, SW) of the basin. We pointed out the existence of five local recharge zones of different altitudes (900 to 1500 m asl.) including the two main mixing zones to the south (SE, SW). Radon-222 showed areas of rapid exchange (upwelling time less than two weeks) between waters of Liasic aquifer and the ones of Triassic origin with high radon activity.The use of PCA on hydrochemical data, allowed us to refine the kind of waters, their transit times and highlighted the existence of several mixing zones between the Triassic aquitard and the Liasic aquifers in more or less faulted structures for the two causses. Our results allow us to obtain a first conceptual model of groundwater circulations between the two causses and the Saïs basin.Previous campaigns of electrical resistivity tomography coupled with electromagnetic measurements (EM34) revealed lateral and vertical variations of electrical conductivity changing with the depth along the North-South axis, and a preferential drain perpendicularly to the causses (EGU 2009). We can note that groundwater flows are conditioned by two major fracture networks, NE-SW and NW-SE directions (Bentayeb & Leclerc 1977, Amraoui 2005). A karstic complex water circulation is especially developed locally at the border of the two hydrogeological units (Qarqori 2015)

Geophysical constraints on the groundwater circulation and its renewal at the main Bittit spring, supplying Meknes and its region (Sais Basin): Characterization & conceptual model.

International audiencePrevious hydrochemical and isotopic studies of the tabular Middle-Atlas karst showed that the Sais basin is mainly fed at its northern border by the carbonated Causses of the fractured Lias, in particular by El Hajeb causse to the east and the Guigo causse west (Miche et al, 2018).In a context of over-exploitation of the water karst system accentuated by the current climatic variations, with recurrent droughts and sparse strong thunderstorms, the karstic spring of Bittit, main water source of Meknes and its region, is strongly solicited. Since we have obtained a conceptual model using hydrochemical and isotopic data, Bittit is mainly involved by the causse of El Hajeb (1500m) and partially by the causse of Imouzzer (1340m). It is necessary to localize the depth of mixing zones and the direction of preferential flows around Bittit spring, for a better understanding of aquifers renewal. The coupling of electromagnetic sounding (EM34) and electrical resistivity tomography (ERT) show a tightening of conductivity zones according to the SE-NW direction. It means preferential flows at depth quasi-perpendicularly to the causses, contrary to what was supposed.This was confirmed by depth estimates of potential sources (drainage area in the porous structure) by using electrical multi-scale wavelet tomography of spontaneous potential (SP) around Bittit. A global conceptual model based on hydrochemical, isotopic and geophysical data is proposed

Hydrochemical constraints between the karst Tabular Middle Atlas Causses and the Saïs basin (Morocco): implications of groundwater circulation

The karst Tabular Middle Atlas Causses reservoir is the main drinking-water supply of Fez-Meknes region (Saïs Basin) in Morocco. Recent analyses showed a decline in associated groundwater chemical quality and increased turbidity. To understand this hydrosystem, four surveys were undertaken during fall and spring, 2009–2011. Hydrogeochemical studies coupled with isotopic analyses (δ18O, δD and 222Rn) showed that the aquifers between the causses (mountains) and the Saïs Basin are of Liassic origin and at the southern extremities are of Triassic origin. Five recharge zones of different altitudes have been defined, including two main mixing zones in the south. Deuterium excess results suggest local recharge, while a plot of δ18O versus δD characterizes a confined aquifer in the eastern sector. 222Rn results reveal areas of rapid exchanges with an upwelling time of less than 2 weeks. A schematic conceptual model is presented to explain the groundwater circulation system and the behavior of this karst system