3 Abstract: One of the major problems in hydrogeology investigations of arid and semi-arid regions is the determination of water that contributed to the recharge of the water table. By considering the hydrodynamic characteristics of the Quaternary aquifers in the Southern sector of the Lake Chad basin, this study has permitted to obtain a quantitative evaluation of the groundwater resources available in a selected area of about 4000 km . The estimated regulating reserve ranges between 0.1 to 0.5 x 10 m /year. Monthly and yearly 2 93 estimation of the water budget, for a period of 16 years, according to the Thornthwaite-Mather method, shows that the main source of regulating reserve is the August rain. For a dry water year (like 1984), there is no effective rainfall, while in a year with a favourable rainfall, for example, we could have effective rainfall ranging between 10 and 15% that could improve the groundwater recharge.
In previous studies, the mean geothermal gradient of the Tlemcen region has been estimated as low (3.7°C/100m). The existing boreholes in the Remchi plain (North of the Tlemcen Mountains) reveal thermal waters (26-48°C) and a remarkable artesianism. To understand the hydrodynamics and origin of thermalism of this groundwater, we merged geophysical and physical-chemical data. Geophysical data allowed us to improve the resolution of the fault network, to estimate the thickness of various sandstone beds connected with the thermal reservoir and to delimit the productive aquifer. Chemical characteristics of waters have been used for a statistical analysis, which displayed two independent water clusters resulting from two hydrodynamic patterns throughout the aquifer. The hydrothermal analysis identifies the depth reached by water and the relationship between water circulation and the fault network. Comparison with previous studies allowed us to build a schematic geological model, which explains hydrodynamics, origin and role of this structure in the groundwater flow.
Other| July 01, 1986 Influence des pluies estivales sur la qualite des reserves de l'aquifere karstique; role du sol et de l'epikarst dans la concentration des chlorures B. Blavoux; B. Blavoux Author Univ. Avignon, lab. hydrogeol., Avignon 84000, France Search for other works by this author on: GSW Google Scholar J. Mudry J. Mudry Author Search for other works by this author on: GSW Google Scholar Bulletin de la Société Géologique de France (1986) II (4): 667–674. https://doi.org/10.2113/gssgfbull.II.4.667 Article history first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation B. Blavoux, J. Mudry; Influence des pluies estivales sur la qualite des reserves de l'aquifere karstique; role du sol et de l'epikarst dans la concentration des chlorures. Bulletin de la Société Géologique de France 1986;; II (4): 667–674. doi: https://doi.org/10.2113/gssgfbull.II.4.667 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyBulletin de la Société Géologique de France Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.
RésuménL'aquifère karstique de la célèbre Fontaine de Vaucluse a fait l'objet de nouvelles investigations hydrogéologiqurs dont les résultats portent sur la définition et le fonctionnement du système.Les données géologiques (lithologie, structure) nous ont permis la délimitation d'un impluvium de 1115 km2 dont l'altitude moyenne. 870 m, est confirmée par les traceurs naturels physico-chimiques et isotopiques.Le bilan hydrique, faute d'un réseau météorologique représentatif, a été calculé à partir d'un modèle climatique par tranches d'altitude, qui utilise les gradients pluviométriques et thermiques régionaux. On montre alors que les 3/4 des pluies efficaces proviennent des zones situées au-dessus de l'altitude moyenne de l'impluvium.Les études hydrodynamiques et physico-chimiques nous ont permis de démontrer la faible inertie du système malgré sa taille, donc sa bonne karstification, ce qui confirme à l'échelle de l'ensemble les observations spéléologiques ponctuelles du plateau et de l'émergence.Malgré leur faible apport au débit total, les précipitations estivales marquent la chimie de l'émergence et interrompent ainsi la phase de tarissement.La contribution de l'eau des réserves au débit est prépondérante : respectivement les 3/4 et les 2/3 des volumes annuels écoulés en 1981 et 1982.Compte tenu de l'absence de sols et de terrains poreux de couverture, ainsi que du développement de la karstification, la vulnérabilité du système est grande. Aussi une attention particulière doit-elle être portée à l'aménagement de la zone montagnarde de l'impluvium dont le rôle est prépondérant dans l'alimentation du système.AbstractThe karst aquifer of the well-known Fontaine de Vaucluse has been recently studied, results have been got about delimitation of the system and its working.Geological data (lithology and structurel have allowed to delimit an 1115 Km2 intake area including Ventoux-Lure north facing range (1,909-1.826 m) and the Plateau which is prolonging it southwards (Fig. 1 and 2). The average altitude of the whole area, obtained by balancing elevation belt surfaces, is about 870 m. This elevation squares with results of tracing tests (Fig. 3), environmental physical, chemical and isotopic tracings, that allow to value a 850 m average altitude for the intake area (Fig. 4).The moisture balance has been computed from an altitude belts climatic model, using local rain an temperature gradients (Fig. 5 and Table II), because the wheather network is not representative. So. rainfalls rise of about 55 mm per 100 m elevation and temperature decreases of about 0.5 0C per 100 m. The consequence of these two antagonist phenomena is the quasi constant value of actuel evapotranspiration on each altitude belt.With the Fig. 7 organigram, curves of effective rainfalls and infiltration coefficient versus elevation can be plotted (Fig. 6). This computation shows that 3/4 of the total and the whole of dry season effective rainfalls are provided by the part of the intake area situated above the average altitude : on the lowest belt, effective rainfalls are only 120 mm per year and increase to 1380 mm on the upper section (Fig. 8 and Table I). The weighted effective rainfalls are about 5 70 mm per year for the whole intake area.Hydrodynamical and physico-chemical studies show, despite its large size, the weak inertia of the system, so proves its good karstification, that confirms for the whole system the pin-point speleological observations. The discharge of the spring, which average value is 21 m3.s-1 (only 18 for the last ten years), can exceed 100 m3.s-1 and the minimum has never been lower than 3.7 m3.s-1 (Fig. 9). When it rains on the intake area, the increase of the discharge is very sudden in a rainy period : one to four days. This short delay is due to seepage through epikarst and unsaturated zone. During dry periods, the spring reaction is deadened, due to storage in the unsaturated zone. The silica content distribution was plotted during several hydrokinematical phases (Fig, 10). It shows : — an almost unimoilal distrihution for the 8 km2 fissured limestone aquifer of Groseau ;— a multimodal one for the 1115 km2 karst aquifer of Fontaine de Vaucluse.This proves that karstification is more important than size in the response of the system.Weak summer rainfalls do not influence the discharge, nevertheless they influence chemistry of the spring water, and so interrupts the water depletion phasis. Then, the decrease of discharge can continue after the end of the chemical depletion phasis, water which is overflowing after summer rainfalls (in a dry period) is influenced by the chemistry of seepage water : on the graph of a principal components analysis, done on chemical variables, an hysteresis phenomenon can he seen (Fig. 11). Λ discriminant analysis (Fig. 12) confirms that these autumn waters, with high ratio seepage tracers, are not reserve waters from the saturared zone.The ratio of reserve water in the total discharge is preponderant : 3/4 and 2/3 respectively of the yearly runoff volumes for 1981 and 1982 (Fig. 13), but an important part of these reserves ean be stored in the unsaturated zone.This storage capacity can be valued by different means: transposing to Vaucluse (1115 km2) the volume measured on another karst system in the Pyrenees (13 km2); it gives about 100 million m2;using setting parameters of Bezes model (1976) on the same aquifer : it gives 113 million m3;using depletion curves, that show, for instance during the 1989 summer and autumn dry period, a 80 million m3 volume.In all cases, we get a value of about one hundred million m for the storage capacity of the unsaturated zone.With a 20 m range of fluctuation for the water table and with a 10-2 specific yield, on a 500 to 1,000 km2 saturated zone, the zone of fluctuation can release about 10 to 20 mil lion m3.Then, the volume of water stored in the whole saturated zone, with a 300 m minimum thickness (depth of the waterlogged pit of the Fontaine), a 500 km2 minimum surface and a 10-3 specific yield, is about 150 million m3 including 2 7 million m3 stored in the channels.So, the unsaturated zone represents a significant part of the whole storage capacity and most of the yearly renewable reserves.Paradoxically, the biggest french spring is not tapped at all ; as its intake area is neither a regional nor a national park, no general protection covers it : because of its good karstification, the vulnerability of the system is important.Good quality of water is attributable to the low population and human activities density on the intake area (4 inh.km2). Λ great part of the intake area is uncultivated (large forest and "garrigues" areas). Due to the lack of surface water and scantncss of soils, agriculture not intensive (lavender, thyme, sage and bulk wheal fields ; meadowlands).On the mountainous zone, roads are salted in winter and snowmelt water can reach a significantly high chloride ratio than in a natural climatic functioning (for instance 2 5 mg.l-1in Font d'Angiou where the ratio would have been 3 mg.l-1).As tourism is developing both on the mountain and on the plateau, the management of the highest intake area must be carefully held : its part is preponderant in the feeding of the system.Mots-clés: KarstImpluviumBilan hydriqueHydrochimiqueHydrocinémiatiqueProtectionKeywords: KarstIntake areaWater budgetHydroki-nematicsHydrochemistryProtection
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