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Vulnérabilité et risque de pollution du système karstique de Ghar Boumaza (Monts de Tlemcen, Nord ouest algérien) Application de la méthode RISK.

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Abstract

The karst system of Ghar Boumaza is constituted of a large network of underground galleries explored over 18km. It is drained by several sources among which the most important is A.Taga. These groundwaters are of a good physic-chemical quality used both for drinking water and irrigation. This system is very sensitive to pollution by its karst constitution. A perimeter of protection is essential to preserve these water resources. The cartography of the intrinsic groundwater vulnerability of this system was realized by applying the R.I.S.K. method (Rock aquifer, Infiltration, Soil and Karst). The resulting map has shown that more than 66% of the surface of the system is in high vulnerability area, about 5% in very high vulnerability, 29% with moderate vulnerability and a small area of 0.02% of the total surface of the system in low vulnerability. The superposition of the map of intrinsic groundwater vulnerability with the map of polluting sources established for Ghar Boumaza karstic system helped to develop a map of pollution risks in which the areas that are most threatened by the pollution are highlighted including the surrounding area of A.Taga. Water bacteriological analysis campaign of this system was performed recently which allowed discovering that some water sources are contaminated and therefore undrinkable. This allowed us to validate the obtained map. At the end of this study recommendations are made for designing protective perimeters of these groundwater catchments.
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The karst aquifers of the Tlemcen Mountains are the region’s main groundwater resource. More than 270 boreholes have been drilled in this region and have a total production capacity of 40 million m3/year. Surface karstic forms are not very developed. However, the numerous boreholes drilled in the region showed that the carbonate reservoirs are well karstified (and this to a depth of more than 500 m). The increasing demand for water, combined with insufficient rainfall over the last few decades, has led to groundwater mining and a significant drop in the piezometric level. This problem was solved by using desalinated seawater as an alternative water source to reduce groundwater abstraction. A more successful Integrated Water Resources Management (IWRM) would aim to abstract less water than the average annual recharge, by modulating abstraction according to seasonal recharge. In addition, it would be advisable to study the feasibility of artificial recharge of these aquifers during periods of heavy rainfall. Groundwater quality is generally quite good, but these aquifers are vulnerable. Mapping intrinsic vulnerability to pollution is therefore necessary to improve water resource protection. Karst vulnerability mapping methods (such as the RISK method) have proven useful in documenting decisions related to drilling location and wastewater collection and treatment. Résumé Les aquifères karstiques des Monts de Tlemcen constituent la principale ressource en eau souterraine de la région. Plus de 270 forages ont été réalisés dans cette région et ont une capacité de production totale de 40 millions de m3/an. Les formes karstiques de surface ne sont pas très développées. Cependant, les nombreux forages réalisés ont démontré que les réservoirs carbonatés sont bien karstifiés (et cela jusqu’à plus de 500 m de profondeur). La demande en eau croissante, conjuguée à l’insuffisance des précipitations au cours des dernières décennies, a entraîné la surexploitation de ces ressources et une baisse significative du niveau piézométrique. Ce problème a été résolu en utilisant l’eau de mer dessalée comme source d’eau alternative afin de réduire les prélèvements d’eau souterraine. Une gestion intégrée des ressources en eau (GIRE) plus aboutie viserait à prélever moins d’eau que la recharge interannuelle moyenne, en modulant les prélèvements en fonction de la recharge annuelle. De plus, il serait souhaitable d’étudier la faisabilité de la recharge artificielle de ces aquifères pendant les périodes de pluies abondantes. La qualité de l’eau souterraines est généralement assez bonne, mais ces aquifères sont vulnérables. Une cartographie fine de la vulnérabilité à la pollution intrinsèque est donc indispensable. Les méthodes de cartographie de la vulnérabilité du karst (telles que la méthode R.I.S.K.), se sont révélées utiles pour documenter les décisions relatives à l’implantation des forages et à la collecte et au traitement des eaux usées.
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