Estimation de la qualité des eaux du Nahr Beyrouth. Utilisation de l'indice diatomique de polluo-sensible (IPS)

Estimation of water quality of Nahr Beirut using polluo-sensible diatomic (IPS) indicator

¹ Commission Libanaise d'Energie Atomique, CNRSL. P.O. Box: 11-8281, Beyrouth, Liban
² Université Libanaise, Faculté des Sciences

Résumé

Pour fournir une estimation fiable de la qualité des eaux d'une petite rivière libanaise le Nahr Beyrouth, nous avons entrepris une étude physico-chimique et algologique. Parmi les algues, les diatomées constituent l'un des meilleurs groupes bioin-dicateurs des eaux de surface continentales. Elles réagissent directement à la pollution organique puisqu'elles présentent des exigences écologiques spécifiques. L'utilisation des indices diatomiques et surtout l'indice de polluo-sensibilité (IPS) est largement répandu pour les réseaux hydrographiques français. Cette métrologie appliquée dans notre biotope montre que l'amont parait d'une bonne qualité et se détériore aux environs de la ville de Beyrouth.

Abstract

The quality of Nahr Beirut's water, supplying water to Beirut and the suburbs, has been studied. A complete algology and chemical study hase been conducted. The diatoms are considered one of the dest bioindicators of the water's surface because they react directly with organic pollutants and they need a very close ecological medium. The use of some diatomic indicators and especially the polluo-sensible ones (IPS) is widespread in the french hydrographic network. The methodology applied in our biotope showed good results around the head-water and dramatic deterioration around Beirut city. Water samples were collected periodically and analyzed for HCO³, Cl, and HNO³, and for the major ions / Ca, Mg, Na and K, and for some trace elements. Atomic absorption spectrometry using flame acetylene was used fo measure the alkaline metals, whereas graphite furnace was used for the defermination of trace elements. It has been found that the major composition in rivers is predominantly influenced by the composition of parent rocks in Lebanon. The slightly elevated values of pH (7.7) in the rivers and the large amount of carbonate HCO³, result from dissolution of calcium carbonate CaCO³, dolomite CaMg (CO³)² and limestone. These compounds lead to elevated concentrations of Ca and Mg in the studied stations. The concentrations of the major ions varied with the cationic dominance order of Ca > Mg > Na > K, which is compatible with the common natural major ion assemblages established for world rivers. The concentrations of the major ions are also predominantly influenced by the chemical weathering of rocks and minerals. The mean concentration of nitrates increased in urban and agricultural areas. the mean concentrations of trace elements in the freshwater increased with increasing distance from the headwater, particularly at Ain el delbe and the end station of Jisr Bacha, a consequence of anthropogenic inputs. In fact the above mentionned stations are located in industrial and popular areas.