Résumé:
Trace metals (TM) are ubiquitous contaminants of the aquatic environments, resulting directly or indirectly from the chemical alteration and mechanical erosion of source rocks, to which are added another origins anthropogenic such as: agricultural, industrial and domestic. TM can accumulate in the sedimentary compartment in dissolved or especially in particulate form, a veritable well for pollutants, they contaminate the plant and animal organisms living in the environment. The lack of data on the ecological health of the Koudiet Medouar dam has motivated our interest in its investigation. The aim of this study is to assess the level of recent contamination by trace metal elements of the surface sediments of the KM dam and its main tributary Oued Rebaa, and their transfer to fishery products, as well as the hydrobiological quality of this ecosystem. This study is carried out on 9 stations, 6 of which are selected from upstream to downstream of Oued Rebaa and 3 at the KM dam. Eight sampling campaigns covering in situ physical measurements of water, sediment sampling, macroinvertebrate and fish were carried out over 2 years from September 2012 to July 2014. The analytical approach concerned the physicochemical characterization of sediments (water content, organic matter, pHs, electrical conductivity and total limestone, determined on the fraction less than 2 mm) and their contents of 8 TMEs (Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn on the fraction less than 63μm) determined by Flame Atomic Absorption Spectrophotometry. The transfer of TM was determined in the gills, liver and muscles of 4 fish species, Hypophthalmichthys molitrix, Cyprinus carpio, Barbus sitifensis and Alburnus alburnus. The biological approach focused on the biodiversity of the benthic macroinvertebrate and the calculation of the biotic index (BI) reflecting the ecological quality of the environment. The sediments of the KM dam and wadi Rebaa, which are slightly alkaline, are characterized by high electrical conductivity, medium organic matter and high carbonate load. The pseudo-total concentrations of TM in the sediments show the following order of abundance: Mn > Zn > Cr > Cu > Co > Pb > Ni > Cd. The spatial and temporal distribution of TMs shows natural background levels of Co, Pb, Ni and Mn. Contamination by Cd, Cr, Cu and Zn is highlighted and affects all stations. The geoaccumulation index (Igeo), contamination factor (CF), pollution load index (PLI), sediment pollution index (SPI) and degree of contamination (Dc) reveal polymetallic contamination dominated by two or more elements at stations where fish swim through the dam; Cd, Cr, Cu and Zn being of greatest concern. The same results are confirmed by the ecological risk factor (Re), the ecological risk index (IR) and the toxicity criteria (TEC: Threshold Effect Concentration and PEC: Probable Effect Concentration) which show that Cd, Cr, Cu and Zn are the elements concerned by frequent and/or occasional effects on benthic fauna. The hydrobiological quality of wadi Rebaa and the KM dam, expressed by the Biotic Index, reflects a remarkable disturbance situation, illustrated by an impoverishment of the faunal biodiversity and the reduction of the IB from 10/10 to 2/10 from upstream to downstream. The quality of the fishery products is assessed through the level of contamination of the fish fauna of the Koudiet Medouar dam. The hierarchy of mean TM levels in Hypophthalmichthys molitrix, Cyprinus carpio, and Alburnus alburnus follow the following sequence: Zn˃ Mn˃ Cr˃ Cu˃ Pb˃ Co˃ Ni˃ Cd. While Barbus sitifensis, the order of abundance is as follows: Zn˃ Mn˃ Cr˃ Cu˃ Co˃ Pb˃ Ni˃ Cd. Levels determined in the gills of C. caprio and H. molitrix exceed critical values for Zn contamination, whereas the liver of C. caprio and B. setifensis accumulates Cd. The critical values for Cd in the muscle of A. alburnus and the risk it poses to human health make it unfit for consumption. The level of accumulation of Cd increases with the age and size of the fish.
