Influence de la marée sur la qualité des eaux du bas Sebou soumis aux rejets de la plaine du Gharb (Maroc)

Abstract

Demographic, industrial and agricultural development of Morocco during the last few years have often altered water quality. The Gharb district and particularly the Sebou river basin are among the most heavily polluted regions in Morocco. Oued Sebou collecting basin is 40,000 km 2, situated in north-western Morocco. Its water flows from the north-western slopes of the Middle Atlas range to the Atlantic Ocean. Its tributaries collect waters from the Rif range (Oued Ouerrha) and from central Morocco (Oued Beht). It is the main purveyor of water of the Gharb plain, a major agricultural region in Morocco, where fertilizers and pesticides are widely used. Along its banks, various industries (paper mills, sugar plants, tanneries, food industries, wool mills, chemical plants, etc.) have settled and contribute to the development of large towns which are not equipped with facilities for the treatment of either industrial or domestic waters. It is a real example of water pollution in a developing country. In order to assess the actual water quality of lower Sebou and to monitor both the effect of the release of pollutants and the influence of other factors, a wide pluridisciplinary research program on the biological, ecotoxicological and microbiological situation is in progress. Eleven stations have been selected: one on Oued Ouerrha (S1), one on Oued Beht (S6), one on Oued R'dom (S5) and eight on Oued Sebou, distributed along about 230 km (Fig. 1). The following paper reports on the physico-chemical quality of surface waters of lower Sebou, submitted to domestic, agricultural and industrial pollution and to tidal movements. Samples were analyzed for 17 parameters following AFNOR norms; 5 were determined on site (temperature, oxygen, pH, conductivity and salt content) while the others (DBO, DCO, MES, NH + 4, NO − 2, NO − 3, Cl 2− 4, SO 2− 4, PO 2− 4, Ca 2+, total hardness and alkalinity) were determined in the laboratory. Temperature does not vary significantly between different stations. Suspended matters levels were mainly dependent on hydrologic sequences of the rivers (low or high waters) and showed a wide range of seasonal changes. Because of decantation, the levels decreased from upstream to downstream, but after Kénitra, they increased again, because of solid wastes from the town and of a strong bottom tidal flow that pulls out deposited solid matter. Organic pollution is severe. Means of BOD 5 vary from 19 to 76 mg/L and of COD from 39 to 180 mg/L. They can reach 135 mg/L for BOD 5 and 834 mg/L for COD. The worst situation is found downstream to Sidi Allal Tazi sugar plant. For dissolved oxygen, the lowest levels were found downstream to the town of Sidi Slimane (22%) and to Sidi Allal Tazi sugar plant (38%). Ammonium was mostly low (0–0.09 mg/L) while at downstream stations (8, 9, 10, 11) there was a small increase, as well as at S6 (downstream to Sidi Slimane town). The presence of ammonium nitrogen is usually considered as figuring an uncompleted degradation of organic matter (Rodier, 1984). Nitrites followed the variations of ammonium. Nitrates showed a wide range of variations, possibly because of strong turbulence enhancing locally the oxygenation. In order to determine which were the main parameters that drove the ecosystem dynamics, a main components analysis was performed on the matrix given by the results of 8 campaigns at 11 sampling stations. The results are given in Table 1 and Fig. 5. It appears that tidal movements, and thus salt levels, are main factors upstream to Kénitra and downstream to Sidi Allal Tazi. The tidal phenomenon has been specifically studied (Fig. 5D and E). The confrontation of fresh and sea water in the Sebou mouth gives birth to a physical stopper which prevents polluted water from reaching the ocean and thus slows the improvement of water quality. Oued Sebou gives a good example under Mediterranean weather conditions of a severe pollution due to hydrochemical conditions strongly influenced by municipal and industrial wastes. Most of the physico-chemical parameters are higher than the values given by Nisbet and Verneaux (1970). Moreover, downstream, tidal influence contributes to further damage water quality. Efforts should be made to reduce toxic substances release in the Gharb plain.