Impact de la décharge publique de Marrakech (Maroc) sur les ressources en eau

Résumé.

Les lixiviats de la décharge publique de Marrakech, qui ne sont ni collectés ni traités, pourraient constituer une source potentielle de pollution des eaux de la nappe phréatique et de celles de l'oued Tensift situé près de la décharge. Le but de cette étude est de caractériser ces lixiviats et de déterminer le degré de pollution des eaux à proximité de la décharge. Les analyses physico-chimiques des lixiviats ont montré qu'ils présentent des concentrations élevées en Cl^–, HCO₃ ^–, Na⁺, K⁺, SO₄ ^2–, Ca²⁺, Mg²⁺, NH₄ ⁺, Cu et Pb. La pollution organique est également importante; la DCO peut atteindre 138.000 mg/l O₂. Les concentrations en certains polluants sont nettement supérieures à celles généralement rencontrées dans d'autres décharges d'ordures ménagères. Le suivi de la qualité des eaux durant l'année 1999, a montré que les eaux souterraines et celles de l'oued Tensift sont de mauvaise qualité pour les besoins domestiques et pour l'irrigation. L'Analyse en Composantes Principales a montré que la pollution des eaux, par les lixiviats, est essentiellement due à la matière organique, HCO₃ ^–, NH₄ ⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^– et SO₄ ^2–.

Abstract.

The Marrakech landfill, Morocco, is a traditional landfill that receives, without rigorous management, household wastes and other solid wastes (industrial, medical, etc.). This landfill, started in 1987 and still in use, receives 250.000 tons of waste annually. The landfill area is 9 ha and has a high topographic gradient. Geological and hydrogeological studies show that shales and granite form the two aquifers of the region and have a hydraulic continuity. The water table in these aquifers is relatively shallow (6–10 m). The landfill is situated on the shales and the Quaternary sediments. The permeable character of the cover and the broken state of shales could facilitate infiltration of the landfill leachates. The purpose of this study was to analyse the Marrakech landfill leachate and to investigate the potential for groundwater and surface water pollution. Physico-chemical analysis of leachates from the Marrakech landfill shows that the pH varies from 5,8 to 8 and the conductivity is very high (26,1–74,6 ms/cm). Because of the semi-arid climate and the different kinds of wastes deposited, the pollutant concentrations are higher than those found in other landfills. The ions, were found to have very high concentrations: Cl^–: 3.203–27.500 mg/l; HCO₃ ^–: 2.600–35.000 mg/l; SO₄ ^2–: 877–6.200 mg/l; Na⁺: 1.780–20.000 mg/l; K⁺: 782–14.100 mg/l; Ca²⁺: 505–6.000 mg/l; Mg²⁺: 545–1.560 mg/l; NH₄ ⁺: 417–5.518 mg/l (accounting for about 80% of the total nitrogen); Cu: 0,5–16,5 and Pb: <0,07 to 30,5. In the same way, organic pollution is also high; the chemical oxygen demand (COD) ranged from 26.880 to 13.8240 mg/l O₂, and the biological oxygen demand (BOD) from 9.200 to 26.000 mg/l O₂. The BDO to COD ratio varied from 0,13 to 0,38 because of the high concentration of biologically refractory organic matter, indicating that the landfill is in the methanogenic phase. Chemical analysis of surface water and groundwater samples, collected respectively from Tensift River situated in the vicinity of the landfill and in the granitic-shales aquifer, have revealed spatial as well as temporal changes in the chemical properties of the groundwater and surface waters. The temporal changes are attributed to dilution and concentration phenomena governed by climatic factors. Principal component analysis indicates that the pollution of the water from the wells situated near the discharge is probably caused by the superficial drainage and infiltration of the landfill leachates. This pollution is essentially of organic matter, ammonium, bicarbonate, sodium, calcium, magnesium, chloride and sulphates.