GEOPHYSICS AND PHYSICO-CHEMICAL COUPLED APPROACH OF THE GROUNDWATER CONTAMINATION. APPLICATION IN A POLLUTION BY THE LANDFILL LEACHAT OF OUJDA CITY (EASTERN MOROCCO)

Abstract

The urban waste produced by Oujda city, mixed with solid waste of industrial and hospital origin, was disposed since 1990 until 2005 on an open-air landfill situated approximately7 kmsoutheast of the city center. The groundwater circulates deeply (25-30 m) in grounds post-Miocene on an impermeable substratum formed by the marls of Miocene. The ground is calcimagnesic not very deep, with a more or less important layer of silts and clays.

To evaluate the impact of the leachate resulting from this landfill on groundwater quality, samples of leachate and groundwater were taken during the year 2005 (five samples of leachate inside the landfill) (three groundwater samples were taken from three wells located near the landfill) and two profiles of recognition by electric imaging were realized according to the device pole–pole two dimensional (2D) inside the landfill. The analyses of the sample leachate revealed   strong content of Chemical oxygen demand (COD max = 30760 mg/l), biodegradable organic matter (DBO₅ max = 653.3 mg/l) and of mineral matter (conductivity: max = 63.73 mS/cm). The analysis of groundwater revealed that NO₃^- presents an average concentration higher than 50 mg/l (Standard suggested by WHO). The concentration of chloride exceeds 1450mg/l (Standard suggested by WHO is 600 mg/l for fresh waters). Concerning heavy metals, the concentrations of Iron, Zn and Ni largely exceed the French values standards fixed for groundwater, while the Cd contents are lower than this standard.

The results of the geo-­­­electric prospecting showed that the very low electric resistivities (5 ohm.m), were observed in the inside and in the direct approval of the landfill site. These results highlight zones electrically very conductive, they indicate the progress of the pollution plume in the under ground. The central part of the landfill site is the place under which the contamination affected depths of 20m. These data plead in favour of a strong mineralization of waters circulating in the trainings approached the site, whose potential source is bound to a contamination by leachate very mineralized and charged in organic matter