Caractérisation physico-chimique, toxicologique et microbiologique des eaux du cours moyen du fleuve Nyong et de l’estuaire du fleuve Wouri ; essai de biodégradation du Métalaxyl-M

Abstract

Pollution disrupts the balance of aquatic ecosystem and reduces the possibilities for water use. Human activities are among the main cause. In order to contribute to the preservation of aquatic environments, a study was carried out on the physico-chemical, toxicological and microbiological characterization of the waters of the Nyong and Wouri rivers. Thus, some physicochemical parameters were measured according to standard protocols for water analysis. The heavy metals such as Lead, Copper, Nickel, Cadmium were determined by X-ray fluorescence spectrometry. The pesticide Metalaxyl-M, used extensively in cocoa farms in the watersheds of the two rivers studied, was tested by high performance liquid chromatography. The heterotroph bacteria were isolated and identified according to the usual techniques of bacteriology. The impact on the health of riparians of the quality of these waters was determined using the WHO guide for recreational waters. The degradation capacities of Metalaxyl-M by some of the isolated bacteria were also evaluated. As main results, the water temperature of the two rivers (25,2 °C ± 0,9 °C and 27,9 °C ± 2,2 °C respectivly for Nyong and Wouri rivers) met the national standard but exceeded the international standard. The values of pH, electrical conductivity, total dissolved solids, suspended solids, dissolved oxygen and nitrates were less than or equal to the threshold values for both standards, while the colour was well above these (212,9 Pt-Co and 208,9 Pt-Co respectivly for Nyong and Wouri rivers). Heavy metal concentrations in Nyong waters were lower than the thresholds for both standards, whereas they were, except for Nickel, higher than the normal values for Wouri river (14,9 µg/L ± 9,7 µg/L for Lead, 124,3 µg/L ± 87,1 µg/L for Copper and 2,6 µg/L ± 4,5 µg/L for Cadmium). Levels of Metalaxyl-M were well above the standards (4,1 µg/L for Nyong river and 11 µg/L for Wouri). Around fifty bacterial species were isolated from Nyong and Wouri rivers including feacal contamination indicators. The genera Bacillus, Staphylococcus, Aeromonas and Pseudomonas were the most isolated in the Nyong river, while the genera Vibrio and Pseudomonas were the most isolated in the Wouri river. Skin infections, followed by febrile gastroenteritis or not, were the most recorded ailments. Bacillus coagulans, Pseudomonas fluorescens, Chromobacterium violaceum and Chryseobacterium indologenes were selected for the Metalaxyl-M degradation test. For all these bacteria have been shown to be able of degrading the pollutant. In fact, their concentration increased from the 2nd day and reached a maximum value on the 10th day. The metabolite appeared on the 2nd day, its concentration steadily increased and reached a maximum value on the 10th day. This bacterial action is markedly improved when the species form a consortium. This study highlighted the presence of various pollutants in the water of the Nyong and Wouri rivers, the sanitary need to handle them with caution and the possibility of using bacteria to resolve the thorny public health problem of surface water pollution.