An Appraisal Of The Impact Of Leachates From A Waste Dumpsite On Coastal Soils And Groundwater System Of The Metropolitan Area Of Lagos, Southwestern Nigeria

Open dump being the common waste disposal practice in most of the urban centres has posed serious challenges to groundwater quality especially within its vicinity. This study assessed the impact of solid waste disposal on the quality of groundwater and soils in Oke-Odo area of Metropolitan Lagos, Nigeria. Field physicochemical parameters such as pH, electrical conductivity (EC), total dissolved solids (TDS) and temperature were measured using PC Testre Model 35. A total of 36 water samples, two leachate samples and 15 soil samples were analyzed for their major ions and trace metal components using titration and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) methods. Results revealed a decrease in concentrations of trace and major elements with respect to their distance from the waste dumpsite. A decrease in concentrations of Fe was observed with distances from the dumpsite with values ranging from 12.6 to 0.03mg/l and 4.3 to 0.03mg/l for samples within 500 m and 500-1,500 m, respectively. With exception of a few locations having higher concentrations of Fe, Al, Br, Cu, Mn and Pb, the groundwater sources in the study area are chemically potable with respect to their chemical compositions and generally conform to the World Health Organization and the Nigerian Standard for drinking Water Quality standards for drinking water; and thus, fit for domestic purposes. Sodium adsorption ratio ranges between 0.82 and 5.80 %, indicating low sodium and medium to high salinity hazards. This implies suitability of the water for irrigation purpose. The leachate from the active part of the waste dumpsite showed high concentrations of heavy metals such as Pb (4 mg/l), Cd (0.7 mg/l), Zn (6.54 mg/l) and Cu (3.57 mg/l), which indicates a potential environmental risk. The absence of these metals in the groundwater can be attributed to the attenuating role of the soil above the water table as confirmed by higher concentration of the measured trace metals in comparison to the control sites, with values ranging from 280 to 636 mg/l for Pb, 2 to 9.9 mg/l for Cd, 1041 to 10,000 mg/l for Zn and 102 to 1,307 mg/l for Cu. A total of 66.6 % of soil samples havevery high degree of contamination while 33.3% is considerably contaminated. Hydrochemical characterization of the water revealed predominant Na-K-Cl end-member water type and Ca-Mg-Cl water type which could be attributed to the influence of leachate from the waste dumpsite and possible contribution of seawater from the adjoining lagoons and cation exchange process. This study has revealed that soils and groundwater quality within the vicinity of Oke-Odo dumpsite has been impacted minimally.