Spatial Variation of the Concentration of Heavy Metals in the Vicinity of a Dumpsite by Finite Volume Analysis

ABSTRACT

The study aimed at investigating groundwater pollution in the vicinity of a municipal solid waste (MSW) dumpsite in order to reduce the risks of groundwater contamination and spread of water-borne diseases. To achieve this, the research tools used include model formulation, finite volume analysis, field collection of soil samples at Enugu Waste Management Authority (ESWAMA) dumpsite at various depths along radial sampling lines during wet and dry season periods. Heavy metals (Cu, Fe, Zn, Pb, Cd, Cr, As, Ni, Co, and Mn) concentrations at various distances and depths were determined using an atomic absorption spectrophotometer (AAS model: AA320N). Soil samples were also analysed in the soil mechanics laboratory of the Department of Civil Engineering in the University of Nigeria, Nsukka for soil parameters viz. moisture content, densities, specific gravity, porosity, permeability, advective velocities, dispersion coefficients etc. By the method of finite volume analysis, one symmetric half of the study area was discretised into 160 nodes and all the nodal concentrations were determined through MATLAB solution of a 160 x 160 square matrix (for each heavy metal) arising from a two-dimensional solute transport equation. The predicted heavy metal concentrations from finite volume analysis were then compared with the laboratory results from field investigations. Baseline concentrations (ppm) of the heavy metals increased in dumpsite soils as follows: Cu: 0.167 – 1.351, Fe: 0.043 – 1.558, Zn: 0.257-0.688; Pb: 0.26 – 1.082; Cd: 0.267 – 1.448; As: 0.093 – 0.776; Ni: 0.057 – 0.444, Co: 0.267 – 1.448; Mn: 0.01 – 0.1403. The field results showed marked differences between the minimum and maximum heavy metal concentrations (ppm) respectively as follows Cu: 0.03 and 1.244; Fe: 0.01 and 2.82; Zn : 0.05 and 1.727; Pb: 0.072 and 1.43; Cd: 0.01 and 0.77; Cr: 0.01 and 0.422; As: 0.01 and 0.99; Ni 0.01 and 0.97; Co: 0.01 and 1.90; Mn: 0.01 and 0.39. From the finite volume analysis, the minimum permissible distance from the dumpsite required to site a well and the coefficient of correlation of the curve were computed and showed respectively as follows: Cu: 350m, 0.593; Fe : 140m, 0.583; , Zn; 2816m, 0.573; Pb: 833m, 0.59; Cd: 263m, 0.596; Cr: 470m, 0.570; As: 328m, 0.595; Ni: 351m, 0.594; Co: 550m, 0.590; Mn: 185m, 0.597. Both field results and finite volume analysis showed that the concentration of pollutants decreased with distance and depth from the dumpsite. The recommended minimum permissible distance from the dumpsite to site a well was 2.82 kilometers which corresponds to the distance from the most persistent heavy metal. Long term dumping of municipal wastes can increase the risks of groundwater pollution and spread of water-borne diseases and therefore continuous assessment and control measures should be put in place.