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.
1.0 INTRODUCTION
1.1 Background Of Study
In Nigeria today, the government is unable to meet the ever increasing water demand. Thus individuals have had to look for alternative groundwater sources such as shallow wells and boreholes. The quality of these underground water are affected by the characteristics of the media through which the water passes on its way to the underground water zone of saturation (Adeyemi et al, 2007). Thus the pollutants discharged by industries, traffic, municipal, hazardous waste sites, landfills as well as from fertilizers for agricultural purposes and accidental oil spillages from tankers can result in a steady rise in contamination of ground water (Vodela et al., 1997; Igwilo et al., 2006).
1.2 Research Problem
Individuals in rural and urban areas nationwide complement governments efforts in the water sector, by providing private water supply schemes mostly shallow boreholes and hand dug wells with their attendant high risks of contamination from waste disposal sites, septic tanks
and soak away pits, untreated industrial effluents, storm water runoffs from agricultural cultivated fields, leakages from municipal sewers etc. This study is therefore undertaken as a contribution to government mitigation measures in solving the problem of groundwater contamination that threatens the teeming population of our rural and urban dwellers living without adequate and safe drinking water, but meet their water supply requirements through underground water sources.
1.3 Scope Of Study
The study used a two-dimensional solute transport equation to investigate the spatial variation of the concentration distributions of heavy metals in the vicinity of a dumpsite by finite volume analysis. Soil samples were collected during the months of August 2012 (wet season) and February 2013 (dry season) within 80 meters radius along sampling lines from the boundaries of the dumpsite. The soil samples were analysed using atomic absorption spectrophotometer in order to determine the pollutants’ concentrations and compare same with the predicted values from finite volume analysis. A laboratory model was constructed and used to determine the transport parameters (advective velocities and coefficients of hydrodynamic dispersion). Other parameters determined included soil permeability, porosity, densities, specific gravity and moisture content. The heavy metals tested were Cu, Cr, Ni, Pb, Co, Fe, Cd, As, Zn and Mn. The minimum permissible distance to site a well on a lateritic soil from a dumpsite was also recommended.
1.4 Aims and Objectives of Study
The study is undertaken with the following aims and objectives :
(i) To determine, by field study, the variation of pollutants concentrations with distance from the dumpsite.
(ii) To model the distribution of heavy metals around the dumpsite by finite volume method.
(iii) To determine the minimum allowable or permissible distance of a water supply well from a point source of pollution.
(iv) To compare predicted pollutant concentration values with the corresponding laboratory test values.
(v) To rank pollutants in order of risk of groundwater contamination.
1.5 Significance of Study
From this study, it is possible to predict with great accuracy, the variation of concentration of pollutants in time and space from a dumpsite, and to determine the minimum permissible distance from a dumpsite to site a safe water drinking well.
2.0 LITERATURE REVIEW
Groundwater pollution may be defined as the artificially induced degradation of natural
groundwater quality. The principal sources and causes of groundwater pollution are under four categories namely municipal, industrial, agricultural and miscellaneous.
Municipal pollution may arise from sewer leakage, leacheates from landfill or dumpsites, domestic uses, industries or storm water runoff.
Industrial pollution emanates from discharge of industrial wastewaters into the environment, leakages from underground pipelines and tanks especially petroleum products and radioactive wastes, mines and mining activities.
Agricultural pollution arises from field application of fertilizers or soil amendments, irrigation return flow, animal waste, leaching and storm water runoffs. Pollution from miscellaneous sources include accidental spills and surface discharges, leakage from septic tanks and cesspools, saline water intrusion, sealing and abandonment of wells.
The physical factors considered to influence pollution include depth to water table, aquifer permeability, water table gradient and horizontal distance. Pollutants in groundwater tend to be removed or reduced in concentration with times and with distance travelled. Mechanisms involved include filtration, sorption, chemical processes, microbiological decomposition and dilution. The rate of pollution attenuation depends on the type of pollutant and on the local hydrogeologic situation (Palmquist and Sendlein, 1975). Pollutants once entrained in the saturated groundwater flow, tend to form plumes of polluted water extending downstream from the pollution source until they attenuate to a minimum quality level.
Leachates from either municipal or industrial waste disposal sites are recognized as important groundwater pollutants. The contaminates from sanitary landfills or municipal dumpsites are major concerns and are released from the refuse to the passing water by physical, chemical and microbial processes and percolate through the unsaturated environment, polluting the groundwater with organic and inorganic matter. The rate and characteristics of leachate produced depend on many factors such as solid waste composition, particle size, degree of compaction, hydrology of site, age of landfill, moisture and temperature conditions and available oxygen (Bharat and Singh, 2009). The concentration of non-conservative contaminants (primarily organic in nature) in the landfill leachate increases in the beginning, reaches a peak and declines thereafter (Kouseli-katsiri et al., 1999; Farquhar, 1980).
Industrial waste disposal sites occasioned by technological progress in industries and agriculture has put enormous pressure on our available natural resources like air, water and land. Local fired thermal power plants produce huge quantities of fly-ash (a solid residue resulting from combustion of coal) the disposal of which is linked to environmental quality issues relating to both terrestrial and aquatic ecosystems. Leaching is the most likely path by which coal bottom ash constituents would become mobile environmental contaminants.
Heavy metals can cause serious health effects with varied symptoms depending on the nature and quantity of the heavy metal digested (Adepoju Bello and Alabi, 2005). They produce their toxicity by forming complexes with proteins, in which carboxylic acid (- COOH), amine (-NH2) and thiol (-SH) groups are involved. These modified biological molecules lose their ability to function properly and result in the malfunction or death of the cells. When metals bound to these groups, they inactivate important enzymes systems or affect properties of enzymes. This type of toxin may also cause the formation of radicals which are dangerous chemical molecules.
Arsenic exposure can cause among other illness or symptoms, cancer, abdominal pains and skin lesions. Cadmium exposure produces kidney damage and hypertension. Lead is a cumulative poison and a possible human carcinogen (Bakare –Odunola, 2005) Iron can impart a bitter taste, brown stains on laundry and plumbing fixture, deposits in a distribution system and further water quality deterioration by producing slimes or objectionable odours. Copper can cause acute gastric irritation, accelerated corrosion in domestic water supply network in domestic water supply fittings. Zinc can cause corrosion of galvanized iron pipings or tanks, dezincification of brass fittings and opalescence. Manganese can cause stains in pipeline network as well as slime. Chromium can cause high blood pressure,
anaemia, liver and kidney damage. It also destroys testicular tissue, red blood cells and it is toxic to aquatic biota. Potential health hazards of cobalt include heart and liver damages when human and animals are exposed to large amounts of it over their lifetime (Waller,
1982).
The process of groundwater flow is generally assumed to be governed by the relations expressed in Darcy’s law and the conservation of mass. The purpose of a model that simulates solutes transport in groundwater is to compute the concentration of a dissolved chemical species in an aquifer at any specified time and space. Eminent scholars had at various times developed flow and solute transport equations which we have exhaustively presented in the main work. For instance Grove (1976) presented a general form of solute transport equation, in which terms are incorporated to represent chemical reactions and solute concentration both in the pore fluid and on the solid surface. Richard (1931) proposed the governing differential equation for transient unsaturated flow through soils. Van Genuchten (1980) proposed the volumetric water content function. Various modifications can be made to the equations of Grove, Richard and Van Genuchten to carter for dimensionality and other field conditions such as: transport in discretely fractured porous media, colloid facilitated transport, density driven flow and transport processes etc. Troldborg, (2010) developed a general approach for deriving water and gas phase analytical solutions describing the downward vertical contaminant transport through the unsaturated zone from a surface-near source to an underlying aquifer.
In order to estimate the impact of pollution on groundwater, the concept of mass discharge, MD, was developed. It is defined as the contaminant mass per unit time that migrates across a hypothetical control plane located downstream of the pollution source and perpendicular to the mean groundwater flow. It is calculated by integrating the contaminant concentration and the groundwater flow over the area of the control plane.
Geoelelectrical methods have an important, albeit difficult role to play in landfill investigations. Power-law relationship are found to exist between some geoelectrically important hydrochemical parameters (fluid conductivity chloride content and total dissolved solids) in leachates and leachate-contaminated groundwater for some landfill sites.
3.0 METHODOLOGY
3.1 Model Derivation
Below is the derivation of a three dimensional advective-dispersive contaminant transport equation in a porous medium based on the principle of mass balance in a representative elementary volume (REV). In this derivation, the following assumptions are made:
(i) The medium is homogenous and isotropic, this means that soil properties at any point is the same; and the permeability at any point is independent of the direction of measurement
(ii) All the fluid particles move at identical velocities along parallel streamlines through the porous medium.
(iii) Fick’s law of diffusion is applicable ,
= average velocity, obeying Darcy’s law.
= solute concentration = coefficient of hydrodynamic dispersion.
This material content is developed to serve as a GUIDE for students to conduct academic research
SPATIAL VARIATION OF THE CONCENTRATION OF HEAVY METALS IN THE VICINITY OF A DUMPSITE BY FINITE VOLUME ANALYSIS
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