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SPATIAL VARIATION OF THE CONCENTRATION OF HEAVY METALS IN THE VICINITY OF A DUMPSITE BY FINITE VOLUME ANALYSIS

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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  â€“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.



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SPATIAL VARIATION OF THE CONCENTRATION OF HEAVY METALS IN THE VICINITY OF A DUMPSITE BY FINITE VOLUME ANALYSIS

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