Ground water resources have been under rapidly increasing stress in large parts
of the world due to pollution. Pollution is primarily the result of irrigated
agriculture, industrialization and urbanization, which generates diverse wastes,
with the attendant impact on the ecosystem and ground water. Wastes may be loosely
defined as any material that is considered to be of no further use to the owner
and is, hence discarded (Allen, 2001). Waste is generated
universally and is a direct consequence of all human activities. They are generally
classified into solid, liquid and gaseous.
In Port Harcourt municipality, solid wastes are mostly deposited into open
dump landfills. This is because landfill is the simplest, cheapest and most
cost-effective method of disposing wastes (Barrett and Lawler,
1995). The landfills are poorly conceptualized in design with no adequate
engineered systems to contain landfill emissions. They are indiscriminately
sited within the municipality without regard to the nature of soil, hydrogeology
and proximity to living quarters. The most dominant wastes in the study areas
are mainly decomposing household, municipal and hazardous (toxic) industrial
wastes (Ehirim et al., 2009a).
The study area is characterized by the proximity of the aquifer to the surface,
high annual precipitation, permeable soil which enhances seepage flow and flat
topography. The implication of the nearness of the aquifer to the surface is
that short traveling time in the unsaturated (Vadose) zone do not allow enough
time for landfill emissions to be consumed before they get into the aquifer
(Ehirim et al., 2009b).
The importance of investigating the vulnerability of the aquifer to surface
pollutants due to solid waste landfills is eminent to the availability of portable
water resources for both present and future generations. This is especially
important due to the nearness of the aquifer to the surface and the permeable
soil media (Ehirim and Nwankwo, 2010). The concept of
aquifer vulnerability derives from the assumption that the physical environment
may provide some degree of protection of groundwater against human impacts,
especially with regard to pollutants entering the subsurface (aquifer). Aquifer
vulnerability thus combines the hydraulic inaccessibility of the saturated zone
to the penetration of pollutants with the attenuation capacity of the strata
overlying the saturation zone (Foster, 1998).
The electrical resistivity method is a unique geophysical tool used in ground
water and landfill studies (Zohdy, 1964; Dahlin
and Zhou, 2002; Rosqvist et al., 2003). The
resistivity method is used for electrical sounding and imaging. The electrical
sounding provides information about vertical changes in subsurface electrical
properties and thus, it is useful in the determination of hydrogeologic conditions
such as the depth to water table, depth to bedrock and thickness of soil (Zohdy,
1964). The electrical resistivity imaging maps ground water contaminant
leachate plumes, contaminant source, migration paths and depth (Griffiths
and Barkers, 1993).
These measurements were used to estimate the Dar zarouk parameters from the resistivities and thicknesses of the layers and hence the hydraulic conductivities and transmissivities used in evaluating the vulnerability of the aquifer to surface contaminants. This study is driven by the desire to investigate the geologic structure of a typical Niger Delta aquifer with a view to assessing the pollutant risk of the aquifers to the seepage of contaminants near two solid waste landfills in the municipality. This became necessary as the inhabitants in the study areas depend mostly on ground water for their water supply needs.
MATERIALS AND METHODS
Location and accessibility of the study area: The study area is delineated
by longitudes 6°55' to 6°58' E and latitudes 4°52' to 4°54'
N and is accessible through networks of linked roads (Fig. 1).
It is characterized by alternate wet and dry seasons (Iloeje,
1992), with a total annual rainfall of about 240 cm; relative humidity of
over 90% and mean annual temperature of 27°C (Udom and
Esu, 2004). The areas are dominated by moderate vegetation cover and slightly
|| Location map of the study area
The geology of the study area: Geologically, the area under study is
a typical Niger Delta environment underlain by the Benin formation classified
as coastal plain sand (Reyment, 1965). It consists of
massive, highly porous and permeable freshwater bearing sandstones with minor
clay intercalation (Fig. 2). The formation is generally water
bearing and hence it is the main source of portable groundwater in the municipality
(Etu-Efeotor, 1997). The aquifers are recharged mainly
by surface precipitation and nearby drainages.
Sediments deposition and groundwater flow are generally in the NE-SW trend
inline with the regional trend of the basin (Ehirim and
Methodology: A total of 7 vertical electrical soundings and four 2-D
imaging surveys were used for the study. Seven Vertical Electrical Soundings
(VES) were carried out in the two landfill locations using ABEM Terameter (SAS)
1000C. A maximum electrode expansion (AB/2) of 400 m and MN/2 of 50 m were utilized
in the survey, using the Schlumberger electrode array. The measured resistance
values (R) were converted into apparent resistivity (ρa) by
multiplying with a geometric factor (K), such that:
|| Geological map of the study area
These values were then entered manually in a recording sheet for computer processing
using Schlumberger automatic analysis software (Henker, 1985).
The 2-D resistivity imaging uses a multi-electrode system with equal electrode
spacing a ranging from 10-60 m for successive measurement (Fig.
3). A 20 electrode Wenner-alpha configuration was adopted for the survey
and successive electrode positions were occupied along the survey path by leap
froging (Loke, 1999).
||The arrangement of electrodes for a 2-D electrical survey
and the sequence of measurements used to build a pseudo section (Loke,
The Wenner-alpha configuration was adopted because of its good signal strength
and continuous coverage. The apparent resistivity values were calculated from
the field resistance values using the equation:
where, a is the electrode spacing and R is the field resistance value.
The values of the apparent resistivities, electrode spacing and the x-locations
were entered in a text file for processing using RES2DINV
RESULTS AND DISCUSSION
The Vertical Electrical Soundings (VES) and 2-D resitivity imaging techniques were integrated in the study of the vulnerability of the coastal aquifers to surface pollutants around two landfill sites within Port Harcourt municipality. The results of the interpretation of the vertical electrical sounding data revealed different geoelectric layers in terms of their resistivities and depths in the study areas.
A total of 4 to 6 geoelectric layers were delineated with varying type curves. The curve types identified ranges from AK to KQ type curves reflecting the lithological variations with depth in the study areas. The nature of the type curves suggests that the measured resistivities vary with depth of investigation in the study sites.
The geological interpretation of the VES results reveal that the geoelectric layers are dominantly sandy formations of varying grain sizes and moisture contents, with little clay/shale intercalations at depth (Fig. 4). These sandy formations constitute the vadose zones at shallow depths and unconfined multi-aquiferous systems in the area. The depth to the aquifers ranges from 23.89 to 73.38 m, revealing the variable nature of the depth to the aquifer systems in the sites with an average depth of 53.05 m.
The quantitative assessment of the Dar-Zarouk parameters obtained from the
results of the vertical electrical soundings revealed that the longitudinal
conductance S (overburden protective capacity) in the study areas ranges from
0.009 Siemens to 0.06 Siemens. These range of values for the overburden protective
capacities are less than the critical value of 1.0 Siemens (Table
|| Geoelectric sections of the VESs
|| Computed values of the Dar-Zarouk parameters from the VES
These suggest that the overburden layers do not have significant amount of
clay/shale impermeable beds. These are interpreted as zones or layers of probable
risks to aquifer contamination. Underlying this bed are porous and permeable
sandy formations of varying thicknesses, grain sizes and moisture content which
constitute the aquifer.
The tranverse resistance (T) of the aquifers varies from 11 to 2.12x102
Ωm2 (Table 1). These were interpreted as zones
of high transmissivity. The high values of the tranverse resistances (Transmissivities)
are due to the lithological nature of the aquifer materials which are porous
and permeable to fluids in accordance with the results of the vertical electrical
sounding. The low values of the protective capacity of the overburden clay/shale
layer and the high transmissivities of the vadose zones and the aquifers, will
aid the seepage and migration of contaminants within and around the landfills
|| Inverted resistivity sections of site 1 profile 1
|| Inverted resistivity sections of site 1 profile 2
|| Inverted resistivity sections of site 2 profile 1
Compounds of anomalously low, intermediate and high resistivities were mapped
and identified within and around the study sites using the 2-D electrical resistivity
tomography techniques (Fig. 5-8). The highly
resistive anomalies were mapped and identified as (pink to purple) with resistivities
ranging from 725 to 4419 Ωm and at depth exceeding 31.3 m. These anomalously
high resistive features could be associated with the presence of landfill gases
(Ammonia, methane, sulphur (IV) oxide, or carbon (IV) oxide) generated as a
result of the anaerobic decomposition of the landfill municipal wastes.
|| Inverted resistivity sections of site 2 profile 2
These gases have been displaced to various degrees with respect to depth in
the study sites due to their low densities and pressure buildup within the landfill
and are migrating to the surface through the permeable and porous sandy formations.
They are relatively soluble in the moisture permeating the pores of the formations,
thereby causing serious contamination of the groundwater aquifer.
The low resistive zones identified as (deep blue) with resistivity ranging from 15.65 to 179 Ωm were interpreted as leachate contaminant plumes containing dangerous pathogens, dissolved organic and inorganic constituents. These features manifests at depths ranging from 1.25 m to more than 31.3 m in the study areas and are observed to have seeped from surface points to depths exceeding 31.3 m. This observed seepage is enhanced by the porous and permeable nature of the dominant sandy formations of the aquifer materials.
Finally, layers of increasing resistivities in the entire sections (green to yellow) with resistivities ranging from 225 to 2155 Ωm were also mapped and identified as porous and permeable sandy layers of varying grain sizes and moisture contents.
Based on the results of the 2-D imaging, no appreciable clay/shale impermeable layer was delineated and hence, the major lithological units are sandy formations of varying grain sizes in accordance with the results of the VES interpretations in the sites.
The results of both VES and the 2-D resistivity imaging revealed the various lithological units in the study sites based on their respective depths of investigation. The geologic interpretation of the VES showed that the study areas are dominantly underlain by sandy formations of varying grain sizes and moisture content with high transmissivities.
The quantitative assessment of the protective capacity of the geoelectric layers overlying the aquifers showed that the longitudinal unit conductance ranges from 0.009 siemens to 0.06 siemens. This implies poor aquifer protection and indicates zones of probable risks to soil and groundwater contamination. The formations do not show good aquifer protective capacity. Thus, they are vulnerable to any near-surface contamination.
With the aid of the tomograms, two distinct contaminant plumes had been mapped and identified with an intermediate resistivity layers within the study sites. They are:
||Highly conductive leachate contaminant plumes seeping from
surface points to the aquifers
||Highly resistive gaseous contaminants that are probably due
to landfill gases (Ammonia, methane etc.) obtained as a result of the anaerobic
decomposition of the landfill organic wastes
The hydrogeologic features of the study areas indicated that contaminants derived
from the waste disposal sites infiltrate through the porous and permeable soil
media into the vulnerable aquifers. This suggests that the soil and the groundwater
system may have been contaminated to depths exceeding 31.3 m in the sites.