The protective capacity of aquifers is a prerequisite for groundwater quality in areas prone to contamination from the ground surface. Aquifers without protective layers are more susceptible to contaminations from point and non-point sources. The protective capacity of an aquifer significantly correlates with the thickness of clay and organic materials that mantled it. To assess aquifer protective capacity, electrical conductivity property of saturated clay was determined from filtered imaginary and real components of Very Low-Frequency Electromagnetic (VLF-EM). The filtered real components against distance and current density pseudo-section produced from real components were concomitantly used for interpretation. Clays capping aquifers were delineated from those without capping from low and high values of filtered imaginary and real components and current density pseudo-section. The distribution of clay laterally across the area indicates that aquifers are poorly protected and susceptible to contaminations from a point and non-point sources. The apparent agreement between inferred geology from VLF-EM interpretation, borehole data and resistivity data underscores the efficacy of VLF-EM as an important tool that can be used or combined with other geophysical methods and borehole information/data for assessment of the protective capacity of the aquifer.
ABSTRACT:Resistivity surveys have been utilized to delineate the sand horizons and possible water bearing zones in the shallow Quaternary deposits that mask the Benin Formation that underlies the Sombreiro-Warri Deltaic Plain at Orerokpe in the western Niger Delta. Fifteen dug wells and shallow boreholes were sampled for water quality analysis. The results show that from the surface to about 70m, there are four layers of water bearing sands that are separated in places by discontinuous clay horizons. Depth to water level ranges from 3.5m to 5.5m. Ground water flow is from northeast to southwest towards the Warri River. Ground water which is used here for domestic purposes without treatment is soft and it is shown that the most important factor contributing to groundwater chemistry is weathering of the aquifer matrix. The order of preponderance of the cations is Naand HCO3 -> SO4 2-> Cl -for the anions; trends that are not only a departure from normal pristine conditions, but also basically different from those observed in nearby Warri and indicative of possible contamination. The relatively low calcium and magnesium content is identified as a potential health problem if water constitutes the main source of ingestion of these ions. The results reinforce the complex geological nature and geochemical conditions that exist in the Quaternary superficial deposits of the Niger Delta environment. @JASEM
Understanding aquifer lithofacies and depth of occurrence, and what factors influence its quality and chemistry are of paramount importance to the management of groundwater resource. Subsurface lithofacies distribution was characterized by resistivity and validated with available subsurface geology. Resistivity values varied from less than 100 Ωm to above 1000 Ωm. Lithofacies identified includes clay, clayey sand, sand and peat. Shallow unconfined and confined aquifers occurred at depths ranging from 0 to 12 m and 18 to 63 m, respectively. Geochemistry and multivariate statistical analysis consisting of principal component analysis (PCA) and cluster analysis (CA) were used for the determination of quality and groundwater evolution. Groundwater types depicted by Piper plots were Ca3+, Cl− and Na+, Cl−, which was characterized by low dissolved ions, slightly acidic and Fe2+. The dominant variables influencing groundwater quality as returned by PCA were organic pollution resulting from swampy depositional environment, anthropogenic effects resulting from septic and leachates from haphazard dumpsites mixing with groundwater from diffuse sources. In addition, the weathering and dissolution of aquifer sediments rich in feldspar and clay minerals have considerable impact on groundwater quality. CA depicted two distinct types of groundwater that are significantly comparable to those obtained from Piper plots.
Results from chemical analyses of forty dug well water samples in the Effurun-Warri metropolis show that mean pH is 7.1 and mean TDS is 193 mg/l. Representative mean levels of cation occurrence include Ca, Mg, Na and K at 6.13mg/l, 4.09 mg/l, 4.89 mg/l and 3.37 mg/l respectively. Mean anion concentration for bicarbonate was 8.20mg/l, 1.27mg/l for sulphate and 23.74mg/l for chloride. Physical and chemical parameters are thus well below WHO and Standard Organization of Nigeria drinking-water quality standards. Piper diagram plots of the data indicate that ground water is predominantly Ca+Mg+Na Chloride facie and that mixing and ion exchange processes control the dominant cation in space and thus at each specific locality. Leachates from the many, widely distributed and unregulated landfills and dumpsites have been identified as possibly the principal sources of major ion loading to groundwater. The ubiquitous onsite sewage treatment soak away pits also contribute major ions to groundwater. These two sources are thus accountable for any observed local spikes in groundwater chloride content rather than sea water intrusion as had been previously suggested.
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