Saltwater intrusion into the coastal aquifer, a phenomenon brought by the flow of seawater into freshwater aquifers originally caused by groundwater extraction near the coast, has long been recognised as a major concern around the world. In this study, we employed geophysical and geochemical techniques to map and provide evidences that the coastal aquifers in the study area have been intruded by saltwater from the adjacent Lagos lagoon. The resistivity data were acquired with an electrode spacing (a) that vary between 1.6 to 8 m, and expansion factor n of 30. The depth inverted models obtained from inversion of the fifteen resistivity data obtained in the area revealed significant impact of the lagoon water on the aquifers indicated as low resistivity usually below 7 Ωm. A combination of four different electrode arrays – Schlumberger, Wenner, Dipole-dipole and pole–dipole, with at least three deployed at each site ( except for three traverses – traverses 13, 14 and 15), yield better horizontal and vertical resolution, having depth range of 36–226 m with 1.6–8 m electrode spacing used. The delineated geoelectric layers were juxtaposed with logs from both boreholes located within the campus. Evidence from geochemical study of borehole and the lagoon water samples corroborated the ERT result. Progressive decrease in total dissolved solute (TDS) and electrical conductivity (EC) from the lagoon to the coastal aquifer buttresses gradual encroachment of the inland aquifers by the intruding lagoon water. In addition, similar trend was observed in heavy metal distribution Pollution Index (PI) plot suggesting possible underground flow of water from the lagoon to the aquifers. From this study, we deduced that excessive groundwater extraction and possibly the reduction of groundwater gradients which allows saline-water to displace fresh water in the aquifer of the investigated area are responsible for the saline water intrusion observed.
Leachate originating from open refuse dumpsite systems can be delineated through an integration of qualitative and quantitative methods. This study was designed to examine extent of leachate and pollution from one of the numerous open refuse dumpsites in Lagos metropolis. Qualitative assessment was determined using electrical resistivity tomography (ERT), vertical electrical sounding (VES) and induced polarization geophysical methods. Both ERT and VES methods revealed persistent low resistivity (1 - 20 Ωm) of leachate to the depth above 35 m. The two methods were projected to produce 3-D view of the site which shows a NW-SE flow pattern of the leachate and possibly, the groundwater. IP values observed over the polluted zone was -2.9 - 8 ms, indicating a sandy layer. Quantitative assessment was achieved by analysis of geochemical substances in the water samples taken from wells and boreholes in the precinct of the dumpsite. Here, we examine the macroelements, salts (sulphates, nitrates and chlorides), heavy metals, radioactive metals contents and physical parameters of the water samples. The analyses reveal the presence of these substances in the water and their strong correlations justified the provenance as the same. As part of the quantitative evaluation, physical parameters (pH, TDS, DO, salinity, total hardness, turbidity, electrical conductivity EC and temperature) of the water samples were also determined. The samples pH plotted in the acidic domain unsuitable for human consumption. Leachate flow direction was generated from the decreasing concentration of measured parameters (geochemical elements and physicals properties) in NW-SE direction which agrees with similar flow pattern deduced from ERT results.
Land reclaimed from water bodies or its equivalent (swamp, flood zones and abandon river channel) usually contain certain mechanically unstable geological formations (e.g. peat and clay) which are inimical to the foundation of engineering structures. Concerted effort must therefore be ensured to determine depth to competent layer across the proposed site as against conventional geotechnical test which is a point test.Complementary non invasive geophysical methods must be carried out for any engineering construction so as to ensure stability of building construction and avoid the devastating effects of building collapse rampant of late. Thus, electrical resistivity tomography and induced polarisation methods, constrained by percussion drilling, were used for the present study. The results of the 2-D and 3-D DC resistivity tomography using 64 multi-electrode systems with gradient electrode configuration along five parallel profiles with smallest electrode separation of 4.0m on a total length of 252 m was interpreted using Earth Imager Inversion software. The subsurface was characterised by Peat/Clay sandwich between upper and lower Clayey-sand/Sandy-clay and Sand. The presence of Peat and Clay were correlated to the cause of chronic differential settlements of some engineering structures within the area. The depths to the sand material were observed between 17-47 m. At shallow depths, the sand materials were underlain by mobile clay/peat materials, which correlates with the depth to competent layer (Sand) achieved by percussion drilling in the range of 20-24 m. This lithological sequence in which the near surface sand deposits are underlain by clay/peat saturated with water, gives a sense of instability to the overlaying sandy layer. To buttress this finding, residential and other public buildings surveyed in the entire area are marked with various degrees of distress, ranging from multiple cracks, sinking of building, partial collapse to complete collapse. It should be noted that the depth to the sand layer achieved during the research varied along the respective profiles. Thus application of 2-D resistivity tomography has revealed both the lateral and vertical variation in depth to competent sand layer within the study area as opposed to point information from CPT or Percussion drilling. This provides a useful guide to the site engineers in designing appropriate foundation structure.
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