Integrated Geophysical and Geochemical investigations of saline water intrusion in a coastal alluvial terrain, Southwestern Nigeria.

Deterioration of water quality in the coastal areas of Lagos due to saltwater infiltration into the freshwater aquifer has become a major concern. Geophysical techniques involving 2D electrical resistivity tomography (ERT) using Wenner and dipole-dipole arrays were incorporated with geochemical evaluation using ICP-OES and 1C analytic methods to study the extent of saline water intrusion and its environmental impacts. The resistivity data were acquired with minimum electrode spacing of 5m (Traverses 1, 2, and 3) and 6 m (Traverse 4). Data level of 8 (maximum electrode spacing of 40.0 m) was achieved in each of the profiles; dipole length with n-factor of 6 was employed for the dipole-dipole array. The depth of inverse models from the geoelectrical resistivity data obtained in the area revealed significant impact of the saline water on delineated aquifers with very low resistivity values uniquely below 4.0 . ERT results also show the lateral invasion and up-coning of saline water within the aquifer systems. Results of the physiochemical analysis indicate that the water is alkaline with pH values range 8.6-8.9. The salinity is also high with the total dissolved solids TDS values and chloride concentration ranges 10405 – 12005 (mg/L) and 432 – 724 (mg/L) respectively. The water has very high electrical conductivity (EC) values range 2005-3013 μS/cm and very Hard with 121 to 180 mg/L range of total hardness thereby falling below the minimum permissible standard of the Indian standard institution (ISI), European union (EU) and world health organization (WHO) for portable water consumption

Geophysical Imaging of Archaeological Materials at Iyekere, Ile-Ife Southwestern Nigeria.

Non-invasive geophysical methods are increasingly being used in archaeological studies. In this study, magnetic and electrical resistivity tomography geophysical techniques were integrated to locate subsurface archaeological materials. The survey consists of four parallel and three perpendicular profiles with station interval of 0.5 m for both magnetic and electrical resistivity tomography. Wenner array with electrode spacing ranging from 0.5 – 3.0 m was used to collect the electrical resistivity data. The results show that high total magnetic intensity anomalies correspond to high inverse model resistivities. The regions with high magnetic and resistivity anomalies were thought to be locations of archaeological materials; the corresponding depths to these materials were inferred from the resulting geophysical images. Test units conducted at the regions of high total magnetic intensity and inverse model resistivity yield archaeological materials including burnt pipes (Tuyere), iron slag, iron smelting, and pottery fragments at approximate depths inferred from the geophysical images

Applications of magnetic methods and electrical resistivity tomography for imaging archaeological structures at Iyekere Ile-Ife Southwestern Nigeria.

Magnetic and electrical resistivity tomography geophysical techniques were integrated to locate subsurface archaeological materials. The magnetic survey comprises seven profiles in N-S and E-W direction with station interval of 0.5 m. [Orthogonal set of] 2D electrical resistivity tomography data consisting of four parallel and three perpendicular profiles were collected using Wenner array with electrode spacing ranging from 0.5 – 3.0 m. Trial pits carried out at regions of high total magnetic intensity and model resistivity yield burnt pipes “TUYERE”, iron slag, iron smelting, and pottery fragments

Preliminary geotechnical characterization of a site in southwest Nigeria using integrated electrical and seismic methods

Geophysical investigation using Vertical Electrical Sounding (VES), Electrical Resistivity Tomography (ERT) and Seismic Refraction at a proposed conference center site along Ajibode-Labani road, Ibadan, southwestern Nigeria has been carried out. The investigation aims at characterizing and delineating the subsurface strata to understand the weathered profile at the site. Understanding the weathered profile is essential in determining the suitability of the site for engineering construction of the future conference center. A total of 25 VES and 10 ERT profiles were acquired in a systematic grid pattern using both Schlumberger andWenner configurations with Allied omega terrameter. TheVES data were processed and analyzed using WinResist and the ERT data were inverted using RES2DINV. The data were combined to form a 3-D data set of the site and RES3DINV was used to produce the depth slices. Seismic refraction data were also acquired with an ABEM seismograph and processed using SeisImager and Fajseis software. Seismic data were used in understanding the velocity distribution and thickness. The results of VES, ERT and seismic refraction show good correlation. Four sub-surface layers were delineated: top layer of reworked sand, clayey sand/ lateritic hard pan, clay/ sandy clay and fracture/ fresh basement. The 3-D model permits a pictorial view of the sub-surface in relation to materials that overlie the basement. The thickness of unconsolidated materials to bedrock varies from 2.7 m to 12.2 m which revealed inhomogeneity in weathering under the shallow sub-surface. It is found that the integrated geophysical tool is well suited to characterize and delineate sub-surface structure (weathered profile) for engineering site characterization

Delineation of Leachate Plume Migration Using Electrical Resistivity Imaging on Lapite Dumpsite in Ibadan, Southwestern Nigeria

A geophysical investigation involving 2D resistivity survey was carried out on Lapite dump site that has been in operation since 1998 in ancient city of Ibadan, Southwestern Nigeria. The aim was to map the conductive leachate plume and extent of migration of leachate plumes in the subsurface for possible groundwater contamination. The 2D resistivity survey was carried out using Campus Tigre model resistivity meter with Wenner array configuration of electrode spacing distance ranging from 5 - 25m. A total of nine (9) 2D resistivity profiles with length ranging between 80 and 120 m were conducted within the dump site. A control profile of 2D resistivity survey was also carried out at about 300 m away from the dump site. The obtained resistivity data was interpreted using both RES2DINV and RES3DINV respectively. The 2D and 3D inverse resistivity models of the subsurface revealed the extent of leachate plumes with resistivity values less than 10 Ωm and allowed the location of leachate, clay soil, bedrock and seepage path from the dumpsite to be clearly delineated. There may be possible contamination of shallow groundwater system in the nearest future if proper mitigation measures are not taken into consideration at the dumpsit

Assessment of groundwater contamination around active dumpsite in Ibadan southwestern Nigeria using integrated electrical resistivity and hydrochemical methods

Investigation of groundwater contamination due to leachate migration in a solid waste disposal site was done using both geophysical and hydrochemical methods. The main goals were to delineate groundwater contamination due to leachate percolation and thus assessment of quality of groundwater from nearby hand-dug wells bordering the dumpsite for drinking purpose. A total of ten resistivity traverses were acquired within and outside the dumpsite using Wenner configuration with constant electrode separation ranging from 5 to 25 m. The 2D resistivity data were processed and inverted using RES2DINV and RES3DINV softwares, respectively. Geochemical assessment of groundwater samples were carried out according to APHA standards while hydrochemical facies of the sampled groundwater was evaluated using Piper Trilinear software. The inverse resistivity models of the subsurface from 2D and 3D imaging revealed low resistivity value less than 10 X m suspected to be leachate while 3D inverse sections allowed delineation of leachate, weathered layer, bedrock and seepage path from the dumpsite. The extent of migration was more pronounced in the southern part of the dumpsite, hence possible contamination of shallow groundwater system as dumpsite ages. The results of physico-chemical analyses showed the groundwater samples to be within the limits of WHO/NSDWQ for drinking purpose. However, higher values of concentrations of most analyzed parameters were noticed in well 1 due to its nearness to dumpsite and well 10 due to agricultural activities, respectively. Interpretation of Piper diagram showed CaHCO3 to be dominant facie in the area while alkaline earth metals (Ca2þ; Mg2þ) and weak acids (HCO3 -, CO3-) are dominant cations and anions during both climatic seasons. Groundwater in the study area is of hard, fresh and alkaline in natur

Investigation of incessant road failure in parts of Abeokuta, Southwestern Nigeria using integrated geoelectric methods and soil analysis

A flexible pavement devoid of discontinuities allows for smooth movement of a vehicle load on the roadway. This study involved the use of integrated geoelectric methods comprised of 1D and 2D Electrical Resistivity Tomography (ERT) as well as soil analysis to investigate causes of unceasing road failures along busy Camp—Alabata Road, Abeokuta, Southwestern Nigeria. Four road sections (two failed portions and one fair section and one good section) were identified along which four resistivity traverses were established along the investigated roadway. Four 1D Vertical Electrical Sounding (VES) points were also carried out on the 2D ERT lines. Apparent resistivity data were measured along the four traverses using Schlumberger and Wenner arrays with the aid of a Campus Ohmega resistivity meter. The VES and 2D resistivity data were processed and inverted using WinResist an

A framework combining geophysical and hydrogeological data for protecting groundwater sources in Nigeria

Groundwater is preferred to surface water as a drinking water source because it is less vulnerable to contamination from pathogens and chemical compounds. However, there is an increasing threat to groundwater health globally and in Nigeria resulting from sources such as uncontrolled use of fertilizers, chemicals and saltwater intrusion along coastal aquifers. To avoid high remediation costs and the health hazards associated with contaminated groundwater, it is preferable to protect groundwater sources. Groundwater source protection approaches include imaging aquifer structure, mapping source catchment area, estimating travel time distribution and parameters controlling groundwater flow and solute transport as well as investigating the mechanisms controlling saltwater intrusion. These studies require the use of numerical models parameterized by field parameter estimates. Field estimates of aquifer parameters at high resolution remains a challenge globally and in Nigeria, the quantification of spatially varying hydraulic parameters necessary to reduce uncertainties in groundwater source protection and assess vulnerability has received minimal attention. This research therefore takes a first step in exploring the use of predictive numerical models and field parameter estimates integrating geophysical and hydrogeological methods for protecting groundwater sources from anthropogenic contamination and seawater intrusion. We utilized refraction seismic and electrical resistivity for delineating the aquifer architecture while resistivity provided a proxy for imaging saltwater intrusion and transient groundwater flow. The multi-geophysical data sets aided high resolution estimates of hydrodynamic and hydrodispersive parameters used for calibrating the groundwater flow and solute transport models using MODFLOW, MT3DMS and SEAWAT. In addition to extending the global state of the art on characterizing aquifer heterogeneity at high resolution, this research produces a framework that can aid policy formulation for protecting groundwater resources in Nigeria

Geoengineering site characterization for foundation integrity assessment

Inefficient near-surface characterizations prior to building construction have largely contributed to the incessant building failures in the form of structural defects and building collapses recently occurring in southern parts of Nigeria. Combined geophysical and geotechnical investigations have been used to select suitable foundation type and depth at a building construction site in part of the Lagos Island, Nigeria. Three geoelectrical resistivity survey profiles of length 150 m each were conducted using minimum and maximum electrode spacing of 5.0 m and 45.0 m, respectively. Boring, in-situ geomaterials samplings, cone penetrating tests, standard penetrometer tests and laboratory tests were among the geotechnical investigations carried out in accordance with the British standard code of practice. The geoelectrical resistivity imaging results provide the lateral and spatial spread of the geoelectrical units stratification within the study area, their clay and water contents. The results of the laboratory and geotechnical tests also reveal the subsoils lithologic units, their compressibility and shear strengths. A deeper piletype foundation on a more competent stable subsoil stratum at the depth greater than 13 m is recommended for the proposed buildings in the study area so as to effectively transmit their loads

Evaluating the groundwater potential of coastal aquifer using geoelectrical resistivity survey and porosity estimation: A case in Ota, SW Nigeria

Geoelectrical resistivity sounding surveys was carried out at two locations (A and B) in the new CanaanCity residential estate of Canaan land Ota, Ogun state in order to locate possible targets for groundwater explorations. A total of twelve (12) vertical electrical soundings were conducted using the Schlumberger array configuration. The interpretations of the VES curves revealed five to six geoelectrical layers at location A which comprises VES1-7. The geoelectric units consist of the topsoil, sandy clay, lateritic clay, confining bed (clayey sand), and main aquifer (sand). The top of the aquifer in this part of the study area is between 40 m and 83.5 m. Likewise, a total of five to seven layers of geoelectrical units were delineated at the location B portion of the study site (VES 8–12) similar to location A with an addition of the shale unit that was interpreted to be that of the Akimbo Formation of the Dahomey Basin. The depth to the top of the productive aquifer within location B is in the range of 40–112.4 m. The delineated basal shale layer is impermeable, serving as the base seal rock for the confined aquifer system. The estimated porosity values range from 28% to 74%, indicating that the highly porous aquifer system in the area is the coastal plain sands mixed with the alluvium sand of the Benin Formation of Dahomey Basin. The study revealed the complexity of the productive aquifer system in the area and its undulating topography