Geophysical, hydrogeological, and hydrochemical investigations have been used to assess the impact on soil and groundwater of some ancient dumpsites in Akure Metropolis. One hundred and seventy one (171) Vertical Electrical Sounding (VES) and dipole – dipole profiling along six (6) profiles were carried out. Seventy (70) water samples were collected from hand dug wells for chemical analysis within a radius of about 600 m from the centre of the ancient dumpsites.

The geoelectric sections and the inverted 2-D resistivity structures delineate three subsurface geological units consisting of the topsoil, weathered layer and the fractured/fresh basement rock. The weathered layer constitutes the main aquifer unit. The 2-D resistivity structures show that within the premises of the dumpsites, the topsoil and the weathered basement aquifer unit are characterized by relatively low layer resistivity values of <40 ohm-m and <20 ohm-m respectively. The relatively low resistivity values are suspected to be due to conductive leachate from the dumpsites. Results of chemical analysis of water samples from hand dug wells in the study area show that water samples from the area around the dumpsites are characterized by relatively higher electrical conductivity (1000 – 2361 μs/cm), chloride concentration of > 250 mg/l and nitrate concentration of > 10 mg/l which confirm groundwater pollution. It can be concluded that the soils and the groundwater in the areas around the investigated dumpsites have been polluted. The polluted zone has a depth extent of about 2.5 – >7.5 m and lateral extent of about 250 m north and up to 500 m south of the dumpsites.

Physico-chemical analysis was carried out on groundwater samples in the area around some selected dumpsites in Akure Metropolis, Southwestern Nigeria. The aim of the study was to determine the impact of the dumpsites on the quality of the groundwater. Seventy (70) water samples were collected from hand dug wells within the immediate vicinity and further away from the dumpsites, for analysis. The samples were analyzed for temperature, pH, conductivity, total dissolved solids (TDS), Ca2+, Mg2+, Na+, K+, HCO3-, Cl- , NO3-, NH3, SO42-, PO4, Fe, Pb2+, and Zn2+.

The temperature, pH and TDS ranged in values from 28.5-35.5oC; 6.0-9.6 and 72.5-1056 mg/l respectively while the concentration levels of Ca2+, Mg2+, Na+ and K+ were 23-179 mg/l; 94-141mg/l; 38.1-47.8 mg/l and 106-175 mg/l; 0.5-189 mg/l; 5.9-138 mg/l; 29.5-47.4 mg/l; 90.8- 136 mg/l, respectively, in the water samples collected from around and outside the dumpsites. The heavy metals (Fe, Pb2+, and Zn2+) gave concentration levels of 0.1-16.9 mg/l; non-detectable – 0.2 mg/l; non-detectable – 0.5 mg/l, and 0.1-15.8 mg/l; non-detectable – 0.1 mg/l and non-detectable – 1.4 mg/l, respectively. The pH, conductivity, TDS values and the concentration levels of Cl- , NO3- were 6.9-9.6; 680-2361 μS/cm; 118.5-1056 mg/l; 103-819 mg/l and 1.4-30 mg/l respectively for water samples collected from within the immediate vicinity of the dumpsites. The upper limits of these values are higher than the WHO (2004) maximum permissible levels of 8.5; 1000 μS/cm; 500 mg/l; 200 mg/l and 10 mg/l respectively, for potable water and higher than the baseline concentration levels of 5.2-7.8; 19-909 μS/cm; 85-509 mg/l; 0.015-394 mg/l and 0.02-22.9 mg/l, respectively, from the basement complex terrain of Nigeria, indicating pollution from the dumpsites leachate. This study concluded that the leachate from the dumpsites had impacted the groundwater making it non-potable.

Geoelectric Survey involving Schlumberger Vertical Electrical Sounding (VES) and dipole-dipole profiling techniques has been used to map some ancient dumpsites in Akure Metropolis, southwestern Nigeria. Thirty three (33) VES soundings and six (6) Dipole-Dipole profiling were carried out in the study area. The geoelectric sections and the inverted 2-D resistivity structures reveal three subsurface layers composed of topsoil, weathered layer and the fractured/fresh basement rock. The weathered layer constitutes the main aquifer unit. Topsoil resistivity and thickness ranges are respectively 6 – 461 ohm-m and 0.4 – 2.4 m. The weathered layer resistivity values vary from 9 – 38 ohm-m while its thickness ranges from 2 – 12.8 m beneath the dumpsites. The depth to the bedrock varies from 2.1 – 14.6 m. The 2-D resistivity structures show that zones with relatively low resistivity (< 20 ohm-m) topsoil/weathered layer are suspected to be due to leachate saturation with depth extent varying from 2.5 - >7.5 m. In view of the relatively high water table of 0.5 – 2.0 m around the dumpsites, the groundwater in the area beneath the dumpsites is suspected to have been polluted. Preliminary results of chemical analysis of water samples from hand-dug wells in the study area (electrical conductivity (EC), chloride and nitrate concentrations of 145 – 2361 µs/cm, 46 – 677 mg/l and 1.4 – 52 mg/l respectively) confirm groundwater pollution.

An integrated geophysical investigation involving the magnetic and electrical resistivity methods was used to investigate a spring in Ibuji, near IgbaraOke, southwestern Nigeria. This was with a view to determining its nature and source and the feasibility of developing it as a groundwater resource. Magnetic profiling and 2-D subsurface resistivity imaging with the dipole-dipole array and thirteen Schlumberger Vertical Electrical Soundings (VES) were carried out along three traverses. The total field magnetic measurements were taken at 5 m interval and corrected for diurnal variation and offset. Magnetic anomaly that is typical of a thin dipping dyke model (suspected to be a fault/fracture or shear zone) was delineated along each of the three traverses. Quantitative interpretation of the magnetic anomalies gave depths to the magnetic basement bedrock of between 13.2 and 20.1 m. The geoelectric sections and the inverted 2-D resistivity structures delineate maximum of four subsurface geological units consisting of the topsoil, weathered layer, the fractured basement and the fresh basement rock. The VES gave depths to basement bedrock of 7.4-25.1 m. Also the resistivity structures identify a major low resistivity vertical discontinuity (F1) typical of a fault zone across the three traverses and another suspected strike slip fault (F2) across fault zone F1 on which the Ibuji Spring is located. The width of the anomalous zone varies from 10-17.5 m. It can be concluded that the Ibuji Spring is structurally controlled. The 10-17.5 m wide suspected fault zone F1 is favourable to groundwater development.

A direct current electrical resistivity survey using the Schlumberger Vertical Electrical Sounding (VES) and dipole – dipole profiling techniques have been used to map structurally controlled pollution plume around the Otutubiosun dumpsite, Akure, southwestern Nigeria. The geoelectric interpretation and the inverted 2-D resistivity structures revealed three subsurface layers: the topsoil, weathered layer and the fractured/fresh basement rock. The fractured/fresh basement constitutes the basement bedrock. The weathered layer and the fractured/faulted zones constitutes the major aquifer units in the study area. Topsoil resistivity and thickness value ranges from 5 – 107 ohm-m and 0.2 – 2.7 m respectively. The weathered layer resistivity values vary from 4 – 35 ohm-m while its thickness ranges from 1.1 – 12.9 m beneath the dumpsite. The depth to the bedrock varies from 2.2 – 13.1 m. The resistivity values of the fractured/fault zones rages from 2 – 55 ohm-m and the thickness varies from 10 – 30 m. The 2-D resistivity structures show that zones with relatively low resistivity values of (< 35 ohm-m) with a depth range of 1.1 – > 15 m for topsoil, weathered layer and fractured/faulted basement zones are suspected to be due to leachate saturation. Due to the observed relatively shallow water table of 0.5 – 2.0 m around the dumpsites the groundwater around the dumpsites is suspected to have been polluted. This study reveals that subsurface geologic structures within the bedrock (fractures/faults) control the migration of leachate plume around the investigated dumpsite

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Fifty one Schlumberger Vertical Electrical Sounding (VES) were carried out In Ejigbo and its environs Southwestern, Nigeria in an attempt to locate suitable sites for potable water supply in the area. The study area is underlain by rock of the Precambrian Basement Complex of southwestern Nigeria. The rock units include gneiss, migmatite, quartz and quartz - schist. Charnockite/migmatite, fine - medium grained granite, quartz diorite and Quartz syenite. The VES curves were interpreted by partial curve matching technique using two layer model curves and corresponding auxiliary curves. The computer Iteration was carried out with the computer programme RESIST VERSION 1.0. The programme was used to compare the computer-generated curves with the field curves. Where a good fit of about 90% correlation was obtained between the field curve and the computer-generated curve, the interpretation result was considered satisfactory.

A maximum of four subsurface geologic units were delineated. They are the topsoil, weathered layer, partly weathered/fractured basement and the fresh basement. The depth to the bedrock varies from 5 to 40m. The weathered layer and the partly weathered/fractured basement constitute the aquifer units. The weathered layer is about 10m to 40m thick in most cases in the study area. These aquifers are presumed to have a tendency for high groundwater yielding capacity which is expected to enhance the success rate of borehole drilling programmes in the area.

The survey revealed a good correlation between the lithological logs from selected drilled boreholes and the geophysically predicted subsurface sequence and the bedrock structural disposition.

Vertical electrical sounding data have been analysed and used to delineate and characterize the aquifers of the basement complex terrain around Irewole and Isokan Local Government Areas of Osun State, Southwest Nigeria.

A maximumof four subsurface geological units were delineated in the study area. The first is the topsoil with resistivity value varying from 19 – 1400 ohm-m and thickness range of 0.4 to 3 m. The second layer is the weathered layer. The resistivity value varies from 10 – 700 ohm-m and thickness of between 0.8 and 29.4 m. The third layer is the weathered/fractured basement with resistivity value ranging from 85 – 400 ohm-m and thickness of between 1.6 and 23.2 m. The last layer is the fresh basement rock with resistivity value ranging from 2400 – infinity ohm-m.

The fractured basement rock and the overlying weathered layer constitute the two main aquifer units in the study area. Three of the five aquifer types delineated by Olorunfemi and Fasuyi, 1993 in the Nigerian basement terrains were also delineated from the area. Borehole logs observed from the study area show that the fracture thickness varies from 3 – 23 m. The pegmatite is more fractured than the other rocks, followed by the fine-grained granite.

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