Basement Complex Of Southwestern Nigeria Research Articles

GEOELECTRIC SIGNATURES FROM A LEACHATE PLUME MAPPED FROM A BASEMENT COMPLEX TERRAIN IN AKURE, NIGERIA

In this study, we investigated the implications of a leachate plume within an aquifer system, unravelling the intricate dynamics that govern contaminant transport and dispersion. An integrated approach of geophysical investigations is used to establish the impact of an open-waste disposal site around Aromed, in a part of Akure, in the Precambrian Basement Complex of Southwestern Nigeria. Investigations using eight (8) Vertical Electrical Sounding (VES) and Double-Dipole Resistivity Tomography (ERT) along two (2) traverses were conducted. The geoelectric interpretation and the inverted two-dimensional electrical resistivity tomography (ERT) images provided insights into the underlying geological composition, identifying three distinct units: the topsoil, the weathered column, and the fractured Basement/fresh Basement bedrock. The geoelectric tomography structures in the 2-D interpretation reveal that the dumpsite area exhibits low apparent resistivity estimated to be between 12 to 71 ohm-m in both the topsoil and the weathered column aquiferous zone. These values closely agree with results from the geoelectric sections, which range between 11 - 68 ohm-m, respectively. The relatively low apparent resistivity results are suggestive to be due to the presence of leachate’s chemical composition from the open waste disposal sites, which are suspected to be generated from the dissolution of ions of iron and other conductive minerals producing the leachate plume. The probable leachate depth of migration as revealed by the 2-D tomography structures varies from about <1 – > 14 m beneath the dumpsites. The leachate migration, which is structurally controlled, has a southward flow, majorly in the orientation with the observed structures. The subsurface depression-relief is also attributed to influence the direction of leachate flow. In conclusion, it is suggested that groundwater in the aquiferous zones in the vicinity of the dumpsites have entered a significant pollution level; hence water in the aquifer is not safe for consumption.

Microstructural interpretation of the garnet-sillimanite-bearing granite gneiss of Ikare area, southwestern Nigeria

This study presents microstructures from the granite gneiss of the Basement complex of southwestern Nigeria. The granite gneiss of Ikare area comprises of the mineral assemblage; garnet, biotite, K-feldspar, quartz, plagioclase, sillimanite and opaque minerals. K-feldspar porphyroblasts have cuspate interstitial quartz grains, also quartz grains are included in K-feldspar. Most K-feldspar grains are highly strained and in some cases are fractured. Prismatic and fibrolite-type sillimanite are both present. Biotite is closely associated with fibrolitetype sillimanite. Sillimanite was observed to mimic the foliation produced by biotite. K-feldspar is the dominant mineral and could have formed by the dehydration reaction of biotite + plagioclase + quartz + sillimanite → garnet + K-feldspar + melt. Microscopic structures such as strained feldspar grains, micro-fractures in Kfeldspar, simple twinning in K-feldspar, myrmekite, mimicking textures, microperthitic intergrowth and cuspate volumes of quartz surrounding K-feldspar are indicative of melt-bearing and solid-state deformation of regional metamorphic rocks.

English

The Osogbo central solid waste dumpsite in Osun State, Nigeria falls within the Ilesha Schist Belt, southwestern basement complex of Nigeria. Thirty-three (33) Vertical Electric Sounding (VES), and 2-D Electric Resistivity Tomography (ERT) data were acquired across various locations in the study area using ABEM Terrameter SAS 300, respectively. Schlumberger configuration with a maximum spread length of 100 m, for the VES was used, while four traverses of ERT were conducted, using Dipole-Dipole Array with average traverse length of 112.50 m. A smoothness-constrained least-squares inversion was then applied to the ERT data using a 2-D inversion algorithm [that is, DIPRO for Windows, version 4.01. Generally, the results defined the contaminated leachate plumes as electrically conductive, with anomalous resistivity values. Both the VES and the 2-D tomography identified zones of low resistivity values, between 24 and 67 ?m, from 10 to 20 m deep, as leachate contaminant plumes. Some of the weathered and fractured basement rocks indicated that the subsurface soil and groundwater have been contaminated by the leachates, which appears to have migrated outside landfill site. Zones within the sub-surface, with resistivity values relatively high (> 300 Ωm) and depth above 30 m might be pollutant free. Solid wastes management calls for the adoption of the best practicable environmental management approach through the governments’ enforcement of circular economic policies, using extended producer responsibility (EPR) model and construction of engineered landfills to replace unsanitary landfills. The conduct of environmental impact assessment (EIA), prior to commencement of operations and phytoremediation of closed sites is also crucial. Key words: Tomography, leachate, pollution, contaminant plume.

Spatio-temporal monitoring of leachates dispersion beneath a solid wastes dump in basement complex of southwestern Nigeria

For real-time monitoring and modeling of leachates dispersion beneath Arada wastes dump, Ogbomoso geologically located within Southwestern Nigeria basement complex, an integrated technique involving time-lapse Electrical Resistivity Tomography (ERT), hydrogeology, and geotechnic was used. ERT was performed using Wenner electrodes configuration in a Northeast-Southwest azimuth on the wastes dump monthly from March to September, and twice in March and September, at the control site, during the wet season. The Total Dissolved Solids (TDS), Electrical Conductivity (EC) and static water level of two monitoring wells on the dumpsite were measured during every ERT surveying. Twenty four soil samples were taken at 0.5 m intervals from 2.0 to 3.0 m deep bores on the anomalously low resistivity spots discovered on the subsurface resistivity distribution of the wastes dump, and sieve analysis and permeability tests were performed in the laboratory on these samples. Using the approach of grain-size analysis, the hydraulic conductivity and porosity were calculated empirically. On the wastes dump and control site, the resistivities of spatially distributed lenses of material ranged within 1.45–37.3 Ωm and 22.5–52.6 Ωm, respectively, suggesting leachates and clay anomalies. The wastes dump subsurface might be regarded as a porous medium since its porosity ranged between 38.2 and 44.2%, and the permeability range of (5.4–9.6) x 10−5 m/s showed that water could freely drain from it. Water samples taken from the monitoring wells on the waste dump had TDS and EC that ranged from 520 to 970 ppm and 1.05 to 2.37 mS/cm, respectively, but decreased in the southwest direction of groundwater flow, indicating leachates dispersion. The hydraulic conductivity ranged within 0.00285–0.176 m/s but varied anomalously and spatially indicating heterogeneous hydraulic conductivity field capable of initiating local variation in velocity that caused leachate to disperse longitudinally and transversely. The rate of longitudinal and transverse leachates dispersion as revealed by subsurface resistivity distribution images ranged within 0.000–0.250 m/day and 0.001–0.045 m/day, respectively. The former rate was 5.6 folds the latter rate and hence fell within the stipulated 5–100 for dispersion process that conformed to natural consequence of dispersivity. The conductivity-time curves (CTCs) of the leachates anomalies' steepness and doubly-peaked properties indicated Fickian dispersion, while different proportion times; t-1.588 and t-2.478, which are not equal to t-0.5, showed that the CTCs described non-Fickian dispersion at a significantly later time.

Frequency-Domain Electromagnetic and VLF Data with An Application to Modeling of Basement Structures: A Case Study Within, Ibadan Area, Southwestern Nigeria

The study area, the church camp is within Ibadan which fall within southwestern basement complex of Nigeria and it is mainly by migmatite-gneiss. The hard rocks are usually characterized by basement fracture which may serve as a conduit for groundwater passage. This has necessitated detail geological and geophysical investigation to accurately and precisely delineate this structure. Geological mapping alongside very low frequency Electromagnetic (VLF-EM) techniques were adopted in which conductivity data were acquired along Fifteen (15) VLF profiles using ABEM WADI. Thirteen (13) of these profiles were in the East-West (E-W) direction which is the dip direction of the structural elements, while the remaining two profiles were in North-South (N-S) direction. The data obtained from the field were processed, filtered and presented inform of profiles and Karous-Hjelt (K-H) filtered pseudo-section to visualise conductivity in (2-D). Pockets of conductive structures were delineated and were interpreted as fractures of various dimension and orientation. Some are single fracture and others are closely-spaced double fractures which some of them joined or fused together at depth with some suspected to contain water while some are dry base on their conductivity/resistivity signature exhibited. In conclusion, VLF-EM has proven to possess the capability to characterize and model the basement fractures and define their dimension, axial orientation and indicates their parallelism which also signifies their origin as regard stress regime that produces the fracture.

Evaluation of soil-gas radon concentrations from different geological units with varying strata in a crystalline basement complex of southwestern Nigeria.

The aim of this study is to determine the variation of soil-gas radon concentrations from different rock formations in Ogbomoso, southwestern Nigeria. The radon concentrations at different five geological domains in Ogbomoso are determined with respect to depth. The measurements varied from the surface (0cm) to 100cm depth, with an interval of 20cm. At all the geological domains (Porphyroclastic, Granite, Quartzite, Migmatite and Banded gneiss), radon has its minimum emission over migmatite at 0cm, while its maximum emissions occured over granite and banded gneiss at 80cm. The overall soil-gas radon concentrations in Ogbomoso varied from 0.06 to 26.5kBq/m3, which is within the natural limit of 0.4 to 40kBq/m3 based on the International Commission on Radiological Protection's recommendation. An F-ratio of 6.989 and a p-value of 0.001 were obtained for the first inferential hypothesis, while an F-ratio of 2.489 and a p-value of 0.076 were obtained for the second inferential hypothesis using ANOVA test. The post hoc (using Tukey HSD and Duncan) tests revealed that at 60 + cm, depth controls the level of radon concentrations being emanated from the subsurface. The pollution index in Ogbomoso is of level 1 at 80cm and level 0 (safe limit) at other depths. In conclusion, the soil-gas radon emission depends on the local geology and lithological sequences (depths). Cracks that could act as passage for indoor radon at the floors of the buildings around the polluted zones should be avoided in order to have a sustainable city.

Investigation of the Source of Groundwater Contamination Near a Farmland Using the VLF-EM and Magnetic Methods

Groundwater pollution continues to be a major problem in society where there is high level of urbanization, industrialization and conventional/inorganic farming. A detailed ground magnetic survey and VLF-EM was carried out on a site just beside a farmland to investigate the possible causes of the groundwater contamination in the area which lies within the Precambrian Basement Complex of Southwestern Nigeria. The survey was carried out using a proton precision magnetometer kept at a constant height during the survey, accompanied by Very Low Frequency - Electromagnetic Method (VLF-EM). Data for the two methods were acquired on five (5) traverses with the length of 100 m each. The distance between each traverse (inter traverse spacing) is 5 m. The VLF-EM data were interpreted qualitatively, using the Fraser and the Karous–Hjelt filters. The Fraser filtered data and relative current density pseudosections from the VLF-EM data indicate the presence of shallow and deep conductive zones that cross the farmland which are indicative of fractures. The quantitative interpretation of the data was done with a 2-D code that performs the inversion of the data. Magnetic maps were generated using Oasis Montaj, and profiles were plotted using Microsoft Excel software. From the results obtained, some major geological features were identified while linear features suspected to be fault/fractured zones, weathered zones, and bedrock depressions were delineated. Low magnetic intensity indicating presence of weak and incompetent zones were discovered. Earlier chemical analysis of both surface and groundwater collected at some depth on the farmland confirms the presence of water contamination. This knowledge supported the interpretation of the anomalies detected by VLF-EM and magnetic data as a possible cause of the contaminated groundwater flowing in connected fractures. The study concluded that the groundwater contamination originates from the farmland where some samples of the soil were found to contain nitrate chemicals resulting from fertilizer and pesticide used to enhance plant growth. Keywords: Nitrate contamination, groundwater pollution, multiple fractures, contaminant transport, fractured zone, inorganic farming DOI: 10.7176/JEES/12-6-04 Publication date: June 30 th 2022

Radiogeochemistry, uranium migration, and radiogenic heat of the granite gneisses in parts of the southwestern Basement Complex of Nigeria

The radiogeochemistry of granite gneisses in Akunu-Akoko (AK) and Ayere (AY) areas of the southwestern Basement Complex of Nigeria was studied using gamma-ray spectrometry, and geostatistical analysis and modeling. This study was aimed at determining the concentrations of radionuclides (K, U, and Th) in these rocks, as well as alteration scenarios, nature and rates of uranium migration (UM), levels of radiological hazards, and rate of radiogenic heat production (RHP). The interquartile (lowest Q1 < highest Q2 < highest Q3) elemental concentrations for 40K, 238U, and 232Th were 0.23 < 3.75<4.06%; 1.00 < 4.20<5.40 ppm; and 4.10 < 16.10<30.20 ppm, respectively, within the minimum and maximum limits of BDL (below detection limit) to 5.23%, BDL–9.70 ppm, and 0.40–60.20 ppm. The average activity concentrations of 40K, 238U, and 232Th with values of 629.28, 38.58, and 49.96 Bqkg−1, respectively, were above their crustal averages in the order of AK3<AK2<AK3<AY3<AY1<AY2. Variability in K-eU-eTh content and alteration in the rocks were affirmed by radioelement ratios and F-parameter. The high radionuclide concentrations may be attributed to high levels of radioactive materials, as well as trace and rare earth elements. Low radionuclide concentration in rocks is due to weathering, metamorphism, and 12–86% outward UM. The absorbed dose rate (ADR) average of 75.09 nGyh−1 was within the allowable range, with regression R-squared (R2) prediction values of 25–92% from the radionuclides. The average radium activity was below the permissible limit of 370 Bqkg−1, and the annual effective doses, hazard indices, gamma activity, and activity utilization index were all far below the safe limit of 1 mSvy−1. However, gonad equivalent and excess lifetime cancer risk were above their permissible limits. RHP average of 1.89 μWm−3 above the crustal range of 0.8–1.2 μWm−3 will considerably contribute to the heat flux in the areas. Based on the prediction accuracy of the R2 results, 36–86% of RHP rates in the rocks are achievable from the radionuclides.

Field Observations, Petrography, and Microstructures of Granite from Abeokuta Southwestern Nigeria

Abstract in English The granite of Abeokuta is part of the Older Granite suite of the basement complex of southwestern Nigeria. Field observation has identified the granite as porphyritic in texture, with a preferred alignment of megacrysts of K-feldspar &gt;5 cm that are mostly euhedral to subhedral in shape. K-feldspar megacrysts are predominantly yellowish, but whitish and pinkish colouration has also been observed. The dark grey porphyritic, mafic rock composition occurs as enclaves within the granite, with phenocrysts having a preferred alignment as the enclosing granite. Also, K-feldspar megacrysts show zoning defined by concentric arrangement of inclusions of biotite relative to the crystal faces. Petrographic study shows the following minerals in order of abundance: K-feldspar, biotite, quartz, and plagioclase, with K-feldspar having a microperthitic intergrowth and biotite crystals aligned in a preferred direction. Evidence such as the crystal shape of K-feldspars, preferred alignment of K-feldspar phenocrysts, and concentric crystallographic arrangements of inclusions of biotite in K-feldspar support a magmatic/phenocrystic origin of K-feldspar megacrysts of Abeokuta granite rather than originating by growing in a solid state as porphyroblasts.

Investigation of Dam Integrity from Electrical Resistivity Methods: A Case of Erelu Dam, Southwestern Nigeria

DOI:10.17014/ijog.8.2.265-274Erelu dam is geologically within the Precambrian Basement Complex of southwestern Nigeria, which serves a principal source of water supply to an increasing population; currently at about 779,318. In an attempt to investigate the integrity of the dam, horizontal resistivity profiling (HRP) of dipole-dipole and Schlumberger’s vertical electrical sounding (VES) approaches were explored to delineate the lithologic layers, facies, and geologic structures in the subsurface of the dam. The HRP revealed changes in subsurface conductivity that depicts lateritic hard pan and saturated rock blankets at shallow depth. The vertical geo-electric section delineated predominant four lithologic units. The regolith is 7 m thick and constitutes the vertical rock fill impervious cores of the dam, which is underlained by saprolitic and fresh bedrock. The region of weathered rock depression coincided the shallow saturated anomalous zone and showed surface manifestation of fractured or unconsolidated terrain within the embankment. The fracture serves as conduit for seepage which could be responsible for possible loss in the reservoir water and increases its susceptibility to failure. It is recommended to keep monitoring the seepage with reservoir levels, by periodical geophysical and geotechnical measurements for the two (dry and wet) seasons.

English

This paper investigates and reports the lithological and compositional features of Igue marble deposit in southwestern Nigeria. The deposit occurs as low-lying heterogeneous units under a humus-laden dark brown tropical soil within Igarra Schist Belt on south eastern flank of basement complex of southwestern Nigeria. The marble deposit occurs as lenses sandwiched between quartz-biotite schist, calc-silicate gneiss and polygenetic metaconglomerate intercalated with mica schist and quartzite all resting on ancient migmatite gneiss. The three colour variants of the marble which are white, grey, and foliated have average SiO2 contents of 2.92, 1.72 and 4.04% respectively. In the same order, average CaO contents are 60.76, 59.35 and 51.7%; MgO contents are 1.62, 1.42 and 2.57% indicating the marble is calcitic in nature. The oxides P2O5, TiO2 and MnO altogether constitute 0.09, 0.11 and 0.41% of the bulk chemistry of the white, grey, and foliated marble, respectively. Average loss on ignition (LOI) in white marble (33.46%), grey marble (36.12%) and foliated marble (33.49%) are relatively high suggesting high volatile contents. Chemical features of the marble deposit are comparable to similar rock types in Obajana and Ososo areas in Southwestern Nigeria. The marble differs significantly from Igbeti marble deposit which is dolomitic in nature. Marble deposits from different parts of Nigeria basement exhibits variable geochemical features that are influenced by their protoliths, mode of formation and associated lithologies and forms the basis for various economic uses. Key words: Igue marble, lithological and compositional features, Igarra Schist Belt, Nigeria, calcitic.

Investigating aquifer structure in a low-latitude crystalline basement complex of southwestern Nigeria using radial vertical electrical sounding

The ability of radial vertical electrical sounding (RVES) to detect anisotropy caused geologically by fracturing, jointing, layering, and rock fabrics was employed to investigate the structure of hard rock aquifers in Oyun-Asa basin. This was facilitated by the need to hydrogeologically characterize the aquifers in an attempt to meet increased demands on the potable water supply due to population explosion, increasing industrialization, and agricultural growth in the area. This was accomplished by using the anisotropy models obtained from the variation of computed coefficient of anisotropy and its orientation with depth to characterize hydrogeological properties of the weathering profile developed over different lithology in the area. The geometric mean resistivity values were also computed and calibrated to assess the degree of weathering. The coefficient of anisotropy values obtained from best-fit ellipses of the polar apparent resistivity plots of RVES data varied between 1.01 and 3.00 along different directions. The lower and upper range values, 1.01–1.20 and 1.21–3.00, were attributed to heterogeneous and homogeneous anisotropy, respectively. The heterogeneous anisotropy revealed pre-existing heterogeneities within the lithology at different depths and could have enhanced the weathering process locally. The homogeneous anisotropy indicated the fissured layers comprise three fissure sets: horizontal or stratified fissured layer (SFL), sub-vertical, and vertical. Six types of homogeneous anisotropy models whose coefficient of anisotropy (homogeneous) values and orientations vary with fracture intensity and depth were also identified. The geometric mean resistivity ranged between 82 and 2337 Ωm indicating varied degrees of weathering and its dependency of different lithologies in the study area. Aquifers in Oyun-Asa basin are localized, depending on extent of weathering and fissuring of the underlying rocks. The presence of SFL known for its contemporaneity with weathering process indicates the hard rocks in Oyun-Asa are in different weathering phases.

Aeromagnetic mapping of fault architecture along Lagos–Ore axis, southwestern Nigeria

AbstractA seismic wave is released when there is sudden displacement on a fault plane. The passage of this wave along the fault plane or within the lithosphere could result in ground shaking or vibration at the surface of the Earth. To provide a geophysical explanation to this phenomenon, the high-resolution aeromagnetic data of the sedimentary terrain and part of the Basement Complex of Southwestern Nigeria were processed and interpreted to provide fault architecture of the area, which could serve as conduit for the passage of seismic energy in the study area. High-resolution aeromagnetic data along the Lagos–Ore axis are processed for fault mapping in the study area. The reduced-to-equator (RTE) residual aeromagnetic data used were enhanced using the total horizontal derivative (THD) and upward continuation (UC) filtering techniques on Oasis Montaj 6.4.2 (HJ) software. The resultant maps were overlaid and compared with the plotted RTE residual maps for relevant interpretations. Varying signatures of magnetic anomalies are grouped into high (57.9–89.1 nT), intermediate (38.2–57.9 nT), and low (4.0–38.2 nT) magnetic intensities, which are associated with contracting basement rocks features. The obtained lineaments from the THD reveal areas of various deformations such as brittle, which is associated with faults/fractures, and ductile deformation, which is associated with folds of geological features. The faults, as depict by the UC map, reveal different depth ranges of 500–2250 m at the western side and 1,500–1,250 m at the northwestern area of the study. Since it has been on record that September 11, 2009, earth tremor of magnitude 4.4, with the epicenter at Allada, Bennin Republic, 128 km west of Lagos, Nigeria occurred within the study area, it can be inferred that the established geologic fault architecture could be responsible for the hazard and be part or synthetic to the Ifewara-Zungeru fault in Nigeria.

Petrography of Allanite-bearing Tonalite from Iwo Region, Osun State, Nigeria

Abstract Primary, secondary and accessory minerals in tonalitic rocks from Iwo region of the Precambrian Basement Complex of Southwestern Nigeria were identified and analysed with the aim of determining the various processes involved during the crystallisation of magma. Thin sections of tonalite were prepared and studied with the aid of a petrographic microscope. The mineral assemblages observed are biotite, plagioclase, alkali-feldspar, amphiboles, pyroxene, quartz, muscovite and chlorite. Allanite, titanite, apatite and zircon occur as accessory minerals. Muscovite and chlorite are found to be secondary minerals. The mineral allanite has a characteristic form of zoning and shows evidence of metamictisation, and is surrounded by dark-coloured biotite having radioactive haloes. Titanite is anhedral to subhedral crystals and forms reaction rim round opaque minerals. Plagioclase shows evidence of compositional zoning as well as plastic deformation of the twin lamellae. The allanite observed is primary in nature and has undergone radioactive disintegration; chlorite and muscovite are formed by secondary processes of chloritization and sericitisation, respectively. The tonalite is formed as a result of rapid cooling of magma close to the Earth's surface.

Integrated geophysical investigation of aquifer and its groundwater potential in phases 1 and 2, Federal University Oye-Ekiti, south-western basement complex of Nigeria

Electrical resistivity method is frequently used in groundwater studies. It is one of the many geophysical methods used in finding, locating, and defining subsurface water. The geoelectrical survey was done in Federal University Oye-Ekiti, phases 1 and 2, south-western Nigeria, to assess the groundwater potential. Fifteen very low frequency electromagnetic traverses, three dipole–dipole profiles and three Wenner profiles and 25 vertical electrical soundings were occupied and investigated with maximum current electrode, were arranged with an equal potential spacing (AB/2) of 1–100 m with the aid of ABEM SAS 300C resistivity meter and presented as VES curves. Twenty-five VES location were evenly distributed within the study area. The VES interpretation results are of different VES curves types, namely QA, HA, KH, AH, H, KHK, KQ and AK types. The geoelectric sections delineated three to four geoelectric layers, which are the topsoil with resistivities and thickness values ranging between 113.4 Ωm and 753.1 Ωm and 0.4 and 4.4 m, respectively. The weathered layer has resistivity and thickness values ranging between 37.4 Ωm and 278.1 Ωm and 1.3 and 9.8 m, respectively. The partially weathered layer resistivity and thickness values ranging 495.1–651.1 Ωm and 10.2–22.2 m; for lateritic zones, it shows the resistivity and thickness values ranging 280.6–730 Ωm and 0.1–6.3 m. The geosections show a relatively thin overburden of less than 2.1 m. The third layer is the fractured basement. The fresh basement has a resistivity ranging between 930.5 and 4660 Ωm with depth starting from about 10 m to infinity (∞). Based on relatively thin overburden and the moderate to high fracture density of the basement bedrock, it was concluded that the groundwater potential of the study area is generally low to moderate. However, the aquifer in the north-eastern and south-eastern parts of the study area was determined to be of good quality. It can yield reasonable quantities of water that can serve the university community.

The influence of bedrocks on groundwater chemistry in a crystalline basement complex of southwestern Nigeria

Groundwater in crystalline basement is highly mineralized. The chemistry of bedrocks in crystalline basement of Ogbomosho in relation to chemical composition of groundwater in the area was, therefore, investigated employing a combined approach involving hydrogeological, geological mapping and geochemical method. A total of seventy five (75) water samples collected from wells; shallow and deep, bore within rock exposures (6) and other locations within the study area were analyzed for elemental composition using Inductive Coupled Plasma-Mass Spectrometry (ICP-MS). Physico-chemical parameters, temperature, electrical conductivity (EC), total dissolved solids (TDS) and acidity (pH), were measured in situ using HANNA HI9813-6 hand-held meter. Six rock samples representative of the main geological units of the study area were also collected and analyzed for major oxides and rare earth elements (REE) concentrations using ICP-MS. Thin sections of the rock samples were prepared and observed under petrological microscope for mineralogical compositions of the rocks. Acidity ions/TDS model of groundwater in the area was also established to determine the reactivity of the rock minerals and groundwater evolution pattern. The results of in-situ physico-chemical tests of the water samples indicate the temperature, EC, TDS and pH ranged within 26.6–31.7 °C, 72–1491 mS/cm, 36–747 ppm and 6.15–9.3, respectively. The petrographical analysis revealed biotite, quartz, potassic feldspar (microcline), perthite, albite, hornblende, plagioclase, myrmekite, topaz and muscovite. The major oxides, SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, TiO2, P2O5 and MnO, varied with median values of 72.21%, 14.99%, 1.17%, 0.375%, 2.245%, 4.215%, 3.46%, 0.16%, 0.045% and 0.015%. These indicate dorminance of SiO2 and Al2O3 suggesting acidic and metamorphic/acidic igneous rocks, respectively. The median percentage oxide compositions of the cations in rocks were, therefore, of the order: Na2O > K2O > CaO > MgO. The dissolved cations consequent upon weathering of the minerals were also of the order Na+ (26.9 mg/l) > K+ (4.69 mg/l) > Mg2+ (4.57 mg/l) > Ca2+ (4.23 mg/l) in groundwater based on their median values. These indicate solute concentration in groundwater is proportional to the reactivity of the bedrock minerals. The major anions, HCO3−, Cl− and NO3−, varied within 5.0–455 mg/l, 14.18–184.34 mg/l and 0.02–0.21 mg/l, respectively, in groundwater. The cross-plots of TDS against Ca2+, Na+, Cl− and HCO3− indicate the groundwater in the area is moderately to highly mineralized. The ionic reactivity based on pH ions/TDS plots indicated six-type models. The ionic concentration at low pH within 6.4–7.2 increases with pH for Type-1 (Ca2+), Type-5 (NO3−) and Type-6 (HCO3−,TDS), decreases for Type-2 (Mg2+, K+) but constant as pH increases for Type-4 (Cl−). At high pH, greater than 7.2 but less than 9, ionic concentration increases with pH for Type-1, Type-2 and Type-4, decreases with pH for Type-6 and constant for Type-5. However, the ionic concentration of Na+ (Type-3) momentarily increased, decreased and subsequently increased through low to high pH. These indicate varied degree of reactivity of the bedrock minerals and groundwater evolution pattern. Thus, the composition of dissolved ions in groundwater is controlled by weathering of Ca-feldspar (plagioclase), K-feldspar (orthoclase), Na-feldspar (albite) and biotite found in host rocks. The similarity in trend of cations in groundwater and their oxides in rock samples suggests the influences of local rock chemistry on the groundwater chemistry and hence, groundwater–rock interaction in the study area. The groundwater evolution pattern in the area depends on pH and reactivity of the ions produced from the weathering of the minerals. The chemical compositions of natural waters are, therefore, a direct indication of the geology of their catchment.

Investigating groundwater corrosion and overburden protective capacity in a low latitude crystalline basement complex of southwestern Nigeria

ABSTRACT The total longitudinal conductance S obtained from the interpretations of vertical electrical soundings data acquired near 40 well bores was calibrated using total dissolved solids (measured in situ) in the sampled wells’ groundwater to determine the overburden protective capacity and groundwater corrosion in a crystalline basement complex of Ogbomoso North, southwestern Nigeria. The acidity (pH) and electrical conductivity (EC) were also measured. Groundwater corrosiveness was determined using the Langelier saturation index (LSI) and the aggressive index (AI). The computed S of predominantly H-, KH-, QH-, HA- and HKH- types resistivity curve was 0.037995– 1.10878 mhos. The TDS, pH and EC of the water samples measured 36–747 ppm, 6.15–9.3 and 74–1491 μS/cm, respectively. The regression equation of the TDS-based S calibration curve indicates a strong positive correlation (R2 = 0.8932) and revealed the overburden protective capacity ranged from excellent to good, fair and poor. The computed LSI and AI were −3.61 to 1.2 and 7.8–12.8, respectively. The high values of both LSI (0.2–1.2) and AI (11.2–12.8) in the southwestern part suggest groundwater there is strongly corrosive, while low values of −3.6 to −1.2 and 7.8–10.2 in the northwestern and southeastern parts indicate non-corrosive groundwater.

Understanding the interrelationship between electrical anisotropy and groundwater yield in a typical Basement Complex area

The rocks in the Basement Complexof southwestern Nigeria exhibit electrical anisotropy as a result of their heterogeneous nature caused by weathering, near-surface effects or presence of fractures. It is known that groundwater yield (Y) in a typical Basement Complex aquifers is related to its coefficient of anisotropy (λ). Therefore, it is essential to study the correlation between Y and λ in the basement complex. Ten (10) vertical electrical sounding (VES) points using Schlumberger array were studied in areas underlain by migmatite gneiss and quartzite rocks. The current electrode spacing (AB/2) varies from 1 m to a maximum spread length of 100 m. The quantitative interpretation of VES curves was done by using partial curve matching and computer assisted program called WinResist version 1.0 software. Dar Zarrouk parameters were estimated from the interpreted VES curves and thus coefficient of anisotropy (λ) was calculated from these parameters. Data on the groundwater yield at the VES points were also obtained. The results show that as λ increases, Y also increases with coefficient of correlation (R2) of 0.86 and 0.79 for migmatite gneiss and quartzite rocks respectively. To further investigate the relationship between Y andλ, a regression analysis was performed. The regression analysis that was performed on the dataset shows that λ contributes significantly to the regression models of the two rocks. The relationship between Y and λ reveals that groundwater yield could be predicted from the values of λ in a given locality.

In situ natural radioactivity and radiological hazard assessments of granite gneiss outcrops in parts of the Southwestern Basement Complex of Nigeria

Radioactive emission (gamma ray) emanating from rock materials constitutes threats to humans and their environment. Hence, the natural radioactivity and radiological hazard indices of some granite gneiss outcrops with their residual soils from two different localities within the Southwestern Basement Complex of Nigeria were assessed using Gamma-ray spectrometry. The study aimed at determining the natural radiation levels of granite gneissic rocks and associated radiological threats to people living in the study area from gamma ray exposure. Data were collected along six geophysical traverses of 200 m each with station spacing of 5 m using the portable hand-held Gamma-ray Spectrometer. The results of weighted mean values of the elemental and activity concentrations for 40K, 238U, and 232Th are 2.00 ± 1.43%, 3.13 ± 0.90 ppm, 12.32 ± 4.99 ppm and 629.28 ± 447.29, 38.58 ± 11.02, 49.96 ± 20.23  respectively. The obtained mean concentration ratios of 1.32 for U/Th and 2.14 for Th/U were higher than 0.26 and lower than 3.5 global standard ratios respectively. The increase in activity concentrations and concentration ratio may have suggested the enrichment of radioactive minerals in the granite gneissic rocks. The annual outdoor and indoor effective doses of gamma ray exposure for people living in the study area were below the world permissible standard of 1 , thus, the area is radiologically safe. However, increase in estimated mean of AGDE and ELCR; 525.620  and 1.273  above the world permissible standards of 300  and  respectively does not translate to significant radiological threats. Therefore, control measure by constant monitoring of the radioactivity levels of the area and rocks should be practiced.

Geoelectric evaluation of groundwater potential within sunshine garden estate, akure southwestern Nigeria

The Vertical Electrical Resistivity surveys in the Sunshine Garden Estate have contributed to a better understanding of the basement complex of Southwestern Nigeria. Nine (9) vertical electrical sounding (VES) using the Schlumberger electrode array were interpreted and the results shows three (3) subsurface geoelectric layers within the study area. These are the weathered layer, topsoil, fresh/fractured basement. Groundwater pockets such as fractured zones, valley fills/basement depressions, and weathered zones were delineated in the study area. Weathered/partially weathered layer and weathered basement/fractured basement were the two major aquifer mapped out and these aquifers are characterized by thick overburden, found within basement depressions. The groundwater potential of the study area was zoned into low, medium and high potentials. Zones where the overburden thickness (which constitutes the main aquifer unit) is greater than 13m and of low clay composition (average resistivity value between 200 - 400Ωm) are considered zones of high groundwater potential. Area where the thickness of the aquifer ranges from 11-13m with less clay composition are considered to have medium groundwater potential and the areas where the thickness of the aquifer is less than 11m are considered to have a low groundwater. The VES station underlained by high and medium groundwater potential zones are envisaged to be viable for groundwater development within the area.