Managing groundwater flow in crystalline basement aquifers (CBAs) remains challenging due to their dependence on secondary permeability fields characterized by high spatial variability. This study combines pumping and tracer tests to estimate the hydraulic properties and connectivity in four bedrock wells within a CBA in Southwestern Nigeria. The pumping tests caused drawdowns up to 4.13 m and 12.60 m in observation and pumping wells, with significant drawdowns only in three of four wells, revealing poor connection with the fourth well. The time-drawdown plots confirm double porosity effects suggesting fracture and matrix flow and release of water from a fractured dyke. Fracture and matrix hydraulic conductivities exceeded 7.9 × 10−7 m/s and 1.00 × 10−10 m/s, while the aquifer yield ranged from 0.08 to 0.34%. Groundwater flow velocity and dispersivity of 5.80 × 10−4 m/s and 2.60 m were estimated from the tracer test, while a Peclet number of 3.25 suggests dominant advective flow. Calculated sustainable yield shows that each well could provide water for up to 1600 people under controlled low pumping at 0.50 l/s with higher rates possible using larger diameter wells. These results confirm high variability in groundwater flow within CBAs, justifying the need to characterize them effectively.

The organic-rich Lokpanta shale in the Anambra Basin is considered a potential unconventional hydrocarbon resource in Nigeria. Exploration of unconventional shales requires the paleoenvironmental reconstruction of organic matter productivity and preservation for sweet spot mapping. However, limited studies have been conducted on the paleodepositional factors governing organic matter accumulation in the Lokpanta Shale. This study used scanning electron microscopic and geochemical analyses to reconstruct the paleodepositional settings. Total organic carbon (TOC) content averages 4.34%, indicating very good hydrocarbon generative potential. Lokpanta shale contains Type II organic matter and planktonic foraminifera without benthos, suggesting a marine but limitedly oxygenated setting. The Sr/Ba (1.28-9.27) and Rb/K (37.18-51.93) ratios indicate high paleosalinity, aiding organic matter preservation. Certain enriched trace elements (Mo, Cd, As, Cu, Ti, Ni, and Fe) are similar to signatures in modern hypoxic (e.g., Namibian shelf) and anoxic-euxinic (e.g., Mediterranean sapropels and Black Sea) marine environments, which are both associated with sulfidation for organic matter preservation. Observed pyrite exhibits a primarily spheroidal framboid morphology with an average size of <5.2 mu m and a standard deviation of similar to 2, indicating a syngenetic origin of an euxinic depositional environment. The paleoredox proxies (Mo/TOC <15 ppm/gTOC, Th/U > 2, V/Sc > 16, V/(V + Ni) > 0.70, and Ni/Co > 6) indicate deposition in highly anoxic and intensely sulfidic waters within a highly restricted sill basin exhibits characteristics of an euxinic setting. In addition, Rb/Sr (0.031-0.132) and Sr/Cu (13.77-28.45) and C-value (<0.1) ratios suggest hot and arid paleoclimate, which aided carbonate productivity. The enrichment of CaO, Si, Ti, Zr, and depletion of Th and rare earth elements (REE) contents suggests low clastic influx aiding in carbonate productivity and organic matter preservation. However, the ratios of Ba/Al (18.5-46.7), Baxs (56.63-489.14), Fe/Ti (4.85-11.29), and Al/Al + Fe (0.58-0.79) suggest that organic matter accumulation was governed by neither primary paleoproductivity nor hydrothermal activities. This euxinic setting may not have extended to coeval shales in other coastal basins of West Africa, even though anoxic conditions existed in those areas.

Characterizing the subsurface distribution of crude oil after a spill in a coastal environment is challenging due to variations in the soil and fluid properties. In situ sampling is limited in capturing the lateral and vertical migration of the crude oil within the vadose and saturated zones. This study presents a laboratory sandbox framework used to assess the effectiveness of electrical resistivity imaging for investigating the spatiotemporal distribution of crude oil in coastal sandy soils. A sandbox with dimensions L = 240 cm, W = 60 cm, and H = 60 cm was constructed using a 10 mm plexiglass and filled to a 40 cm height with 2 mm medium to fine-grained sand. At each stage of the experiment, 20 kg of sand was mixed with 1 l of water to create moist sand, after which the mixture was flushed over 12 h to remove suspended fine particles. Both saturated and unsaturated conditions were simulated by setting the water table at 10 cm and draining a fully saturated system overnight. Two liters of crude oil were spilled and monitored for 30 h. A surface array of 98 electrodes, with a unit electrode spacing of 2 cm, was installed along two transects 12 cm apart. Resistivity measurements were collected using a dipole-dipole array before, during, and after the simulated crude oil spill. The time-lapse electrical resistivity results revealed an initial gravity-induced vertical migration under both saturated and unsaturated conditions; over time, lateral migration of crude oil became apparent. In the saturated zone, there was a noticeable reduction in the percentage difference in resistivity from 700 % to 400 % after 24 h, depicting a spatial and temporal redistribution of the crude oil attributed to variation in pore geometry. This highlights the sensitivity of electrical resistivity measurements to subtle but measurable anisotropy in the distribution of soil pores. Overall, electrical resistivity proved successful in imaging the non-ideal behavior of crude oil pollutants and the associated spatial changes in the pore-size distribution of subsurface sediments.

Monitoring crude oil spills in coastal areas is challenging due to limitations in traditional in situ methods. Electrical resistivity imaging (ERI) offers a high-resolution approach to monitoring the subsurface spatial distribution of crude oil, but its effectiveness in highly-resistive, unsaturated coastal sands with varying salinity remains unexplored. This study assessed the effectiveness of ERI for monitoring crude oil spills in sandy soil using a 200 × 60 × 60 cm 3D sandbox filled with medium-fine-grained sand under unsaturated conditions. Two liters of crude oil were spilled under controlled conditions and monitored for 48 h using two surface ERI transects with 98 electrodes spaced every 2 cm and a dipole–dipole electrode array. The influence of varying salinity was simulated by varying the pore-fluid conductivities at four levels (0.6, 20, 50, and 85 mS/cm). After 48 h, the results show a percentage resistivity increase of 980%, 280%, 142%, and 70% for 0.6, 20, 50, and 85 mS/cm, respectively. The crude oil migration patterns varied with porewater salinity as higher salinity enhanced the crude oil retention at shallow depth. High salinity produces a smaller resistivity contrast, thus limiting the sensitivity of ERI in detecting the crude oil contaminant. These findings underscore the need to account for salinity variations when designing remediation strategies, as elevated salinity may restrict crude oil migration, resulting in localized contaminations.

The demand for economical means of evaluating soil nutrients' unpredictability triggered the use of physical factors against the costlier, laborious, and time-consuming chemical approach. This drive led to resolving its capability in evaluating intricate soil properties as a productivity checker. This study aimed at assessing the efficiency of apparent electrical conductivity as a useful alternative to the conventional chemical examination of available nutrients. A petrographic examination was conducted on four rock samples for their classification as the source of soil formation. Apparent Electrical Conductivity (ECa ) measurements were seasonally executed in the wet (912-station) and dry (906-station); the ArcGIS 10.2 programme was used in generating the EC(a )maps for classifying the ECa into low, moderate, and high sections. Ten cored ( in-situ ) soil samples were subjected to permeability test to ascertain water infiltration rate and retention. Twenty soil samples were examined for pH, Electrical Conductivity (EC), available phosphorus, acidity, Na, Mg, K, and Ca using standard soil science procedures. The mineralogical composition of six samples (two samples per ECa region) was determined with X-ray diffraction. The soils EC(a )were 10-344 mu S/cm, categorized as low (1-49 mu S/cm), moderate (50-99 mu S/cm), and high (>100 mu S/cm). The ECa distribution varied from moderate (61%) to high (64%), suggesting a heterogeneous pattern of soil attributes. The infiltration rate was slow in high ECa (5.56x10 -5 -1.67x10 -4 cm/s), signifying good retention capability, whereas the low and moderate ECa (moderate-moderately rapid) sections promote nutrient leaching. 'Positive correlation was observed between CEC and ECa in the low ECa (2.99 cmol/kg), the same was observed in the moderate and high ECa (3.30-4.85 cmol/kg) segments'. The base cation saturation varied from high (81.38%), moderate (73.34%) and low (71.89%), characterizing the high ECa as the higher fertility status. Soils with moderate and high EC(a )have a greater ability to adsorb cations onto their surfaces than soils with low ECa; similarly, the deduction from base saturation showed that more cations are available in soils with high ECa . The high ECa region had low quartz (41.3%) and microcline (15.7%), but high kaolinite (31.1%) had an affinity to adsorb more cations compared to other

The demand for economical means of evaluating soil nutrients’ unpredictability triggered the use of physical factors against the costlier, laborious, and time-consuming chemical approach. This drive led to resolving its capability in evaluating intricate soil properties as a productivity checker. This study aimed at assessing the physical parameters as a useful alternative to the conventional chemical examination of nutrient inconsistency. A petrographic examination was conducted on four rock samples for their classifications. Apparent Electrical Conductivity (ECa) measurements were seasonally executed in the wet (912-station) and dry (906-station). Ten cored soil samples were subjected to a permeability test. Twenty soil samples were examined for pH, Electrical Conductivity (EC), available phosphorus, acidity, Na, Mg, K, and Ca using standard soil science procedures. The mineralogical composition of six samples was determined with X-ray diffraction. The rock is biotite granite gneiss containing plagioclase (22%), microcline (24%), orthoclase (4%), quartz (25%), biotite (7%), and others (18%). The soils ECa were 10-344 µS/cm; categorised as low (1-49 µS/cm), moderate (50-99 µS/cm), and high (>100 µS/cm). The ECa distribution varied from moderate (61%) to high (64%) suggesting a heterogeneous pattern of soil attributes. The infiltration rate was slow in high ECa (5.56x10-5-1.67x10-4 cm/s) signifying good retention capability whereas the low and moderate ECa (moderate-moderately rapid) sections promote nutrient leaching. The cation exchangeable capacity was low (2.99 cmol/kg) in the low ECa and moderate (3.30-4.85 cmol/kg) in the moderate and high ECa; with varying basic cation saturation in the high (81.38%), moderate (73.34%) and low (71.89%) ECa regions and high ECa had higher fertility status. The high ECa had low quartz (41.3%) and microcline (15.7%), but high kaolinite (31.1%) had an affinity to ads orb more cations compared to other ECa regions. ECa variability is practicable in predicting the spatial distribution of soil properties and delineating the management zones. Key words: Granite gneiss, electrical conductivity, permeability, soil composition, mineral assemblages

This study focuses on the comprehensive reservoir characterization of the 'Kukih' Field within the onshore northeastern Niger Delta region, Nigeria. The absence of its detailed description with delineated reservoir properties, lateral continuity, and their use to identify potential reservoir quality and heterogeneity necessitated this study. Integrating well log and 3D seismic data, the investigation aims to elucidate reservoir properties, lithofacies, and depositional environments to unravel hydrocarbon potential. The geological setting, encompassing the Agbada Formation of Early and Middle Miocene age, is scrutinized through detailed geologic analysis. Petrophysical evaluation of four well logs (Kukih-1, Kukih-2, Kukih-3, and Kukih-4) facilitated the determination of key parameters such as shale volume, effective porosity, and water saturation. Seismic interpretation further enriched the structural characterization of the field. Results showcase three predominant reservoir sands (A, B, and C) with distinct lithofacies and thickness variations. Effective porosity ranges from Fair to Excellent, with permeability exhibiting high values for hydrocarbon reservoir potential. Water saturation trends, lithofacies distributions, and structural features were illuminated through iso-parametric maps and seismic analyses. Depositional environments were inferred through facies analysis, revealing the presence of funnel-, cylinder-, and bell-shaped successions that hint at intricate marine sedimentary processes. Challenges owing to limited core data were acknowledged, and the integration of methodologies emerged as a pivotal strategy for enhanced reservoir understanding. This study underscores the 'Kukih' Field's hydrocarbon potential, accentuating the significance of multidisciplinary approaches in deciphering complex reservoir systems. In light of the petrophysical analysis derived from the well logs and the identification of structural highs through the structural maps, this study recommends the drilling of unexplored zones exhibiting promising structural characteristics.

Petrological, mineralogical and magnetic susceptibility measurements have been studied within the southwestern Nigeria Precambrian basement rocks. Detail geological mapping and core sampling was done for mineralogical characterisation and magnetic susceptibility of the rock samples. Results of the petrographic analyses revealed: quartz, feldspars, hornblende, biotite and opaque (magnetite, ilmenite, maghemite, titanomaghemite) minerals; with minor component of ferromagnesian minerals like hornblende. Quartz, Microcline and Plagioclase alone constitute up to volume fractions 70 % of the rock in the thin section with plagioclase being the most dominant. The magnetic measurements have revealed that the volume susceptibilities K from the investigated samples range from 0.2 to 56,419 X10-6 [SI]. Charrnockites, Granite, biotite granite gneiss, banded gneiss, gneiss, aplite dike, granite, diorite granite gneiss ranges from: 10,059-56,419; 247-12856;0.2-129; 20.4-39.4; 14.1-22,366; 20-8250; 14-30233;4467-6641; and 632-26,921 X 10-6 [SI] respectively. Differential results in the frequency distribution is due varied composition of the paramagnetic (low magnetic susceptibility) and ferromagnetic (high magnetic susceptibility) grains in the rock specimen.

The study involved the determination of strength parameters and in situ stresses in two wells pseudo-named H005 and H008 in a hydrocarbon field located offshore of Rio Del Rey basin. The work was aimed at revealing the stability of formation rocks and fault stress regimes associated with the study wells. The parameters determined employed only empirical correlations and tools since core samples were not available for laboratory experiments. The unconfined compressive strength for the two wells was relatively low varying from 1500 to 3300 psi in well H005 and from 1000 to 4000 psi in well H008. The friction angle varies from 21.0° to 23.0° in well H005 and from 21.0° to 27.0° in well H008. The results of the in situ stress analysis in well H005 indicate a reverse fault stress regime where maximum horizontal stress ( S H ) is greater than the minimum horizontal stress ( S h ), which is in turn greater than the vertical stress ( S V ) in the shallow part of the well (1028.5–1249 m), while a normal fault stress regime where the vertical stress is greater than the two horizontal stresses was interpreted at a depth from 1250 to 1390 m. Unlike well H005, the entire depth of well H008 under interpretation revealed a normal fault stress regime. The outcome of the 3D geomechanical model closely agrees with the 1D model. It was concluded that the formation rocks are weak, and the wells are dominated by normal fault stress regimes.

Naturally occurring bitumen seeps migrating within the shallow subsurface contaminates soil and groundwater in communities within the eastern segment of the Dahomey Basin in southwestern Nigeria. Managing these contaminations require an understanding of the distribution of the bitumen seeps to isolate contaminated regions to avoid farming activities or drilling shallow drinking water wells within them. Several studies have assessed the shallow occurrence of bitumen in the region, focusing on their economic potentials but ignores the challenges with soil and groundwater contamination in these communities. This study focuses on using electrical resistivity imaging techniques with limited soil cores to delineate the distribution of bitumen seeps within the shallow subsurface in Imakun-Omi community in southwestern Nigeria. We used both 1D and 2D numerical and field approaches in this study. 13 vertical electrical soundings using a Schlumberger electrode array were first acquired to obtain the bulk resistivities and layer thicknesses. The 1D resistivity alongside well lithologic data served as prior data for a forward modelling study to assess the capability of resolving the bitumen seeps within electrically resistive coastal sands. Results of the forward modelling studies guided the acquisition of five 2D electrical resistivity profiles used to image the distribution of bitumen seeps using a Wenner electrode array with a unit electrode spacing of 2 m. Results of this study show that bitumen seeps with resistivities greater than 3000 Ohm-m are distributed within the top 4 m in the area and extends down to 10 m in some locations. Our numerical studies show that despite difficulties in imaging bitumen seeps within sediments overlain by higher electrical resistivity layers, reliable results can be derived, thus proving the versatility of electrical resistivity in delineating areas contaminated by bitumen seeps within shallow coastal sands.

Precision agriculture requires soil nutrient evaluation via chemical assessment, which is costly and requires painstaking laboratory analysis, whereas the geophysical technique has gained acceptance worldwide as an economical alternative. This research focused on the effectiveness of physical parameters in determining soil quality. The investigation was limited to 0.3-m soil depth at the kola farm of the Cocoa Research Institute of Nigeria, Ibadan. Seven hundred sites were occupied with earth resistivity and VG-meter-200 moisture meters measuring apparent electrical conductivity (ECa) and volumetric water content, respectively. Falling-head permeability was performed on ten cored soil sample. Forty-two samples were analyzed for textural variations using the Bouyoucos method. The soil ECa (12–545 μS/cm) was classified into 1–49 μS/cm, 50–99 μS/cm, >100 μS/cm as low ECa (LECa), moderate ECa (MECa), and high ECa (HECa), respectively. The deduction showed that soil ECa has high (82.83

Management of soil and groundwater resources has been recognized as essential to meeting the sustainable development goals of Agenda 2063 of the African Union. As Africa's fastest growing population with over 200 million people, Nigeria is responsible for leading the continent's environmental sustainability goal. Nigeria has seen a sizable number of crude oil spillages that have contaminated its soils and groundwater resources, and several of these contaminated sites are to be cleaned up yet. There needs to be more scientific data to design an effective cleanup and to manage the soil and groundwater resources effectively. So far, the only extensive crude oil-contaminated site remediation project documented is on Ogoniland in the Niger Delta region of Nigeria. However, this project resulted in less effective, albeit temporary, cleanup solutions. This review presents a state-of-the-art synthesis of research on soil and groundwater contamination by crude oil. It includes sections on processes, measurements, predictions, and management, as well as an analysis of the state and challenges in Nigeria. In-depth field, laboratory, and computer models for crude oil contamination investigation have been developed with over 60 years of significant research. However, studies and case projects in Nigeria have relied on point sampling to determine the concentration of crude oil contaminants in soil and groundwater. This method offers limited information on the solute concentration and hydraulic distribution, which regulates pollutant mobility within the subsurface. The absence of baseline and high-resolution subsurface characterization data has also resulted in a need for more process-based knowledge to direct the development of site-specific remediation strategies. As a result, it is challenging to design a conceptual model that is detailed enough to help with predictions of the flow dynamics of crude oil contaminants in the unsaturated and saturated zones. It is anticipated that this review will stimulate further multidisciplinary research through site evaluation and monitoring to provide reliable information that can be used to develop appropriate model-based remediation solutions.

This study investigated the effects of seasonal variation of resistivity on the regoliths in the basement complex setting in Ibadan, southwestern Nigeria, to quantify the variation in the geoelectrical parameters and geohydraulic properties, as well as the aquifer prospects and protective capacity of the bedrocks. Sixty-four vertical electrical soundings (VES) were conducted in the dry and wet seasons (32 VES for each season). The VES revealed resistivity variations in the thin topmost layer, with the variations occurring at 2.4–3.2 m in quartzite–quartz Schist, 1.3–1.8 m in undifferentiated schist, and 1.3–4.2 m in migmatite gneiss. The top layer resistivity varied from 19 to 1385 Ωm (dry season) and 29 to 444 Ωm (wet season), with thicknesses ranging from 03 to 3.4 and 0.2 to 2.5 m, respectively. The longitudinal conductance ( S ) ranged between 0.01 and 0.5 m (wet), and 0.03 and 0.6 m (dry); while, the transverse resistance ( T ) averaged 1943.5 and 2409.5 Ωm 2 in the wet and dry seasons, respectively. The anisotropy coefficient ( λ ) ranged between 0.58 and 2.30 (wet), and 1.00 and 2.03 (dry). The paired t test analysis revealed that the p values for most of the geoelectrical parameters are greater than the significance level ( p value > 0.05), implying that the model does not explain the seasonal variations in the parameters. However, the exponential regression models ( R 2 ) and p values for the topsoil resistivity and λ with corresponding values of R 2 = 0.62, p value < 0.05, and R 2 = 0.8, p < 0.05, suggest a model that explains a good part of the seasonal variations. The aquifer prospects and protective capacity of the bedrocks indicated moderate to good for both seasons, while the quartzite bedrock revealed poor groundwater prospects in the dry season and moderate prospects for the wet season. The study concluded that only the topsoil resistivity and the λ show seasonal variability.

The impact of bitumen components on soil and groundwater resources is of environmental importance. Contaminants' influx into the environment from bitumen components through anthropogenic activities such as exploration, mining, transportation, and usage of bitumen in all its forms have been reported globally. However, gaps exist in the geogenic occurrence of bitumen in the shallow subsurface such as in southwest Nigeria, contaminating the soil and groundwater resources. This review presents in situ bitumen seeps as a source of geogenic soil and groundwater contaminants in southwestern Nigeria. We conducted a systematic review of literatures based on defined selection criteria. We derived information on the state of knowledge about bitumen seep occurrences and distribution in southwestern Nigeria. Also, the processes that exacerbate bitumen contaminants' influx into soil and groundwater were enunciated. At the same time, case examples highlighted areas for possible in situ bitumen contamination studies in Nigeria. The results of this review showed that a multidisciplinary approach has been employed to assess and monitor the contaminants resulting from the various activities involving the exploitation and application of bitumen in Nigeria. These studies emphasize bitumen contaminants as emanating from anthropogenic sources. The results also suggested that bitumen studies have been mainly exploratory to improve the understanding of the economic potential of the hydrocarbon reserve. Also, recent advances in bitumen contaminants studies accounted for the heterogeneous nature of the bitumen. This allows for the optimized categorization of the mechanism and processes undergone by the different bitumen components when released as environmental contaminants. However, a knowledge gap exists in characterizing and understanding the effects of in situ bitumen seeps as a geogenic source of soil and groundwater contamination. This review identifies the possibility of geogenic soil and groundwater contamination by in situ bitumen seeps in the coastal plain sand of the Dahomey basin in southwestern Nigeria. The impact of the bitumen contaminants on the environment was discussed, while methods for accessing the occurrence and distribution of the bitumen contaminants were highlighted.

There exists little or no core laboratory data needed for dynamic measurements-to-static elastic moduli calibration in most of the offshore fields in Niger Delta Basin. Wells are planned or drilled with little or no geomechanics inputs. Oftentimes, this has led to wellbore instability issues, non-productive time, and abandonment. To overcome this challenge, Gassmann's fluid substitution model that connects with the rock volume's geomechanical properties is therefore presented. This approach involves draining the rocks of its initial pore fluid, so that in-situ dry rock moduli (bulk and shear) and bulk density of the porous frame are estimated. Shale anisotropy, which has effect on wellbore stability analysis especially in highly deviated and shale-rich wells is corrected for in the process. The in-situ static relationship for dynamic-to-static elastic moduli transform is provided and has proven to be optimal in mud-weight design in the offset wells. The static model was compared to existing empirical formula to prove the reliability and otherwise of the model. The results showed that pore pressure exceeded 5200 psi at depth > 8000 ftss with corresponding overpressure >2000 psi. Sudden pressure ramp was observed between 7400 - 7600 ftss and corresponded to the interval kicks which were recorded in the offset wells. At these depths, it was observed that the initial mud-weights window was selected without recourse to geomechanics, i.e., the in-situ stresses and static rock elastic properties interplays were ignored thus, leading to an underbalanced drilling. The ranges of the obtained static rock mechanical properties, the in-situ stresses, the resulting mud-weights, and shear failure profiles at the corresponding depths are presented as reference. The most significant part of this static model is that it has been applied to drill five nearby wells successfully for the first time in this field and can be adapted for future well planning and drilling optimisation. This rock physics approach though rigorous but novel in the sense that it can reduce uncertainties inherent in the characterisation of subsurface stresses and static elastic properties, especially in the absence of core or in-situ laboratory measurements.

Crystalline basement aquifers are important drinking water sources in Nigeria and several sub-Saharan African countries. However, an understanding of their local flow and transport processes and pathways is missing due to limited research. The implication has been their suboptimal management, with frequently reported dry wells and groundwater contaminations. To address this challenge, the Ibadan Hydrogeophysics Research Site was established in 2019 as the first field-scale hydrogeological research laboratory in Nigeria to advance understanding of the geologic, hydraulic, and hydrogeochemical variabilities within crystalline basement aquifers. The over 22,500 m2 research site with a 50 m × 50 m area used for active hydraulic testing is located within the University of Ibadan campus and is instrumented with four initial test wells extending through the weathered and fractured zones to a depth of 30 m each. Preliminary hydrogeological and geophysical studies focused on obtaining a conceptual model and knowledge of hydraulic heterogeneities to aid in detailed experimental and numerical studies. A combination of lithological logs and electrical resistivity revealed areas with subvertical fractures as low-resistivity zones (<200 Ωm), and a pumping test revealed a hydraulic conductivity range of 1.9 × 10−10 to 7.2 × 10−6 m/s. The drawdown–time curve shows flow from single-plane vertical fractures. The results of this study will serve as a basis for further targeted field and numerical studies for the investigation of variability in groundwater flow in complex crystalline basement aquifers. The presented field site is posed to support the adaptation and development of field methods for studying local heterogeneities within these aquifers in Nigeria.

In situ bitumen seeps within the shallow subsurface of a section of the coastal Dahomey basin of southwestern Nigeria act as a geogenic source of polycyclic aromatic hydrocarbons (PAHs), which contaminates soil and groundwater resources in the area. This study focuses on a hydrogeochemical characterization of PAHs con-taminants from the bitumen seeps and assesses their distribution using a groundwater flow and transport model. To do this, we used geophysical results from a previous study to determine the location of three water wells drilled to depths of 15 m each. Soil and groundwater samples were retrieved from different depths during drilling and used for sedimentological and geochemical analysis. We also performed pumping tests on all 3 wells to determine the hydraulic properties of the aquifer in the area and, lastly, developed a MODFLOW-based groundwater flow and transport model to simulate the distribution of the contaminants. Bitumen-bearing sedi-ments retrieved during the groundwater well installation confirmed earlier interpretations of geophysical results, which mapped subsurface lithologies that host the bitumen seeps. These lithologies, which act as PAH contaminant sources, occur at depths below 11 m towards the southern part of the study area. Hydraulic con-ductivity and porosity of the sandy aquifer were estimated to be 0.25 m/day and 46%, respectively. The United States Environmental Protection Agency's priority PAHs, including Naphthalene, Acenaphthylene, Fluorene, and Pyrene of geogenic sources, were identified with concentrations ranging from Below Detection Limit (BDL) to 5 ppm in both sediment and groundwater samples. We simulated the distribution of Naphthalene in groundwater in the area using the contaminant transport model, considering its high dissolution tendency. Hydrodynamic dispersion was observed as the dominant transport mechanism of the contaminants in groundwater. A distri-bution rate at the concentration of significant risk to human exposure was predicted to be 0.022 m/day and 0.015 m/day in the horizontal and vertical directions, respectively. This study confirmed groundwater contamination by PAHs from in situ bitumen seeps and provided a framework for future groundwater remedi-ation within the area.

Human activities on the environment have been the major cause(s) of change in the ecosystem but the negative effect is responsible for its degradation. Effluents/oil spills generated from the petroleum products depot centre are often discharged into environment without proper clean-up mechanism in place. This study is focused on evaluating the protective capacity of the earth material above the aquifer and mapping of hydrocarbon plume. An approach engaging Vertical Electrical Sounding (VES) and remote sensing data was carried out with a view to developing groundwater potential and aquifer vulnerability maps of the study area. A total of twelve Vertical Electrical Soundings (VES) involving Schlumberger array was established. The Tigre earth resistivity meter was used in data acquisition, the current electrode (AB/2) separation ranged from 1 to 55 m and the data were processed using Win RESIST. The remote sensing involving processing and interpreting of Landsat-8 EMT + image covering the study area and the data were used in producing the lineament map of the area using the application of Geographic Information System-based multi-criteria technique ArcGIS software. The inverted resistivity graphs indicated two to three geo-electric sequences; topsoil (35-1558 Ωm), weathered layer(23–580 Ωm) and fractured/fresh bedrock (608–3217 Ωm). The two linear structures run in the northeast-southwest direction. The longitudinal conductance maps delineated areas with <0.1 mho as poor (67%) and 0.1–0.19 mho as weak (33%) protective capacity above the aquifer, which is susceptible to contamination. The resistivity technique was able to map the contaminated section and delineate its subsurface spread.

The physical examination of a functioning cacao farm revealed varying pod production rates in its area. Agricultural soil nutrients assessment is usually through soil geochemical/chemical analyses which are laborious and expensive, necessitating faster/cheaper alternatives. This investigation assessed the physical properties that can substitute for geochemical analysis of soil nutrient. The study was executed at 0.3 m depth. The Volumetric Water Content-VWC and Apparent Electrical Conductivity-ECa of the soils were determined using VG-meter-200 moisture-meter and resistivity earth-meter. 912 ECa/VWC points were measured. Soil textural classes (51-sample) were established using Bouyoucos method. Falling head permeability test was conducted on nine cored soil samples for water infiltration assessment. The soils ECa (10-344 µS/cm) and VWC distributions (2-69%) showed similar variation, increase in VWC corresponds to rise in ECa value; soil moisture aids the mobility of ions in solution and a rise in ECa connotes presence of more dissolved ions. The soils were classified as sandy loam, loamy sand and sandy clayey loam. Soils’ permeability ranged from 4.11x10-5-3.97x10-3 cm/sec; infiltration rate varied inversely with the ECa accounting for the moisture variation. Low permeable soil has high nutrient retention and water-holding capacities. Soil physical properties were effective in evaluating the nutrient inconsistency.