Crystalline Basement Aquifers Research Articles

Hydro-geochemical evolution, quality, and health risk assessment of groundwater in crystalline basement aquifer in Keffi, Nigeria

Hydro-geochemical evolution, quality, and health risk assessment of groundwater in crystalline basement aquifer in Keffi, Nigeria

Water–Rock Interactions Driving Groundwater Composition in the Pra Basin (Ghana) Identified by Combinatorial Inverse Geochemical Modelling

The crystalline basement aquifer of the Pra Basin in Ghana is essential to the water supply systems of the region. This region is experiencing the ongoing pollution of major river networks from illegal mining activities. Water management is difficult due to the limited knowledge of hydrochemical controls on the groundwater. This study investigates its evolution based on analyses from a previous groundwater sampling campaign and mineralogical investigation of outcrops. The dominant reactions driving the average groundwater composition were identified by means of a combinatorial inverse modelling approach under the hypothesis of local thermodynamical equilibrium. The weathering of silicate minerals, including albite, anorthite, plagioclase, K-feldspar, and chalcedony, explains the observed median groundwater composition in the transition and discharge zones. Additional site-specific hypotheses were needed to match the observed composition of the main recharge area, including equilibration with carbon dioxide, kaolinite, and hematite in the soil and unsaturated zones, respectively, and the degradation of organic matter controlling the sulfate/sulfide content, thus pointing towards kinetic effects during water–rock interactions in this zone. Even though an averaged water composition was used, the inverse models can “bridge” the knowledge gap on the large basin scale to come up with quite distinct “best” mineral assemblages that explain observed field conditions. This study provides a conceptual framework of the hydrogeochemical evolution for managing groundwater resources in the Pra Basin and presents modelling techniques that can be applied to similar regions with comparable levels of heterogeneity in water chemistry and limited knowledge of aquifer mineralogy. The combinatorial inverse model approach offers enhanced flexibility by systematically generating all plausible combinations of mineral assemblages from a given pool of mineral phases, thereby allowing for a comprehensive exploration of the reactions driving the chemical evolution of the groundwater.

Hydrochemical Characterization of Surface Water and Groundwater in the Crystalline Basement Aquifer System in the Pra Basin (Ghana)

The quality of groundwater resources in the Pra Basin (Ghana) is threatened by ongoing river pollution from illegal mining. To date, there are very limited data and literature on the hydrochemical characteristics of the basin. For the first time, we provide regional hydrochemical data on surface water and groundwater to gain insight into the geochemical processes and quality for drinking and irrigation purposes. We collected 90 samples from surface water (rivers) and groundwater (boreholes) and analysed them for their chemical parameters. We performed a water quality assessment using conventional water quality rating indices for drinking water and irrigation. Cluster and factor analysis were performed on the hydrochemical data to learn the chemical variations in the hydrochemical data. Bivariate ion plots were used to interpret the plausible geochemical processes controlling the composition of dissolved ions in surface water and groundwater. The water quality assessment using Water Quality Index (WQI) revealed that 74% of surface water and 20% of groundwater samples are of poor drinking quality and, therefore, cannot be used for drinking purposes. For irrigation, surface water and groundwater are of good quality based on Sodium Adsorption Ratio (SAR), Wilcox diagram and United States Salinity (USSL) indices. However, Mn and Fe (total) concentrations observed in most surface water samples are above the acceptable limit for irrigation and therefore require treatment to avoid soil acidification and loss of availability of vital soil nutrients. Manganese and iron (total) are identified as the main contaminants affecting the basin’s water quality. The hierarchical cluster analysis highlights the heterogeneity in the regional hydrochemical data, which showed three distinct spatial associations based on elevation differences. Groundwater composition chemically evolves from a Ca–HCO3 to a Na–HCO3 and finally to a Na–Cl water type along the flow regime from the recharge to the discharge zone. The bivariate ion plot and the factor analysis underscore silicate weathering, carbonate dissolution and ion exchange as the most likely geochemical processes driving the hydrochemical evolution of the Pra Basin groundwater. Going forward, geochemical models should be implemented to elucidate the dominant reaction pathways driving the evolution of groundwater chemistry in the Pra Basin.

Assessing regional variation in yield from weathered basement aquifers in West Africa and modelling their future groundwater development and sustainability

A data-driven modelling approach was applied to quantify the potential groundwater yield from weathered crystalline basement aquifers in West Africa, which are a strategic resource for achieving water and food security. To account for possible geological control on aquifer productivity, seven major geological domains were identified based on lithological, stratigraphic, and structural characteristics of the crystalline basement. Extensive data mining was conducted for the hydrogeological parameterisation that led to the identification of representative distributions of input parameters for numerical simulations of groundwater abstractions. These were calibrated to match distributions of measured yields for each domain. Calibrated models were then applied to investigate aquifer and borehole scenarios to assess groundwater productivity. Considering the entire region, modelling results indicate that approximately 50% of well-sited standard 60-m-deep boreholes could sustain yields exceeding 0.5 L/s, and 25% could sustain the yield required for small irrigation systems (> 1.0 L/s). Results also highlighted some regional differences in the ranges of productivities for the different domains, and the significance of the depth of the static water table and the lateral extent of aquifers across all geological domains. This approach can be applied to derive groundwater maps for the region and provide the quantitative information required to evaluate the potential of different designs of groundwater supply networks.

The Ibadan Hydrogeophysics Research Site (IHRS)—An Observatory for Studying Hydrological Heterogeneities in A Crystalline Basement Aquifer in Southwestern Nigeria

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.

Groundwater recharge and dating in crystalline basement aquifers of Vea catchment: An integrated environmental tracers’ approach

The crystalline basement aquifers host significant groundwater resources for the communities in Vea catchment for public and irrigational water supply. As part of measures to understand the hydrogeological processes of the area, multi-tracers (SF6, CFC-12, CFC-11, 3H, δ18O and δ2H) were employed to elucidate the groundwater residence time, recharge mechanism and mixing process of the shallow groundwater. Ambient concentrations of these tracers were taken from 10 sampling locations in the Vea catchment of Upper East Region of Ghana. A purge and trap method was used to take the samples and electron capture detector gas chromatography (GC-ECD) was used to analytically measure the SF6 and CFCs gases in the groundwater. The SF6, CFC-11 and CFC-12 dating complimented with 3H revealed that the groundwater age in the catchment was relatively young ranging from 5 to 42 years with a mean age of 23 years. This corroborates well with the stable isotopes data , which showed that the catchment receives modern recharge from meteoric origin. The results also indicated that the estimated SF6 ages were younger compared to the CFCs ages, suggesting additional inputs of SF6 from terrigenous sources. Similarly, a comparison of 3H and CFCs showed little variations in ages in some wells, a possible indication of CFCs degradation. The conceptual groundwater flow model developed showed three groundwater flow regimes along the topographical line. Despite some uncertainties in age interpretations, the multi-environmental tracers have been successfully used to date shallow groundwater in the Vea catchment. To reduce the uncertainties in the age determination, the use multi tracer approach is highly recommended.

Borehole siting and construction in rural Malawi: A sociotechnical process?

Choosing the site for a new water well in rural southern Malawi is essentially a political process with competing priorities and stakeholders. For a new well (or borehole) to be sustainably used and maintained, the relevant stakeholders must be fully engaged in the siting process and must be given meaningful responsibility for the final siting decision. However, without sound technical information, a siting decision based solely on stakeholder priorities, such as proximity to the headman’s compound or accessibility to the center of the population, may not result in a satisfactory borehole. Instead, in addition to stakeholder interests, we have used a process that includes electrical resistivity tomography (ERT) as a tool to guide and constrain the local decision-making process. Within the region of the crystalline-basement aquifer, ERT profiles indicate variations in weathering thickness, hence aquifer storage. In a lacustrine setting, the ERT profile delineates a zone of moderately large resistivity associated with a deposit of freshwater saturated sand. This ERT-derived technical information becomes one element in a comprehensive sociotechnical approach to the location of sustainable water resources. We use this sociotechnical approach to complete boreholes for all four villages in our project, and we have a high confidence that the villagers will be motivated to use and maintain these resources.

Natural Groundwater Recharge Response to Climate Variability and Land Cover Change Perturbations in Basins with Contrasting Climate and Geology in Tanzania

The response of aquifers with contrasting climate and geology to climate and land cover change perturbations through natural groundwater recharge remains inadequately understood. In Tanzania and elsewhere in the world, studies have been conducted to assess the impact of climate change and variability, and land use/cover changes on stream flow using different models, but similar studies on groundwater dynamics are inadequate. This study, therefore, examined the influence of land use/cover and climate dynamics on natural groundwater recharge in basins with contrasting climate and geology in Tanzania, applying the modified soil moisture balance method, coupled with the curve number (CN). The method hinges on the balance between the incoming water from precipitation and the outflow of water by evapotranspiration. The different parameters in the soil moisture balance method were computed using the Thornthwaite Water Balance software. The potential evapotranspiration (PET) was calculated using the daily maximum and minimum temperatures, utilizing two-temperature-based PET methods, Penman–Monteith (PM) and Hargreaves–Samani (HS). The rainfall data were obtained from the gauging stations under the Tanzania Meteorological Agency and some additional data were acquired from climate observatories management by water basins. The results show that there has been a quasi-stable CN in the Singida semi-arid, fractured crystalline basement aquifer (74.2 in 1997, 73.64 in 2005, and 73.87 in 2018). In the Kimbiji, humid, Neogene sedimentary aquifer, the CN has been steadily increasing (66.69 in 1997, 69.08 in 2008, and 71.42 in 2016), indicating the rapid land cover changes in the Kimbiji aquifer as compared to the Singida aquifer. For the Kimbiji humid aquifer, the PET calculated using the Penman–Monteith (PM) method for the 1996/1997, 2007/2008, and 2015/2016 hydrological years were 1156.5, 1079.5, and 1143.9 mm/year, respectively, while for the Hargreaves–Samani (HS) method, the PET was found to be 1046.1, 1138.3, and 1204.4 mm/year for the 1996/1997, 2007/2008, and 2015/2016 hydrological years, respectively. For the Singida semi-arid aquifer, the PM PET method resulted in 2083.3, 2053.6, and 1875.4 mm/year for the 1996/1997, 2004/2005, and 2017/2018 hydrological years, respectively. The HS method produced relatively lower PET values for the semi-arid area (1839.4, 1814.7, and 1710.2 mm/year) for the 1996/1997, 2004/2005, and 2017/2018 hydrological years, respectively. It was equally revealed that the recharge and aridity indices correspond with the PET calculated using two temperature-dependent methods. The decline of certain land covers (forests) and increase in others (built-up areas) have contributed to the increase in surface runoff in each study area, possibly resulting in the decreasing trend of groundwater recharge. An overestimation of the PET using the HS method in the Kimbiji humid aquifer was observed, which was relatively smaller than the overestimation of the PET using the PM method in the Singida semi-arid aquifer. Despite the difference in climate and geology, the response of the two aquifers to rainfall is similar. The combined influence of climate and land cover changes on natural groundwater recharge was observed to be prominent in the Kimbiji aquifer, while only climate variability appreciably influences natural groundwater recharge in the Singida semi-arid aquifer. El Nino and the Southern Oscillation as part of the climate variability phenomenon dwarfed the time lags between rainfall and recharge in the two basins, regardless of their difference in climate and geology.

Litho-structural conditioning in the exploration of fractured aquifers: a case study in the Crystalline Basement Aquifer System of Brazil

Productive regions in the Crystalline Basement Aquifer System (CBAS) in Paraná state, Brazil, were identified qualitatively and quantitatively through spatial correlation of wells and geological conditions such as lineaments, hydrography, aeromagnetometry and lithology. Different methods applied in Precambrian metamorphic and igneous aquifers around the world and some Brazilian states were integrated and applied in CBAS with the aim of understanding the best scale and approach to productivity. The median productivity of the 224 wells analyzed is 0.29 m3/h/m. Under a multi-scale regional assessment, the results showed that the best condition is associated with the distance of 350 m from the lineaments (1:100,000), especially those with directions N40W, N10E and N70E. Considering the hydrolithological units, gneisses are the most productive, especially where lineaments coincide with regional structures such as shear zones, foliations and Cenozoic tectonic reactivations. Quartzites, granitoids, schists, phyllites, and rhyolites were also favorable when close to important rivers and not necessarily coinciding with regional lineaments, the high density of lineaments, or with vertical fractures. The areas of intersection of the lineaments and the weathering mantle did not serve as a discriminatory parameter. As the median depths of the water wells reach 90 m from the surface, the extraction of oriented magnetic gradients referred to magnetic sources down to 800 m depth, thus corroborating surface mapped lineaments and new nonoutcroppable structures. Considering the complexity of the environment and the global use of groundwater from fractured aquifers, this work contributed by discriminating geospatial parameters to decrease the exploratory risk in CBAS.

Integration of hydrogeophysical and geological investigations in enhancing groundwater potential assessment in Houtriver gneiss crystalline basement formation of South Africa

Groundwater exploration in crystalline basement aquifers such as the Houtriver gneiss formation in the Limpopo province of South Africa is often described as complex. This is because groundwater availability in such aquifers is largely a consequence of the interaction of several processes related to recharge, underlying geological features and fracture connectivity of the aquifer rock matrix. In this study, an integration of geophysical and geological investigations is applied in inferring potential drill targets within the Houtriver gneiss crystalline basement aquifer system. Results from the magnetic and frequency domain electromagnetic surveys were combined with geological investigations to identify sites where vertical electrical resistivity sounding was applied to infer the thickness and layering of weathered and fractured zones, as well as to identify potential targets where test boreholes were drilled. Constructed geo-resistivity pseudo-sections suggested that groundwater occurrence within this formation is described by a heterogeneous multiple-layered and fractured aquifer system with the main groundwater bearing zones ranging from a depth of 30–72m in most cases. Ten potential drill sites were identified from which three test boreholes were drilled and used to validate the results through a lithostratigraphic conceptual model developed from the correlation of the geophysical results with drill logs. The integration of hydro-geophysical and geological methods thus provided a comprehensive approach for resource assessment in the Houtriver gneiss formation.

Application of geoelectrical resistivity for delineating crystalline basement aquifers in Basiri, Ado-Ekiti, Southwestern Nigeria

Delineating crystalline basement aquifers with adequate capacity to store and transmit sufficient groundwater is quite challenging as substantial spatial variability predominantly marked the basement aquifers. Electrical resistivity is a non-invasive geophysical technique for delineating weathered and fractured zones that permit groundwater accumulation in the basement rocks. This study uses two-dimensional (2D) electrical resistivity imaging (ERI) to delineate the weathering profile developed above the crystalline basement rocks and thus assesses the groundwater potential of the aquifer systems in Basiri, Ado-Ekiti, southwestern Nigeria. The study evaluated the reliability of the 2D ERI images by computing the depth of investigation (DOI) index to predict the depth extent at which the 2D resistivity models are no longer reliable. This approach can be used to minimise the mismatch between the resistivity models and subsurface targets, particularly in complex and subtle hydrogeological conditions as in the crystalline basement complex. This study indicates that the weathering profile developed into a two-layered aquifer system—a low yield shallow saprolite aquifer overlying a deeper fractured bedrock (saprock) aquifer with greater capacity to support adequate drawdown for high yield.

Hydraulic characteristics of a fractured crystalline basement aquifer in Nzhelele area, Limpopo Province, South Africa

Knowledge on hydraulic characteristics of fractured crystalline basement aquifers is limited resulting to lack of reliable information for effective groundwater management. This is crucial as these aquifers provide good sources of potable water in most rural communities. Heterogeneous nature of these aquifers requires detailed understanding and accurate estimation of hydraulic characteristics. This study estimated hydraulic characteristics of a fractured crystalline basement aquifer and inferred their influence on groundwater storage potential and flow. Aquifer Test Solver was used for automatic curve matching to identify appropriate aquifer models and test solutions for estimating hydraulic characteristics. Root-mean-square error (RMSE), mean absolute error (MAE) and correlation coefficient (R) were used to evaluate the performance of the fitted models. Plotted derivative curves were used to identify the presence of fracture dewatering in the aquifer. The R, RMSE and MAE ranged from 0.702 to 0.995, 0.31–3.45 m and 0.23 to 3.06 m, respectively. The fits between measured and estimated drawdowns were good and performance mostly acceptable and comparable to those of related studies. Storativity, transmissivity and hydraulic conductivity ranged from 0.0003 to 0.0680, 0.78 to 12.3 m2/day and 8.56 × 10−7 to 5.32 × 10−6 m/s, respectively. The storage potential of the aquifer varied from low to high, ability to transmit water per unit area was mostly medium though the ability to transmit water through its entire thickness was mostly low. The study area is dominated by leaky aquifer and fracture dewatering. Groundwater use should be monitored and managed effectively to avoid fracture dewatering and its associated risks.

Open-Source Software Application for Hydrogeological Delineation of Potential Groundwater Recharge Zones in the Singida Semi-Arid, Fractured Aquifer, Central Tanzania

This study attempted to delineate and map potential groundwater recharge zones of the Singida, semi-arid, fractured crystalline basement aquifer using open source remote sensing and GIS software. Various thematic maps such as lithology/hydrogeology, soil, land-cover/use, slope, lineament density, drainage density and rainfall distribution were integrated in QGIS software. Vector input layers were rasterized and resampled using QGIS wrap projection function to make sure that the grid cells are of the same size. Reclassification using SAGA and GRASS reclass algorithms in QGIS was carried out to realign the factor classes in a consistent scale, and reclassification to a scale of 1 to 5 was carried out to harmonize the results. The study identified a number of potential areas for groundwater recharge, groundwater exploration, groundwater development and potential areas for artificial groundwater recharge. Potential groundwater recharge zones for the Singida semi-arid fractured aquifer are restricted to areas with high lineament density, cultivated areas, grassland and flat to gentle slopes. The potential of groundwater recharge is also observed in areas with low drainage density. The delineated zones provide a good understanding of the potential recharge zones, which are a starting point for recharge zone protection. This blended approach can be utilized for carrying out suitability analysis using the weighted overlay analysis approach. Areas designated good and very good are recommended for artificial recharging structures as an alternative technique for enhancing groundwater recharge through rainwater harvesting. This will help to augment groundwater storage in this semi-arid environment.

Hydrogeological characterization of crystalline aquifer in the Hout River Catchment, Limpopo province, South Africa

This study attempted to conceptualize the hydrogeological setting of the Hout River Catchment, located in the Limpopo River Basin, using multiple methods that include groundwater flow patterns, structural analysis, stable (18O, 2H and 13C) and radiogenic (14C) isotopes of water and Water Table Fluctuation methods. The hydrogeological system of the catchment is represented by fractured crystalline basement aquifer as the main host for groundwater and is overlain by weathered rocks that act as a vadose zone and shallow aquifer in various places. Groundwater from the fractured basement rocks is the main source of water for large-scale irrigation and domestic use. Potential aquifers in the area are evident within the Hout River granitic gneiss and the Goudplaats granitic gneiss besides the younger granites as a result of fracturing and weathering. Groundwater flow map shows a flow pattern from the southern part of the catchment towards the north-eastern part of the catchment dictated by dolerite dykes and tectonic lineaments that trend in the ENE and E direction (088° and 075°) with the dip angle of 50° to 55°. The deeper aquifer in the southern and central part of the catchment contain old groundwater with high salinity due to long residence time. The stable isotopes further confirmed the limited possibility of local recharge, with rather dominance of regional groundwater circulation into the catchment. The northern part of the catchment seems to be receiving recent recharge with the groundwater of high 14C content derived from the mountains that border the catchment.

Utilizing indicator kriging to identify suitable zones for manual drilling in weathered crystalline basement aquifers

Abstract: Manual drilling offers a practical and affordable method of increasing access to groundwater supply in regions struggling with economic water scarcity. However, manual techniques are limited to specific hydrogeological contexts and must be sited appropriately. Indicator kriging is proposed as an interpolation method that builds upon previous efforts to identify suitable zones for manual drilling, particularly in weathered crystalline basement aquifers. This approach allows for heterogeneity within weathering profiles and provides probability mapping of success for regional planning. Modeling was conducted in the Upper East Region of Ghana using available borehole-log data, including: well yield, static water depth, laterite thickness, depth to hard rock, water quality parameters, and the degree of weathering. Indicator kriging interpolations for depth to hard rock predicted binary variables with over 90% accuracy. The model predicts that drilling into highly weathered layers will often be necessary to reach the required depths for groundwater use, emphasizing the importance of percussion techniques. The results suggest that suitable zones occur near Bolgatanga, Bawku, and Zebila, which coincide with historical drilling efforts in the central and eastern portions of the region.

DISOLV: A Python Package for the Interpretation of Borehole Dilution Tests

Single borehole dilution tests (SBDTs) are an inexpensive but effective technique for hydrogeological characterization of hard-rock aquifers. We present a freely available, easy-to-use, open-source Python package, DISOLV, for plotting, analyzing, and modeling SBDT data. DISOLV can significantly reduce the time spent interpreting field data by helping to identify flowing fractures intersecting the borehole and estimate the corresponding flow rates. DISOLV is successfully benchmarked against two analytical solutions. We also present an example application to real data collected in a borehole in a crystalline basement aquifer in southern India.

Análise da compartimentação estrutural do sistema aquífero embasamento cristalino no município de Bagé (RS) com base no uso de lineamentos (SRTM e magnéticos) e de dados hidrogeológicos

Esse artigo apresenta o estudo de avaliação da compartimentação e do comportamento hidrogeológico do Sistema Aquífero Embasamento Cristalino, no município de Bagé (RS). Para realização desse estudo, foram utilizados dados hidrogeológicos de 174 poços tubulares e dados de lineamentos obtidos de imagens SRTM e de levantamento aerogeofísico. Para realizar a avaliação estrutural da área, foram traçados lineamentos morfoestruturais nas imagens SRTM, em diferentes escalas (1:1.000.000; 1:500.000 e 1:250.000), bem como foram identificadas estruturas por meio da análise e interpretação dos dados aeromagnéticos. Esses dados foram utilizados visando identificar a existência de compartimentos estruturais. Os dados dos poços tubulares foram analisados e utilizados na caracterização hidrogeológica da área de estudo e dos compartimentos estruturais. Os resultados indicaram a ocorrência de cinco estruturas principais, que possuem extensões entre 8,62 e 30km. Com base nessas estruturas, foram definidas cinco zonas (Z1 a Z5) que possuem características distintas. A Zona 1 apresenta os principais trends para N80-90E e N60-70E, capacidade específica de 0,005 m3/h/m e vazão de 0,46 m3/h. Na Zona 2 os principais trends são para N20-30E, N50-60E e N30-40E, a capacidade específica máxima é de 6,93 m3/h/m, com vazão máxima de 16,5 m3/h. Na Zona 3 os trends são N50-60E, N40-50E e N30-40E, com capacidade específica máxima de 3 m3/h/m e vazão máxima de 20,37 m3/h. Na Zona 4 os principais trends são para N70-80E e N20-30E, com capacidade específica máxima de 2,13 m3/h/m e vazão máxima de 12 m3/h. Por fim, na Zona 5 os trends principais são para N70-80E, N20-30E e N00-10W, com capacidade específica máxima de 1,5 m3/h/m e vazão máxima de 30 m3/h. Esses resultados demonstram que a região do município de Bagé se apresenta compartimentada, com zonas apresentando diferentes produtividades e trends estruturais, sendo que a Zona 2 apresenta estruturas que estão associadas a maior produtividade de poços. Assim, a análise de lineamentos magnéticos e morfoestruturais, demonstra ser um estudo que tem importância na avaliação da compartimentação estrutural e hidrogeológica de áreas com ocorrência de aquíferos fraturados.

Application of 2D-Electrical resistivity tomography in delineating groundwater potential zones: Case study from the voltaian super group of Ghana

This study assesses the efficiency of a key geophysical groundwater exploration technique, Electrical Resistivity Tomography (ERT) in part of the Voltaian Supergroup in Northern Ghana. In the Northern Ghana, groundwater has been the main source of water for domestic and other uses. Much of the terrain is underlain by Neoproterozoic sedimentary rocks which have lost their primary porosity to partial metamorphic episodes during the Pan-African Orogeny. As a result, groundwater acquisition in this terrain is challenging compared to crystalline basement aquifers. Borehole success rates (yields ≥ 13 L per minutes) have generally been low (50%) as the terrain has been difficult to characterize with standard geophysical techniques. Previously, when the electromagnetic and vertical electrical sounding (VES) techniques were used to explore for groundwater resources, the success rate was less than 50%. In order to have an improved yield, the 2D Electrical Resistivity Tomography (ERT) approach was used for the investigation to provide information about the lateral and vertical extent of weathering and fracturing in the sub-surface. A total of 30 ERT profiles were run across strike of the underlying lineaments at a maximum profile length of 800 m. The Schlumberger, Dipole-dipole and the Wenner array protocols were used concurrently for the data collection and the GEOTOMO RES2D Inversion software was used for the inversion. Analysis of data on 40 boreholes in 22 selected communities in different Voltaian rock units of drill depths ranging between 92 and 192 m indicates a 80% drilling success rate when drilling is guided by ERT surveys, providing a significant improvement over previous approaches recording a less than 50% success rate. Comparison between the drilling logs and the ERT revealed a strong correlation between the inferred resistivity boundary and lithology changes. On average, the resistivity patterns that indicate a promising zone for drilling a wet well ranges between 10 and 300 Ω-m. This investigation shows that, in hydrogeologically difficult terrains such as the Voltaian Supergroup, a careful application of the ERT with clear understanding of the underlying geological conditions would significantly improve success.

Determination of Groundwater Potentials in Crystalline Basement Areas of Bauchi Local Government Area, Bauchi

Developing groundwater is generally an excellent option for sustainable water supplies in Bauchi due to the ephemeral nature of surface water. However, the large variability in geological and hydrological conditions have a profound influence on the availability of groundwater across the study area. The rationale for this research is to contribute at least, in terms of providing empirical data towards sustainable development of groundwater resource, particularly in the Crystalline Basement hydrogeological unit of Bauchi area. Aquifer properties of wells within the Crystalline Basement were estimated from a constant-rate pumping test of 10 sampled wells by fitting mathematical models (type curves) to drawdown data through curve matching. Generally, yields of sampled boreholes within the crystalline basement aquifer range from 8.64 m 3 /d to 120.96 m 3 /d with an average of 52.36 m 3 /d. The total yield of the boreholes is about 661.83.m 3 /day. This can sustain a population of 26,473 based on water supply standard of 25 litres per day for rural communities (Babatola, 1997). Considering the total population of 12,218 people (NPC, 2014) that are currently using the water points in comparison to water supply standard of 25 litre per person per day, this shows that the aquifers from the 10 sampled boreholes have relatively enough groundwater in terms of yield to meet the water need of the population. However, any significant increase in population growth may result in water scarcity in these areas, as the Crystalline Basement aquifers generally have low yielding capacity. Also, the crystalline basement aquifer properties evaluated reveal that the transmissivity values range from 2.55 m 2 /day to 111.0 m 2 /day with an average of 38.94m 2 /day (Table 5). According to Offodile (2002), a transmissivity range of 5 to 50 m 2 /day could be regarded as high potential in crystalline rock situations. By the above standard, the crystalline basement aquifers in the area are classified as aquifers of negligible to high potentials. The specific capacities of boreholes in the study area were computed and the results show that they vary between 1.30 to 691.2 m 2 /d/m. Keywords: Crystalline Basement, Groundwater, Pumping Test, Aquifer, Transmissivity, Bauchi State, Nigeria DOI : 10.7176/JEES/9-7-02 Publication date :July 31 st 2019

Review: Approaches to groundwater exploration and resource evaluation in the crystalline basement aquifers of Zimbabwe

Assessment of the groundwater potential of crystalline basement aquifers is challenging. These systems can be highly spatially variable, as indicated by the drilling of numerous dry boreholes and seasonal variation in discharge rates. This paper reviews methodologies applied for the evaluation of groundwater occurrence and yield estimation in the crystalline basement aquifers of Zimbabwe. These aquifers underlie much of the country and are described in terms of low yield owing to low recharge potential in the semiarid climate. In such regions, exploitable groundwater forms a strategic supply of potable water used to meet the socio-economic needs of the local population. Case studies are used to show how remote sensing and geophysical methods are integrated to improve borehole success rates in the basement aquifers of Zimbabwe. Potential threats to groundwater resources and quality within crystalline basement aquifers are discussed. It can be concluded that major issues remain to be addressed if sustainable use of the water resources of crystalline basement aquifers in Zimbabwe is to be achieved, especially with respect to borehole-siting approaches and prevention of groundwater contamination. The key recommendation is to address the paucity of primary groundwater monitoring data within the crystalline basement aquifers at the national level, thereby creating a technical groundwater management framework.