Due to complex and erratic nature of groundwater occurrences in crystalline basement terrains, groundwater development in form of boreholes/wells without the necessary pre-drilling hydrogeological investigations usually results in failure. Therefore, there is the need for adequate characterization of aquifers and delineation of groundwater potential zones in such crystalline basement setting. This study employed the integration of multi-criteria decision analysis (MCDA), remote sensing (RS) and geographical information system (GIS) techniques to delineate groundwater potential zones in crystalline basement terrain of SW-Nigeria and validation of the result with existing borehole/well yield data. The study approach involved integration of nine different thematic layers (geology, rainfall geomorphology, soil, drainage density, lineament density, landuse, slope and drainage proximity) based on weights assignment and normalization with respect to the relative contribution of the different themes to groundwater occurrence using Saaty's analytic hierarchy approach. Following weigh normalization and ranking, the thematic maps were integrated using ArcGIS 10.0 software to generate the overall groundwater potential map for the study area. The result revealed that the study area can be categorized into three different groundwater potential zones: high, medium and low. Greater portion of the study area (84,121.8 km 2) representing about 78 % of the total area, fall within the medium groundwater potential zone which are generally underlain by medium-porphyritic granite, biotite-hornblende granite and granite gneiss bedrock settings. About 18,239.7 km 2 (17 %) fall under high groundwater potential zone which are characterized by weathered/fractured quartzite, quartz-schist, amphibolite schist and phyllite bedrock settings. However, areas of low groundwater potentials constitute only 3 % (3,416.54 km 2) of the total study area and are mostly underlain by migmatite, banded and augen gneiss bedrock settings. Subsequent validation with boreholes/well yield data revealed a good correlation with respect to the observed groundwater potential zonation. Wells/boreholes with yields greater than [150 m 3 /day are generally characteristic of areas with high groundwater potential while those with yields of 75-150 and \75 m 3 /day are typical of areas with medium and low groundwater potentials, respectively. The validation clearly highlights the efficacy of the integrated MCDA, RS and GIS methods employed in this study as useful modern approach for proper groundwater resources evaluation; providing quick prospective guides for groundwater exploration and exploitation in such crystalline basement settings.
Groundwater pollution is imminent in most developing countries as a result of increased anthropogenic activities apart from possible natural pollutants. This study reviewed groundwater pollution and discussed possible remediation measures. Sources of pollution can be categorized into two major types: point source pollution and non-point source pollution. Point source pollution (e.g. municipal sewage treatment plant and industrial plant, intense evaporation in shallow aquifers, degradation of water sources in areas located in geothermal/volcanic fields, and rock oxidation) is a single identify localized source while non-point source pollution (diffuse sources such as human land use, land use changes, chemical reactions of elements in the air or in the water and pollutes runoff from agricultural areas draining into a river) is characterized by multiple discharge point. Point source is relatively easy to identify, quantify and control. On the other hand, non-point source is difficult to monitor and control because the pollution cannot be traced to a single point of discharge. Pollution occurrence depends on the level of contaminant transported. Contaminants can be transported through filtration, sorption, chemical processes, microbiological decomposition and dilution. Groundwater pollution may cause ecosystem imbalance apart from severe sickness which may lead to death. Prevention of groundwater pollution is more appropriate than remediation. Such preventive measures include proper waste disposal, monitoring of hazardous materials, conducting environmental audit periodically and intensifying health education while remediation includes stream stripping, oxygen sparging, bioremediation, chemical oxidation and thermal treatment. This study revealed two main sources (point source and non-point source) of pollution with non-point pollution more difficult to remediate due to extent of spread. In addition, most pollution of groundwater is anthropocentric and can be prevented through intensive health education.
Assessments of hydrochemical quality and recharge source are critical components of sustainable groundwater resources evaluation and management, especially in complex weathered/fractured crystalline bedrock terrains. Assessment of the weathered/fractured basement aquifer of Ekiti area, SW-Nigeria was undertaken with emphasis on hydrochemical quality and stable isotope characterization of shallow groundwater system. The study approach involved field sampling and in situ measurements of physico-chemical parameters followed by hydrochemical and stable isotope analyses of the water samples. In situ measurements revealed EC value of 43-995 lS/cm (av 431.96 lS/cm) and total hardness of 31.3-295.1 mg/L. The pH of the water sample ranged from 7.0 to 8.9 (av 8.00). The concentrations of the major cations are in the order of Ca 2? [ K ? [ Na ? [ Mg 2? with their respective average values of 33.35, 25.61, 24.91 and 8.7 mg/L respectively while that of the anions are in the order of HCO 3-[ Cl-[ SO 4 2-[ NO 3 with their respective average values of 165.44, 31.85, 21.27 and 5.44 mg/L respectively. However, the results of stable isotope analyses revealed d 18 O value of-4.2 to-2.1 % and dD of-22.3 to-10.2 %, suggesting meteoric water source (recent precipitation water recharging the associated shallow basement aquifer) while the estimated deuterium excess of 6.80-11.00 % (av 9.6 %) suggested little or no imprint of kinetic evaporation. Furthermore, the overall hydrochemical profiles revealed the main hydrochemical facies as Ca(Mg)-HCO 3 (dominant) and Na(K)-HCO 3 (minor) water types which can be attributed to dissolution of minerals in the various rock units in the study area. Conclusively, the overall hydrochemical and stable isotope evaluations revealed a recharge predominantly derived from meteoric (rainfall) source while the corresponding low TDS is a reflection of low water-rock interaction and limited migratory history of the shallow groundwater system in the study area.
The petrographical and physical properties of major Precambrian basement rocks in Ado-Ekiti, Nigeria, were evaluated with a view to determine their suitability as road construction aggregates. Fifty rock samples were collected and subjected to petrographic evaluation and laboratory tests (water absorption capacity, specific gravity, Los Angeles abrasion value, aggregate crushing value, aggregate impact value and ten percent fines, flakiness index and unconfined compressive strength). Petrographical analysis revealed that the rocks (migmatite gneiss, quartzite, granite and charnockite) were predominantly acidic, rich in quartzo-feldspathic minerals. Opaque minerals in the rock units ranged between 0.6 and 2.3 % with maximum value in fine-grained charnockite. The specific gravity of the rock aggregates ranged between 2.65 and 2.74. All water absorption capacity values conform to recommended standard value of water absorption capacity \2 %. The aggregate crushing value of the rocks ranged between 17.6 and 28.4 % with minimum from quartzite (17.60-18.40 %) and highest in migmatite gneiss (27.70-28.40 %). The aggregate impact value ranged between 11.0 and 19.0 %, with the lowest value (11.0-11.6 %) from quartzite and highest in charnockite (18.4-19.0 %). Other properties including Los Angeles abrasion, ten percent fines, flakiness index and unconfined compressive strength ranged 17. 4-29.8 %, 96.96-131.63 kN, 13 and 24.9 and 138.98-167.52 MPa, respectively. Based on physical and mechanical properties of the rock aggregates in this study, quartzite, and fine-medium grained granites were considered most suitable for road aggregates production as the tests were within the acceptable limits for general road aggregates purposes. Porphyritic granite, the migmatite gneiss and charnockites showed results for which most values were outside, at the margin or slightly within the acceptable limits for general road stone purpose. The geotechnical assessment provided by this study revealed that not all the Precambrian basement rocks would yield aggregates that are satisfactory for use as road aggregates.
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