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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 (0 cm) to 100 cm depth, with an interval of 20 cm. At all the geological domains (Porphyroclastic, Granite, Quartzite, Migmatite and Banded gneiss), radon has its minimum emission over migmatite at 0 cm, while its maximum emissions occured over granite and banded gneiss at 80 cm. The overall soil-gas radon concentrations in Ogbomoso varied from 0.06 to 26.5 kBq/m3, which is within the natural limit of 0.4 to 40 kBq/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 80 cm 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. [ABSTRACT FROM AUTHOR]

The present study investigates geological structures in parts of the crystalline basement and the adjoining middle Niger Basin of Nigeria using high-resolution airborne magnetic and radiometric data. The purpose is to provide additional insights into the contrasting structures of the magnetic basement within the crystalline and the sedimentary terrain. The magnetic and radiometric data of the study area were subjected to standard interpretation techniques. The results obtained showed the entire crystalline basement to be overprinted by younger igneous rocks at extremely shallow depths. The sedimentary basin on the other hand is marked by basement subsidence and volcanic intrusions. Two major shear zones traverse the crystalline basement and the adjoining sedimentary basin. The shear zones cross-cut the entire study area in a NNE direction and exhibit three major dipping contacts, namely, vertical-, north-western- and south-eastern-dipping contacts in specific locations. The shear zones exhibit multiple surface faults mostly within the crystalline basement and are characterized by linear magnetic highs that indicate magnetic minerals precipitate in specific areas. Some parts are characterized by a low-amplitude magnetic anomaly that coincides with high potassium and fairly low thorium concentrations that indicate the presence of younger volcanic rocks and possibly gold mineralization in the shear planes. The zones are parallel to the trans-current Ifewara–Kalangai–Zungeru shear zone already linked to the South Atlantic transform fault system. The structural architecture of the study area and perhaps the entire basement complex displayed a positive prospect for economic minerals in specific locations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]