Geoelectric Evaluation of Groundwater Potential and Vulnerability of Overburden Aquifers at Onibu-Eja Active Open Dumpsite, Osogbo, Southwestern Nigeria

Electrical resistivity method was used to assess groundwater potential and vulnerability of overburden aquifers at Onibu-Eja active open dumpsite, Osogbo, Southwestern Nigeria. Eighteen Vertical Electrical Sounding (VES) points and five 2-D imaging profiles established in five traverses at the periphery of the dumpsite were surveyed and analysed. The subsurface comprised of thin topsoil (resistivity 65 - 998 Ωm);heterogeneous weathered layer with resistivity 63 - 333 Ωm and thickness 0.7 - 8.5 m;weathered basement (resistivity 31 - 1253 Ωm and thickness 0.7 - 27.0 m) and fractured/fresh basement (resistivity 36 - 6213 Ωm). The 2-D inverse model of the profiles delineated low resistivity values ranging from 5 to 100 Ωm at a depth range of 10 - 20 m along traverses TR1-TR3 which is attributed to leachate percolation close to the dumpsite. The weathered basement was inclined relative to the dumpsite. The total overburden thickness varies from 6.9 to 33.7 m, with 20 and 40 m generally recommended as productive for groundwater abstraction in Southwestern Nigeria occurring in 61% of the area. Further, about 85% of the weathered layer resistivity values fall within medium groundwater potential (100 - 250 Ωm) and high groundwater potential (>250 Ωm). The ranking of groundwater potential as a function of saprolite (weathered basement) resistivity showed that 72% of the study area is characterized by optimum weathering (20 - 100 Ωm) and is classified as good groundwater potential. Fractured basement covered <30% of the study area. The evaluation of aquifer protective capacity has helped to classify the area into moderate, weak and poor protective capacities with moderate protective capacity zone covering 72%.

Geophysical and Hydrogeological Groundwater Prospectivity Mapping in the Kraaipan Granite-Greenstone Terrain, Southeast Botswana

Application of regional geophysical methods for hydrogeological purposes has increased over the last two decades especially in arid and semi-arid areas. A project to map the Kraaipan granite-greenstone terrain in southeast Botswana has recently been undertaken using regional aeromagnetic and gravity data with the aim to map the rocks at depth to understand the geology while the secondary objective was to subsequently assess the mineralization and groundwater potential in the area. An integrated analysis of the aeromagnetic and gravity data and their derived/processed products is hereby investigated for groundwater for drinking and agricultural purposes. The studies include: subsurface characterisation and delineation of structural framework suitable for groundwater exploration and determination of petrophysical relationships used to link the geophysical properties (e.g., density) to hydrological properties (e.g., porosity). The results of interpretation indicate that the rocks are under ~50 m of Kalahari cover and the study area is composed of three aquifers: the extensive hard rock aquifer (granitic and volcanic), the important (fractured) karst aquifer and the minor sedimentary aquifer. The area is dissected by an ENE-to-EW-trending dyke swarm visible on the regional aeromagnetic data and much clearer on high resolution aeromagnetic data. Minor fault and/or dyke elements of NW-SE and NE-SW trend are observed. Spectral analysis reveals three main average ensample interfaces at depths of 0.7 km, 1.99 km and 4.8 km. The linear Euler solutions maps reveal that the majority depths to top of magnetic bodies range from 40 m to 400 m throughout the survey area. The shallowest depths are the most significant one in this case as they probably relate to depth of bedrock and thickness of regolith or thickest sediments. For 2695 existing boreholes analysed, maximum borehole depth is 482 m (mean 108 m), and almost half (1263) were dry with another 972 having low yield (1 - 5 m3/hr) and 432 yielding 6 - 49 m3/hr and only 28 above 50 m3/hr (maximum ~160 m3/hr) and an average water strike of 64 m. There is very little correlation between interpreted hydrogeological features and the existing borehole locations. The study shows the importance of preliminary geophysical investigations before ground borehole siting and drilling in order to improve borehole success rates and/or reduce costs inherent in groundwater projects.

Gravity and Aeromagnetic Studies of the Filabusi Greenstone Belt, Zimbabwe Craton: Regional and Geotectonic Implications

The Filabusi greenstone belt (FGB), Zimbabwe craton, has been geologically remapped relatively recently but its regional tectonic setting and subsurface structure have, until now, remained unresolved. This paper presents gravity and aeromagnetic studies that have been undertaken to provide this important information, and also extend mapping to areas of poor exposure. Several new NNW-trending dykes and structures cutting across the greenstone belt have been revealed, as well as a major extension of one of the metakomatiitic-BIF units, the Shamba Range. ESE-trending dykes identified in the southeast appear on a regional scale to be part of the giant Okavango dyke swarm in northern Botswana. An ~3 km wide NNE-striking magnetic low occurs over the Irisvale-Lancaster shear zone (ILSZ) on the extreme west of the FGB where it roughly marks the boundary with the Bulawayo greenstone belt. Magnetic anomaly trends over ultramafic schists are consistent with strike-slip movement along the ILSZ, and together with the gravity anomalies, support northeasterly directed detachment of the adjacent Fort Rixon belt from the Bulawayo-Filabusi belt. The Bouguer gravity anomaly map shows that the FGB is characterised by a well defined positive anomaly up to 37 mGal, whose symmetry and extent confirm the postulated synclinal structure of the belt. Isolated oval shaped small gravity lows generally correlate with sub-/out-cropping K-rich post-volcanic granite plutons. 2.5D gravity models along three profiles across the greenstone belt show a simple “basin shape” with a possible maximum depth extent of only 4.5 km, compared to an estimated stratigraphic thickness of about 9.0 km. This suggests a truncation at shallow depth of the structurally repeated lithologies. Gravity data and models support the proposed FGB model;deposition of volcanics in an extensional, structurally determined, evolving basin. This autochthonous setting is consistent with other greenstone belts in the Zimbabwe craton and other parts of the world.