Abstract
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%.
Highlights
The city of Osogbo and its environs in southwestern Nigeria are experiencing population growth and urbanization with its resultant pressure on the natural resources including both land and potable water supply
The aquifer protective capacity characterization is based on the values of the longitudinal unit conductance of the overburden rock units in the area
Clayey overburden, which is depicted by relatively high longitudinal conductance, gives protection to the underlying aquifer. [36]-[38] classified the protective capacity of the overburden into excellent, very good, good, moderate, weak and poor protective capacity zones (Table 4)
Summary
The city of Osogbo and its environs in southwestern Nigeria are experiencing population growth and urbanization with its resultant pressure on the natural resources including both land and potable water supply. According to [4], the overburden materials have high porosity, contain a considerable amount of water and exhibit low permeability due to its relatively high clay content. Aquifers in this basement terrain often occur at shallow depths, subjecting the water within to environmental hazard, susceptible to surface or near-surface contaminants. Some dumpsites that were sited far from city centre are being habited because of urbanizations An example of such dumpsites is the Osun state main active open dump, Onibu-Eja Dumpsite, Osogbo (Figure 1(a) and Figure 1(b)) with residential buildings some few meters away
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