Abstract:
The assessment of groundwater vulnerability was carried out in north-western part of
Akure, Southwestern Nigeria where presence of automobile workshops is the major
source of pollution. One hundred and eighty-seven (187) Vertical Electrical Sounding
data were acquired using Schlumberger array technique with (AB/2) ranging from 65 -
150 m. Fifteen (15) water samples were obtained from wells for the chemical analysis
across the area. The VES results delineated 3 to 5 geoelectric layers across the area
which corresponds to five subsurface layers; topsoil, weathered layer, partially
weathered basement, partially fractured basement and the presumed fresh basement.
The subsurface layer resistivities vary respectively as 4.7 - 790.1 Ω-m, 3.4 - 3409.9 Ω-
m, 51.1 - 2597.8 Ω-m, 144.2 - 509.3 Ω-m and 695.5 - ∞ Ω-m in the topsoil, weathered
layer, partially weathered basement, partially fractured basement and the fresh
basement. The thickness of the layers varies respectively as 0.4 - 4.1 m, 0.9 - 18.6 m,
7.7 - 13 m and 1.2 - 58.2 m. The overburden thickness across the area varies from 6.7 -
61.4 m across the study area. Six parameters of hydrogeological importance were used
to develop groundwater vulnerability model for the study area. The parameters consist
of surface elevation, lithology, aquifer overlying layer resistivity, aquifer overlying
layer thickness, coefficient of anisotropy and hydraulic conductivity. These factors were
subjected to the Fuzzy Analytical Hierarchy Process (FAHP) method of weightage
determination to assign weights to each criterion of the groundwater vulnerability
conditioning factors. Groundwater vulnerability map (GWVM) was produced using the
determined groundwater vulnerability index. The GWVM of the area classified the area
into four (4) vulnerability zones; very low, low, moderate and high. Two parameters
were used to validate the developed GWVM; namely physiochemical parameters
determined from water samples collected across the area (Lead, Pb) and longitudinal
conductance derived from the geo-electric parameters. The validations were done by
superimposing the values of the validating parameters on the GWVM. The validation
using Lead (Pb) and longitudinal conductance values gave accuracy of 73% and 74%
respectively and they both proved the efficacy of the model.
