Abstract:
The aim of this study is to determine the source of saline water in the inland aquifers of
Agbabu, Southwestern Nigeria. A combination of aeromagnetic data covering the eastern
Dahomey basin, 23 Vertical Electrical Soundings data and hydrochemical analysis of 11
subsurface water samples were utilized in this study. Analysis of the aeromagnetic maps
which included the magnetic intensity maps, the derivative maps and the depth solution maps
revealed the inland extension of the Chain Fracture Zone (CFZ). This NE-SW trending
fracture zone cut across the entire Eastern Dahomey Basin including Agbabu and its
environs. Four geoelectric layers were delineated across Agbabu, namely, the topsoil, clayey
sand, clayey and bituminous sand layers. The resistivity of these layers varied respectively
from 4 - 2806 Ωm, 3 - 1280 Ωm, 3 - 161 Ωm and 1 - 10390 Ωm while the thickness of the
upper three layers varied respectively from 0.3 - 3.9 m, 3 - 8.1 m and 0.9 - 178.1 m. Within
the third layer, zones of anomalously low resistivity (1 - 55 Ωm) considered to be saline
water zones were observed across the traverses. The Salinity Extent Chart generated from
the VES results revealed a possible boundary between saline and non-saline water zones in
Agbabu and indicates that the saline source is from the eastern end of the town. Similarly,
water analysis results confirmed the presence of saline water zones. Based on the
combination of the deductions made from interpretations of all the methods used and the
identification of the inland extension of the Chain Fracture Zone (CFZ), two possible sources
or origin of the saline water zones in Agbabu are suggested. First, flow of saline bottom
water from the ocean through the CFZ and second, connate water trapped in the interstices
of the sedimentary rocks
