Assessment of Saltwater Intrusion into Coastal Aquifer using Electrical Resistivity Method: A Case Study of Ojo, Lagos State

Abstract

The migration of saline water into freshwater aquifers is known as saltwater intrusion, and
it has long been a source of concern around the world. Coastal regions are home to about
60% of the world's population. It is critical to assess the impact of saltwater intrusion on
groundwater potential along the Ojo shoreline in Lagos State in order to determine suitable
location for groundwater extraction for domestic use. The 2D Electrical Resistivity Imaging
(ERI) survey was complemented with twenty Vertical Electrical Sounding (VES)
measurements conducted along four traverses (TR) covering a total spread length (AB/2) of
120 m. The sand delineated in the second layer of the 2D electrical resistivity images across
TR (1, 3 and 4) with low resistivity response revealed the lateral and vertical extent of
contamination. The 2D results also coincided with the anomaly observed in the VES results.
The sand identified in the second and third layers along TR1 (VES 3 and 4), TR3 (VES 11–
13) and TR4 (VES 18 and 19) with associated low resistivity values of range 1.3–10.6 Ωm at
depth range 0.5–44 m showed the relative degree of saltwater intrusion when compared to
the identified sand in TR2 away from the Beachline. However, a probable confined
freshwater aquifer with resistivity range 100.5–278 Ωm for VES (1 and 2) along TR1, VES
(16) along TR4 at depth 30 m and VES (8) along TR2 at depth range 14–32 m was identified
at fourth and fifth geoelectric layers. This study has shown the possible infiltration of salt
water into the groundwater along the coastline. Hence, it is essential to monitor the saline
contamination to preserve the quality of groundwater in such areas.