DELINEATION OF SALINE WATER INTRUSION AND FLOW DIRECTION USING GEOPHYSICAL LOGS IN PARTS OF LAGOS STATE, NIGERIA

Abstract

Study on Saline water intrusion was conducted in parts of Lagos Metropolis; Southwestern Nigeria. The study was aimed at delineating the Saline water flow direction while the objectives of the study included; identifying lithological units and fluid contents, and determining depth and thickness of the Saline water and freshwater aquifer. Thirteen (13) borehole logs were obtained from different locations across six Local Governments Areas of Lagos state. The logs, consisting of natural gamma and electrical resistivity logs with depth ranging from 120 to 280 m. Response from the Gamma ray logs ranged from 5 – 115 cps, while the long normal resistivity logs response ranged from 2 - 1400 Ωm across the study area. The depth to the fresh water zones along the West - East direction ranged from 82 to 150 m except for BH 11 and BH 12 which had a fresh water at the surface from 0 m to a depth range of 20 and 54 m. The Thickness of the fresh water zone ranged from 20 to 174 m. The aquifer types are generally confined aquifers except for BH 11 and 12 that had unconfined aquifers. The depth to the fresh water zones along southeast to northwest direction ranged from 110 to 125 m, except for BH 2 which had a fresh water at the surface from 0 to 80 m. The Thickness of the fresh water zone ranged from 65 to 125 m. The aquifer types are unconfined aquifer except for BH 2 that has confined aquifers. The depth to the fresh water zone along southwest to northeast direction ranged from 0 to 126 m. The thickness of the fresh water zone ranged from 52 to 154 m. The aquifer types are confined aquifer across the direction. The depth to the fresh water zones along southwest to northeast direction ranged from 110 to 126 m, except for BH 1 which had fresh water at the surface from 0 to 52 m. The thickness of the fresh water zone ranged from 0 to 94 m. The aquifer types are unconfined aquifer except for BH 1 that had confined aquifers. Petrophysical parameters (porosity, permeability and water saturation) from the logs in the study area were used to produce isomaps. Porosity and permeability were very high in the extreme western and eastern flanks of the study area. The generated maps of the water saturation, Saline water thickness, and depth to Saline water zone revealed that Saline water was flowing from the southern part of the study area to the northern, northwestern and northeastern parts of the study area. Also the map of depth to Saline zone shows that the wells closer to the Atlantic Ocean were first affected and had a very shallow depth to Saline zone than wells further away from the ocean. Based on these, the Saline water was believed to have been sourced from the Atlantic Ocean, and the Saline water flows from the Atlantic Ocean to the northern, northwestern, and northeastern direction of the study area.