Sub-surface Geology And Groundwater Distribution Pattern In Lagos Island Environs, South Western Nigeria
Accepted on December 22, 2010
Salt-water intrusion was reported to be prevalent in some parts of Lagos Island. To address this problem, sub-surface geological mapping of the affected area was carried out using gamma ray and resistivity logs, ditch cutting samples, and geophysical logs of some existing wells in the area. The work aimed at determining the probable origin of saline water as well as assessing the depth to the saline/fresh water interface. The suite of logs used facilitated the delineation of the aquifers and the saline/brackish and fresh water interfaces. A complex lithology of alternating sequence of sand and clay deposits was observed up to a depth of about 270m. Also, seven aquifer horizons were delineated. The depths ranges to the tops of these aquifers are: 3-10m; 40-70m; 60-100m; 110-140m; 150-180m; 178-210m and 212-240m, corresponding to aquifer thicknesses of 15-25m, 15-30m; 10-45m; 20-40m; 10-42m; 10-30m and 20-45m respectively. It is also observed that brackish/saline water occurrence is a major feature of the first four aquifer horizons; this implies that freshwater can only be encountered below 126m, unlike on the Lagos mainland where fresh water is encountered first and at a shallower depth. While it is apparent that the aquifers are highly susceptible to salt-water pollution, anthropogenic and biogenic influences, however, water contained in the upper four aquifers are probably lagoonal in origin and must have retained the salinity that characterized lagoonal environment, while water in the last three aquifers are of fluvial origin.