Abstract
Saltwater intrusion, erosion, and periodic flooding during severe storms are some of the critical problems bedeviling the precarious coastal environment of Nigeria. A time-lapse study aimed at monitoring the dynamics of saltwater intrusion within the coastland surrounding Ibeno, southeastern Nigeria is presented. Three (3) vertical electrical soundings (VES) and four time-lapse electrical resistivity tomography (ERT) profiles with minimum and maximum electrode spacing of 5 m and 40 m and profile lengths of 120 m and 150 m respectively were acquired in 2016 and 2017 using the Wenner and Schlumberger arrays. Data acquired from the field surveys were modeled with WINRESIST and RES2DINV software to generate 1D and 2D resistivity images of the subsurface. The VES results with a maximum survey length of AB = 300 m revealed a four-layer KH curve. The 2D geo-electrical sections delineated contrast between the high resistivity values obtained for the saturated freshwater zones and the low resistivity values obtained for the saturated saltwater zone. Expectedly, the intrusion was dominant nearer the coastline than inland. The salinity changes are believed to have been caused by both incremental tides passing through highly porous materials in the active-salinity change area and over-pumping of groundwater. The time-lapse resistivity measurements showed significant, active lateral salinity changes rapidly intruding the freshwater aquifers. The study demonstrates the application of time-lapse 2D ERT and vertical electrical sounding in identifying the spatial and temporal changes of saltwater intrusion in the coastal aquifer.
Highlights
The economic significance of coastal regions and its consequent rapid increase in population cannot be overemphasized as major cities of the world are situated there
The region of low resistivity values underlying the sandy clay unit of the profile corresponds to the sandy aquifer intruded by saline water, and it extends from depth 8 m to more than 20 m
The sandy aquifer intruded by saline water is underlain by a high resistivity layer which corresponds to a sand pocket with resistivity values above 390 Ωm with depth ranging from 19 m to more than 30 m
Summary
The economic significance of coastal regions and its consequent rapid increase in population cannot be overemphasized as major cities of the world are situated there. The need for their development has led to increasing emphasis on developing available resources including groundwater (George et al 2014). The challenge, has been that of poor groundwater quality due to increase in rate of potable water abstraction and uncontrolled land-use practices among others These practices have negative impact on groundwater quality, and one of such is saltwater intrusion (Edet and Okereke 2001a, b; Udom et al 1999, 2002)
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