Assessment of seasonal variation of saltwater intrusion using integrated geophysical and hydrochemical methods in some selected part of Ogun Waterside, Southwest, Nigeria

This study presents the seasonal variation of saltwater intrusion in Ogun Waterside using a combine electrical resistivity tomography (ERT) and groundwater physicochemical analysis. The ERT survey was completed with 22 profiles using dipole–dipole array method covering seven major communities (Imakun-omi, Irokun, Isekun, Ode-omi, Iwopin, Itebu-elero, and Ayila) east of Lagos. The field survey was carried out between August 2016 and March 2017 with a minimum electrode spacing of 2.5 m. The inverted model generally revealed a resistivity range of 1.0 Ωm–100,000 Ωm delineated through a depth range of 37 m–197 m. The model obtained for Imakun-omi, Irokun, Isekun, and Ode-omi revealed significant impact of saltwater intrusion from nearby lagoon, sea, or creeks indicated as low-resistivity structures generally below 20 Ωm. The intrusion is more pronounced during the dry season due to decrease in groundwater level associated with low precipitation and possibly aggravated by high rate of groundwater extraction. Groundwater physicochemical analysis obtained in the study area revealed a general increase in the ionic concentrations with a corresponding increase in the water quality index (WQI) during dry season. The calculated WQI for the wet and dry seasons revealed good-to-excellent water quality in most samples taken in the study area except for samples taken in the southern part which were found within the poor-to-very poor water quality category. Evidence from the physicochemical analysis of groundwater samples corroborates the ERT result viz-a-viz the aquifer contamination status of Imakun-omi, Irokun, Isekun, and Ode-omi.