Vol.:(0123456789) 1 3 Environmental Earth Sciences (2021) 80:99 https://doi.org/10.1007/s12665-021-09379-y ORIGINAL ARTICLE Assessment of seasonal variation of saltwater intrusion using integrated geophysical and hydrochemical methods in some selected part of Ogun Waterside, Southwest, Nigeria Ganiyu Olabode Badmus 1,5  · Olukayode Dewunmi Akinyemi 2,5  · Adewole Michael Gbadebo 3,5  · John Adebayo Oyedepo 4,5  · Gbolahan Muyiwa Folarin 5 Received: 23 May 2020 / Accepted: 11 January 2021 © The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature 2021 Abstract 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. Keywords Saline water · Pollution · Geoelectric layer · Coastal aquifer · 2-D ERT models Introduction Freshwater quality and its accessibility are one of the essen- tial environmental and sustainability issues of the twenty- first century. Since groundwater represents more than 90% of the accessible freshwater resources, it can be seen as the largest reservoir of freshwater readily available to humans. Groundwater became the primary source of freshwater in many African countries including Nigeria owing to the fact that government gives less importance to building of dams to produce potable water for public use (Adeyemo et al. 2017). This perhaps led to the uncontrolled siting of boreholes by many Nigerians. Coastal dwellers within 100 km of the coast represent about 37% of the global population. In developing nations, coastal zones form the centre of attention for economic activity and human settlement. The major occupation pro- viding income for a large percentage of coastal dwellers is * Ganiyu Olabode Badmus ogbadmus@abuad.edu.ng 1 Department of Mathematical and Physical Sciences, Afe Babalola University, P.M.B. 5454, Ado-Ekiti, Nigeria 2 Department of Physics, Federal University of Agriculture, P.M.B. 2240, Abeokuta, Nigeria 3 Department of Environmental Management and Toxicology, Federal University of Agriculture, P.M.B. 2240, Abeokuta, Nigeria 4 Institute of Food Security, Environmental Resources and Agricultural Research, Federal University of Agriculture, P.M.B. 2240, Abeokuta, Nigeria 5 Centre of Excellence in Agricultural Development and Sustainable Environment, Federal University of Agriculture, P.M.B. 2240, Abeokuta, Nigeria Content courtesy of Springer Nature, terms of use apply. Rights reserved.