Cases of salt water incursion into coastal aquifer and its consequential effect on the quality of portable fresh water as well as lives and properties, have received worldwide attention and has been reported in several parts of the world (Qi and Qiu 2011[1]; Werner et al, 2012[2]; Edwards and Evans 2013[3]; Elsheikh and Zeinelabdein 2015[4]; Ahmed and Askri 2016[5]; Gururaja et al.,’2019[6]; Behera et. al.’ 2019[7]; Idowu and Lasisi 2020[8]; Vann et. al., 2020)[9]. This is because it has caused a number of hazards not only to the environment but also to lives and properties. The effects of this has been highly devastating to mos people living in the coastal environment. Lee et. al., (2002)[10] employed the use of geochemical and geophysical techniques to detect and monitor saline water intrusion in Korean paddy field while Batayneh (2006)[11] employed electrical resistivity methods to detect subsurface fresh and saline water and delineated their interfacial configuration in eastern Dead Sea coastal aquifers of Jordan. Al Barwani and Helmi (2006)[12] studied the seawater intrusion in coastal aquifer in the area between Seeb and Suwaiq in the Sultanate of Oman, Barlow and reported a great devatstating effect on the immediate environment. Barrow and Reichard (2010) [13] reported saltwater intrusion in coastal regions of North America while Siddha and Sahu (2020) [14] reviewed the status of seawater intrusion in coastal aquifer of Gujarat, India. Kumar et. al., (2015) [15] did a critical evaluation of Index-based groundwater vulnerability mapping models using hydrogeological settings, to mention a few.
In Nigeria, this challenge has affected over 1000 km of the coastline of Nigeria extending from Rivers and passes through Bayelsa, Akwa Ibom, Cross Rivers State, Delta, Ondo, Ogun and Lagos States, all of which share their southern border with the Atlantic Ocean and has been reported in several literatures. [Adeyemo et al., 2017[16]; Ohwoghere-Asuma, and Oghenevovwero 2017[17]; Oloruntola et al., 2019[18]; Yusuf et al., 2021[19]; Atuluchukwu et al., 2022[20]; Affiah et al., 2018[21]; Edet 2017[22], Akerele et al., 2023[23], Inim et al, 2020[24], Obunadike et al., 2024[25]]. The works of Oteri and Atolagbe (2003)[26] reported that salt water incursion has been occurring especially in coastal areas of the Niger Delta, Nigeria. This has also been supported by the works of Adepelumi et. al., (2008)[27], Bayode (2018)[28], all of who worked on the delineation of saltwater intrusion into the freshwater aquifer of Lekki Peninsula in Lagos, Nigeria, Opobo town in Rivers state Nigeria and Warri Nigeria respectively. Others include the works of Ayolabi et. al., (2010)[29] who worked on integrated assessments of possible effects of hydrocarbon and salt water intrusion on the groundwater of Iganmu area of Lagos Metropolis, Southwestern Nigeria and Adeyemo et. al., (2017)[16] who carried out geoelectric soundings for delineation of saline water intrusion into aquifers in part of eastern Dahomey basin, Nigeria.
One of the major problems of dwellers of coastal areas is the problem of salt water intrusion into coater aquifers. This is because it has contributed in no small way to groundwater challenge in those regions. When saline water intrudes into coastal aquifers, it mixes with the groundwater and reduces the quality of the groundwater. Most people who dwell around coastal areas depend on fresh portable groundwater because the saline water that is available in large amount in lagoons and oceans are not useful because of the amount of cations and anions that is dissolved in them exceed the normal value and this caused a lot of health challenge. Umara et al., (2021)]30] stated that the use of desalinization of sea water using treatment plants could be highly capital intensive and so, may not be affordable, hence the dependence on fresh groundwater from the subsurface since it requires little or no purification before it can be used for achieving sustainable development (Coker, 2020)[31].
Most saline water incursion into coastal aquifer are usually geogenic and anthropogenic in origin, although anthropogenic factors are the most dominant factors (Oloruntola, 2019)[18]. Kumar et. al., (2015)[32] observed that seawater intrusion do occur when fresh groundwater withdrawal in coastal aquifers causes a decrease in the level of groundwater thereby causing lateral and/or vertical movement of saline water, hence causing deterioration of groundwater quality. United Nations (2017)[33] stated that more than 40% of the world population lives within 100 km of the coastline and are usually large cities that are located along the coastlines while 10% of the population reside in coastal areas that are less than 10 meters above sea level.
In the past, saline water intrusion into coastal aquifers have been studied with the use of geochemical methods. For instance, Petalas and Diamantis (1999)[34] was able to employ geochemical analysis technique to understudy the origin and distribution of saline groundwaters in the upper Miocene aquifer system, coastal Rhodope area, northeastern Greece. Also, Petalas and Lambrakis (2006)[35] was able to simulate intense salinization phenomena in coastal aquifers-the case of the coastal aquifers of Thrace while Polemio et al., (2002)[36] characterized the Ionian-Lucanian coastal aquifer and seawater intrusion hazard. Others include the works of Somay and Gemici (2009)[37] and Koukadaki et al (2007)[38] and Kruse et al., (1998)[39]. All these workers employed geochemical methods to study saline water incursion based on measures of electrical conductivity of chloride concentration, and other cation and anion concentrations, parameters which generally highlight seawater contamination. However, the works of Oteri and Atolagbe (2003)[26], Ayolabi et al., (2013)[40], Oteri and Rahseed (2019)[41] and Adepelumi et al., (2009)[27] amongst others, have proven that geophysical technique is more than adequate to decipher the depth to saline water incursion. However, when geophysical techniques are integrated with borehole logging and hydrogeological water quality studies, a better result with accurate and high precision is able to detect and conclude the depth to incursion and extent of the incursion. Badagary is located towards the western part of Lagos State and share its borderline with the neighboring country, Republic of Benin and its southern part with the Atlantic Ocean. This coastline is not exempted from this devastating phenomenon. Although a number of workers have identify the saline water incursion along the coast of parts of Lagos Nigeria, especially at Apapa, a few have been reported at Badagary, a surburb of Lagos in Nigeria. In this research, an attempt is made to establish the most recent vertical extent and the lateral extent of the incursion using conclusions from hydrochemical analysis as well as available borehole logs coupled with data obtained from geophysical vertical electrical soundings. This is because over the years, local residence where the effect have not been pronounced are beginning too report evidences of incursion in their shallow well as as well as deep abandoned wells.