Seawater intrusion is gradually becoming an inevitable problem of the coastal aquifer, especially in coastal zones of the world [
1]. Coastal regions have been described as the area of the world that is fast becoming home to high and growing populations which are consequently undergoing environmental degradation [
1,
2]. High population contributes significantly to this environmental decline. As at the year 2003, approximately three billion people, about half of the world’s population, lived within 200 km of the coastline, a figure which will likely double by the year 2025 [
2]. Coastal areas account for approximately 40% and 45% of the world’s and Nigeria’s population, respectively [
3,
4].
Rural–urban migration associated with economic, industrial and agricultural opportunities among other factors leads to this rapid population growth. In most of the developing countries around the world, there is associated infrastructural deficiency which includes erratic pipe-borne water supply among others. This, therefore, puts significant pressure on coastal freshwater aquifers which remain the only alternative source of water, to meet water required for domestic, agricultural, industrial and recreational purposes.
Eastern Dahomey Basin in Nigeria comprises two major inland river basins which include the Ogun-Osun and Benin-Owena river basin, with the addition of the coastal aquifer. This basin contributes significantly to the water resources of Nigeria and is also the major source of freshwater supply to various households. There are several reports from different sectors, ranging from government agencies, non-governmental organisations, research and academic institutions to “ad-hoc” and uncoordinated individuals and private companies, on the alarming degradation and the threat to groundwater quality, especially in the coastal area. Here, the groundwater level is generally higher, which permits constant interaction with the polluted surface water. The daily water demand in Lagos, the most populous city in Subsaharan Africa underlain by this basin is about 724 million gallons with production standing at 317 million gallons, this leaves a gap of about 407 million gallons which is complemented through the groundwater from shallow hand-dug wells and boreholes. Worse still, some of the pipe-borne water never reached households due to constant defects from burst transmission pipes and old trunk lines [
5]. This further emphasises the importance of this invaluable resource to the economic and health status of inhabitants of the country’s southwest coast. Indiscriminate waste disposal and management also pollute surface water which recharges the groundwater. This direct interaction of contaminated and polluted surface water with groundwater degrades the quality of groundwater in the unconfined top aquifer in this coastal area as observed in the work of [
6,
7,
8]. In the search for potable freshwater, most wells and boreholes in this area are drilled to the second aquifer, which is a confined aquifer (
Figure 1) underlying the coastal area of the basin which is most densely populated, in order to meet their daily water demand. This development has put pressure on groundwater and led to saltwater intrusion in the freshwater aquifer through up conning wedge shift resulting from over-abstraction of groundwater [
3,
9,
10,
11,
12]. Proximity to the sea also encourages saltwater intrusion into the freshwater of the coastal aquifer due to sea-level rise (SLR) [
13]. Another school of thought has also suggested possible dissolution of evaporite minerals trapped within the basin lithology during a past event of ocean transgression and regression into groundwater causing their enhanced salinity and electrical conductivity [
14,
15]. The work of [
3,
7,
9,
10,
16,
17,
18,
19,
20,
21] have employed different methods such as electrical resistivity tomography (ERT), induced polarization (IP) and hydrochemistry on groundwater in some selected and specific locations within this basin. Some of these studies which have identified saltwater intrusion in the upper aquifers mainly in the eastern coast of Lagos, such as Lekki, Ajjah, Victoria Island, Sagontedo. The deterioration of water quality in parts of Lekki phase 1 and Oniru environs of Lagos metropolis due to saltwater infiltration in the freshwater aquifer has become a significant concern [
10]. Despite all this, the report still shows that systematic and detailed groundwater quality assessment and monitoring are insufficient for sustainable management of groundwater in the face of the menace of salty water. This challenge results from a combination of factors, such as costs associated with water-quality monitoring, the relatively low levels of funding for research in Sub-Sahara Africa over the past decade and limited national regulation of high-intensity rainfall events which pose a risk to shallow and poorly protected groundwater sources [
9,
22]. Also, groundwater is a dynamic resource whose quality requires constant regimented monitoring for early detection of contamination and pollution to ensure a sustainability and health safety measure of life, and possibly plant and animals which depends on it for survival. Moreover, there is a vital need to monitor the possible risk of saline water intrusion of the coastal aquifers because, once saline intrusion into coastal aquifer has occurred, it is challenging to overcome and to improve the management of the water resources based on long-term strategy. Within this location, several boreholes have had to be abandoned, and other water sources sought, often at a high cost. Furthermore, subsurface data are generally scarce because invasive methods such as borehole drilling and associated aquifer tests are expensive and time-consuming. The current lack of hydrogeological information of the existing commercial boreholes is also a factor. Therefore, non-invasive hydrogeophysical methods of subsurface data acquisition provide an alternative, or a complement to, direct observations. Protection of groundwater resources in coastal areas and their sustainable management, in conjunction with other water resources (e.g., surface water, seawater), require an understanding of the coastal aquifer hydrogeology.
This study, therefore, focuses on the delineation of origin and distribution of saltwater intrusion in the coastal freshwater aquifer of Eastern Dahomey Basin using a physicochemical approach through the electrical conductivity (EC) and geophysical method of ERT and IP complemented with borehole logs with the objective of identify saltwater bearing lithological units that are responsible for salinisation of freshwater. Complementary use of these methods improves the extensive coverage of hydrogeological investigations due to the difference between their characteristics and capability.
1.2. Geology and Hydrogeology of Eastern Dahomey Basin
The lithological character of the sediments was dictated by the regime of transgressions and regressions of the sea since the Cretaceous age, and the transgressions are found to have been coming from the south. The stratigraphic description of the sediments has been provided by various authors, including [
23,
24,
25,
26], as presented in
Table 1. Some of these authors have proven the area to be composed of the following stratigraphic units from youngest to the oldest. The Alluvium Deposits and coastal plain sands consist of soft, very poorly sorted clayey sands, pebbly sands, sandy clays and rare thin lignite of Oligocene to the recent age. This is underlain by Ilaro formation which consists of massive, yellowish, poorly consolidated, cross-bedded sandstones, which are fine to medium-grained and poorly sorted [
23]. This is followed by the Ewekoro formation which consists of predominantly Paleocene fossiliferous limestone which becomes arenaceous towards the base [
26] and the Abeokuta formation which consists of lower Cretaceous sandstone and grits with interbedded mudstone unconformity overlain the basement complex fine detrital sandstone, siltstone and shale overlying the formation in the upper parts. The youngest sets of strata are marginal to fully marine sand and shale of the Maastrichtian Age.
The Coastal Plains Sands represents the main aquifer in the southern parts of the basin which most of the well and boreholes exploit. This has resulted in a multi-aquifer system consisting of three aquifer horizons separated by silty or clayey layers [
16]. The aquifer shows high thickness at the northern part of Abeokuta, through Ewekoro, Ilaro, and thins out into the coastal plain sands, in locations closer to the coast in the south. The percentage of sands in lithology also increases towards the south [
16]. The geological map of the basin is presented in
Figure 2, showing the distribution of rock units within the basin.
The coastal plain and alluvium deposits of the Eastern Dahomey Basin are characterised by a multi-layer aquifer, which is classified into three types [
16]. The first aquifer is a water table aquifer which is prone to pollution because of its nearness to the ground surface. The second and third are confined aquifers composed of an alternating sequence of sand and clay. They are harnessed through boreholes and are the basis of mini water-works in the Lagos area and other parts of the Basin. These aquifers belong to the continental Ilaro Formation. The third aquifer seems to be the most productive and faces the highest level of groundwater exploitation within which most boreholes terminated. Within it, groundwater exists confined to the semi-confined condition [
16,
19,
27]. Generally, the water-table ranges from 2.0 to 15.0 m below ground level (b.g.l) in the area. Also, the study area is well drained by rivers and streams that flows southerly into the lagoon and the Atlantic Ocean. The average annual precipitation is above 1700 mm and serves as a primary source of groundwater replenishment.