Time-Domain Electromagnetic Survey for Saline Water Intrusion in Coastal Lagos

This study applies time-domain electromagnetic surveying to map saline water intrusion in coastal aquifers of Lagos State, South West Nigeria. Lagos is Nigeria's most populous city and its principal economic hub, with extensive coastal and estuarine water bodies that create pathways for saltwater-freshwater interface displacement. Intensive groundwater abstraction from shallow coastal aquifers has accelerated saline water intrusion in many areas, threatening the quality of water supply to millions of residents. This study deploys a Geonics TEM47 system using a 40 by 40 metre transmitter loop with a central receiver at 20 stations distributed across Lagos Island, Ajah, and Badagry coastal areas. Transient electromagnetic decay curves are recorded and inverted using TEMIX software to produce resistivity-depth profiles. The positions of freshwater-saltwater interfaces are identified from changes in formation resistivity with depth. Findings reveal highly conductive zones with apparent resistivities below 2 ohm-metres at depths between 8 and 25 metres in all three study areas, interpreted as saline water-saturated sands. The freshwater lens is thinnest in Lagos Island (less than 5 metres), where abstraction intensity is highest, and thickest in Badagry (12 to 18 metres). Resistivities of 3 to 15 ohm-metres in transitional zones indicate mixing of freshwater and saline water. The saline water interface maps produced confirm and extend findings from the limited available monitoring well data. The study recommends managed aquifer recharge schemes and abstraction regulation for coastal Lagos to prevent further freshwater lens depletion.

Keywords: time-domain electromagnetic, saline intrusion, coastal aquifer, Lagos, freshwater lens.