Geophysical Investigation of Submarine Groundwater Discharge Along the Nigerian Coastline: Implications for Coastal Hydrogeology and Ecosystem Health

Submarine groundwater discharge, the process by which terrestrial groundwater enters the coastal ocean through the seabed, is a poorly constrained component of the Nigerian coastal freshwater and nutrient budget, with direct implications for coastal fisheries productivity, mangrove ecosystem health, and the management of aquifer connectivity between onshore and offshore systems in the Niger Delta and western Nigerian coast. This research develops the first quantitative estimate of submarine groundwater discharge flux along the Nigerian coastline using a geophysically driven methodology incorporating continuous resistivity profiling, natural radon tracer measurements, and temperature anomaly mapping of nearshore seafloor sediments. Towed marine resistivity profiling over 450 kilometres of nearshore transects between Lagos and the Calabar River estuary maps the depth and extent of freshwater-saline water mixing zones beneath the seafloor at horizontal resolutions of 5 to 15 metres. Continuous radon-222 concentration measurements in surface water collected simultaneously provide an independent tracer of subsurface groundwater input that is calibrated against the resistivity-derived freshwater fraction. Seafloor temperature anomalies mapped by autonomous underwater vehicle thermal imaging identify localised zones of enhanced upwelling consistent with high-permeability pathways in the seafloor sediment. Integration of the geophysical flux estimates with seasonal rainfall and groundwater level records produces a first-order annual SGD budget for the Nigerian coastline of between 0.8 and 2.4 cubic kilometres per year, distributed non-uniformly with highest discharge in the Niger Delta sector. The ecological significance of the derived nutrient flux is assessed against primary productivity observations from satellite ocean colour data. Keywords: submarine groundwater discharge, coastal hydrogeology, marine resistivity, radon tracer, Niger Delta