Assessment of Seawater Intrusion in Coastal Aquifers Using Integrated Hydrochemical Indices and Geophysical Techniques: A Case Study of the Niger Delta, Nigeria

Abstract

Seawater intrusion threatens groundwater availability in the coastal Niger Delta, Nigeria. This study assessed its extent by integrating hydrochemical indices with electrical resistivity techniques. Groundwater samples were collected from eight boreholes along an inland-to-coastal transect and analysed for physicochemical parameters, major ions, and heavy metals. The Sodium Adsorption Ratio, Kelly Ratio, Magnesium Hazard, Permeability Index, Chloro-Alkaline Index, and Revelle Index were calculated, while electrical resistivity surveys were used to delineate subsurface saline-water zones. Groundwater pH ranged from 6.05 to 6.92, electrical conductivity from 498 to 1,845 µS/cm, total dissolved solids from 305 to 1,182 mg/L, salinity from 0.18 to 1.47 ppt, chloride from 58 to 458 mg/L, and sodium from 41 to 224 mg/L. These parameters generally increased towards the coastline. The hydrochemical indices indicated increasing sodium enrichment, active ion exchange, and moderate-to-severe saline influence. Iron, manganese, lead, cadmium, and chromium concentrations also increased towards the coast. Low-resistivity zones identified by electrical resistivity imaging corresponded to saline groundwater and corroborated the hydrochemical results. Strong positive correlations among electrical conductivity, total dissolved solids, chloride, sodium, and seawater-intrusion indices (r > 0.96) indicated that marine water encroachment was the principal control on groundwater salinisation. Groundwater quality classification identified 25% of samples as excellent, 25% as good, 25% as poor, 12.5% as very poor, and 12.5% as unsuitable for drinking. The integrated approach provided a consistent basis for delineating saline intrusion and groundwater vulnerability.