Geoelectrohydrogeological indices of evidence of ingress of saline water into freshwater in parts of coastal aquifers of Ikot Abasi, southern Nigeria

• Geoelectrohydrogeologic indices show intrusion of saltwater into freshwater aquifer.
• Aquifer depths >105 m and ρ>190Ωm are non-saline in the study area.
• Aquifer with ρ   slightly 〈or〉 190Ωm but <105 m deep are prone to salinity in the area.
• For good aquifers, distance–depth ratio of borehole from river is ⩽⩽30 in the area.
• Shallow depth borehole facilities show chemical effect of salinity in the area.

Saltwater–freshwater contamination affects groundwater in the coastal belts of Ikot Abasi Local Government Area thereby causing deterioration in groundwater quality and discourages the uses of groundwater in the area. To investigate the ingress of saltwater from river sources into fresh groundwater sources, we deployed the indices of geoelectric and geohydrogeological outliers to assess the intrusion of groundwater into freshwater. The resulting Dar-Zarrouk parameters generated from constrained one dimensional (1D) inversion of vertical electrical sounding (VES) and geohydrogeological indices from measured water electrical conductivity (WEC), total dissolved solids (TDS), temperature and pH value indicate that hydrogeological units with depths >105 m deep and resistivity ρ ∼ >190 Ω m are none saline as the WEC and the TDS values are respectively <700 mS/cm and 500 mg/L. However, aquifers with resistivity slightly <190 Ω m or >190 Ω m but with depth of burial <105 m are all prone to salinity as their measured WECs and TDS exceed 700 mS/cm and 500 mg/L respectively. The ratio of the radial saltwater source distance to depth of borehole has to be ⩽⩽30 for a successful exploitation of freshwater from the mapped area. Box and whisker plots and contour maps have been deployed to show the variability of the measured parameters. The results show that invasion of salinity from saltwater sources into freshwater in the study area is humongous and predominant in the shallow depths of the saturated layers. The visual observations of boreholes drilled within the shallow depths show chemical effect of corrosion in metal parts, plastic tanks and the raised concrete parts which are evidences of ingress of saline water into shallow formation in the mapped area.