2D electrical resistivity imaging of the effect of tide on groundwater quality in Ogulagha Estuary, western Niger Delta, Nigeria
The flow of saltwater from the ocean at high tide to the shoreline has substantial effect on groundwater quality of aquifer adjoining the sea. During high tide, saltwater from the sea inundates the ground surface close to the sea and infiltrates into the subsurface. The flow often terminates at the high tide mark or upper shoreface with resultant effect of degrading aquifers adjacent to the ocean. The objective of this study is to investigate the effect that saltwater precipitated by tide has on groundwater quality. Two profiles parallel to the ocean and one further inland were carried out with the aid of a 64 multi-electrodes SAS 4000 ABEM Terrameter. A spacing of 5m and 2.5m were used for the acquisition of 2D electrical resistivity data respectively. The profiles parallel to the ocean and placed at the high tide mark revealed resistivity values that ranged from 0.375 Ωm to 4.2Ωm with saltwater/freshwater depth greater than 51m deep. Resistivity increases gradationally with depth, which significantly suggests increasing freshness of the groundwater depth. Further inland resistivity ranged from 7.4 Ωm to 494Ωm, which indicates presence of clay and freshwater respectively. The study concludes that quality of groundwater beneath the high tide mark is evidently affected by saltwater enhanced by tide. The thickness of the saltwater is more than 50.8m as revealed by the resistivity low of 4.2 Ωm for the aquifer close to the sea. The effect of tide is most pronounced on the upper most geologic layers situated at the hightide-mark but with no significant impact on aquifer situated further inland from the hightide-mark. Further study on the exact position of saltwater/freshwater boundary is recommended for possible placement and subsequent monitoring of intrusion.
Keywords: Saltwater, high tide mark, 2D ERI, groundwater quality, Ogulagha