Studies of Salt Water intrusion in Opobo/Nkoro Rivers State Using Geophysical and Hydrochemical Methods

Abstract

Salt water intrusions into fresh water aquifers in Opobo town in Nkoro L.G.A, Rivers state is becoming an issue of concern to the community, and as such requires detailed analysis through an integration of qualitative (geo-electric) and quantitative (hydrochemical) methods. Twenty vertical electrical soundings (VES) labelled as O/MSC/01 to O/MSC/20 and three 2D electrical resistivity tomography (ERT) data were acquired in the area. Results from interpretation of the twenty VES profiles showed 4 geo-electric layers, which are topsoil, sand, sandy clay and clay. In VES geo-electric sections, it was observed that salt intrusions occur at depths of 43.135m, 55.20m, 43.528m, 51.63m, and 28.357m along O/MSC/02, O/MSC/05, O/MSC/07, O/MSC/11 and O/MSC/15 profiles respectively. No salt intrusion was observed in O/MSC/01, O/MSC/04, O/MSC/09, O/MSC/10, O/MSC/12 and O/MSC/14 within the depth investigated. Results of 2D imaging along VES profiles 10, 11 and 12, show salt intrusions observed along the three profiles at different depths within the subsurface. Along profile O/MSC/10, saltwater intrusions (with resistivity range of 59.2Ωm-60.1Ωm) was observed at depth of 7.59-9.59m at lateral distance of 34.0-36.0m and depth of 5.59-7.59m at lateral distance of 94-96m. Along profile O/MSC/11 salt intrusions (with resistivity range of 58.8Ωm-59.2Ωm) was observed at depth of 2.59-5.59m at lateral distance of 28-40m, and 110-128m, while along profile O/MSC/12 salt intrusions (with resistivity range of 14.5-20Ωm) was observed at depth of 3.59-5.59m at lateral distance of 112-118m. These findings were not observed in VES surveys along similar profiles where 2D ERT was carried out. 3D map computed to connect aquifers within the third geo-electric layers for VES points 1-20, showed that within the third geo-electric layers for O/MSC/02, 05, 07, 11, 16, 17, and 20 lies saline water. The reliability of these results was validated by results from quantitative assessment of physiochemical parameters (P^H, electrical conductivity (EC), and total hardness) and hydrochemical contents (HCO₃^-, Na⁺, K⁺, CL^-, and SO4^2-) used to evaluate the ionic abundance of water samples taken from seven wells/boreholes in the area. P^H values of 6.40-6.90 were predominant in the samples which indicates slight acidity and unsuitable for human consumption, EC values in the order of 1705μS/cm, & 1853.74μS/cm were obtained and total dissolved Solids (TDS) was in the order of 700.30mg/l & 750.02mg/l. Elevated EC values in water is indicative of high ionic abundance, and is diagnostic to salinity of the water as reflected in the 1D and 2D geo-electric surveys. However, the study area also contains fresh water aquifers lying within the third geo-electric layers for VES profiles O/MSC/01, 03, 06, 08, 09, 10, 12, and 18, hence a water supply tube well can be drilled in these areas for groundwater production but the water should be subjected to hydrochemical test to ascertain its level of contamination and prior treatments before human consumption.

Keywords: Saltwater Intrusion, Groundwater, Vertical electrical sounding (VES), Electrical resistivity  tomography (ERT), Geo-electric section.