NVESTIGATION OF SALINE WATER INTRUSION IN SELECTED COASTAL AREA OF RIVERS STATE, SOUTH-SOUTH NIGERIA

Abstract

The study was aimed to investigate the intrusion of saline water in some selected areas of Rivers State Nigeria. The objectives of the present study were summarized by the use of electrical resistivity signatures of the area’s subsurface to study the occurrence of Saltwater-freshwater interface, determining the depth and thickness of different subsurface layers for groundwater exploitation and delineate saline-water intruded sands in the study area using the borehole geophysical logging tool. The study scoped at producing geologic maps of the study area to gather information concerning the geomorphologic features, application of the electrical resistivity method (VES) to obtain necessary data to investigate the occurrence of seawater intrusion by determining the resistivity, depth and thickness of the various lithologic layers and delineating saline-water intruded sands in the study area by exploring the potential of borehole geophysical logging tool. A total of 11 geoelectric (VES) surveys and four (4) downhole logging was carried out in selected locations within the River State Metropolis to investigate the saline water intrusion in the area. The obtained results reveal that from the Electrical resistivity sounding, the predominant lithologies in the area includes clay, clayey sand, coarse sand, consolidated sand, gravelly sand, consolidated gravel sand, gravelly sand, silty sand, freshwater bearing sand and saltwater bearing sand. Geoelectric layers identified across the area ranged from 3 to 6 layers. Resistivity ranged from 29.60 to 364058.00 Ohm.m for lithologic units, 113 to 181 Ohm.m for freshwater sands and 0.11 to 28.50 Ohm.m for saline water sands. Resistivity field type curves recognized included; H, QQ, AA, KHA, QH, HKH, QQHK, A, HK, KHK and KQH depicting the heterogenous nature of soils in the area. Six of eleven VES points encountered salt water at varying depths and include; Ogonokom, Eagle Island, Okirika, Assarama, Ikuru and Opobo sounding points. Thickness of the saline sandy layers ranged 3.10 m to 71.48 m. Downhole logging (SP and Resistivity) conducted in four coastal communities of the study area revealed four lithologic units which includes clay, clayey sand, coarse sand and saline water bearing sand. Saline zones were identified as having negative SP values and low resistivities. Clay were mapped as areas having positive SP values and low resistivities. The thickness of the saline zones ranged from 16 to 74 m. The largest saline layer thickness was obtained at Bonny. This study has shown that boreholes in Bille and Bonny communities exceeding depths of 15 m are likely to encounter salt water. The saltwater intrusion is predominantly concentrated around the southern part of the study area around Bonny and Opobo communities. The presence of Opobo River, Bonny River and a suite of creeks which are open to the Atlantic Ocean area responsible for the salt intrusion into these coastal boreholes. The outcome of the study yielded the importance that boreholes in Bonny, Opobo and Bille communities that has encountered saline water should be prevented from further use until treatment for salt water intrusions are conducted and water exploitation plan should be developed by the government and enforced to be utilized by residents in Buguma, Degema, Abonnema and NLNG to prevent saline intrusion because these communities are the most vulnerable to saline intrusion from over-exploitation of freshwater in the area.