Geoelectric Survey for Mapping GroundwaterFlow Pattern in Okigwe District, SoutheasternNigeria

摘要

Aims: This study was carried out to determine the groundwater flow pattern in order to identify the converging centres with high yield expectations.Study Design: The study area, Okigwe District in Imo State Nigeria, lies between latitude 5o30′N to 5o57′N and longitude 7o04′E to 7o26′E and covers an area of approximately 1,824 km2.Methodology: One hundred and twenty (120) vertical electrical sounding (VES) results, using the Schlumberger configuration were acquired in order to map the ground water flow pattern within the study area. The maximum current electrode spacing for the survey was 900 m.Results: Twelve of the VES stations were sited near existing boreholes to enhance interpretation. The resistivity of the aquiferous zones varied across the study area ranging from 33.1Ωm obtained at Umuedi (VES 7) in the northern part to 32600Ωm at Otoko (VES 93) in the southern part of the area. Using an average transmissivity of 1032.0848 m2/day determined from pumping test data of the boreholes in the area a mean conductance value of 91.222 m/day was obtained for the area. Analysis of VES data shows that the groundwater flows from the Northern part of the district towards the Southeastern part, South central and South Western parts. It also flows from the South-South area to Southeastern and South central parts of the study area forming two main collecting (converging) centres C1 and C2 having approximately West-East trend. C1 lies within the valleys of Efuru and Eze Rivers while zone C2 lies almost parallel to the Onuinyi River and the Abadaba Lake.Conclusion: The survey results show that the Southern and Northeastern parts of the district are more promising for siting borehole with high yield expectations than the North Western part. The occurrence of aquifer in this area is linked to the presence of fractures in the shale members.