Aquifer Hydraulic Characteristics and Vulnerability Estimation from Vertical Electrical Sounding: Case Study of Oru and Environs, South-Eastern, Nigeria

ABSTRACT

Aquifer Hydraulic Characteristics and Vulnerability Estimation from Vertical Electrical Sounding: A Case Study of Oru and its Environs, South-Eastern, Nigeria was carried out to delineate the different aquifer units with a view to determine the vulnerability of groundwater in the area. The study area is underlain by the Benin Formation which is within the Niger Delta Basin. Twenty-Seven (27) VES data were acquired using a Schlumberger array with a maximum electrode separation of AB/2= 500 meters and interpreted using 2D-inverse interpretation resistivity software. The resistivity’s of the geoelectric layers vary between 1100Ωm to 28200Ωm which revealed that the lithology of the area is mainly composed of sands and alternating sequence of clays and silts. The thickness of sands increases from 22.2m to 42.7m at Awo-Omamma and from 32.1m to 258m at Ohakpu while it decreases towards Ura-Akatta community from 32m to greater than 60.3m and at Amagu community from 18m to greater than 50m. The aquifer hydraulic characteristics were estimated using the concept of Dar-Zarrock parameters while the aquifer vulnerability index assessment was determined using the DRASTIC model. Results show that the value of the apparent resistivity ranges from 2000Ωm to 22,000Ωm. The aquifer conductivity ranges from 0.00020 Siemens per meter to 0.00052 Siemens per meter. For transverse resistance, it ranges from a low value of about 89175 Ω/m2 to3,000,000 Ω/m2 as it occurs at Ibiasoegbe/Ofeahia, Otulu, Awo-Omamma to a much higher value of about 1,000,000 Ω/m2 to 3,200,000 Ω/m2 and it occurs as such at Umuokwe Awo-Omamma, Ubachima, Amaji, Akuma etc. The storativity of the area ranges from a moderate high value of about 0.0001 (1.0 x 10-4 ) to 0.00025 (2.5 x 10-4 ) at Akuma, Umuabiahu, Amadehi-Ubulu to a high value that ranges from 0.00025 (2.5 x 10-4 ) to 0.0006 (6.0 x 10-4 ) as it occurs at Awo-Omamma, Ibiasoegbe/Ofeahia. The aquifer thickness in the study area varies from one location to another. In terms of depth of occurrence of the aquiferous layer/medium, there are shallow aquifers at Amagu and Amiri at 16m and 15m respectively; while at Ura-Akatta, Akwada-Aji, Ubachima etc the aquifers occur at 32m, 39.9m and 38m respectively; but, at Umuokwe Awo-Omamma, Eziani-Mgbidi, EziamaUbulu, Umuezike Amadehi Oburu etc. the aquifers occur at 50.3m, 54m, 56.5m, and 63.5m respectively. Based on the information obtained from the iso-resistivity models, it showed that the groundwater potential in the study area is very promising and this was based on the fact that the study area has a lot of reservoir sands and sandy aquiferous units and that the depth to aquifer in both its thickness and lateral extent i.e. the volume is large. In terms of the aquifer vulnerability assessment, it was revealed that the study area is moderately high to high in vulnerability to groundwater contamination and pollution and based on this fact, it is very susceptible to contamination and pollution from the ground surface where infiltration and percolation often take place each time rain falls in the study area