APPLICATION OF ELECTRICAL RESISTIVITY METHOD IN QUANTITATIVE ASSESSMENT OF GROUNDWATER RESERVE OF UNCONFINED AQUIFERS

Abstract

Application of Electrical Resistivity Method in Quantitative Assessment of Groundwater Reserve of

Unconfined Aquifers was carried out at Port Harcourt and Obio/Akpor Local Government Areas,

Rivers State, Nigeria, in order to locate, delineate subsurface water resources and estimate its

reserve. A Wenner electrode configuration was employed. The field survey was conducted along

thirteen profiles providing continuous coverage. Color-modulated sections of resistivity versus

depth were plotted for, giving an approximate image of the subsurface. The field survey was

accompanied by a laboratory work. Resistivity of rock and soil samples taken from the field

were measured and the resistivity formation factors were obtained. The porosities and hydraulic

conductivities of the same samples were calculated. A relationship between the porosity and

the formation factor has been established. The laboratory established relationship was applied

to the data obtained in the field to calculate the porosity values of the formation present within

the investigated area. The porosity values were contoured and plotted. Depth to the bedrock

for each line was obtained. A commercial computer software was used to enable the

computation of groundwater reserve within the investigated area. The results showed that the

layers associated with the aquifers have resistivities between 345 Ω.m and 8286 Ω.m, and are

located at depths varying from 6 to 51 m. The layers have porosity between 18% and 35%. The

results obtained from the electrical resistivity profiles indicate that the aquifers occupy a surface

area of about 15977900 m2 with a mean depth and net volume of 29 m and 204610000 m3

respectively. The average aquifer porosity was revealed to be 30%, suggesting a usable capacity

of about 61,383,000 ± 7,980,440 m3 reserve of groundwater.