Hydrogeophysical Analysis of Vertical Electrical Soundings for Groundwater Potential and Aquifer Vulnerability Evaluation in the Federal Capital Territory, Abuja, Nigeria
According to the United Nations World Water Development Report, groundwater accounts for 26% of the world's renewable freshwater, with around 2.5 billion people relying primarily on it for basic water needs. The most realistic and cost-effective strategy to increase universal access to clean water, meet the 2030 sustainable development goals (SDG), and minimize climate change impacts is broad exploitation and management of groundwater. The study area is Nigeria's capital Abuja, generally characterized by moderate precipitation and few surface water sources. The water treatment plant, designed with a capacity of 10,000 cubic meters per hour of treated water, was aimed at supporting a population of 500,000 people 34 years ago. However, due to population growth and urbanization, the water supply is no longer meeting demand. Groundwater demand and consumption in Abuja have increased significantly over the last decade due to fast population expansion, urbanization, and industrialization. Understanding groundwater potential and aquifer vulnerability is critical for sustainable resource management.
Geologically, Abuja is underlain by Precambrian rocks of the Nigerian Basement Complex, which cover approximately 85% of the land surface, and sedimentary rocks, which cover approximately 15%. In the study area, four significant lithologic units are visible; these include the Older Granites, the Metasediments/Metavolcanics, the Migmatite-Gneiss Complex, and the Nupe sandstones of the Bida Basin, which occupies the southwestern region of the territory.
This study aims to map groundwater potential and aquifer vulnerability zones using Hydrogeophysical method, which incorporates geoelectrical resistivity through vertical electrical sounding (VES) and geographic information system (GIS) approaches. With a maximum current electrode separation (AB/2) of 100m, the Schlumberger electrode configuration was used to acquire the field resistivity data in 823 locations across the study area using a DC resistivity meter (Campus Ohmega Ω).
The resistivity method works by passing an electric current into the ground through two electrodes and measuring the consequent potential difference across two other electrodes. The electrode spacing gradually increases while the electrode array's center point remains fixed. However, as the current electrode spacing grows, the current penetrates deeper into the ground, and the apparent resistivity reflects the resistivity of the deeper layers as well. The resistance is estimated as the ratio of potential difference to current in ohms(Ω). Using a global positioning system (GPS), the absolute coordinates of the survey points (VES) were determined.
Three to five subsurface geoelectrical layers were identified in the research area with the aid of IPI2Win software. Vertical electrical sounding (VES) data are often interpreted using the IPI2Win software, which is a user-friendly geophysical software designed to process resistivity data and generate one-dimensional models of subsurface layers Layer resistivity and thickness were estimated using the software by iterating the model with the observed field data acquired using the Schlumberger array. The H-type sounding curve is the most dominant among the identified curve types.
The interpreted data were used to determine parameters including Depth to Bedrock, Transverse Resistance, Longitudinal Conductance, Reflection Coefficient, and Layer resistivity. Using scaling criteria, the longitudinal conductance was used to determine the aquifer protective Capacity (Vulnerability), and the result revealed the dominance of moderate vulnerability across the study area.
The groundwater potential zones in the research area were characterized based on the following criteria as established by previous authors in this field: Areas with overburden thickness ≥ 30m and reflection coefficient < 0.8 were classified as very high groundwater potential; Areas with overburden thickness ≥13m and reflection coefficient < 0.8 were classified as High groundwater potential; Areas with overburden thickness ≥ 13m and reflection coefficient ≥ 0.8 were classified as moderate potential while areas with overburden thickness <13m and reflection coefficient ≥ 0.8 were classified as low potential, and finally, areas with overburden thickness >13m and reflection coefficient < 0.8. were classified as very low potential. These criteria were written as Python codes that classify the area into five groundwater potential zones. The area covered by each zone was calculated after the geospatial analysis: the very high GPZ occupies about 19.70% of the study area, high 20.30%, moderate 20.0%, low 19.74%, and very low 20.31%.
Ordinary Kriging (OK) interpolation algorithm was used to generate the layer resistivity map, layer thickness map, depth to bedrock map, aquifer vulnerability map, and groundwater potential zone maps using the smart-map QGIS plugin. Smart-Map is a QGIS plugin that allows the generation of interpolated maps in the QGIS environment. Kriging is an unbiased linear interpolation technique that uses a weighted average of nearby samples to estimate unknown values in specific areas. It is deemed the best interpolation method for spatially varying data. For this study, the resistivity (VES) data was randomly distributed over a large area, and the sampling distance between one VES data and the other ranged from 0.5km to 10km.
This study evaluated groundwater parameters in the study area based on the geo-electric properties of the earth material. The results reveal that weathered/fractured basement and sandstone formations in the study area are substantial aquifer systems that host potable water. Data from some drilled boreholes across the study area were used to cross-validate the VES results against borehole log records. This knowledge aided in a better understanding of aquifer disposition, vulnerability, and potential consequences. The study's findings will provide a geo-database for groundwater potential zones in the Federal Capital Territory (Abuja), with significant implications for sustainable groundwater resource design and management.