QUALITATIVE CHARACTERIZATION OF DELINEATION OF UPPER AND DEEP AQUIFER INTERVALS AND VERTICAL VARIATION OF GROUNDWATER QUALITY IN OGONILAND, NIGER DELTA, NIGERIA

Groundwater remains the primary source of potable water in Ogoniland, located within the Niger Delta of southern Nigeria, where increasing anthropogenic activities particularly oil exploration, waste disposal, and urbanization pose significant threats to water quality. Understanding the vertical variation in aquifer characteristics and groundwater chemistry is therefore essential for sustainable water resource management. This study assessed aquifer characterization and groundwater quality variation across depth in Ogoniland, with the aim of evaluating hydrostratigraphic conditions, hydraulic properties, and hydrochemical processes influencing groundwater systems. A combination of field and laboratory methods was employed. Boreholes were drilled using rotary drilling techniques, followed by wireline and manual logging to delineate lithology and aquifer boundaries. Pumping tests were conducted to determine hydraulic parameters such as hydraulic conductivity, transmissivity, and storativity. Groundwater samples were collected from both shallow and deep aquifers and analyzed for physical, physicochemical, and microbiological parameters using standard laboratory procedures. Hydrochemical facies were determined using Piper diagram analysis, while water quality was evaluated using the Water Quality Index (WQI). Results revealed significant spatial and vertical variability in aquifer characteristics. The upper aquifer thickness ranged from 15 m to 80 m, while the deep aquifer varied from 20 m to 85 m. The depth to the top of the deep aquifer ranged widely from 16.00 masl to 109.27 masl, indicating strong heterogeneity in subsurface conditions. Hydraulic conductivity values ranged from 15.31 to 264 m/day in the upper aquifer and 7.92 to 264 m/day in the deep aquifer, while transmissivity varied from 371 to 20,880 m²/day and 158.39 to 7,776 m²/day, respectively. Storativity values (10⁻⁴ to 10⁻⁷) confirmed semi-confined conditions in both aquifers. Hydrochemical analysis showed that groundwater is predominantly of the calcium–chloride (Ca–Cl) type, indicating the influence of marine processes, ion exchange, and water–rock interactions. Physicochemical parameters revealed generally low mineralization, with total dissolved solids ranging from 3 to 58 mg/L in the shallow aquifer and 2 to 38 mg/L in the deep aquifer. Electrical conductivity ranged from 7 to 116 µS/cm, while pH values varied between 5.50 and 7.25. Chloride concentrations ranged from 0.05 to 9.77 mg/L, with higher values observed in coastal communities such as Bodo and Onne, suggesting saline influence. Most heavy metals were below detectable limits, although elevated iron concentrations (up to 1.026 mg/L) were recorded in some locations. Microbiological analysis showed minimal contamination, with no detection of E. coli and only moderate total coliform presence in a few shallow wells. The Water Quality Index classified most groundwater samples as “excellent,” with only isolated cases of reduced quality. The study concludes that groundwater in Ogoniland is generally of good quality but is influenced by complex hydro stratigraphic conditions and is vulnerable to contamination due to the semi-confined nature of the aquifer system and the absence of a continuous protective clay layer. Vertical hydraulic connectivity between aquifers increases the risk of contaminant migration, particularly in areas affected by oil-related activities. It is recommended that groundwater protection strategies be strengthened through regular monitoring, proper siting and design of boreholes, and control of pollution sources. Protection of recharge zones and enforcement of environmental regulations in oil-producing areas are essential. Further studies should incorporate long-term monitoring and advanced contaminant tracing techniques to better understand groundwater dynamics and ensure sustainable water resource management in the region.