Assessment of Aquifer Protective Capacity, Groundwater Vulnerability, and Heavy Metal Pollution in Gully Erosion Landscapes of Abia State, Nigeria

Abstract

Groundwater resources in gully erosion-prone areas are increasingly threatened by contaminant infiltration due to the removal of protective overburden and exposure of aquifer systems. This study assessed aquifer protective capacity, groundwater vulnerability, and heavy metal pollution in selected gully erosion landscapes of Abia State, Nigeria, using an integrated hydrogeophysical and hydrochemical approach. Measurements were obtained at twenty Vertical Electrical Sounding (VES) stations and along six Electrical Resistivity Tomography (ERT) profiles to delineate aquifer characteristics and evaluate protective capacity using Dar Zarrouk parameters. Groundwater samples were collected from twenty boreholes and analysed for physicochemical parameters and selected heavy metals using standard analytical procedures. The Heavy Metal Pollution Index (HPI), Hazard Quotient (HQ), Hazard Index (HI), Pearson correlation analysis, Principal Component Analysis (PCA), and Geographic Information System (GIS)-based spatial analysis were employed to evaluate groundwater quality and contamination risk. Aquifer resistivity ranged from 138.4 to 982.6 Ωm, whereas aquifer thickness varied between 22.6 and 72.4 m. Longitudinal conductance values ranged from 0.18 to 1.76 S, indicating aquifer protective capacities ranging from weak to good. Groundwater vulnerability assessment showed that 30.0% of the investigated locations were highly vulnerable, 45.0% were moderately vulnerable, and 25.0% exhibited low vulnerability. Iron recorded the highest concentration (0.08–1.26 mg/L), and concentrations exceeding World Health Organization guideline values for iron, lead, and cadmium were observed at several sampling locations. Heavy Metal Pollution Index values ranged from 42.8 to 156.7, with 35.0% of groundwater samples classified as highly polluted. The Hazard Index values were 2.11 for adults and 3.76 for children, indicating greater non-carcinogenic health risks among children. Pearson correlation analysis revealed strong positive relationships among electrical conductivity, total dissolved solids, and selected heavy metals, while PCA showed that the first three principal components explained 84.6% of the variance in the dataset. The findings indicate that gully erosion has reduced the natural protective capacity of aquifers and increased groundwater vulnerability to heavy metal contamination in parts of Abia State. The integration of hydrogeophysical and hydrochemical techniques was effective for groundwater resource assessment and provided valuable information for groundwater protection, environmental monitoring, and sustainable water resource management in erosion-prone regions.