Integrating GIS-Based DRASTICA Model and Aquifer Protective Capacity for Hydrogeospatial Evaluation of Groundwater Vulnerability to Contaminants in Owerri, Southeastern Nigeria.

Abstract

Owerri, the capital of Imo State, depends almost exclusively on groundwater abstracted from shallow to intermediate-depth aquifers developed within the Benin Formation, a lithologic unit dominated by unconsolidated sands and sandstone with limited natural protection in most areas. This study integrates a GIS-based DRASTICA model with aquifer protective capacity evaluation derived from VES (longitudinal conductance values) to provide a comprehensive hydrogeospatial assessment of groundwater contamination risk in Owerri and its environs. 70 VES stations were interpreted to characterize subsurface lithology, determine vadose zone media, and aquifer protective potential, while digital elevation models, soil maps and LULC data were synthesized into a spatial database using ArcGIS 10.5 software. DRASTICA vulnerability indices range from 108 to 330 and delineate three dominant vulnerability classes: moderate (87.04%), high (12.42%), and very high (0.54%). Zones of elevated vulnerability are concentrated in densely urbanized areas and along the Imo River/Otamiri River zones, where shallow water tables, sandy vadose zones, minimal slopes collide with intense human activities to promote rapid contaminant infiltration. Longitudinal conductance values range from 0.004 to 31.003 Ω⁻¹, indicating predominantly poor to moderate aquifer protective capacity across greater parts of the Owerri, and consistent with areas of dominated by mostly sandy overburden. Areas of high longitudinal conductance correspond spatially with zones of low DRASTICA vulnerability, while areas of low conductance coincide with moderate to high vulnerability. The findings reveal the sensitivity of Owerri’s aquifer system to contamination from poorly managed waste disposal. The integrated DRASTICA–Aquifer Protective Capacity framework provides a replicable tool for groundwater protection planning, environmental monitoring, and sustainable urban development in rapidly urbanizing sedimentary environments.