Groundwater vulnerability assessment using the modified DRASTIC model in the upper Fafan catchment, Wabi Shebelle Basin, Ethiopia

Abstract

Abstract Groundwater is the primary service provider in the semi-arid Upper Fafan catchment of northeastern Ethiopia, where increasing anthropogenic pressure threatens water quality. This study evaluated groundwater vulnerability using four approaches: the conventional DRASTIC model, land use integrated specific DRASTIC, rate-modified DRASTIC, and DRASTIC coupled with Analytic Hierarchy Process (AHP). It provides comparative vulnerability assessment in the catchment, and demonstrated the potential of statistical validation combined with multi-criteria decision analysis to improve prediction. Seven hydro-geological parameters combined with land-use data were used to produce intrinsic and potential vulnerability analysis for the catchment. The intrinsic DRASTIC index ranged from 97 to 185. The high vulnerability zones were associated with shallow groundwater depths (less than 4.5 m), high recharge area accounting 88% of the area, and permeable sand and gravel aquifers accounting for 67% of the catchment. Model Validation using nitrate concentrations from 28 boreholes depicted weak correlations for the conventional intrinsic and specific DRASTIC models ( r = 0.24 in both cases). The rate modification approach improved model performance ( r = 0.44 for intrinsic and r = 0.50 for specific). A moderate correlation was obtained from the AHP-based DRASTIC model ( r = 0.56), and 34% of the area was categorized as moderate to high and high vulnerability classes. The result suggested that integrating land use, statistical validation, and multi-criteria decision analysis can enhance groundwater vulnerability assessment. Future work needs to consider obtaining additional borehole data to further improve vulnerability prediction.