Aquifer vulnerability mapping is essential for protecting groundwater resources from contamination. This study aimed to enhance the accuracy of groundwater contamination risk zone delineation by introducing the use of segregated subsurface lithology. The study was conducted in Tochi watershed, in Northern Uganda. The vulnerability mapping using segregated lithology utilizes groundwater tables, land use, rainfall, and lithological data subdivided into soil, laterite, saprolite, and granite derived from borehole drill logs. The result of the vulnerability mapping based on segregated lithology was then compared with the one derived based on unsegregated lithology. Statistical analysis (T-test and one-sample t-test) revealed that the two methods produced statistically different vulnerability maps (P > Chi-square = 0). The segregated lithology approach demonstrated significant improvement in delineating high vulnerability zones (P-value = 0.0295) compared to the unsegregated method (P-value 0.47). It was also noted that the use of segregated lithology produced vulnerability zones with distinct partitioning with P-values = 0.00103 compared to unsegregated lithology approach, P-value = 0.07122. Overall accuracy, evaluated using the Area Under the Curve (AUC) based on the Groundwater Quality Index (GQI) of borehole wells for segregated lithology approach, showed good to very good model performance, accounting for 75% and 100% of areas under high and low potential, respectively. In consideration of factors such as Analytical Hierarchy Process (AHP) and criteria, the findings indicate that integrating segregated subsurface lithology provides a more distinct aquifer vulnerability partitioning that better conforms to GQI trends, offering a superior method for developing aquifer vulnerability maps and improving groundwater resource management. While GQI data suggest water sources are generally safe for the year ranging between 2003 and 2016 with 22% low risk, and 70.6% very low risk, the semi-confined nature of the aquifer and increasing anthropogenic activities necessitate improved mapping for future protection.
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