In the face of the global groundwater crisis and climate change challenges, the groundwater potential (GWP) and quality (GWQ) assessment for tackling the depletion and contamination of aquifers are imperative for India's sustainable development of groundwater resources. This study explores the GWP and GWQ in the southwestern part of Bihar state, located in the declining groundwater of the lower Indo-Gangetic plain, where population growth, rapid urbanization, and escalating agricultural demands have strained aquifer systems. This study uses the Analytical Hierarchy Process and weighted average techniques in the GIS framework to assess groundwater vulnerability. Our results reveal that over 82% of the area exhibits good groundwater potential, except for the southern part. The presence of Vindhyan mountains in the southern part has different geology, elevation, and land use and land cover, leading to poor groundwater potential. Considering the desirable and acceptable limits by BIS (2012) and Who (2022), it has been found that 89% and 18% of the study area falls under the poor to unsuitable groundwater category, affecting a staggering 7.7 million and 1.6 million individuals, respectively despite substantial groundwater potential. Findings from this investigation indicate that groundwater contamination resulted from anthropogenic and geogenic factors; however, it remains suitable for agricultural purposes. Our research will assist policymakers in implementing sustainable management strategies to ensure a lasting and reliable supply for domestic and agricultural uses of groundwater.