Groundwater is of paramount significance for sustaining the socio-economic development and ecological health of arid endorheic intermountain basins. However, its availability is severely limited by hydrochemical composition, which remains inadequately quantified. This study takes the Chaka Basin on the Tibetan Plateau as an example to elucidate the spatial patterns, driving forces, and their quantitative contributions of groundwater geochemistry in such arid endorheic basins. Results indicate that groundwater is generally alkaline with substantial spatial variability in salinity. Near the mountain pass, groundwater remains fresh (TDS <1 g/L), retaining favorable hydrochemical characteristics inherited from mountain-sourced recharge. Salinity increases progressively along the groundwater flow path, with a notable contrast between phreatic and confined aquifers in the middle-lower reaches. Phreatic groundwater shows high TDS (>10 g/L near the terminal lake), while confined groundwater remains relatively fresh under the same settings, largely due to evaporation. Hydrogeochemical compositions are predominantly contributed by water-rock interactions, including evaporite dissolution (40.18 %), silicate weathering (16.31 %), and fluoride mineral dissolution/ion exchange (13.08 %). Human activities, such as agricultural practices (16.31 %) and domestic sewage discharge (13.41 %), also exert considerable contributions. A conceptual model was developed to elucidate the spatial patterns and quantitative formation mechanisms of groundwater chemistry. These findings provide a systematic framework for understanding the hydrogeochemical evolution and availability of groundwater quality in arid endorheic basins worldwide.