Abstract: The northern foothills of the Qinling Mountains constitute a vital water source conservation area for the Yellow River. Examining the hydrogeochemical evolution patterns and models in this region holds paramount importance for the development and preservation of groundwater resources in the Qinling Mountains, and also helps advancing ecological protection and facilitating the high-quality development of the Guanzhong Plain and the broader Yellow River Basin. Utilizing data collected during the hydrogeological survey in the Weihe River Basin spanning 2019 to 2021, this study delves into the hydrogeochemical evolution patterns and models of groundwater from the Qinling to the Weihe River Valley. Employing multivariate statistics, and hydrogeochemical reverse simulation methods, the analysis reveals a shift from HCO₃-Ca to HCO₃·SO₄-Ca·Na in the hydrochemical composition of groundwater from the rocky substrate to the fine soil plain, with subtle zoning observed, accompanied by a rise in Total Dissolved Solids (TDS) concentration from 254 mg/L to 889 mg/L. The flow of groundwater indicates a shift in water-rock interaction dominance, transitioning from the weathering and leaching of carbonate rock to that of silicate rock and rock salt. Concurrently, cation exchange intensifies. Despite the rapid circulation and replenishment of groundwater, evaporation and concentration remain relatively insignificant. Groundwater age, as measured by ¹⁴C, predominantly falls within the range of 2 450 years. As a critical groundwater conservation area and runoff channel, the Qinling piedmont proluvial fan stays as a pivotal player in safeguarding the quality of both unconfined and confined water in the Guanzhong Plain. Due to the proluvial fan's limited resistance to pollution, it is imperative to bolster the protection of groundwater resources amidst ongoing groundwater development, industrial activities, and agricultural practices.

Key words: groundwater protection, northern piedmont of Qinling mountains, hydro-geochemistry, total dissolved solids (TDS), ion ratio, evolution law