| Abstract: To reveal the hydrogeochemical evolution differences and vertical hydraulic connectivity between groundwater depression cones and groundwater mounds in the piedmont alluvial-proluvial plain of Baoding, 72 groundwater samples were collected along two hydrogeological profiles. Comprehensive analyses were conducted using hydrochemistry, δD-δ¹?O isotopic tracing, and CFCs/¹?C dating techniques. The results show that from the alluvial-proluvial fan to the plain discharge area, the TDS of shallow groundwater increases to over 1 g/L, and the hydrochemical type evolves from HCO?-Mg·Ca to Cl-Na, indicating salinization. For deep groundwater, pH increases from 7.37 to 9.27, and the hydrochemical type evolves to HCO?-Na, indicating alkalinization. The δD-δ¹?O signatures indicate that the groundwater originates from meteoric precipitation. Shallow groundwater exhibits significant evaporative fractionation, while deep groundwater is primarily recharged by precipitation from high altitudes or cold periods. Along the flow path, groundwater ages increase: CFCs-derived ages of shallow groundwater range from 23 to 54 a, and the corrected ¹?C ages of deep groundwater range from 5.3 to 34.45 ka. In the depression cone, δ¹?O is more negative and groundwater ages are older. In the Baoding northern depression cone, vertical leakage from shallow to deep aquifers exists but the recharge amount is limited, while in the Gaoyang depression cone, mixing between shallow and deep groundwater occurs. In the groundwater mound areas, along the Caohe River channel, groundwater has the youngest age and more positive δ¹?O, with a hydrochemical type of HCO?·Cl-Mg·Ca, indicating strong vertical hydraulic connectivity and a risk of water quality deterioration. In the Baiyangdian groundwater mound, the age of deep groundwater decreases to 19.30 ka, but the hydrochemical differences between shallow and deep groundwater suggest limited vertical leakage intensity. This study reveals the evolution mechanism of groundwater circulation in the piedmont alluvial-proluvial plain under the influence of human activities, providing a scientific basis for the management of groundwater overexploitation and the sustainable utilization of water resources in the region. |