Hydrogeochemical mechanism and interconversion processes of groundwater-surface water in the Chan River Basin: A new perspective from hydrochemistry and isotopes

Rivers and groundwater are the main water sources for cities. The mutual transformation between river water and groundwater makes it difficult to accurately evaluate and rationally utilize water resources. Scientifically quantifying the interaction of surface water and groundwater remains challenging. Taking Chan River Basin as an example, this study aimed to determine the interconversion processes of groundwater and surface water by hydrogeochemical genesis mechanism analysis, isotope tracing, and end-member mixing analysis (EMMA). 28 surface water samples and 23 groundwater samples were collected and analyzed during December 2023 and January 2024. Results showed the water bodies in the study area were dominated by the HCO₃-Ca, HCO₃-Ca·Mg·Na, and HCO₃·SO₄-Ca·Na types, with hydrogeochemical processes controlled by the weathering and dissolution of both carbonate and silicate rock minerals. The river water in the upper reaches of the study area is mainly recharged by groundwater, with the average recharge ratio of 54.10 %. Similarly, river water is still recharged by groundwater in the middle reaches, the average recharge ratio changes to 28.61 %. In the downstream area, where Xi'an City located, due to the heavy exploitation of groundwater, the river water recharges to groundwater with an average recharge ratio of 85.23 %, although in the immediate middle reaches, groundwater still replenishes surface water with a recharge rate of 75.00 %. The results laid the bases for the reasonable utilization of water resources in the Chan River Basin and also served as a reference in other regions of the world.