Mechanisms of water exchange in the Chagan Lake Basin, Songnen Plain: hydrochemical and isotopic constraints from August sampling

The Chagan Lake Basin lies in a typical arid–semi-arid transition zone in northern China. Understanding its hydrological connectivity is essential for aquatic ecosystem conservation and sustainable water use. This study integrates major-ion chemistry, stable water isotopes (δ²H and δ¹⁸O), and the Bayesian mixing model MixSIAR to investigate water exchanges among precipitation, shallow groundwater, rivers, and the lake, together with associated hydrogeochemical processes. Precipitation (TDS = 28 mg L⁻¹) represents a low-mineralization recharge endmember. River water is predominantly of the HCO₃–Ca type, with its chemistry controlled mainly by groundwater baseflow. Shallow groundwater exhibits marked spatial heterogeneity, while the co-enrichment of Na⁺, Cl⁻, and SO₄²⁻ in some samples suggests the combined effects of evaporite dissolution and anthropogenic inputs. Lake water displays a transitional HCO₃–Ca–Na facies and pronounced evaporative enrichment in δ²H and δ¹⁸O. MixSIAR identifies groundwater as the dominant water source to Chagan Lake (55.4%), followed by river water (27.8%) and precipitation (16.8%). River-water isotopes differ substantially from those of shallow groundwater and follow the lake-water evaporation trend, indicating inputs of evaporatively enriched lake water to some river reaches. However, major ions retain a strong groundwater-baseflow signature, revealing decoupled hydrochemical and isotopic responses. Overall, the system is governed by precipitation-fed groundwater recharge, groundwater discharge to rivers and the lake, bidirectional river–lake exchange, and in-lake evaporative concentration and cation exchange. These findings support regional water-resource management and pollution control.