Abstract: The Liyuan River Basin is located in an arid inland in northwest China and it is an important agricultural area in the Hexi Corridor, Gansu Province. As a tributary stream of the Heihe River, the Liyuan River is of significance in promoting economic and ecological development in the Zhangye Basin. The ecosystem in the region is highly vulnerable and strongly affected by the hydrochemical composition of water. For this reason, it is necessary to monitor the hydrochemical evolution in this area so as to avoid the degeneration of the local ecological environment. As surface water resources are limited, groundwater has gradually become as an indispensable water source in people’s daily life and agriculture. A lack of understanding of the mechanisms of groundwater evolution may lead to unwise utilization of groundwater, and thus have a negative impact on local sustainable development may be resulted in. In this paper, the relationships between hydrochemical properties and stable isotopes were analyzed. The transformation of surface water and groundwater was lucubrated so as to provide the government with decisional support concerning groundwater conservation. It was assumed that groundwater flowed in the same path from the southwest to the northeast based on considering the flow direction of the Liyuan River. The results showed that the TDS values of groundwater increased from the southwest to the northeast. The highest TDS value was 1 258 mg/L, which was too high for drinking use; this was the result of water-rock interaction and the salinization caused by the infiltration of irrigation water. Along with the flow path, the hydrochemical type of deep groundwater changed from Ca-HCO₃ to Ca-Mg-HCO₃, Mg-SO₄ and Na-SO₄, that of shallow groundwater changed from Ca-HCO₃ to Mg-HCO₃-SO₄ and Mg-SO₄, and that of surface water remained as Ca-HCO₃ without change. Among the hyrdochemical changes, the increasing content of Na⁺ was the most apparent. There were two reasons for the increase of Na⁺ content along the flow path: the first was that Ca²⁺ in groundwater exchanged with Na⁺ in minerals in the cationexchange process, and the second was the weathering of silicate minerals that produced Na⁺. The results simulated with Phreeqc software identified that there were the main water-rock interactions in groundwater along the flow path. In deep groundwater, calcite was precipitated at first, and then dissolved; whereas CO2 gas, gypsum, dolomite and halite were dissolved. In shallow groundwater, calcite was dissolved at first, and then precipitated, while gypsum and halite were dissolved. The modeling results were the evidences of the hydrochemical processes. The tracer elements of ²H and ¹⁸O in water molecules were used in this paper to analyze the water resources and recharge patterns. By comparing the δ¹⁸O-δ²H relation line of surface water and groundwater, global meteoric water line (GMWL) and local meteoric water line (LMWL), it was concluded that groundwater was recharged by precipitation or snowmelt water via surface runoff from the mountain area. Deep groundwater migrated upward to recharge shallow groundwater and surface water.

Key words: groundwater, hydrogeochemistry, Phreeqc software, stable isotope, Liyuan River Basin, Gansu