Objective Linzhou County is the core area of Lhasa River Basin in Xizang. Groundwater and surface water are important drinking water sources and ecological factors in this area.In recent years, the impacts of human activities and climate change on the regional water environment have become increasingly pronounced. This study aims to elucidate the hydrochemical formation mechanisms in Linzhou County, assess current water quality conditions, and provide a scientific basis for water resource protection in plateau regions.

Methods A total of 37 water samples were collected, and 31 hydrochemical parameters were measured. Comprehensive analytical methods, including mathematical statistics, Piper diagrams, Gibbs diagrams, and ion ratio coefficients, were employed to investigate hydrochemical characteristics and formation mechanisms. Water quality was evaluated using both the Single Indicator Highest Category Method and the Fuzzy Mathematics Method.

Results The water quality in the study area is characterized as weakly alkaline freshwater, with dominant ions being HCO₃⁻, SO₄²⁻, Ca²⁺and Mg²⁺ ions. The hydrochemical types of water are mainly HCO₃−Ca and HCO₃•SO₄−Ca, demonstrating a strong correlation between groundwater and surface water chemistry. The hydrochemical composition is predominantly governed by rock weathering processes, with silicate and carbonate weathering serving as the principal sources, linked to the geological origins of magmatic, metamorphic, and carbonate rocks in the region.Based on the Fuzzy Mathematics Evaluation Method, water quality classification revealed that 87% of samples met Class Ⅰ standards, 5% were Class Ⅱ, and 8% were Class Ⅲ, with no samples falling into Class Ⅳ or Ⅴ. The Single Indicator Highest Category Method identified nitrate, ammonia nitrogen, manganese, and arsenic as the dominant controlling factors. Elevated nitrate and ammonia nitrogen levels were primarily attributed to agricultural activities, while manganese and arsenic exceedances were associated with natural geochemical background conditions.

Conclusions The overall water quality in Linzhou County remains excellent; however, localized agricultural pollution and geogenic heavy metals require targeted attention.These findings provide a theoretical foundation for water resource management and ecological conservation in high−altitude regions.