Chemical Characteristics, Genesisand Water Quality Assessment of Groundwaterand Surface water in Linzhou County,Tibet

In order to study the hydrochemical characteristics, formation mechanism, controlling factors and quality of water environment in Linzhou County, Tibet, 27 groups of groundwater samples and 10 groups of surface water samples were collected in July 2022, and 31 indicators were analyzed and tested. The hydrochemical features and controlling factors of the study area were analyzed and discussed by applying such methods as mathematical statistics, hydrochemical identification, Piper trilinear diagram, correlation analysis, Gibbs diagram, ion endmember plot, and ion ratio coefficient. In addition, the water quality of the research area was also evaluated by using the single index highest category method and fuzzy mathematics method. The results showed that the pH of groundwater and surface water in the study region was weakly alkaline, with an average TDS value of 174.74 mg/L, belonging to low mineralized water, dominated by HCO₃^-、SO₄^2-、Ca²⁺andMg²⁺ ions. The hydrochemical types of water were mainly HCO₃-Ca and HCO₃•SO₄-Ca, and there was a strong correlation between surface water and groundwater. The main controlling factor for the composition of groundwater and surface water is rock weathering, and the alternating adsorption of cations is also an influencing factor on the chemical composition of groundwater. The weathering and dissolution of silicate and carbonate minerals are the main sources of hydrochemical composition in the area, and their genesis is related to the geological origins of magmatic, metamorphic, and carbonate rocks in the area. According to the fuzzy mathematics evaluation, the overall water quality standard in the area is first category, which accounts for 86.49%. and Class II water quality accounting for 5.41%; Class III water quality accounts for 8.11%, There is no Class IV or Class V water. Nitrate, ammonia nitrogen, manganese, and arsenic are the main controlling indicators in the study area. Local excessive nitrate and high content ammonia nitrogen are mainly affected by human agricultural activities, while local excessive manganese and high arsenic water are mainly affected by primary geochemical processes.