Chemical characteristics and evolution of groundwater in northeastern margin of the Tibetan Plateau, China.

With the excellent water quality, abundant water quantity and convenient and economical exploitation conditions, groundwater has become an important water source for the social and economic development and people's livelihood in the northeast margin of the Tibetan Plateau in China. This study employed geostatistics, mineral saturation index, Gibbs diagram, ion ratio coefficient, chloralkali index and other methods to reveal the chemical distribution characteristics, evolution law and hydrogeochemical formation mechanism of groundwater in the northeastern margin of the Tibetan Plateau. The results showed that the contents of main chemical components of groundwater in Beichuan increased continuously from 1980 to 2020 complicating the types of hydrochemistry due to intensive groundwater exploitation and potential pollution from chemical plants. In contrast, Xinachuan, Xichuan, and Nanchuan witnessed an initial increase followed by a decrease in chemical components, simplifying hydrochemical types. The groundwater exhibited a spatial pattern of widespread high-quality water with sporadic banded and island brackish water. Chemical concentrations gradually rose along the groundwater flow direction. The leaching intensity of minerals by groundwater follows the order: halite > gypsum > calcite > dolomite. Leaching, cation exchange, and human activities are identified as the primary drivers of the chemical field evolution in the Xining area. The presence of Tertiary strata, rich in soluble salts like gypsum and halite, influences water-rock interactions, leading to downstream TDS increases and gradual salinization. Centralized pumping well exploitation altered groundwater runoff intensity and direction, contributing to high TDS areas near water sources and industrial parks, exacerbated by artificial pollution. The above conclusions are of great theoretical and practical significance to realize sustainable utilization of water resources and important for urban development in the northeastern margin of the Tibetan Plateau.