Hydrochemical Characteristics of Groundwater of the Heihe Basin in the Hexi Corridor, Gansu Province

The Heihe basin is located in the inland arid district, northwest China. Controlled by surface runoff evaporation and percolation, the hydrochemistry of groundwater is characterized by the horizontal zonation and vertical upside-down differentiation from the supply sources to the discharge area. Horizontally, from the top of pluvial fans in the front of the Qilian mountains to the salt marsh (salt land) in the north of the alluvial-pluvial plain made up of by soil, the mineralization degree of the top layers of groundwater becomes higher as the runoff lengt hincreases, accompanied by fresh water to saltwater and even to brinestage. Accordingly, the chemical components gradually change from HCO₃^--Na⁺-Ca²⁺ type to SO₄^2--HCO₃^--Na⁺-Ca²⁺ and SO₄^2--Cl^--Na⁺-Ca²⁺ types, and then became Cl^--SO₄^2--Na⁺ and Cl^--Na⁺ types, and during the change, eachhy drohemical zone is closely related to a certain geomorphical unit. Vertically, in fresh water of unconfined aquifer, the mineralization degree decreases as the depth increases, in the top is about 0.5～4.0g/L, while in water of confined aquiferit becomes lower than 1g/L.