Hydrochemical characteristics and brine evolution paths of Lop Nor Basin, Xinjiang Province, Western China

This study was carried out in the Lop Nor basin, a large arid closed drainage basin in Western China. The objective was to contribute to the understanding of the hydrological and hydrochemical processes of the Lop Nor basin by analysis of the chemical composition of different water sources and associated mineralogical characteristics of the playa sediments. The dominant river inflow waters to the Lop Nor basin are of the Na–Mg–Ca–SO4–Cl–HCO3 type. Spring inflow is dominated by Na+ and Cl−. Present-day concentrated groundwater brines vary little in the study area and are consistently rich in Na+ and Cl− and poor in Ca2+ and HCO3- + CO32-, but also contain a considerable amount of Mg2+ and SO42-. EQL/EVP (equilibrium/evaporation), a brine equilibrium model, simulated evaporation of inflow water and groundwater brines in an open system and showed good agreement between theoretically predicted and observed minerals in the Lop Nor basin. Brine chemical modeling cannot however explain the massive amounts of glauberite (Na2SO4·CaSO4) and polyhalite (K2SO4·MgSO4·2CaSO4·2H2O) deposits found in a 230 m deep core ZK1200B from the Lop Nor basin. EQL/EVP simulations under a closed system allowed brine reactions with previously formed minerals and indicate that glauberite forms by back reaction between brine, gypsum and anhydrite and polyhalite forms by reaction between brine and glauberite. Diagenetic textures related to recrystallization and secondary replacement were commonly observed in core ZK1200B, indicating significant mineral–brine interaction during crystallization of glauberite and polyhalite. Mineral assemblages predicted from the evaporation of Tarim river water match closely with natural assemblages and abundances, which can explain the unusual glauberite deposits in the Lop Nor basin. It is suggested that the Tarim river inflow is the dominant source over the lake’s history. The distribution of minerals in the cored sediments documents the history of inflow water response to wet and dry periods in the Lop Nor basin.

Research highlights
► The paper describes the principal hydrochemical features of the Lop Nor basin, the terminal point of China’s largest endorheic basin (Tarim basin), with emphasis on the chemical composition of inflow waters and groundwater brines.
► The paper applies principles of brine evolution and chemical divides to calculate brine evolution pathways and mineral sequences, using the computer program EQL/EVP.
► The results are compared with the mineral assemblages found in a 225 m-long core from the Lop Nor basin, to interpret the primary inflow water sources to the Lop Nor basin and the history of inflow water response to regional climatic changes.