Isotope and hydrochemistry reveal evolutionary processes of lake water in Qinghai Lake

Qinghai Lake is the largest lake in China and is one of more than 500 brackish lakes on the Tibet Plateau. This study investigated stable isotopes (δ18O and δ2H) and hydrochemistry of lake water in Qinghai Lake and surrounding lakes to understand the lakes' evolutionary processes. These processes help explain the formation and evolution of the brackish and salt lakes on the Tibetan Plateau. The original isotopic and chemical compositions of lake water were controlled by a mixture of water from river water, groundwater, and precipitation. The values of δ18O and δ2H of most of the lake water samples were greater than zero due to continuous strong evaporation. The cations were in the order Na+ > Mg2 + > K+ > Ca2 +, and anions were in the order Cl− > SO42 − > HCO3−. The hydrochemical types of Qinghai Lake and surrounding lakes were Na+-Cl−. The lake water, excluding Erhai Lake, was saturated with CaCO3. There was significant aragonite precipitation, and the Ca2 + concentration was relatively low. Haiyanwan Lake and Gahai Lake, which have no surface water inputs, had experienced stronger evaporation than Qinghai Lake since the lakes were isolated from Qinghai Lake due to the dropping lake level and the arid climate. Erhai Lake water had relatively low salinity due to the fresh water coming in from the Daotang River. With the continued strong evaporation of lake water, Qinghai Lake and surrounding lakes should become increasingly saline. However, the salinity of lakes on the Tibetan Plateau is likely to slowly decrease in future decades due to the increasing precipitation and runoff, which will dilute lake water. In the longer term, however, Qinghai Lake and the brackish lakes will likely still become more saline, given the continued contribution of chemical ions from river water and groundwater and strong evaporation from the lake surface.