Water-soluble ions and oxygen isotope in precipitation over a site in northeastern Tibetan Plateau, China

A total of 30 precipitation samples were collected at a remote site of Qinghai Lake in the northeastern Tibetan Plateau, China, from June to August 2010. All samples were analyzed for major cations (NH₄⁺, Na⁺, K⁺, Ca²⁺, and Mg²⁺) and anions (F⁻, Cl⁻, NO₃⁻, and SO₄²⁻), electric conductivity (EC), pH, dissolved organic carbon (DOC), and oxygen isotopic composition (δ¹⁸O). The volume-weighted mean (VWM) values of pH and EC in the precipitation samples were 7.2 and 19.0 μs cm⁻¹. Ca²⁺ was the dominant cation in precipitation with a VWM of 116.9 μeq L⁻¹ (1.6–662.9 μeq L⁻¹), accounting for 45.7% of total ions in precipitation. SO₄²⁻ was the predominant anion with a VWM of 32.7 μeq L⁻¹, accounting for 47.1% of the total anions. The average precipitation DOC was 1.4 mg L⁻¹, and it shows a roughly negative power function with the precipitation amount. The values of δ¹⁸O in the rainwater in Qinghai Lake varied from −13.5‰ to −3.9‰ with an average of −8.1‰. The enrichment factor analysis indicates that crustal materials from continental dust were the major sources for Ca²⁺ in the precipitation samples. The high concentration of Ca²⁺ in the atmosphere played an important role in neutralizing the acidity of rainwater in Qinghai Lake area. Cluster analysis of air-mass trajectories indicates that the air masses associated with northeast and east had high values of NH₄⁺, SO₄²⁻, and NO₃⁻, whereas large Ca²⁺ loading was related to the air mass from west.