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

Abstract

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 SO₄^2?), 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. SO₄^2? 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⁺, SO₄^2?, and NO₃^?, whereas large Ca²⁺ loading was related to the air mass from west.