Sulfur isotope and chemical composition of the rainwater at the Three Gorges Reservoir

Abstract

Rainwater samples were collected at the Three Gorges Reservoir (TGR) from June 2009 to July 2010 and the pH, major ion concentrations and δ34S values of sulfate were measured. The results indicated that the rain in this region was seriously acidized, 60% of the rain events' pH were lower than 5.0. SO42− and NO3− were the main anions, and their volume weighted mean (VWM) values were 161.9μeq/L and 65.2μeq/L, respectively, which accounted for 71%–99% of the total measured anions. Ca2+ and NH4+ were the major cations with VWM values of 108.3μeq/L and 88.4μeq/L. Sum of Ca2+ and NH4+ accounted for 25%–94% of the total cations. Analysis of the seasonal variations of the pH showed that the rainwater was more acidized in summer than in the other seasons. Investigations of neutralization factors (NFs) indicated that the acidity of rainwater in winter and spring was neutralized by NH4+ and Ca2+. Studies of the origins of major ions showed that SO42− and NO3− were from coal combustion and fossil fuels, and Na+ and Cl− were from sea sources, while Ca2+, Mg2+ and K+ were from the continental sources.The δ34S values of SO42− in rainwater ranged from −2.1‰ to 6.1‰ (mean value of 2.1‰). The δ34S values of SO42− in winter were much higher than those in other seasons. Analysis of the δ34S values vs. SO42− of rainwater combined with air mass trajectory showed that atmospheric sulfur at TGR was mainly associated with coal combustion and fossil fuels. The lower δ34S values in summer at TGR could be explained by higher contribution of biogenic sulfur.