Chemical composition of rainwater in a karstic agricultural area, Southwest China: The impact of urbanization

The chemical composition of rainwater has been studied in a karstic agricultural area, Southwest China. Rainwater pH and major ions were measured in samples collected from the town of Puding, Guizhou province, during the rainy season, between March and October 2008. The pH of samples varied between 4.6 and 7.1, with a volume-weighted mean (VWM) of 5.4. These relatively high-pH values do not signify a lack of acid components in the precipitation; instead, they demonstrate neutralization of acidity. Ca2+ and NH4+ were the dominant cations in the rainwater, with VWM concentrations of 156 and 33 μeq/L, respectively. SO42− was the predominant anion, with a VWM concentration of 152 μeq/L, followed by NO3− (17 μeq/L). Our results show that most of the acidity in the collected samples was neutralized by Ca2+-rich alkaline soil dusts. We compared our findings with other areas of China that experience acid rain. The concentration of ions in Puding rainwater generally exceeded that reported in southern China but was lower than in northern China. This is probably due to karst rock desertification, which could provide the atmosphere with a source of alkaline dust. From back-trajectory analysis, correlation studies, and a comparison of major ion composition with other selected sites worldwide, we conclude that the rainwater ion composition of Puding is more significantly influenced by terrestrial and anthropogenic sources (mainly due to industrial and traffic emissions in surrounding large cities), rather than by marine sources.