[Hydrochemical Characteristics and Sources of Ions in Precipitation at the East Qilian Mountains]

Abstract

Precipitation of the northwest China is different from that in other regions of China. The vapor reaches this region after long distance transportation with little precipitation, and the ratio of precipitation variation is large. Wushaoling at the east Qilian Mountains is an important divided line of climate in China. The east region of it is affected by South Asia and East Asia monsoon, while the west region of it is influenced by Westerly circulation. So ion combinations in precipitation are complex for the trajectories of water vapor transportation, the natural environments and the development levels of social and economy in different regions. Precipitation samples were collected at Heisongyi located at the east Qilian Mountains. Hydrochemical characteristics and sources of ions were analyzed by factor analysis, Enrichment factor analysis and back trajectory analysis. EC values in precipitation ranged from 29.20 to 892.00 μS·cm^-1, which were controlled by alkaline element. The precipitation was weak alkaline with pH values ranging from 7.02 to 8.89. EC values in precipitation were higher in autumn and winter than in spring and summer, opposite to pH values. The type of precipitation was SO₄^2--Ca²⁺ for the cation concentrations following the order of Ca²⁺ >Mg²⁺ >Na⁺ >NH₄⁺ >K⁺ and the anion concentrations following the order of SO₄^2- >Cl^- >NO₃^-. The concentrations of K⁺, Mg²⁺, Ca²⁺, NH₄⁺, Cl^-, NO₃^- and SO₄^2- were the highest in autumn, but the peak value of Na⁺ concentration appeared in winter. As a whole, ion concentrations were higher in autumn and winter than in spring and summer. Enrichment factor indicated that Na⁺and Cl^- mainly came from marine source but Na⁺ partly originated from crust source in winter and Cl^- partly originated from anthropogenic source in winter, and that K⁺, Mg²⁺ and Ca²⁺ mainly came from crust source except that Mg²⁺ partly originated from anthropogenic source in winter, and that NO₃^- and SO₄^2- mainly came from anthropogenic source. The trajectories of water vapor transportation from directions of northwest, north, north with southeast, northwest with southeast, northwest with southwest, northwest with north and southeast were ion original sources in precipitation, and the major trajectory came from northwest direction. Among these trajectories of water vapor transportation, that from northwest, north and north with southeast appeared in each season, but that from northwest with southeast appeared in spring and summer, that from northwest with southwest and northwest and north with southeast appeared in summer. Though ions originated from marine transport by Westerly and monsoon, from crust provided by desert and Gobi in central Asia, Xingjiang and Mongolian plateau, and from humanity activity related to cities pollution and industrial and agriculture production of oasis at all trajectories, the concentrations of ions in precipitation were effected by the strong and weak variations of different weather systems.

Keywords: Qilian Mountains; ion concentration; ion source; precipitation; water vapor transportation.