[Seasonal Variations in River Water Chemical Weathering and Its Influence Factors in the Malian River Basin].

In order to discern temporal variations, sources, and controlling factors of river water chemistry in the Malian River Basin, time series samples were collected from the Yuluoping hydrological station in 2016. The compositions of major cations and anions were analyzed and a forward model was used to calculate the weathering rates of evaporite, silicate, and carbonate. Results showed that river water was brackish with average total dissolved solids of 1154.0 mg·L, indicating significant differences from other main rivers in China. Na, Ca, Mg, and SO were the major ions present in water, with mean concentrations of 202.8, 86.0, 78.6, and 431.2 mg·L respectively. Water chemistry exhibited distinct seasonal variations, with major ions gradually declining during the pre-monsoon period and increasing in the post-monsoon period. An abrupt rise in concentrations of major ions during the ice melting interval was observed, as well as a sharp drop during stormy events. Dissolved loads were mainly derived from chemical weathering, with the contribution ratios of evaporite, silicate, and carbonate being 67.1%, 13.7%, and 19.2% respectively. Chemical processes showed different responses to climate forcing, attributed to variations in mineral content in the watershed and dissolution kinetics. The dominant contribution of evaporite in the monsoon season was due to its rapid dissolution, while silicate weathering increased during the pre-monsoon period, with longer water rock interaction times when water discharge was lower. During the post-monsoon season, carbonate weathering was enhanced due to its high content in loess and due to more CO absorption by rain from soil. The average chemical weathering rates of evaporite, silicate, and carbonate were 30.6, 6.2, and 8.7 kg·(km·d), respectively. A strong correlation between evaporite weathering rates and river discharge was evident; a correlation was also observed between carbonate weathering rates and river discharge, indicating that water discharge played a dominant role in chemical weathering, rather than temperature or precipitation.