Seasonal variation in river water chemistry of the middle reaches of the Yellow River and its controlling factors

• First time tracing seasonal variations of water chemistry in the middle Yellow River.
• Modern loess weathering dominates the river chemistry of the Yellow River.
• Water discharge and erosion control seasonal loess weathering rates.
• Densely time-series data catch the impact of hydrological events on water chemistry.

In order to reveal the seasonal variations of river water chemistry and its controlling factors in the middle reaches of the Yellow River, river water samples were collected weekly from the Longmen hydrological station over the whole year of 2013. Major cation and anion compositions were measured. The major ionic compositions of the dissolved load exhibited distinct seasonal variability over the one year period, reflecting seasonal differences in relative inputs from various sources and weathering reactions within the catchment. A forward model was used to calculate the apportionments of rain, anthropogenic, silicate, evaporite, and carbonate origins on a seasonal basis. The contributions of both rain and anthropogenic sources to the middle Yellow River were less than ~ 12% in average, with little seasonal variations in 2013. The calculations showed that silicate sources had a relatively large impact on water chemistry during the pre- and early-monsoon seasons, though both silicate and carbonate weathering increased greatly during the monsoon season. The high silicate input during the pre- and early-monsoon seasons was attributed to the waters potentially derived from plateau headstream, glacial melting, and soil pore. Meanwhile, the dominated contributions of evaporites and carbonates in the late- and post-monsoon seasons were resulted from their rapid dissolution from the loess plateau through physical erosion acceleration along with high water discharge, because widespread loess is characterized by easy denudation and high carbonates and evaporites. Our densely time-series data also highlighted a significant impact of extreme hydrological events on water chemistry in semi-arid area. During the ice melting interval in the springtime, more silicate-origin ions entered the river from melting water, whereas during the stormy event interval in the peak monsoon, more carbonates and evaporites reached the Yellow River from the loess plateau via accelerated physical erosion. Our data shed further light on processes responsible for seasonal variation of river chemistry of the Yellow River and modern loess weathering.