Study On The Evolution Of Hydrochemistry And Chemical Weathering Of The Weihe River In Guanzhong Plain

Rivers are important links between continents and oceans.It is an important topic in the global carbon cycle issue to understand the chemical weathering of watershed based on hydrochemistry and elemental geochemistry proxies.As the largest tributary of the Yellow River basin,Weihe River is the main channel for material transport from the Loess Plateau and Qinling Mountains to the middle reaches of the Yellow River.It is of great significance to study the hydrochemical characteristics and chemical weathering characteristics of the Weihe River for the understanding of carbon sinks in the middle reaches of the Yellow River Catchment.The Weihe River watershed is characterized by complex lithology and diverse topography.In order to determine the carbon sink effect in the Weihe River,it is necessary to fully understand the effects of geological,meteorological and hydrological factors on the water chemical characteristics of the Weihe River.Therefore,supported by the the National Natural Science Foundation of China and the Fundamental Research Funds for the Central Universities,this paper study the the temporal and spatial evolution mechanism of water chemistry and the characteristics of chemical weathering.,using hydrogeochemistry,isotope and element geochemistry.The following conclusions are obtained:（1）The spatial and seasonal variations of hydrochemistry and stable isotopes of hydrogen and oxygen in mainstream and tributaries of the middle and lower reaches of the Weihe River were determined using a geochemistry and isotope dataset of high spatial resolution.The results show that Na⁺,Ca²⁺and HCO₃^–predominate in the main stream of the Weihe River,and the average TDS in wet season and dry season are 494 mg/L and 676 mg/L,respectively,which are close to the average TDS of the Yellow River and much higher than the average TDS of Yangtze River and world rivers.The hydrochemical characteristics of Weihe River belong to the typical river characteristics of evaporative salt and carbonate rocks.Spatially,the dissolved solutes of the streams follow the order north tributaries>the mainstream>the south tributaries.The stable isotopes of hydrogen and oxygen in Weihe River water are enriched in the wet season and slightly depleted in the dry season.The main stream of the Weihe River exhibites a mixture of major elements and stable isotopes from tributaries and groundwater.（2）Based on water balance,isotope mass conservation and base flow separation,the water source composition of the main stream is determined.The results show that groundwater is a major source of the Weihe River main stream.Based on the Gibbs diagram,ion ratio and mineral saturation index,it is concluded that the water-rock interaction is the controlling mechanism of the river chemistry,while the effects of atmospheric precipitation and evaporation are insignificant.The major chemical elements in the river water are closely related to the dissolution and precipitation of carbonate and evaporite,such as calcite,dolomite,gypsum and halite.（3）Based onα_E^Al index,in sediments of main stream and north bank tributary,Na is depleted and Ca and Mg are enriched with respect to UCC.While the sediments in the south bank are significantly depleted of element Ca and Mg and slightly enriched in element Na.The CIA index,K/Na,Al/Na,K/Ca,Al/Ca ratios reflect that the weathering intensity of Weihe River sediments is weak.Within the watershed,the weathering intensity of carbonate in the south bank is stronger than that in the north bank.According to the calculation results of Forward model,the dissolved loads of the main stream water are mainly dominated by evaporite and carbonate weathering,the contribution of silicate is low.In particular,the contribution of carbonate weathering for south bank river water is higher than that of north bank tributaries.It can be seen that the chemical composition of river water is consistent with the chemical weathering intensity of rocks and the activity of sediment elements.（4）The total CO₂ consumption（?CO₂）consumed by chemical weathering at Huaxian station of the Weihe River watershed is 1748.87×10⁶ mol/month during the wet season,of which the middle and lower reaches of the Weihe River contributes 35%.The CO₂ consumed via carbonate weathering(?CO_2carb)and silicate weathering(?CO_2sili)are 1026.74×10⁶mol/month and 722.13×10⁶ mol/month,respectively.During the dry season,the?CO₂ of at Huaxian station of the Weihe River watershed is 1191.46×10⁶ mol/month,81%of which come from the middle and lower reaches of Weihe River.The?CO_2carb and?CO_2sili are 750.26×10⁶mol/month and 441.21×10⁶ mol/month,respectively.The CO₂ consumption of chemical weathering in Weihe River Basin is higher for carbonate than silicate,and higher in wet season than in dry season.At the annual scale,the?CO₂ at Huaxian station of the Weihe River is14.30×10⁹～20.99×10⁹ mol/yr,which contributes 24～36%to the CO₂ consumption in the middle reaches of the Yellow River.The CO₂ consumption of chemical weathering in the Weihe River Basin can be underestimated by 16～24%without considering artificial water diversion.