Study On Geochemical Characteristics Of Dissolved Inorganic Carbon In Lakes Of Karst Plateau Area

Dissolved inorganic carbon(DIC)is an important part of carbon migration and transformation in a river basin,and the transportation of DIC in rivers has important biogeochemical significance.The karst region in southwest China is an important place for global carbon cycle research,especially for rivers,lakes,reservoirs and other water ecosystems with strong carbon source and sink capacity,and their carbon migration has received more attention.In order to more accurately and comprehensively understand the characteristics of DIC changes in reservoirs in karst areas,and to understand its influencing factors,this study selected the Pingzhai Reservoir,a deep-water reservoir in a typical karst area,as the research object.In January(spring),July(summer)and November(autumn),field monitoring and laboratory experiments were conducted on the rivers and reservoir areas of the Pingzhai Reservoir.A total of 14 indicators of DIC and its isotope(?¹³C_DIC),anion(cation)ions and water quality physical and chemical parameters are selected.The characteristics of temporal and spatial changes in rivers and the surface layer of the reservoir area are analyzed,and the seasonal stratification of each indicator in the deep water sampling points of the reservoir area is analyzed.Combining the correlation with the water environment and the characteristics of?¹³C_DIC,the influencing factors and sources of DIC in the water body are explored.Based on the CO₂ partial pressure difference at the water-gas interface,the CO₂ exchange flux at the water-gas interface is estimated,and the CO₂"source"-"sink"status of the Pingzhai Reservoir is explored.The main research conclusions are as follows:(1)In the physical and chemical indexes of the surface of Pingzhai Reservoir and the water body of the river entering the reservoir,T,EC,DO and TDS all show obvious seasonal characteristics,the seasonal fluctuation of p H is small and it is weakly alkaline;the vertical spring water temperature starts to appear stratified,the most obvious in summer,it gradually disappears in autumn and is completely mixed in winter.p H and DO decrease with the increase of water depth.Except for winter,the surface water DO is saturated in the rest of the season.EC and TDS both increase with the increase of water depth.The ion content ratio shows that the water body of Pingzhai Reservoir is HCO₃^-—Ca²⁺type water,which generally shows the characteristics of excess cations;Na⁺and K⁺are mainly derived from the dissolution of silicate,and Na⁺,Cl^-and SO₄^2-have the same homology,mainly from the upstream sulfide oxidation input;the reservoir as a whole is affected by carbonate weathering carbonate rock,and the water chemistry is mainly controlled by the weathering of the rock,and is not affected by atmospheric precipitation and evaporation.(2)The DIC concentration of surface water bodies and rivers entering the reservoir shows seasonal characteristics of winter>autumn>spring>summer,and river>reservoir area,?¹³C_DIC is spring>summer>winter>autumn,river<reservoir area.The concentration of DIC in the vertical direction increases with the increase of water depth,and?¹³C_DIC is negative with the increase of water depth.DIC concentration is affected by water quality parameters such as T,p H,EC and DO,its source is mainly carbonic acid and sulfuric acid co-weathering carbonate rocks,followed by respiration of aquatic organisms and atmospheric CO₂ release,but it is related to soil CO₂ input little relevance.(3)The average CO₂ partial pressure of the rivers and surface waters in the reservoir is 459.36?atm,which is larger than atmospheric CO₂(328.38?atm).The seasonal change is winter>autumn>summer>spring;the vertical direction of the reservoir area increases with the increase of water depth.The average value of the CO₂ exchange flux at the water-air interface on the river entering the reservoir is 24.02mmol·(m²·d)^-1,which is the"source"of CO₂ and releases CO₂ to the atmosphere throughout the year.The average value on the reservoir area is-0.53 mmol·(m²·d)^-1,it is a CO₂"sink"throughout the year,slightly absorbing atmospheric CO₂,but in winter it will release CO₂ to the atmosphere,becoming the"source"of CO₂,which is mainly affected by the relative intensity of the respiration of photosynthesis by aquatic organisms,The influence of soil CO₂ input and precipitation dilution in the watershed.