Study On CO₂ Release Flux And Its Influencing Factors At The Water-air Interface Of Karst Rivers Under The Influence Of Acidic Coal Mine Drainag

River ecosystems are an important part of inland water bodies and also serve as a link between the two major carbon pools of terrestrial and marine ecosystems,playing a crucial role in the global and regional carbon cycle.When rivers transport terrestrial carbon sources to marine ecosystems,most of the carbon fails to reach the ocean,and the lost carbon is mainly fixed,stored and released as CO₂.Coal mining often discharges large amounts of Acid Mine Draige（AMD）,and when AMD is discharged,it leads to a decrease in the p H of water bodies and dewatering of dissolved inorganic carbon（DIC）in the water bodies,which accelerates the release of CO₂ from receiving rivers and causes an increase in the flux of carbon release from rivers.Karst is an important part of the global carbon cycle,which can consume CO₂ in soil or atmosphere,and thus has the effect of increasing sink and reducing source to the atmosphere.Therefore,it is important to investigate the partial pressure of carbon dioxide（pCO₂）in the water column,the CO₂ exchange flux at the water-gas interface and the migration and transformation processes of DIC stable isotopes for the study of karst carbon cycle process and its carbon sink effect stability.Guizhou is a typical region where AMD and karst coexist,and a lot of research work has been carried out on the ecological and environmental effects of AMD and its management in karst areas,but less attention has been paid to the effects of AMD emissions on the carbon cycle of karst rivers.Therefore,this study selects the karst rivers in the Gedangzhai and Gubingcun watersheds,which are affected by AMD in Majiang County,Guizhou,to reveal the characteristics of karst river water chemistry changes under the influence of AMD,the spatial and temporal distribution of river pCO₂ and its influencing factors,and the migration transformation of DIC and its stable isotopes in the watershed affected by AMD,and to clarify the influence of AMD on CO₂ release fluxes at the water-air interface of karst rivers.The results of the study will be used as the basis for the study of the carbon emissions in karst regions.The results of the study will serve as an important supplement to the study of carbon cycle in karst regions and provide a reference for understanding the carbon cycle processes in karst watersheds under the disturbance of mining activities.The main results are as follows:（1）The environmental quality of karst river water is significantly influenced by AMD.Ca²⁺,Mg²⁺ and SO₄^2-concentrations in water bodies of Gedangzhai watershed and Gubing village watershed increased significantly and EC increased after being influenced by AMD.HCO^3-concentration,p H and Alk all decreased to different degrees.The concentrations of Ca²⁺ and Mg²⁺ in the karst river water could be restored to the unaffected levels within 0.1⁰.8 km after the impact of AMD,while the concentrations of HCO^3-and SO₄^2-could not be restored within 1km after the impact,but they could be restored to the original concentration levels more quickly through the process of vigorous water drop aeration.（2）The pCO₂ of karst river water bodies are all higher than the partial pressure of atmospheric CO₂,and the overall performance is a source of CO₂ emission.The river pCO₂ in the Gedangzhai river section before and after the influence of AMD ranged from 776 1862 μatm and 794 45709 μatm,and in the Gubing village river section before and after the influence of AMD ranged from 1175 10471μatm and after the influence of AMD,the river pCO₂ ranged from 1230 15136μatm,respectively.The pCO₂ of the karst river water increased significantly at the beginning of the period of receiving mine acid water,and although the pCO₂ of the river water tended to decrease gradually with the flow direction,it was still significantly higher than the atmospheric CO₂ partial pressure.（3）The influence of AMD sink causes the main body of DIC in the water bodies of Gedangzhai watershed and Gubing village to be converted from HCO^3-ions to CO₂（aq）and then to CO₂（g）,which escapes from river dewatering to the atmosphere,and makes the enrichment of δ13CDIC in the water bodies occur about 1‰.（4）The Gedangzhai River section is 5.2 to 203.1 times larger than before it was affected,and the Gubing Village section is 3.0 to 4.5 times larger than before it was affected.The CO₂ exchange flux in karst rivers increased sharply and reached the peak,and then decreased gradually along the river flow direction,but it was still higher than the CO₂ flux before AMD pollution for a long time.（5）CO₂ output at the water-gas interface of karst rivers is mainly influenced by p H and carbonate acid-base balance,and less influenced by water temperature,DO and other parameters.At the same time,it is also subject to the hydrodynamic conditions such as the degree of aeration of the water body after mixing.