Study On The Chemical Characteristics Of Surface River Water And CO₂ Release Flux At The Water Air Interface In Karst Urban Areas

Rivers are the main circulation pathways in the global carbon cycle even though they only make up 0.47 percent of the earth’s land surface.They serve as a link between the ocean and land carbon stores.The majority of the carbon that rivers carry from terrestrial sources to marine environments is fixed,stored,and emitted as CO₂.Dissolved inorganic carbon（DIC）in river water moves and changes dramatically with the intensity of the global urban process due to the discharge of domestic and industrial wastewater and the continual entry of organic carbon and nutrients（TN,TP）during urbanization.Karstification processes exist in water bodies as DIC and can collect CO₂from the atmosphere or soil through water-rock interaction.Karstification is thus a crucial part of the global carbon cycle.There are few studies on the carbon cycle of karst surface rivers in urban environments,despite the fact that urban growth frequently pollutes waterways,particularly in karst regions where karst pipes and cracks form.Understanding how DIC（containing CO₂）migrates and transforms in karst urban surface rivers can help us better understand how the carbon cycle in karst basins reacts to human activity.The Nanming River in Guiyang City,Guizhou Province,which is affected by urbanization,is used in this study as the research object in order to reveal the chemical change characteristics of karst river water under the influence of urbanization process,the temporal and spatial distribution law of river p CO₂and its influencing factors,and the migration and transformation process of water body DIC affected by urbanization process.The following are the key conclusions:（1）In the three hydrological seasons,the p H value and conductivity（EC）of the Nanming River basin did not significantly change;The difference of dissolved oxygen（DO）and alkalinity（Alk）in each hydrological period is in opposition to the change of water temperature,with the wet season having the highest water temperature（T）,the flat season having the second-highest temperature,and the dry season having the lowest temperature.The urbanized river reach’s p H,DO,and Alk all fell to variable degrees in space,but its EC increased.The Nanming River basin’s river water ion concentrations are generally highest in the dry season,second highest in the wet season,and lowest in the flat season.The two ions in the basin with the highest concentrations are HCO₃^-and Ca²⁺.The complicated variations in their spatial distribution that occur during each hydrological season differ from the EC change rule.（2）The concentration of TN and TP in the water of the Nanming River basin are low in the flat season,while DOC is high in the flat season.In space,the TN and TP in the water of the river section flowing through the urban area are higher than those in other river sections,while the DOC changes are relatively complex and irregular.Through the comparative analysis of the tributaries in the basin,it can be seen that the rivers with high nutrient content mostly flow through cities or construction and development areas,while the rivers with low nutrient content mostly flow through areas with less human activities.In general,the water quality of the Xiaohuang River and Shixi River is the worst,followed by Toubao River,while the water quality of the main stream of Nanming River has improved due to the treatment in recent years.（3）The hydrochemical type of the studied area is Ca²⁺-HCO₃^-type,and a small number of water samples at the sampling sites are Ca²⁺-Cl^-type and mixed type.In general,the main ions(Ca²⁺,Mg²⁺,HCO₃^-)in the Nanming River basin play a leading role in the control of rock weathering,and mainly contribute to the weathering of carbonate rocks,while NO₃^-,Cl^-ions are mainly affected by human activities.（4）The mean value ofδ¹³C-DIC in the wet season of the Nanming River basin is（-9.45±1.05）‰;The average value ofδ¹³C-DIC in dry season is（-10.10±0.95）‰;The average value ofδ¹³C-DIC is（-7.95±1.40）‰in flat season.On the whole,it shows the characteristics of"positive"in the normal season and"negative"in the dry season,showing significant seasonal differences.According to the DIC isotope tracer method,the main source and contribution of DIC in river water samples in this study are carbonated weathered carbonate rocks,meanwhile,the discharge of sewage also has an impact on its source.（5）The p CO₂of the water body in the Nanming River basin is the highest in the wet season（1852.58±983.87μatm）,followed by（1437.34±958.29μatm）in dry season,the lowest in flat season（1055.40±796.46μatm）,the mean value of p CO₂in the three hydrological periods is p CO₂in the standard atmosphere（410μatm）.In space,the p CO₂value of the main stream of the Nanming River and its largest tributary,Toubao River,has a trend of first increasing and then decreasing from upstream to downstream,but the main stream has no significant correlation with land use type,while the change of the p CO₂value of Toubao River has a significant positive correlation with the proportion of urban land.（6）The CO₂diffusion flux（Fc）at the water-air interface in the Nanming River basin is the highest in the wet season（147.02±100.30 mmol/（m²·d））,the second in the flat season（119.28±168.22 mmol/（m²·d））,and the lowest in the dry season（76.02±67.45 mmol/（m²·d））.The spatial variation characteristics are basically the same as that of p CO₂.Compared with the CO₂diffusion flux of natural background rivers,the CO₂diffusion flux value of Nanming River is higher and its range of change is larger,reflecting that the karst rivers have greater CO₂emission potential under the influence of urbanization.