Buffering Of Acid Mine Drainage By Carbonate Systems And Dissolved Inorganic Carbon Transport Processes In Karst Rivers

The development of karstic medium and high sulphur coal mines produces large quantities of AMD,which is discharged into surface river systems.On the one hand,the low p H acidic water will dissolve the carbonate rocks through which it flows and activate a large amount of geological’old carbon’to participate in the epigenetic carbon cycle,thereby increasing the carbon cycle load on the surface water system.On the other hand,the buffering of AMD by the carbonate system of the surface water significantly alters the reservoir state of the carbonate fraction in the water,thereby enhancing the CO₂ degassing rate at the water-gas interface.Considering the scale of mining of sulphide deposits and the large quantities of AMD,the buffering of these acidic waters by surface water carbonates may cause serious disturbances to regional karst carbon sink processes.Therefore,it is necessary to re-evaluate the DIC transport processes and CO₂ source-sink relationships in watersheds under the influence of mining of high-sulphur coal in karst.In this paper,the Huatan River Basin in eastern Jinsha County,Guizhou Province,is selected as a typical study area,and the buffering process and mechanism of the carbonate system of the watershed water to AMD were systematically analysed to elucidate the DIC source and transport evolution process of karst rivers under the influence of AMD input from karstic medium and high sulphur coal mining areas.The results of studies on the buffering effect of surface water carbonate systems on AMD show that the Revelle factor reflects not only the buffering capacity of weakly alkaline waters for atmospheric CO₂ uptake,but also the buffering of H⁺by CO₂degassing during acidification of the water.In surface freshwater systems,the maximum value of the Revelle factor occurs at p H=8.38.At this point,the buffering capacity of the water carbonate system is at its weakest.When p H>8.38,the Revelle value reflects the uptake of CO₂ from the atmosphere by the water;when p H<8.38,it shows the buffering of H⁺by the CO₂ degassing process.The upstream tributary of the Huatan River,the Chetian River,has p H<8.38 and Revelle variation range between1.00 and 51.96 due to the influence of AMD input,which shows buffering of H⁺by CO₂ degassing.The buffer factorsγ_DIC,β_DIC,ω_DIC,γ_Alk,β_Alk andω_Alk based on the binary equations of p H and DIC concentration further refined the relative variation of the components CO₂（aq）,H⁺and CO₃^2-on DIC concentration and alkalinity.It can be found that the six buffer factors respond well to the dynamic transformation of carbonate components during the acidification of the water.At p H=8.38,all six factors show extreme values,reflecting the lower buffering capacity of the water.At p H>6.35,β_Alkcorrelates linearly with the concentration of CO₂（aq）;as acidification increases,β_DICresponds well to the buffering process of H⁺by the chemical equilibrium of the carbonate system.When p H<6.35 is accompanied by a gradual increase in the proportion of CO₂（aq）in the water,the water carbonate system is no longer in a closed environment,and carbon transport processes at the water-gas and water-rock interfaces are enhanced.When CO₂ degassing dominates,the absolute value of the buffer factor becomes larger;when H⁺to carbonate rock dissolution processes dominate,the absolute value of the buffer factor becomes smaller.Under the four scenarios of RCPs,the degassing rate of water bodies in the Huatan River Basin will decrease by 4.96%,14.73%,25.64%and 47.65%,respectively.Although the Huatan River Basin exhibits CO₂degassing characteristics in winter and spring under current conditions,it has the potential to eventually become a sink for atmospheric CO₂ as atmospheric CO₂concentrations continue to increase.The analysis of water chemistry type and weathering process shows that the water chemistry type of Huatan River Basin is HCO₃^--SO₄^2--Ca²⁺-Mg²⁺type,and sulphuric acid is significantly involved in the weathering process of Huatan River Basin rocks.Combined analysis based on the characteristics of CO₂ excess and apparent oxygen consumption changes,the non-synergistic evolution of alkalinity and DIC,andδ¹³C-DIC and DIC concentration changes,the evolution of the DIC in Huatan River is controlled by AMD input,CO₂ degassing,OM degradation,and the balance of dissolution and precipitation of carbonate minerals in different seasons.In spring,the mineralization of OM from terrestrial and domestic sewage gradually enhances and replenishes DIC in the water.As the p H increases in this season,the capacity for buffering CO₂ increases.Meanwhile,OM degradation generates a large amount of CO₂in summer,and carbonic acid begins to dissolve limestone.In autumn,the p H decreases due to the enhanced weathering of sulfide-rich coal measures and the mass input of AMD.Thus,the river shows the ability to drive CO₂ outgassing.In winter,CO₂degassing gradually weakens,DIC concentration is at its lowest,andδ¹³C-DIC reaches the heaviest value.Further analysis of the relationship between the degassing process andδ¹³C-DIC in the Huatan River Basin reveals that the CO₂ degassing process in the Huatan River Basin is the main cause of isotope fractionation,which is mainly Rayleigh fractionation.