The non-coevolution of DIC and alkalinity and the CO degassing in a karst river affected by acid mine drainage in Southwest China.

Dissolved inorganic carbon (DIC) in mine water generated during coal mining is a large and potential source of atmospheric CO, however its geochemical behaviors under the influence of AMD in relation to CO degassing and carbonate buffering are not well known. In this study, water temperature, pH, DO, alkalinity, Ca concentration, and the carbon isotope of DIC were measured monthly from November 2020 to November 2021 and carbonate chemistry and CO emission flux were calculated to reveal the processes of DIC evolution and CO degassing from the Chetian River draining a karst region, which is materially affected by the input of large quantities of AMD. The results showed that carbonate erosion, the mineralization of terrestrial organic matter, and domestic sewage input are all identified to contribute DIC to different degrees to the river. Throughout the year, the Chetian River undergoes high-intensity CO degassing, which is dominated by HCO-neutralized degassing and proton-enhanced degassing in different reaches. The pCO in the river under the influence of AMD is as high as 237,482 μatm, while the F-CO approaches 316.9 g C m d. Meanwhile, the carbonate system in the downstream karst river buffers an average of 85.2 % of DIC release at the river's outlet. The input of AMD significantly altered the carbon cycle of the surface watershed in the headwaters of tributaries, and greatly enhanced the release of CO from surface water to the atmosphere; meanwhile, the buffering of carbonates on acidity in the water of main streams causes pCO to rapidly reduce over a short distance. Obviously, the carbon emission effect generated by the interaction between AMD and carbonate mainly occurs in the tributary water system. Considering the huge amount of AMD worldwide, this large potential source of atmospheric CO requires a specific and precise quantitative analysis based on actual observations.