Buffering effect of chemical equilibrium of surface water carbonate system on acid mine drainage in small karst watershed

The dynamic changes of different components in water carbonate system (CO₂+HCO$_3^{-}$+CO$_3^{2-}$) can be characterized by Revelle factor which can not only reflect the buffering capacity of weak-basicity water to absorb atmospheric CO₂, but also reflect the buffering effect of CO₂ degassing on H⁺ during water acidification. Compared with the marine system, the Revelle factor in the surface water carbonate system has a larger variation range. However, the study on the variation of buffering factors in the dynamic transformation of carbonate components in freshwater system is still very limited. This study selected the Chetian river located in Eastern Jinsha county, Guizhou Province as the research area. Through the analysis of multiple buffering factors, the buffering effect of the surface water carbonate system on AMD input was discussed. The results will help to further understand the DIC cycle process and the CO₂ source-sink relationship in surface water in the karst area of medium-high sulfur coal mine. Based on the 13-month sampling analysis from November 2020 to November 2021, the equations of Revelle factor—γ_DIC, β_DIC, ω_DIC, γ_Alk, β_Alk and ω_Alk—were established to characterize the relationship between acid-base chemical balance of water and the dynamic variation of carbonate components. Results show that when the Revelle factor is at the maximum, the buffering capacity of the water carbonate system is the weakest. In the marine system, the maximum value of Revelle factor appears at pH 7.50, and the seawater sample data are mainly distributed on the right side of this factor, reflecting the absorption and buffering capacity of the ocean to atmospheric CO₂. When the pH is in the range of 6.35-8.38, the carbonate balance in surface water mainly reflects the conversion between CO₂ (aq) and HCO$_3^{-}$, and CO$_3^{2}$⁻ is almost negligible. Due to the influence of AMD input, all data in the Chetian river fall on the left side of the maximum value, with a variation range of 1.00-51.96, which is shown as the buffering of CO₂ degassing on H⁺. In acidic water with pH<6.35, DIC is gradually dominated by CO₂ (aq), and the sensitivity of Revelle factor is reduced. The buffering factors such as γ_DIC, β_DIC, ω_DIC, γ_Alk, β_Alk and ω_Alk, based on the binary equations of pH and DIC concentration, can be used to further elaborate the relative variation of CO₂ (aq), H⁺ and CO$_3^{2}$⁻ on DIC concentration and alkalinity. It can be found that the six buffering factors show a good response to the dynamic transformation of carbonate components during water acidification. When pH is equal to 8.38, the six factors have extreme values, reflecting the low buffering capacity of water carbonate system. At pH>6.35, β_Alk is linearly related to the concentration of CO₂ (aq). With the improvement of acidification degree, β_DIC can respond well to the buffering of chemical equilibrium of carbonate system to H⁺. When pH is less than 6.35, with the gradual increase of the proportion of CO₂ (aq), the water carbonate system is no longer in a closed environment, and the carbon transport at the water-gas interface and water-rock interface is enhanced. When the CO₂ degassing is dominant, the absolute value of these buffering factors becomes larger; when the erosion of carbonate rocks by H⁺ is the dominate process, the absolute value of these buffering factors become smaller.