The evolution of dissolved inorganic carbon (DIC) and δ13CDIC in acid mine drainage polluted karst rivers: Preliminary findings from laboratory experiments

[Display omitted] •The water chemistry evolution in AMD-karst river mixture water are main influenced by DO and Fe2+.•Mean loss of 41.0 ± 11.8 % occurred in the initial DIC in AMD mixed with karst river water.•δ13CDIC was gradually enriched with increasing rate of DIC loss.•The main effect mechanism of DIC in AMD-karst river mixture water are HCO3− dehydration and exchange of CO2(aq). Acid mine drainage (AMD) has caused serious pollution in the surface water environments of karst areas, but the effect of AMD on the dissolved inorganic carbon (DIC) dynamics in the receiving karst rivers remains poorly understood. The AMD and karst river water were sampled from the Yangliujie River watershed in Southwest China, and mixed different proportions of AMD and karst river water in a laboratory experiment to simulate seasonal hydrological conditions in the river, in an attempt to explore the impact of AMD on the loss rate and evolution of DIC in karst river water. The results showed that AMD can cause a rapid pH drop in karst river water, resulting in approximately 41.0 ± 11.8 % DIC loss in the early stage of mixing as CO2(g) release to the atmosphere. With the biological or chemical oxidation of Fe2+, and followed by Fe3+ biogenic formation of schwertmannite or hydrolysis, the pH decreased and the DIC concentration further decreased in the mixed samples, the degree of decrease was related to the oxidation rate of Fe2+. With decreasing DIC concentration, δ13CDIC enrichment gradually increased. When the mixing ratio of AMD was large, the total H+ in AMD and H+ generated by the oxidation of Fe2+ exceeded the HCO3− content in karst river water, the isotopic enrichment coefficient caused by the initial mixing and the end of the reaction was small, and the DIC transformation was mainly dominated by the loss of CO2(g). When a small proportion of AMD was mixed in, the HCO3− concentration in the mixed water sample was in excess of the H+ produced by Fe2+ and the total H+ in AMD, the isotopic enrichment coefficient caused by the initial mixing and the end of the reaction was larger, and the DIC conversion was mainly affected by HCO3− dehydration. These preliminary findings support evidence showing DIC conversion and loss in AMD-affected karst rivers increase the transfer of carbon dioxide to the atmosphere, which diminishes net carbon sink fluxes in karst river basins, so reasonable AMD treatment methods should be applied to minimize these negative impacts.