Effect of progressive acidification on stable carbon isotopes of dissolved inorganic carbon in surface waters

Acidification of surface waters by acid mine drainage (AMD) contamination or atmospheric deposition perturbs the carbonate equilibrium, with unknown effects to the isotope ratio of dissolved inorganic carbon (DIC). We progressively acidified samples of NaHCO3, stream water, groundwater, and spring water contaminated by AMD (AMD spring) to a pH < 3 using H2SO4 under open conditions (exposed to the atmosphere) and closed conditions (isolated from the atmosphere). The δ13CDIC of the acidified samples were enriched by 0.7‰ to 5.0‰ during the HCO3− dehydration phase and depleted by 0.6‰ to 2.3‰ during the phase after HCO3− was exhausted. The δ13CCO2 showed enrichment and depletion trends that were similar to those of DIC. The positive enrichment in δ13CDIC during the HCO3− dehydration phase was driven by 1) kinetic fractionation of CO2 during diffusion, or 2) a combination of fractionation accompanying HCO3− dehydration to CO2(aq) followed by isotopic exchange of carbon between CO2(aq) and HCO3−. We conclude that carbon isotope values in conjunction with concentrations of DIC species (CO2(aq), HCO3−, and CO32−) can be used to provide evidence for the effects of acidification on DIC in surface waters.