Dissolved inorganic carbon concentrations and stable carbon isotope ratios in streams polluted by variable amounts of acid mine drainage

SummaryDissolved inorganic carbon (DIC) concentrations and stable carbon isotopes of DIC (δ13CDIC) were determined in streams polluted by acid mine drainage (AMD). The aim was to assess the effects of variable AMD contamination on DIC dynamics and δ13CDIC. The stream with relatively high metal (e.g., Fe, Al, Mn) concentrations exhibited downstream decreases in pH because production of protons by the chemical evolution of AMD exceeded the stream’s buffering capacity. DIC dynamics in this stream was driven by proton-enhanced CO2 degassing. In the stream with lower metal concentrations, the protons were neutralized by HCO3- and pH increased in the downstream direction. In this stream, DIC dynamics was driven by CO2 loss due to higher partial pressure of CO2 (pCO2) in stream water compared to atmospheric. In both contaminated watersheds, CO2 loss resulted in seasonal exports of <1–47% of stream DIC. The decreases in DIC concentrations were accompanied by a variable enrichment of δ13CDIC which depended on the extent to which HCO3- was dehydrated to CO2(aq), how CO2 was lost from the streams, and if carbon in DIC was exchanged with atmospheric CO2. The δ13CDIC was enriched by <3.0‰ when CO2 loss was proton enhanced and isotopic fractionation was controlled mostly by diffusion. The δ13CDIC was enriched by >3.0‰ when CO2 loss was neutralization induced and CO2 loss was accompanied by partial exchange of carbon between DIC and atmospheric CO2. We conclude that DIC loss and δ13CDIC enrichment in AMD-contaminated streams depends on the rate of production and amount of protons produced by metal hydrolysis, the stream’s buffering capacity, and the mechanism of CO2 loss.