Dissolved inorganic carbon evolution and stable carbon isotope fractionation in acid mine drainage contaminated streams: Insights from a laboratory study

Samples of groundwater, spring water and stream water contaminated by acid mine drainage (AMD), and uncontaminated stream water were collected and allowed to evolve in contact with air in the laboratory for 15–88 days. The objective of this study was to (1) document temporal changes in dissolved inorganic C (DIC) concentrations and stable isotopic composition (δ13CDIC) and (2) to determine the reaction mechanism and resulting isotopic fractionation (13C/12C) accompanying the chemical evolution of AMD. The contaminated spring and stream samples and one groundwater sample (with no image) showed temporal decreases in pH, Fe2+, alkalinity, and DIC, and enrichment in δ13CDIC. One contaminated groundwater sample (with image between 529 and 630 mg/L) showed a temporal increase in pH despite observed decreases in Fe2+, alkalinity and DIC, and enrichment in δ13CDIC. The uncontaminated stream samples showed a continuous temporal increase in pH, relatively constant alkalinity and DIC, and enrichment in δ13CDIC. The results suggest that proton production related to Fe2+ transformation is the driving force for DIC loss in AMD-contaminated samples, and that DIC loss can be described by first order kinetics. The C isotope enrichment rates associated with DIC loss in the contaminated samples varied between 1.0‰ and 1.8‰ for stream water, 2.1‰ and 2.6‰ for the spring, 1.0‰ and 1.2‰ for groundwater with no image, and 7.6‰ and 9.3‰ for groundwater with high image. Variations in 13C enrichment in the contaminated samples are attributed to differences in the initial Fe2+:image ratio. The effect of proton production on 13C enrichment in the AMD-contaminated samples was modeled as a Rayleigh-type distillation, whereby isotope fractionation was constant and occurred in an “equilibrium closed system”. In the uncontaminated stream samples, C exchange between DIC and atmospheric CO2 resulted in an overall enrichment in δ13CDIC of not, vert, similar6‰. It is concluded that C isotope enrichment induced by the chemical evolution of AMD in contaminated streams should range from 1.0‰ to 3.0‰ in the absence of in-stream processes that may affect DIC.