Effects of Acid Mine Drainage on Dissolved Inorganic Carbon and Stable Carbon Isotopes in Receiving Streams

Dissolved inorganic carbon (DIC) constitutes a significant fraction of a stream\u27s carbon budget, yet the role of acid mine drainage (AMD) in DIC dynamics in receiving streams remains poorly understood. The objective of this study was to evaluate spatial and temporal effects of AMD and its chemical evolution on DIC and stable isotope ratio of DIC (δ13CDIC) in receiving streams. We examined spatial and seasonal variations in physical and chemical parameters, DIC, and δ13CDIC in a stream receiving AMD. In addition, we mixed different proportions of AMD and tap water in a laboratory experiment to investigate AMD dilution and variable bicarbonate concentrations to simulate downstream and seasonal hydrologic conditions in the stream. Field and laboratory samples showed variable pH, overall decreases in Fe2+, alkalinity, and DIC, and variable increase in δ13CDIC. We attribute the decrease in alkalinity, DIC loss, and enrichment of 13C of DIC in stream water to protons produced from oxidation of Fe2+ followed by Fe3+ hydrolysis and precipitation of Fe(OH)3(s). The extent of DIC decrease and 13C enrichment of DIC was related to the amount of View the MathML source dehydrated by protons. The laboratory experiment showed that lower 13C enrichment occurred in unmixed AMD (2.7‰) when the amount of protons produced was in excess of View the MathML source or in tap water (3.2‰) where no protons were produced from Fe3+ hydrolysis for View the MathML source dehydration. The 13C enrichment increased and was highest for AMD-tap water mixture (8.0‰) where Fe2+ was proportional to View the MathML source concentration. Thus, the variable downstream and seasonal 13C enrichment in stream water was due in part to: (1) variations in the volume of stream water initially mixed with AMD and (2) to View the MathML source input from groundwater and seepage in the downstream direction. Protons produced during the chemical evolution of AMD caused seasonal losses of 50 to \u3e98% of stream water DIC. This loss of DIC in AMD impacted streams may have implications for CO2 transfer to the atmosphere and watershed DIC export