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's 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 (delta C-13(DIC)) in receiving streams. We examined spatial and seasonal variations in physical and chemical parameters, DIC, and delta C-13(DIC) 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 delta C-13(DIC). We attribute the decrease in alkalinity, DIC loss, and enrichment of C-13 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 C-13 enrichment of DIC was related to the amount of HCO3- dehydrated by protons. The laboratory experiment showed that lower C-13 enrichment occurred in unmixed AMD (2.7 parts per thousand) when the amount of protons produced was in excess of HCO3- or in tap water (3.2 parts per thousand) where no protons were produced from Fe3+ hydrolysis for HCO3- dehydration. The C-13 enrichment increased and was highest for AMD-tap water mixture (8.0 parts per thousand) where Fe was proportional to HCO3- concentration. Thus, the variable downstream and seasonal C-13 enrichment in stream water was due in part to: (1) variations in the volume of stream water initially mixed with AMD and (2) to HCO3- input from groundwater and seepage in the downstream direction. Protons produced during the chemical evolution of AMD caused seasonal losses of 50 to >98% 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. (C) 2007 Elsevier Ltd. All rights reserved.