IMPACTS OF ACID MINE DRAINAGE ON DISSOLVED INORGANIC CARBON AND STABLE CARBON ISOTOPES IN STREAMS

Perturbation of dissolved inorganic carbon (DIC) pool in streams by acid mine drainage (AMD) has important consequences for biotic activities, chemical reactions, and carbon dynamics. We examined variations in DIC and stable carbon isotope ratios (δ¹³C) in a 1.6 km reach of a headwater stream to determine how AMD affects DIC dynamics. Variations in pH, dissolved oxygen (DO), Fe²⁺, DIC, and δ¹³C of stream water were compared to water samples in laboratory reactors collected from the AMD source and two downstream stations and allowed to evolve under ambient conditions for 400 hours. The AMD source had a pH of 5.8, DO of 0.3 mg/l, Fe²⁺ of 147 mg/l, DIC of 95.2 mg C/l and a δ¹³C of -11.5⁰/₀₀. The AMD impacted stream water was nearly saturated with DO (7.8 to 9.8 mg/l) and showed minor downstream change in pH (6.4 to 6.7) and Fe²⁺ (85 to 58 mg/l). DIC decreased (49 to 13 mg C /l) and δ¹³C increased (-8.4 to -3.9 ⁰/₀₀) downstream. For water samples in the laboratory reactors, pH decreased to 2.9, Fe²⁺ to 1 mg/l, and DIC to 0.2 mg C/l from their respective field values. The δ¹³C increased from field values to -8.9 ⁰/₀₀ for the AMD source and -2.6 ⁰/₀₀ for stream water as pH decreased to 3.2. For pH values below 3.2, the δ¹³C decreased to -10.6 and -11.6 ⁰/₀₀ for the AMD source and stream samples, respectively. The laboratory data indicated that Fe²⁺ was oxidized to Fe³⁺ resulting in low pH. The higher and nearly constant pH of stream water was due to groundwater influx, which buffered pH decreases from Fe²⁺ oxidation. Decrease in DIC (73%) and enrichment of δ¹³C (4.5 ⁰/₀₀) in stream water over the 1.6 km reach studied indicate CO₂ evasion to the atmosphere similar to laboratory samples. This suggests that the pH buffering mechanism was not carbonate related. The decrease in δ¹³C at pH below 3.2 suggests input of a light source of CO₂ possibly from microbial respiration of organic matter in the water samples. Thus AMD evolution and its potential impact on stream DIC pool can be overlooked if only pH is used to characterize the stream system.