2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 26
Presentation Time: 8:00 AM-12:00 PM


FONYUY, Ernest W. and ATEKWANA, Eliot A., Department of Geological Sciences and Engineering, University of Missouri-Rolla, 125 McNutt Hall, 1870 Miner Circle, Rolla, MO 65409, few437@umr.edu

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.