Quantifying denitrification in a field-scale bioremediation experiment

Nitrate (NO₃-) in mine waste rock derived from undetonated NH₄NO₃ can contaminate receiving waters. An in-situ bioremediation experiment was conducted at a coal mining operation in Elk Valley, British Columbia, Canada to remediate NO₃- from large volumes of mine water. Over the test period (201 d), 5000 to 7500 m³ d^-1 of NO₃--rich (mean concentration 22 mg N L^-1) mine water was injected into saturated waste rock along with methanol, nutrients, and a conservative tracer (Br-). Complete denitrification (<0.5 mg N L^-1) was recorded in monitoring wells located 38 m from the injection wells after 114 to 141 d of operation. Plots of δ¹⁵N- and δ¹⁸O-NO₃- versus NO₃--N concentrations for monitoring wells yielded isotopic enrichment factors (ε) for δ¹⁵N- and δ¹⁸O-NO₃- of -25.7 and -13.2 ‰ for high C/C₀ NO₃- concentrations (>10.5 mg N L^-1) and -5.5 and -3.6 ‰ for lower C/C₀ values. The fraction of NO₃^- denitrified (Dp) calculated using bi-linear ε values for δ¹⁵N- and δ¹⁸O reproduced the Dp determined independently using a conservative tracer indicating that stable isotope tracers of the NO₃- reducing processes in bioremediation are invaluable to determine Dp. Based on the success of this ongoing bioremediation experiment, the technology is being applied at other sites.