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 m(3) 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 delta N-15- and delta O-18-NO3- versus NO3-N concentrations for monitoring wells yielded isotopic enrichment factors (epsilon) for delta N-15- and delta O-18-NO3-of -25.7 and -13.2 parts per thousand for high C/C-0 NO3-concentrations (> 10.5 mg N L-1) and -5.5 and -3.6 parts per thousand for lower C/C-0 values. The fraction of NO3- denitrified (Dp) calculated using bi-linear epsilon values for delta N-15- and delta O-18 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.