Nitrification of exchangeable ammonium as a source of nitrate in coal mine rock.

Waste rock from coal mining in the Elk Valley, Canada primarily consists of mudstones and siltstones, and contains two natural nitrogen (N) reservoirs: organic N and exchangeable ammonium in clay minerals (NH4+-ex). Although the mean concentration of organic N is greater than that of NH4+-ex (713 ± 615 vs. 31.0 ± 24 mg N/kg), laboratory experiments show NH4+-ex undergoes rapid (< 50 d) nitrification to nitrate (NO3-) while organic N is nonreactive over the time frame of the experiments (393 days). A conceptual model was developed, based on the results of batch experiments to describe the release of NH4+-ex within individual mine-rock particles and its diffusion to particle surfaces where it is nitrified to NO3-. Oxic experiments with various parent rock particle sizes (< 0.71 to 75 mm), along with N form analyses on aged mine-rock particles (< 2 to 1000 mm; estimated to be 22 years old at the time of sampling in 2022), show NO3- release occurs rapidly (< 1 year) in particles less than several tens of millimetres in diameter. However, larger particles exhibit slower NO3- release due to delayed migration of NH4+ to particle surfaces. The results show nitrification of NH4+-ex can be a major contributor to the NO3- load to surface waters from mine-rock piles and potentially lead to prolonged NO3- release. Such a process may affect aquatic systems associated with other mine-rock piles. Residual ammonium nitrate used as blasting agents provide an additional source of NO3- and NH4+ in the mined rock. Based on the current results, it is not possible to distinguish the relative contributions of NO3- from blasting agents and NH4+-ex.