Nitrate release from waste rock dumps in the Elk Valley, British Columbia, Canada

Abstract

The origin, distribution and leaching of nitrate (NO3−) from coal waste rock dumps in the Elk Valley, British Columbia, Canada were defined using chemical and NO3− isotope analyses (δ15N- and δ18O-NO3−) of solids samples of pre- and post-blast waste rock and from thick (up to 180m) unsaturated waste rock dump profiles constructed between 1982 and 2012 as well as water samples collected from a rock drain located at the base of one dump and effluent from humidity cell (HC) and leach pad (LP) tests on waste rock. δ15N- and δ18O-NO3− values and NO3− concentrations of waste rock and rock drain waters confirmed the source of NO3− in the waste rock to be explosives and that limited to no denitrification occurs in the dump. The average mass of N released during blasting was estimated to be about 3–6% of the N in the explosives. NO3− concentrations in the fresh-blast waste rock and recently placed waste rock used for the HC and LP experiments were highly variable, ranging from below detection to 241mg/kg. The mean and median concentrations of these samples ranged from 10–30mg/kg. In this range of concentrations, the initial aqueous concentration of fresh-blasted waste rock could range from approximately 200–600mg NO3−-N/L. Flushing of NO3− from the HCs, LPs and a deep field profile was simulated using a scale dependent leaching efficiency (f) where f ranged from 5–15% for HCs, to 35–80% for the LPs, to 80–90% for the field profile. Our findings show aqueous phase NO3− from blasting residuals is present at highly variable initial concentrations in waste rock and the majority of this NO3− (>75%) should be flushed by recharging water during displacement of the first stored water volume.