NITRATE IN COAL WASTE ROCK DUMPS, ELK VALLEY, BRITISH COLUMBIA, CANADA

Abstract

Explosives used for blasting during mining operations contain nitrogen (N) compounds. The release of N from blasting residuals can result in elevated nitrate (NO3ˉ) concentrations in freshly blasted waste rock, which can be subsequently leached from waste rock dumps. The distribution and leaching of NO3ˉ through coal waste rock (rates and efficiencies) in the Elk Valley, British Columbia, Canada were studied using multiple datasets of NO3ˉ, collected from pre-blast and fresh-blast rock, an unsaturated waste rock dump deposited between 1982 and 2012, and humidity cell (HC), and leach pad (LP) experiments. Chemical analyses showed NO3ˉ concentrations in fresh-blast rock (27.6±47.4 mg/kg; n=36) to be greater than pre-blast rock (0.02±0.10 mg/kg; n=22). 15N-NO3ˉ and 18O-NO3ˉ analyses confirmed that the source of NO3ˉ in the dump was the N used in the explosives. Laboratory aqueous leach tests on fresh-blast rocks showed the mass of N released from fresh waste rock equated to 3.4% N loss from the blasting residuals. 15N-NO3ˉ and 18O-NO3ˉ data suggested limited to no denitrification occurred in the unsaturated dump . Total NO3ˉ-N mass released from humidity cells, leach pads, and boreholes indicated the leaching efficiency (EL) of NO3ˉ to be scale dependent at about 20, 40 and 80%, respectively. Scale dependency of EL suggested that the findings from smaller scale experiments (HCs and LPs) are not applicable to the field-scale dump, but can be valuable for understanding the flushing mechanism of NO3ˉ from the dump . Since 1982 the total NO3ˉ-N mass flushed from the WLC dump was estimated to be about 1.1 × 106 kg.