Assessing the fate of explosives derived nitrate in mine waste rock dumps using the stable isotopes of oxygen and nitrogen

Abstract Ammonium nitrate (NH 4 NO 3 ) mixed with fuel oil is a common blasting agent used to fragment rock into workable size fractions at mines throughout the world. The decomposition and oxidation of undetonated explosives can result in high NO 3 – concentrations in waters emanating from waste rock dumps. We used the stable isotopic composition of NO 3 – ( δ 15 N- and δ 18 O-NO 3 – ) to define and quantify the controls on NO 3 – composition in waste rock dumps by studying water-unsaturated and saturated conditions at nine coal waste rock dumps located in the Elk Valley, British Columbia, Canada. Estimates of the extent of nitrification of NH 4 NO 3 in oxic zones in the dumps, initial NO 3 – concentrations prior to denitrification, and the extent of NO 3 – removal by denitrification in sub-oxic to anoxic zones are provided. δ 15 N data from unsaturated waste rock dumps confirm NO 3 – is derived from blasting. δ 15 N- and δ 18 O-NO 3 – data show extensive denitrification can occur in saturated waste rock and in localized zones of elevated water saturation and low oxygen concentrations in unsaturated waste rock. At the mine dump scale, the extent of denitrification in the unsaturated waste rock was inferred from water samples collected from underlying rock drains.