The Diavik Waste Rock Project: Persistence of contaminants from blasting agents in waste rock effluent

Abstract

During mining operations, explosives are used to fragment rock into workable size fractions. Mine-water chemistry can be affected by blasting agent residuals, including NH3, NO2-, NO3-, Cl−, and ClO4-. At the Diavik diamond mine, Northwest Territories, Canada, waste rock generated from open-pit and underground mining is stockpiled on site. Three large-scale test piles measuring 60 by 50m at the base and 15m in height, along with four 2×2m lysimeters each 2m in height, were constructed at Diavik as part of a comprehensive research program to evaluate the quality of water emanating from waste rock stockpiles. Ongoing monitoring of the water chemistry since 2007 shows that blasting residuals comprise a large proportion of the dissolved constituents in the initial pore water and effluent. Leach tests conducted on freshly blasted rock from Diavik indicate the mass of N released corresponds to a 5.4% N loss from the blasting explosives; this mass is in the range for N loss reported for blasting operations at Diavik during the period when the test piles were constructed. The total mass of N released from the lysimeters was also within this range. The three large-scale test piles have only released a small fraction of the N estimated to be contained within them. Blasting of waste rock contributes SO42- to effluent through the oxidation of sulfide minerals in the rock during the blast. During the initial flush of water, the test pile that contained waste rock with the higher S content was observed to release higher concentrations of SO42- than the test pile with lower S content waste rock. Mass-balance calculations based on the ratios of SO42- to total N can be used to estimate the relative contributions of sulfide oxidation within the test piles and SO42- released when S in the host rock is oxidized during blasting. These calculations provide an estimate of S mass released during the first flush of the test piles.