Simulating nitrate release from an unsaturated coal waste rock dump

Knowledge of the controls affecting the release of contaminants from waste rock dumps is critical for developing strategies to mitigate downstream impacts on water quality. In this study, a three-dimensional model of a large coal waste rock dump constructed in the Elk Valley, British Columbia, Canada was developed to capture the impact of construction history (1981-2012) and solute transport on nitrate (NO₃^-) release over a 100-year timeframe. The model consisted of 21, one-dimensional finite element models that represented the temporal evolution of the dump. Nitrate, derived from undetonated blast products, was assumed to be present at the time of waste rock placement and was simulated as a conservative species. The simulated pattern of NO₃^- release to the surface water receptor occurred approximately 8 years before its measured arrival. This time lag is attributed to displacement of the water within a basal alluvial aquifer by dump effluent. The simulated patterns of historic releases corrected for the 8-year time lag, compare favourably with monitoring data and suggest the dominant hydrogeological and geochemical mechanisms are captured in the model. The model indicated the flushing of NO₃^- from the dump should be complete by about 2042 with a peak effluent concentration of NO₃^- in 2008. The addition of reclamation covers to the model resulted in an immediate decrease in the annual NO₃^- loading rate but extended the time frame for NO₃^- release from the dump relative to the no cover case. The model also showed that the timing of cover placement had little impact on NO₃^- release relative to the no cover case due to long duration of waste rock placement (~30 years) over a relatively large footprint.