A CONCEPTUAL MODEL FOR THE FLUSHING OF CONSTITUENTS OF INTEREST FROM COAL WASTE ROCK IN THE ELK VALLEY, BRITISH COLUMBIA

Abstract

Coal mining in the mountainous regions of the Elk Valley, British Columbia has resulted in the formation of rock drains. Rock drains are ex-pit landforms composed of waste rock, resulting from the placement of mine waste rock in and about water courses, while allowing streamflow to pass through a basal layer of the waste rock. These rock drains act as under drains to ex-pit waste rock piles, collecting and transporting water that migrates through the overlying unsaturated waste rock, and mixing it with water flowing through the rock drain from higher in the watershed. Our understanding of the impact of the waste rock piles on both surface and groundwater from the long-term flushing of constituents of interest (CIs) including nitrate (NO3), selenium (Se), and sulphate (SO4) is evolving. Previous research has begun to characterize the effluent chemistry of small-scale waste rock test piles and laboratory cells; however, the long-term monitoring of flow rates and chemistry of rock drains underlying full-scale waste rock piles presents an opportunity to further our understanding of the impact of these waste rock piles on the receiving environment.

Historical databases of water flow and geochemistry were provided by Teck Coal Ltd. for eleven rock drains from all five mines in the Elk Valley, B.C. These data were augmented with site specific information including waste rock pile construction dates and volumes over time, as well as hydrological data. The rock drains represented waste rock of varying ages and depositional methods, key factors in defining the impact of waste rock pile construction on CI release. The data sets were investigated to identify patterns in the production and release of CIs with time. Patterns in release of sulphate, nitrate, and chloride were assessed, and used to group waste rock piles by age. Notable fluctuations in concentration of CIs, such as a change from an increasing to a decreasing trend, were found to be significant, and provided insight into internal processes within the waste rock piles. The ratio of sulphate to nitrate release was key in characterizing the loading associated with ongoing pyrite oxidation within the waste rock piles, separate from the flush of soluble sulphate present in the piles following placement of the waste rock.

System dynamics models for water and CI release were developed for two waste rock piles that reflect different methods of pile construction. The impact of parameters that control CI release from waste rock piles was evaluated using the models. These parameters included waste rock volumetric water content, initial concentration, and leaching efficiencies. The parameters controlled flushing time, maximum concentration, and amplitude of the CI release curve. A comparison of the estimates from the system dynamics models with monitoring data showed that the conceptual models were able to reproduce similar patterns of CI release as those observed for both nitrate and sulphate.

These two initial site models may be expanded upon to include other waste rock piles in the Elk Valley where there is sufficient hydrological and historical waste rock pile construction information, and to additional mine sites with differing geology to test the validity of assumptions in estimating outflow concentrations.