USE OF LABORATORY HUMIDITY-CELL MEASUREMENTS TO ESTIMATE WASTE-ROCK DRAINAGE QUALITY AT AN ARCTIC MINE SITE

Estimates of the lag time and the longevity of poor quality effluent, from acid mine drainage for example, are standard calculations required for permitting, implementation and monitoring of new and closed mining sites. Water quality prediction models typically use small-scale laboratory experiments, such as humidity cells, to estimate the total mass loadings of elements from waste-rock stockpiles and tailings management facilities. There are many challenges and site specific parameters that increase the complexity of extrapolating geochemical mass loadings from small-scale laboratory experiments to larger-scale field-based experiments, and ultimately, to full-scale operational waste-rock stockpiles. Results from scale-up calculations from the Diavik Test Piles Research Program, located at the Rio Tinto Diavik Diamond Mine 300 km northeast of Yellowknife, Northwest Territories, Canada, will be discussed. Waste rock at Diavik consists of granite and granite pegmatite with irregular xenoliths of biotite schist. These host rocks are cut by diabase dikes. Waste Rock humidity-cell experiments (1 kg sample) conducted at 4°C and 22°C were used to estimate the effluent quality and release rates of oxidation products from small-scale (6 t) field-based lysimeters for a 5-year period between 2007 and 2011. Measured values for temperature, S content and surface area were used in these estimates. The release rate of SO₄^2- from humidity cells with varying S content (ranging from 0.02 to 0.18 wt. % S) were normalized to the surface area of S (mol m^-2 S^-1 sec^-1) from each humidity cell and used to estimate the release rates from the field lysimeters. A comparison between estimated and measured total mass loadings from field lysimeters showed the variability in S-release rates and metal release rates (i.e. Cu, Ni and Zn) for all waste-rock types fell within one order of magnitude. In addition, the results suggest that metals with solubility controls, such as Fe and Al, were either over estimated or under estimated depending on the annual precipitation (dry year versus wet year). The scale-up approach developed in the study suggests that humidity-cell results can be used to estimate the water quality from larger scale experiments if a thorough characterization of the waste rock accompanies the analysis to refine the release rates.