Scaling Geochemical Loads in Mine Drainage Chemistry Modeling: An Empirical Derivation of Bulk Scaling Factors

摘要

The development of water quality predictions for seepage from mine waste facilities is an integral component of the environmental assessment of minesites. Direct scaling of geochemical loads measured by laboratory kinetic tests such as humidity cells relative to the mass of a full-scale mine waste facility will lead to concentration predictions for mine drainage that are unrealistically high for many dissolved constituents. As a result, "scaling factors" are applied to account for discrepancies in parameters such as grain size, temperature and water/rock ratio between the laboratory experiments and the field scale waste rock facilities.In this study, waste dump seepage chemistry data from two minesites are compared to upscaled, representative laboratory test cell leachate data in an attempt to better constrain bulk scaling factors. Additionally, larger scale field experimental data (field bins) were considered as an intermediate scaling step.It was observed that geochemical loads for major ions from the sites are commonly more than two orders of magnitude lower than those predicted by direct scaling of laboratory kinetic test loads, yielding bulk scaling factors of <1%. In these models, many dissolved trace ions that may be of concern in mine drainage (e.g., As, Cu, Cd, Se, etc.) may still be significantly overpredicted if the model is calibrated to major ions, likely as a result of solubility limits and other attenuation mechanisms. Unlike loading rates, concentrations in field bin and waste dump drainage were commonly found to be on the same order of magnitude for both neutral and acidic sites suggesting that geochemical equilibrium may be attained at relatively small scales in waste piles, highlighting the importance of intermediate-scale field experiments.