British Columbia Mine Reclamation Symposium

Process-driven design and piloting of a site-specific constructed wetland for copper and selenium treatment in the Yukon, Canada Haakensen, M.; Pittet, V.; Spencer, J.; Rodgers Jr., J. H.; Castle, J. W.


When designed and executed in a scientifically guided manner, constructed wetland treatment systems (CWTSs) can treat various contaminants in water. A process-driven treatment plan was developed to convert contaminants in the water to minerals in the soil, improving water quality and decreasing risk to the environment. This site-specific CWTS is being developed for Capstone Mining Corporation’s Minto Mine (Yukon), using a scaled phased approach to allow for improvement, optimisation and flexibility for modifications along each step. These phases are: (1) site assessment and information gathering, (2) technology selection and conceptual design, (3) pilot-scale testing and optimisation (controlled environment), (4) onsite demonstration-scale confirmation and optimisation and (5) full-scale implementation. The first three phases have been completed successfully, with Phase 4 underway. Performance highlights include successful application of microbial profiling technologies (genetic and growth based) to guide system design in a site-specific context. Microbiome technologies applied in a site assessment and throughout pilot-scale testing allowed for identification of natural copper- and selenium-attenuating microbial communities and ecosystems at the mine site, correlation with native wetland plant species and confirmation of the enhancement of beneficial microbial populations through design of the CWTS. The pilot-scale CWTSs confirmed plant amenability to transplantation, and the design selected for further testing on site achieved 92% removal of copper (mean influent 146 μg/L, outflow 11.3 μg/L) and 41% removal of selenium (mean influent 10.2 μg/L, outflow 6 μg/L) using synthetic influent designed to mimic the worst-case water chemistry of a long-term closure scenario. A mass-balance of the pilot-scale systems confirmed that the elements were sequestered to sediments of the CWTS, with less than 0.5% of the copper and 2% of the selenium transferred to the plant leaves. The pilot-scale system allowed for selection of the optimal design (from three different designs) and for different water chemistries to be tested for treatment, mimicking early closure (containing ammonia and nitrate) and long-term closure (no longer containing ammonia and nitrate). At the conclusion of testing, the pilot-scale CWTS was converted to a hybrid bioreactor/CWTS to evaluate its potential to function as a semi-passive treatment system, should the need arise (e.g. change in regulatory requirements or change of influent water chemistry). Conversion to the hybrid bioreactor/CWTS allowed identification of critical aspects of the transition period and improvement of selenium removal, having influent of 12 μg/L and outflow of 3.9 μg/L, with lowest recorded outflow concentration of 1.9 μg/L (84% removal). Data from the pilot-scale testing were used to calculate system-specific removal rate coefficients, allowing for more accurate sizing estimates for full-scale and modelling effects of seasonal flow-rate and water chemistry variations. The optimised design was applied to construct a demonstration-scale CWTS at the Minto Mine during fall 2014 to refine findings and calculations, including seasonal variations.

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