Bench- and field-scale trials of in-situ biological treatment of cadmium and zinc in flooded mine workings, Northern Canada Gault, Andrew G.; Kleinberger, Rachelle M.; Friesen, Vanessa; Woloshyn, Kai S.; Harrington, Jim M.
Passive and semi-passive treatment of underground mine waters provides a low cost, low maintenance alternative to active treatment; however, such non-traditional approaches often require pilot testwork to ensure regulator and stakeholder acceptance. Here we describe two examples of laboratory- and field-based studies of in-situ treatment of mine workings water. To support closure planning of an unnamed mine in British Columbia, 200 L mesocosms comprising synthetic mine water (SMW), waste rock, and cemented tailings that will backfill the mine at closure were prepared. These were amended with glycerol or ethanol, resulting in sulphate-reducing conditions under which constituents of concern (COCs) such as antimony (average initial concentration of 1.9 mg/L), cadmium (0.088 mg/L), selenium (0.039 mg/L), and zinc (5.0 mg/L) exhibited marked peak removal (>99%, >99%, 99%, and 95%, respectively relative to control trial). Sustained treatment below the discharge standards was achieved by the end of the 26-week experiment. Glycerol-amended trials exhibited the greatest treatment, likely related to the higher abundance and diversity of sulphide-producing and selenium-reducing bacteria observed in these trials. At the historical Silver King mine (Yukon), in-situ treatment has been ongoing since October 2014 and provides a field-scale exemplar for this treatment approach in a remote, cold climate setting. Molasses injection to the flooded underground mine workings resulted in rapid removal (>90%) of COCs cadmium and zinc. Multiple injection events in the first year of operation created sustained sulphate-reducing conditions with an attendant shift in the microbial community structure towards the naturally occurring sulphide-producing bacteria key to the treatment process. Following such initial ‘commissioning’, biennial methanol injections have maintained metal concentrations in the adit discharge below effluent quality standards since 2016. Such laboratory- and field-based demonstrations provide important information regarding the inclusion of in-situ treatment in the toolbox of long-term treatment options for mine closure.
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