Abstract
Since abandonment in April 2015, Government of Yukon (YG) has been responsible for the Ketza River mine. Between 1988 and 1990, mining and milling operations deposited approximately 342,000 tonnes of oxidized arsenic-bearing tailings in a tailings storage facility (TSF). The TSF consists of an emergency spillway, several creek diversions and two dams that retain tailings and arsenic-contaminated pH-neutral supernatant. Groundwater continually discharging into the TSF pond requires active management to prevent spillway overflow. At abandonment, the installed arsenic water treatment plant (WTP) was nonfunctioning, and YG did not have any information regarding TSF flood storage or routing capabilities. YG retained a site operations contractor that focussed on rectifying operational and safety deficiencies associated with the TSF dewatering system. Engineering consultants supported YG in setting the pond’s normal operating water level (NOWL) 0.5 m lower than the pre-abandonment NOWL. Geochemical assessments indicated that despite increased subaerial tailings exposure from the lower NOWL, supernatant arsenic concentrations would be unlikely to rise or exceed the influent parameters of the WTP. By maintaining the operating range of the pond below the NOWL, the TSF could store a flood larger than a 1-in-200 return period environmental design flood. During the inflow design flood (IDF), the TSF would have minor deficiencies related to wave run-up on the dam, and to under-sized rip rap armouring the emergency spillway. As YG considered implementing earthworks upgrades to the TSF, it was notified that five bridges on the site access road required replacement. Due to limited resources, bridge replacement was advanced because loss of site access was an immediate issue when compared to the risk of an unlikely IDF event. In nearly a decade of site management, YG’s approach to TSF management has protected the downstream receiving environment and the health and safety of area users.