The Vangorda Plateau at the Faro Mine Complex contains two open pits that, starting in 2013, will both require dewatering. Since mine abandonment in 1998, water levels in Vangorda pit have been actively maintained below a maximum recommended elevation, whereas Grum pit has been filling at a rate of approximately 3 m per year due to average annual inputs of 400,000 m³. In late 2011, Grum pit levels reached a threshold elevation requiring dewatering to begin in 2013 to prevent exceeding the maximum recommended elevation. Continued filling of Grum pit could allow contaminated pit water to enter groundwater in as little as two years, and reduced pit capacity may prevent its use for contingency surface water storage during extreme flood events. Water from the pits cannot be discharged directly to the environment because it is noncompliant with respect to metals. Zinc (Zn) is one of the main contaminants of concern, with a site discharge limit of 0.5 mg/L. Vangorda pit water samples have contained up to 235 mg/L Zn (at pH 3 to 5), and Grum pit water samples have contained up to 6 mg/L Zn (at pH 7.5 to 9).
The Vangorda Water Treatment Plant (WTP) was constructed in 1992 to treat acidic and metal-contaminated water from Vangorda pit utilizing a low-density sludge lime treatment process. Contaminated water from Vangorda pit is pumped uphill to the WTP through a 24" HDPE pipeline. Treated water is directed to a polishing pond where sludge is settled, and compliant effluent is decanted for release to the environment. The pond's maximum sludge storage capacity is reached after treatment of 350,000 to 500,000 m³ of Vangorda pit water over a four week summer period. Sludge removal is a laborious process that occurs in mid-winter when frozen sludge is excavated and hauled by truck to a nearby overburden dump. Thus, the current system has no excess sludge management capacity available to allow for treatment of an additional 400,000 m³ of water that will need to be removed annually from Grum pit. To ensure a sustainable water management solution for 2013 and beyond, a pump and 8" HDPE pipeline will be installed to transfer water from Grum pit to Vangorda pit, and a dredging system will be installed over the polishing pond so that sludge removal can occur intermittently during the treatment season. Sludge will be pumped out of the pond and flow by gravity down the 24" influent pipeline to Vangorda pit for subaqueous disposal. These upgrades will allow for seasonal treatment of 750,000 to 900,000 m³ of contaminated water, and will provide excess treatment capacity should extreme storm or flood events necessitate emergency pit dewatering. This paper reviews the upgrade requirements and compares the possible alternatives based on their cost, time required to implement, and site-specific operational parameters.