Field performance evaluation of soil-based cover systems to mitigate ARD for the closure of a potentially acid-generating tailings storage facility

This paper summarizes field performance results of three engineered soil-based cover systems and one control system intended to mitigate potential acid rock drainage and metal leaching at a tailings disposal site. The field performance evaluation commenced in 2002 on the tailings beach of the Premier Gold Project (PGP) site near Stewart, British Columbia, Canada (~ 56˚05’N, 130˚00’W). Four 15 m by 15 m test plots were constructed using the barrier layers of either 1) a geosynthetic clay liner (GCL) Bentomat® ST, 2) 0.6 m of local sand and gravel plus 6% EnviroGel® 8 Wyoming sodium bentonite by eight, 3) 0.8-m loose till, or 4) no barrier (control system). Two years of monitoring results indicate that the barrier layers did not freeze which may be attributed to the insulating effect of approximately 2 m of snow pack at the site. Of the four systems studied, the sand-bentonite (S-B) system performed best when considering performance indicators such as percolation, interflow (lateral drainage above the barrier), and oxygen diffusion. Field and laboratory results indicated that the ‘as-is till’ layer would have to be more than 2 m thick to reduce oxygen diffusion adequately. Field oxygen concentrations and oxygen flux modeling indicated that the GCL system would have to be upgraded substantially (e.g. by incorporating another layer of GCL or a low permeability soil layer) to adequately reduce oxygen diffusion. Unit costs for the as-built S-B cover system without instrumentation were 1.9 and 2.0 times as expensive as the as-built till and the as-built GCL cover systems, respectively.Non UBCUnreviewedOthe