Long-term Evaluation of a Composite Cover Overlaying a Sulfidic Tailings Facility

A high density polypropylene (HDPE) and bentonite composite cover, used to reduce oxygen ingress into a sulfidic tailings facility, was evaluated over a 7 year post-installation period between 1998 and 2005, and found to be a reliable long-term barrier to oxygen ingress and rainfall infiltration. The evaluation was based on multiple lines of evidence and included (i) changes in metal and inorganic concentrations in the regional ground water immediately underlying the tailings residue; (ii) oxygen concentrations from probes installed below the composite cover; (iii) changes in pore water levels within the tailings residue, and; (iv) changes in temperature of the vadose zone below the composite cover. While no systematic reduction in metal and inorganic concentrations were observed, oxygen concentrations from most probes installed below the composite cover showed no measurable oxygen (< 0.01%) and indicated that oxygen ingress through the composite cover was slow, compared to oxygen consumption in the tailings residue. The decline and stabilization in pore water levels over time within the tailings residue provided evidence that the cover effectively reduced infiltration of rainfall-recharge through the tailings residue. Temperatures below the composite cover were offset and attenuated with depth, and showed a decrease in temperature (especially at the deepest location just above the tailings residue), suggesting that the exothermic sulfide oxidation reaction process was not maintained after the cover was installed.