Deep water STD at the Misima gold and silver mine, Papua, New Guinea

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Abstract
>The Misima Mine is the pioneer of deep‐water submarine tailings disposal (STD) and discharges at 112‐m depth, well below the zone of major biological productivity (euphotic zone), onto a steep seafloor slope that leads directly to a deep ocean basin. The system discharges approximately 18,000 tonnes of tailings solids per day and has a mix tank with seawater intake from 82‐m depth. Prior to discharge, each tailings part is diluted with approximately seven parts of seawater. Two validation surveys have shown that the system complies with regulatory conditions and meets ambient standards for contaminants well within a submerged mixing zone that extends to 1200‐m radius from the mix tank location. Video records obtained by remotely operated vehicle (ROV) show the tailings slurry descending the steep slope as a coherent bottom‐attached density current to at least the limit of the ROV tether at 160‐m depth. Geophysical survey and ocean floor sediment sampling has confirmed that accumulation of tailings solids are confined to the gently sloping floor of a deep basin between 1000‐ and 1500‐m water depth. Oceanographic investigations during the 3‐year mine planning and impact assessment phase showed that the ocean at Misima was permanently stratified, and it was predicted that tailings would not rise to the surface providing the outfall was deeper than 100 m. The prediction has been proven correct because after 5 years of continuous operation, no tailings material has ever been observed in the surface waters.

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