Deep-sea tailings placement (DSTP) involves the discharge of mine tailings at depth (usually >100 m) at the edge of an extended drop off, with ultimate deposition of the tailing solids on the deep-sea bed (>1000 m), well below the euphotic zone. DSTP discharges consist of a slurry of finely crushed rock, mud, silt, sand, water, low concentrations of target metals (e.g. copper, nickel), measurable concentrations of other metals and metalloids, and process liquor such as flotation agents, flocculants, surfactants, sodium cyanide and other acids. The DSTP discharge therefore, needs to be considered as a wastewater mixture, with different components potentially affecting both pelagic and benthic biota inhabiting the continental shelf and the sea floor. There is a general lack of information on the potential long-term impacts from DSTP, largely due to the remoteness of deep-sea systems, the study of which is both costly and technologically challenging. The major impact of DSTP is smothering of benthic biota, which has been shown to reduce species biodiversity and abundance over large areas, even outside the predicted depositional zones. The timescales for recovery are slow, but limited evidence suggests that benthic sediment recovery will take from years to decades and may lead to different biological communities than those that existed prior to tailings deposition.
We developed conceptual models for DSTP in both tropical and temperate environments and used these to establish a qualitative risk framework for DSTP in the marine environment. This environmental risk assessment framework will allow decision makers to better understand the likelihood, severity, spatial extent and duration of impacts associated with DSTP in the marine environment.