A key environmental management issue associated with mining projects is tailing disposal. In high rainfall environments this is particularly challenging and generally requires the discharge of excess water from a tailing dam to adjacent streams or coastal waters. Such discharges can potentially have adverse impacts on downstream water quality and the environmental values associated with these waters, such as providing a drinking water supply for local villagers, maintaining a fisheries resource or protecting a fish species of high conservation significance. An alternative to the construction and operation of a conventional tailing dam is Deep Sea Tailing Placement (DSTP). The key features of such a system are:Discharge of the tailing slurry at considerable depth (approximately 120 - 150 m) below the ocean's surface. This requires a pipeline to be laid from the process plant to a mix tank, where the tailing is mixed with seawater and entrained air released. The pipeline then continues down the ocean floor to the required discharge depth, which is below the surface mixed layer and the euphotic zone.After discharge, formation of a density current which continues to descend along the ocean floor as long as the slope is sufficiently steep.Final deposition of the tailing in an area where the slope of the ocean floor is insufficient to maintain the density current. This typically occurs at depths of more than 800 m. Water quality issues associated with DSTP include:Impacts on the oceanic water column due to the formation of subsurface plumes which break away from the descending density current.The occurrence of elevated concentrations of contaminants in the interstitial waters of the deposited tailing solids.Fluxes of contaminants from the tailing deposits into the overlying oceanic water column.This paper discusses the water quality aspects of DSTP with particular reference to a case study in Papua New Guinea. The evaluations undertaken as part of the DSTP impact assessment included consideration of both the short- and long-term chemical behaviour of contaminants and bioassays, within the appropriate water quality assessment framework.