Deep Sea Minerals - Vol 2 - Manganese Nodules

3.6

The potential extent of impacts

The footprint or spatial scale of extraction will vary with each site. Whether a site is likely to be mined, and the extent of a commercial mining operation, will depend on the region, nodule distribution and density, ore grade, and topography/bathyme- try. Initial estimates from the CCZ suggest extraction could be on the order of several hundred square kilometres per year and tens of thousands of square kilometres over the commercial life of an operation (Smith et al . 2008). The area to be mined, com- In addition to the potential impacts from normal operation, it is important to consider accidental events and natural haz- ards. These include possible spills and oil leaks on the ves- sel/platform that then enter the sea and leaks from the sea- floor to vessel lifting equipment or from sea-floor equipment (e.g., hydraulic oil leaks). Although unlikely, extreme events – such as a ship sinking or collisions between vessels or with marine mammals – are possible. Commercial operators and national management agencies must reduce the risk of such events occurring in the first instance, and they must be pre- pared to respond if they do happen. Such precautions are generally covered under national and/or international (in- cluding maritime) regulations and are not detailed here. Accidental Events and Natural Hazards

pared to the area that will be untouched, may be an important consideration when developing management strategies.

There is little doubt that some amount of sediment plume will be created by the extraction of nodules. Because of dispersal by currents, the plume may have a larger footprint than the physical mining area. There is also potential for such plumes to extend into the bottom parts of the water column. Several direct mea- surements of actual plumes generated by ploughing the sea- floor in the CCZ showed the plumes visibly lasting up to six hours (Bluhm 1994). Modelling indicates that dispersal and resettle- ment rates could be very low (Rolinski et al . 2001) and result in coverage of the surrounding 100 km by a layer of fine sediment that can have a smothering effect on the benthic ecosystem, even if the layer is less than 1 cm deep (Glover and Smith 2003). The area affected by discharge of the waste water and fine sedi- ments will need to be considered. Discharge plumes in the water column will disperse with distance, and this progressive dilution will mean there is a gradient of impact, with effects lessening as distance from the mining site increases. The likelihood that resedimentation could extend over a con- siderable area adjacent to the physical operation means that care is needed when defining conservation sites. Such areas will have to be far enough away frommining operations to avoid any potential “downstream” effect.

MANGANESE NODULES 38