Dumped wastes

2016

By volume, dredging contributes the largest pressure associated with dumped wastes (not including marine debris, toxins, pesticides and herbicides) on the marine environment. In 2011–15, 90 million cubic metres of sediment from dredging were disposed of in the marine environment (Ports Australia 2014, 2015). Dredging associated with the development or maintenance of port facilities around the coast of Australia is a necessary and unavoidable activity, which is vital to the continuity of the Australian economy (see also pressures associated with dredging, as detailed in the Coasts report). Disposal of dredged material is most commonly by placement on the sea floor in specially designated areas.

Pressures arising from this practice include direct burial of biota and less direct impacts arising from resuspension of sediments placed on the sea floor. Although the footprint of disposal sites is relatively small on the scale of Australia’s continental shelf, the area affected by remobilised material is potentially larger and less well known (McCook et al. 2015).

The impacts of placing sediment in these areas are closely monitored, and effects in the disposal areas are well documented. Impacts are usually relatively short lived, particularly when sites are in shallower waters, where natural sediment regimes and the organisms that are found within them are generally highly dynamic (Jones 1986, Engler et al. 1991, Harvey et al. 1998, Neil et al. 2003, Bolam et al. 2006). Areas occupied by benthic primary producers, such as seagrass and other macrofauna, are avoided. Fauna likely to be affected are infaunal polychaete worms, molluscs and crustaceans. Most dredging projects report that the impacts detected by their monitoring programs are within expectations and approved levels (Ports Australia 2014, 2015).

The amount of dredged material disposed of at sea in the past 5 years has increased markedly compared with that reported up to 2011 (Figure MAR22). This is largely associated with 2 large port developments in north-western Australia’s Pilbara region: Gorgon and Wheatstone. Generally, there appears to have been a strong increase in the volume of dredged material being disposed of in the Pilbara region in the past decade. Project approvals involving dredging and disposal in the region have been granted for 34 million cubic metres at Anketell Port and 42 million cubic metres at Port Hedland. Given the decline in resource development since 2014, this trend may abate in the short term. Additionally, there appears to be an increased willingness on the part of port operators to dispose of dredged material on land for later re-use (potentially driven by national guidelines), which may also reduce impacts on the marine environment.

Statistics relating to the volume of dredged material disposed of at sea annually and the cumulative area footprint of these dredge material placement areas (DMPAs) are not routinely made available. This limits any assessment of the footprint of dredging pressures and how this might be changing over time. It is possible to monitor dredge plumes using satellite imagery; however, these images do not provide information on plumes in subsurface waters, and it is difficult to differentiate between dredge plume and benthic features in shallow, clear water (e.g. Evans et al. 2012). In addition, the processes of resuspension and transport of sediment, particularly fine sediment, placed on the sea floor are relatively uncertain (SKM 2013, McCook et al. 2015). This uncertainty has led to contentious debate about the more general impacts of at-sea placement of dredged material beyond the immediate DMPA footprint. On a national basis, the impacts of dumped wastes on the Australian marine environment are currently considered to be low with an increasing trend, largely driven by disposal of dredged material in specific, limited regions (e.g. the North-west Marine Region).

Evans K, Bax NJ, Smith DC (2016). Marine environment: Dumped wastes. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/marine-environment/topic/2016/dumped-wastes, DOI 10.4226/94/58b657ea7c296