Mardie Minerals proposed change to dispose dredge spoil material to sea – and not to land
Mardie Minerals is currently constructing a greenfield salt and sulphate project, including an export facility, located at Mardie, 80 kilometres southwest of Karratha. The construction of the marine structures, including the jetty, is now 74 per cent complete.
A key part of the project involves dredging to achieve the necessary depth for both the berth pocket and a 4.9-kilometre channel.
Initially, Mardie Minerals was approved to dredge up to 800,000 cubic meters and dispose of the dredge spoil on land. However, they have now revised their plans. The dredging volume has been reduced to 320,300 cubic metres, and the schedule has been condensed to approximately five months. As a result, Mardie Minerals has informed WAFIC that they now propose to dispose of the spoil offshore rather than on land.
It is WAFIC’s understanding that the shift to offshore disposal is driven by the higher costs and impracticality of land disposal, as well as concerns about potential harm to Minuria tridens (Minnie Daisy) at the originally approved land disposal site. The proposed offshore disposal site, “DMPA 4,” is located in waters deeper than 16 metres, 14 nautical miles from the Mardie Project site, and 5.6 nautical miles from Sholl Island and 5.3 nautical miles from Stewart Island. A map for reference can be found here.
Mardie Minerals has informed WAFIC it plans to carry out dredging and disposal during the dry season (April-September) using split-hull hopper barge. An assumption of the disposal modelling was a disposal rate of three hopper loads per day with each hopper load holding 1,200 cubic metres (i.e. 3,600 cubic metres per day). Their modelling suggests that the sedimentation depth at DMPA 4 could reach a maximum of 1.85 metres after 98 days of dredging, with spoil sediment expected to consist of sand, fine sand, silt, and clay.
Mardie Minerals has also conducted a benthic communities and habitat field survey of the site and reported that the seafloor is largely featureless, with no significant habitats or exposed reef systems. They have proposed management measures, including in-situ monitoring stations to track plume impacts at DMPA 4. However, WAFIC retains concerns with monitoring impacts after they occur as it is not an effective way to mitigate and manage potential damage, given the irreversible consequences of dumping spoil at sea.
WAFIC has previously raised serious concerns to the EPA and Mardie Minerals regarding the original Optimised Mardie Project proposal, particularly in relation to its cumulative impacts on nearshore habitats along the Pilbara coastline. These habitats are crucial for the recruitment of key fisheries and endangered species, such as blue-spotted emperor, green sawfish and various prawn species, some of which are vital to commercial fisheries.
Expanding the disturbance footprint further offshore will lead to significant cumulative impacts, and we believe these potential consequences have not been fully considered by Mardie Minerals.
For this reason, WAFIC does not support this proposed change and will be formally objecting to the proposal.
If this proposal concerns you, or if you have any feedback or questions, please contact [email protected].
In the meantime, WAFIC will be keeping a watching brief on this project and will continue to provide updates to relevant licence holders as more information becomes available. If you are not a licence holder operating in the Mardie area, but wish to get updated, please contact WAFIC.