Project “Don Diego” utilizes the expertise of the world’s best environmental consultancies and scientists. Numerous and extensive studies have been carried out that support the information contained in the EIA and Non-Technical Summary.
Type of dredging for the “Don Diego” Project
The type of Trailer Suction Hopper Dredging (TSHD) that will be used for removal of phosphate-rich sands on the seabed at the “Don Diego” site has been carried out for many years in coastal waters, including those of Mexico.
What is a Trailer Suction Hopper Dredger?
A Trailer Suction Hopper Dredger is a vessel which removes seabed sediments through a ‘draghead’ which is fitted with a suction pump that transfers material to the cargo hold of the TSHD. In the case of dredging for phosphate-rich deposits, part of the dredged material is composed of sand and shells of no phosphate content. This non- phosphatic material will be returned to the seabed. For the “Don Diego” project, processing of dredged deposits and return of excess material to the seabed will take place on a second vessel, rather than on the dredger itself. It is important to note that during this proposed phosphate extraction process, chemicals are not added at any stage of the process. Phosphate is extracted solely by mechanical means and only shells, oversized material and fines are returned to the seabed. Once the seabed deposits are transferred to the TSHD, the phosphate-rich components are separated from the fine sands that comprise about 50% of the dredged material. Residual sediments with low phosphate content will then be returned to the seabed.
Direct (primary) impacts of dredging
The main effect is the alteration in seabed topography and the removal of organisms that are directly under the path of the draghead. However, it is important to note that the proposed dredging area supports remarkably low biodiversity compared to deeper waters to the west and shallower waters towards the coast.
Suspended solids and deposition
According to a series of complex studies modeled by one of the world’s leading consultancies for this matter, HR Wallingford in England, any significant increases in suspended sediment concentrations that result from the dredging activities will be confined to 2.5 kilometers from the activity site and will not go further than 4 kilometers in a high-energy event such as a storm or hurricane. After the dredging process, the excess material will be returned to the seabed through a ‘green valve’ located in the hull of the vessel and will be mainly below the surface waters to minimize any effects on phytoplankton production. Various studies predict that the suspended particles in the water, once the excess material has been returned, will create a fine layer of sediment along the seabed, similar to which the resident organisms are naturally adapted to. It is therefore unlikely to have a damaging effect on the fauna. According to extensive computer modeling, the deposition rates near the operating vessels and in the far field of the deposition plume is within the range estimated for natural deposition in the area.
A comprehensive series of toxicity tests have been carried out on a range of sensitive marine invertebrates according to EPA standards to provide information on whether the dredged material deposited on the seabed is likely to have an impact on organisms. These tests show that there is no evidence of acute toxicity of either the dredged sediments from the “Don Diego” site or of the seawater that had been vigorously mixed with the sediments when the excess material is returned to the seabed.
A detailed assessment of the potential impacts of sound from the dredging and processing vessels on a variety of species of conservation significance has been carried out using newly-developed techniques that incorporate sound propagation models and information on the responses of marine animals to sound. The results indicate that the sound frequencies and decibel level of sound emanating from a typical Trailer Suction Hopper Dredger (TSHD) that will be used at the “Don Diego” site are similar to that of other vessels that currently transit off the west coast of Baja California for whale watching and commercial activities.
Additional detail on the research and analysis undertaken in the preparation of the Environmental Impact Assessment is available in a non-technical summary available here.