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Environmental Impacts of Mid-water and Deep-water Mining Operations (>30m)


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6.5.4 Environmental Impacts of Mid-water and Deep-water Mining Operations (>30m)

Tailings Plumes

Impact: Fine tailings can remain in suspension for long periods forming plumes that are advected away from the mining vessel by ambient currents. Most of the silt sinks rapidly (minutes) during the initial convective descent phase; entrainment of seawater resulting in dilution of both the dissolved and particulate constituents of the discharge. Ultimately the density of the diluted discharge becomes neutrally buoyant, with the remaining particulate matter spreading and settling further through passive diffusion (hours). The potential impacts on water column processes of the temporary redistribution of slow sinking silts and clays originating from mining activities has received much attention in both Namibia and South Africa. A structured approach to the assessment of suspended sediment plumes, as recommended by the Environmental Protection Agency of the United States, has been adopted by the mining companies concerned. This includes desktop studies and predictive modeling in the pre-mining phase, followed by more intensive in situ biological and toxicological evaluations requiring field sampling, laboratory testing and rigorous data analysis.

Although the plumes may extend several kilometers, the potential impact on phytoplankton communities through reduction of light, nutrient enrichment, remobilization of contaminants, and deep oxygen consumption through decomposition of silt particles during descent, is generally very limited and localized. The extent of the impact of the plume depends largely on the proportions of silts and clays in the targeted sediment, and the sea surface conditions during disposal. Measurements of dissolved nutrient concentrations (NH4, PO4, NO3 and NO2) in the tailings discharge were within the specified limits set by the “Water Quality Guidelines for the South African Coastal Zone”. Rapid descent and dilution of dissolved nutrients and contaminants in the convective descent and passive dispersion phases resulted in ammonium concentrations declining to below detection levels within an hour of release. There was no evidence of the release of NO3 and NO2 from interstitial waters as is to be expected from anoxic sediments. Due to their low solubility in seawater, elevated concentrations of trace elements (Cd, Co, Cr, Mn and Pb) have, however, been recorded in the tailings plumes; those of Cd, Cr, Cu and Zn breaching the recommended water quality guidelines. Evidence suggests that pesticide levels resulting from resuspended biogenic muds are unlikely to exceed recommended guideline levels. Remobilization and subsequent uptake of contaminants by marine organisms has important implications for bioaccumulation down the food chain. Assessments of the potential oxygen consumption through decomposition of silt particles during descent have indicated that rates are low and no measurable impacts on the typical bottom water concentrations are expected.

Ferrosilicon loss in the tailings has been reduced through the introduction of vertical impact crushers and sophisticated recovery systems, but an average of 127 t (in 1995) per vessel are still lost annually. In areas with iron deficiency, this could potentially increase primary productivity and alter the phytoplankton community structure.
Mitigation: No mitigation is warranted.

Impacts on Benthic Fauna

Impact: The mining process removes unconsolidated sediments, resulting in the destruction of benthic fauna, and modification of the benthic habitat in the mining path and in adjacent areas where disturbed sediments are re-deposited. This causes direct mortality of organisms through the dredging and discharging process, potential smothering of organisms affected by the fallout, and possible aggravation of oligoxic conditions causing migration or even death. Consequently, a significant change in abundance and diversity of benthos has been observed in mined areas and their immediate vicinity. Substantial restratification of sediments occurs thereby influencing the rate of recolonisation as well as the structure of the developing benthic community. The recovery rate of a perturbed area has been estimated to take as long as eight years, but habitat modifications may be permanent resulting in a persistent environmental impact and change in the associated communities. This may potentially affect the food chain and have important implications for the distribution and abundance of other marine organisms such as rock lobsters and fish. However, without baseline data on the natural variability and patchiness in benthic community structure with sediment type, depth, and transient changes in water quality, it has not been possible to provide a cumulative assessment of mining damage. Furthermore, spatial heterogeneity of benthic communities has precluded the application of results across wider areas.

Mitigation: Leaving lanes of sediments undisturbed will facilitate the recovery of benthic communities.

Impacts on Fish and Fisheries

The fish fauna of the west coast of South Africa and Namibia has a low diversity and contains few endemic species. Most species are widespread and to some extent migratory and are thus able to escape from oligoxic incursions as well as disturbance by mining tools. Although baseline knowledge exists of the abundance and distribution of the commercially important species in the areas of mining activity (hake, monkfish, sole, kingklip, and rock lobster), attempts to quantify these on a sufficiently fine scale to determine the impacts of offshore mining have not been attempted. Rock lobster and sole are the most restricted in their distributions and migratory capacity and thus the most likely to be at risk. The main spawning areas of the commercially important fish species are primarily north (<25°S) of current mining activities in Namibia, and south (St Helena Bay) of activities in South Africa. The shallow shelf region between St Helena Bay and the Olifants River appears to be utilized as recruitment grounds by most of these species. Desktop studies have identified the main potential impact of mining on fish to be on the breeding success rather than on the adult stocks themselves. The impact is thought to be greatest in the water column below the thermocline where the vulnerable early life-history stages may be negatively influenced by oligoxic conditions in the suspended sediment plumes. No quantitative data are available, however, and the coincidence between the sediment plume and fish egg and larval distributions needs to be investigated further.

The principle risk of offshore mining activities to rock lobster is the aggravation of oxygen poor conditions and the disruption of seasonal offshore migrations as part of their breeding and moulting cycles. Most of the rock lobster fishing activities occur within 5 nautical miles of the coast; overlap with offshore mining is thus limited. However, information on rock lobster behaviour in deeper water and the role of migrations is lacking and should be researched further before conclusions concerning the impact of mining can be made.

Impacts on Other Marine Fauna

The primary sources of noise associated with mining operations are the sounds caused by equipment and machinery, and sonar and seismic equipment. The latter frequencies overlap with the spectrum of frequencies used by marine mammals to communicate and have the potential to cause injury and discomfort.
Some crew changes involve aircraft flying low over wetlands, which has been found to cause disruption to the waterbirds. Reactions to fixed-wing aircraft and helicopters varies between bird species but foraging, roosting and breeding activities are generally negatively affected, potentially reducing reproductive success. Although no similar studies have been conducted on the effects of low flying helicopters on seabird and seal populations on offshore islands, short-term disturbances are to be expected.

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