A major goal for the FIMITA-project was to investigate the short- and long-term impacts of submarine tailing deposits on benthic organisms.

The mining industry has major challenges related to the handling of vast amounts of non-processed overburden rock and processed tailings. Tailings can be placed on land or in subsea tailing deposits. Although the number of mines conducting submarine tailing disposals is low at the global scale this is common practice in Norway. Despite the relatively long tradition of subsea tailing deposits in Norway, our knowledge regarding ecosystem impacts and recovery potential is limited.

Subsea tailing deposition may affect the coastal environment through many different mechanisms, such as metal-, radionuclide- and process-chemical-toxicity, hyper-sedimentation, changes in grain size and angularity, changes in topography, sediment plumes and turbidity. It is also well known that subsea tailing deposits cover and decimate the seafloor environment close to the outflow pipe, and will therefore significantly modify the local benthic ecosystem in this area. However, it is more uncertain how soft- and hard-bottom organism living on the fringe of a deposit are affected.

A major goal for the FIMITA-project was to investigate the short- and long-term impacts of submarine tailing deposits on benthic organisms.

This was done through a combination of laboratory and field-studies. The field-studies were conducted in Repparfjorden, northern-Norway. Copper-mining took place in Repparfjord from 1972 to 1978, with subsea tailing deposition in the inner part of the fjord, offering a unique opportunity to study long-term recovery processes. New copper-mining activities with related deposition of tailings are planned, starting from 2019-2020. Laboratory studies were conducted at the Institute of Marine Research laboratory facilities in Austevoll.

Characterization of old deposit

The bathymetry of the inner part of the fjord, in the area where the old subsea tailing deposit is located, was mapped and sediment samples were collected. The deposit occurs as a series of cone-like shapes, protruding off the seafloor, forming an irregular ridge shape. The series of cones most probably represent openings in the pipeline every 100 meters over a 600-meter length, resulting in a ridge of tailings material.

These cones reach up to approximately 10 meters above the natural sea bottom. Analytical results of surface sediments show that only small amounts of natural sediments have settled on top of these cones. This indicates that the deposited tailings still can spread either through sediment slides along the cone sides or by water currents.

Sampling the inner fjord sediments proved to be challenging. In parts of the inner fjord (outside the cones), a hard layer at a depth of approximately 20 cm below the sediment surface stopped the sampling equipment from penetrating the sediment column further. This layer turned out to be schist-like material, which consists mainly of iron oxide. This layer was probably formed during the time of tailing-deposition (1972-1978), which would indicate a net sedimentation rate of 20 cm in this area in approximately four decades. Chemical analyses of sediments showed that the mine tailings are clearly enriched in many elements, including Cu. On the other hand, the tailings are depleted, relative to natural sediments in the fjord, on other elements.

Effects on benthic soft-bottom communities

An interesting question is how well the benthic community that was buried by tailings nearly 40 years ago has recovered. As already mentioned the chemical analyses of the sediments showed that the metal concentrations in sediment from the old deposit area deviate from those in reference sediments collected in other parts of the fjord. The benthic community was sampled at various distances from the old deposit.

Preliminary results show that there are no clear disturbance effects on the fauna with regard to diversity or species composition. This indicates that the benthic community has recovered after the deposition stopped.

A recolonization experiment was performed to study which impacts fresh tailings from the new copper-mine may have on benthic fauna in the fringe of the deposit.

Experimental trays filled with defaunated sediment and layer of mine tailings on the top. Photo: Hilde Cecilie Trannum, NIVA.

Sixteen experimental trays filled with defaunated sediment (i.e. frozen to remove existing fauna) and fresh tailings (produced according to methods that will be used in the new mining operations) on top (0 cm (control), 6, 10 and 14.5 mm) was placed at the seabed and subject to colonization for 15 months.

A major finding from this experiment was that trays treated with mine tailings showed a lower colonization than the controls, evidenced by lower species number, lower abundance and lower biomass.

This effect can be due to toxicity caused by e.g. elevated levels of copper, and/or effects of particle properties, like grain size, angularity, or low organic content. Indeed, the experimental trays had a higher sediment fine fraction, which may decrease sediment heterogeneity and niches diversity.

Effects on benthic hard-bottom species

To study how hard-bottom organisms may respond to elevated levels of particles in the water or on the body surface laboratory experiment were performed at Austevoll. In these experiments, the bivalve Acesta excavata, the coral Primnoa resedaeformis and the sponge Geodia barretti, were exposed to suspended solids (mimicking tailing particles). The results showed that exposure to sediment did not affect the oxygen consumption and metabolism in these species. However, elevated levels of ammonia in the water during incubations and a reduced conditions index indicates that the organisms used stored energy to maintain metabolism during times of sediment exposure.

This is probably not sustainable in the long run (laboratory study was limited to 40 days). It is, therefore, possible that long-term effects on populations of these species will occur in areas with continuous discharge of particulate material. Since mine-tailings may contain elevated levels of copper (Cu) toxicity tests were performed using the coral Lophelia pertusa, the bivalve A. excavata and the sponge Phakellia ventilabrum. Sub-lethal stress responses on cellular biomarkers and respiration were observed at concentrations as low as 1 mol Cu/l, and mortality at 100 mol Cu/l, but the responses and threshold for effects varied between species.

The FIMITA-study has provided new information about chemical processes in old deposits and impacts of subsea tailing deposition on benthic species that will be important for environmental managers in future assessments of subsea tailing deposition.

The results indicate that subsea tailing deposition may have negative effects on benthic species living outside the central deposit area, but also that recovery may be expected within some years after the closure of the mine. However, more research is needed to determine the speed and extent of the recovery processes.

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