Publication

Summary

Sibelco's mining operations at Stjernøy in Finnmark produce a nepheline feldspar concentrate based on a local occurrence of the rock mineral nepheline syenite. The outlet is discharged as dry matter in the littoral zone from which it is transported down a steep slope and spread in a 1.5 km2 fan-shaped area on the fjord bottom at more than 400 m depth. The tailings were one of the three tailings types compared in different test setups in the NYKOS research project. The ore has a low content of heavy metals and no chemicals are used during the preparation. Therefore, the tailings were expected to provide the least harmful effects on the seabed. The tests included ecotoxicological tests on marine organisms exposed in seawater elutriates of the tailings and whole sediment mixed with the tailings. In addition, experiments were conducted to assess the effects of increasing sedimentation of tailings on natural, benthic communities in box core samples (mesocosm) and field experiments with recolonization in tailings in boxes located at 30m deep in the Oslofjord. The objectives of this report were to provide an overall assessment of potential effects of the tailings from Sibelco Stjernøy based on the results of these experiments and from field studies in the Stjernsundet during the period 1993-2016. Surprisingly, the ecotoxicological tests on water elutriates of the Sibelco tailings had larger effects than corresponding elutriates of the two other tailings types tested. Unlike the other test products types, Sibelco’s tailings are not equilibrated with seawater prior to discharge. This may have resulted in some initial release of exchangeable metal fractions to the small volumes of the elutriates. In all tests where organisms were in direct contact with tailings (ecotoxicological whole-sediment tests, mesocosm experiments and field trials in the Oslofjord), the results confirmed that harmful effects of the Sibelco tailings were small beyond the effects expected from increased sedimentation. The mesocosm experiment showed that sedimentation of 0.3-6 cm tailings over a four-week period had significant effects on the bottom fauna and on the exchange of nutrients between the seabed and the water above. In particular, fluxes of silicate and phosphate were sensitive to small amounts (0.3-2 cm) of discharge, probably due to reduced bioturbation (irrigation) and adsorption/desorption exchange with tailings particles. The effects on the bottom fauna were comparable to the small or moderate effects observed in tailings affected area in the deep part of Stjernsundet. According to investigations performed by the Norwegian Geological Survey (NGU), the sedimentation of tailings in this area is on average 6 cm of tailings per year. Frequent avalanches of tailings accumulated in the steep slopes and the formation of turbidity currrents spreading out on the flat fjord bottom are probably an important mechanism for how the tailings settle in this area. In areas where the turbidity flow strikes, an immediate negative impact with reduced biomass and numbers of individuals and complete disappearance of some species is likely to occur. However, even after episodes with extensive sedimentation, both field monitoring studies and the recolonization experiments indicated that viable benthic communities will recover within a few months. Substantial deterioration of the bottom fauna was observed only in the immediate vicinity of the discharge in Lillebukt, where there are also major effects on the hard-bottom communities. The cause of the effects on the fauna may be food shortage or disruption of important life functions such as respiration, nutrient uptake and protection for organisms adapted to a seabed with very slow sediment growth. High content of Sibelco tailings have also been found to result in a very compact sediment, which can affect some species and reduce oxygen penetration.