Evaluation of dispersion of particles from Aksla quarry

Summary

Dispersion of mineral particles from a planned quay at Bårdvikneset in Frøysjøen is modeled in this project with the ROMS ocean model and using the OpenDrift tool that calculates particle tracks. Particle concentration in the water masses and sedimentation rate along the bottom are calculated. The results are scaled against environmental investigations previously made in the sea area outside the existing quay at Dyrstad. The sea area outside Bårdvika, where it is expected that the dispersion of mineral particles from the loading of vessels consists steep sloping bottom down to more than 300 m deep, and with such topography it is probable that rocky bottom dominates. Such steep bathymetry with rocky bottom are usually dominated by seaweed, kelp and other macro algae down to about 20-25 m deep and with moving benthic animals as the only aquatic organisms deeper than the belt of seaweed and kelp. Both the algae and most of the wildlife are attached to the substrate consisting of rock and recruitment of these organisms depends on a solid substrate without sediment cover to attach and with increasing sedimentation from the activity at the quay, such hard bottom organisms will be harmed. In the tidal zone wind-driven waves and water movement wash out particles and they settle in the bottom areas below. It has been observed poor ecological state for benthic animals and for seaweed and kelp in the area of the inner coast of Western Norway, which also applies to Sogn og Fjordane. This is due to the fact that thread algae overgrown and suffocate the original algae and lead to a reduced ecological state. Such conditions are often associated with increased sedimentation, but unlike the sedimentation from mineral particles that originate from quarries, this sludge will have larger elements of organic matter that can be nutrition for filtering animals. Particles that disperse from loading of vessels can to some degree increase sedimentation on the rock bottom, thus contributing to the increasing risk that organisms will not find suitable substrates to settle down. This applies to macroalgae, but also different animals that will have reduced opportunities to attach to the bottom. The extent of this effect will depend on the size of the affected area that will be covered with a layer of mineral particles. The OpenDrift results show that the sedimentation rate can be over 2 mm/year below the quay, and in a belt along the bottom towards the mouth of Hennøysundet. This will have a negative impact on benthic animals and for seaweed and kelp in these areas. The particle concentration in the water masses in Hennøysundet or at the nearest aqua culture site at Løypingneset will not be increased to such levels that this can be a problem for fish in cages. If an aquaculture site is located close to the quay the fish in the cages might experience elevated particulate concentration. The model calculations indicate that the concentration may reach 6 mg/L at approximately 500 m and less than 1 mg/L at a distance of 1500 m from the quay.