Dispersion of Mine Particles in Førdefjorden

Abstract

Førdefjorden is the site of a newly permitted submarine tailing disposal (STD) in Norway, and it is nationally debated. Major implications of an STD can be toxic compounds of the tailing, severe impacts on the benthic infauna, and dispersion of the tailing in the water masses. The most fine-grained particles have low sinking velocity and can potentially disperse far away from the STD site. It is crucial to understand the dynamics at the STD site to determine the spread of the tailing. In this thesis, I study the dispersion of fine-grained particles in Førdefjorden using data from the hydrodynamic fjord model Førdefjord160, which applies ROMS, and the particle tracking model LADIM. I evaluate the hydrodynamic model's performance compared to observations, and investigate the effect of changing the bathymetry and the release depths on the particle dispersion through four scenarios. The two first scenarios represent the early stages of the permitted STD with today's bathymetry and release depths close to the basin bottom. The third scenario represents the end of the STD, while the last scenario is an extreme case. The latter two scenarios have release depths higher up in the water column and a shallower bottom at the STD site. The validation implies that the model does not entirely capture the higher current speed values, nonetheless, it presents a realistic current field in the fjord. I find that particles drift further with decreasing release depths and a shallower bathymetry. I consider the fine fraction in regard to the regulations of the permitted STD and find that there is a pronounced amount of annual sedimentation in all scenarios. Daily concentrations of particles in the water masses are frequently close to or exceeding the set limits. The fine fraction of the annual tailing discharge can lead to high values of sedimentation and concentration, and it should be investigated further prior to the start of the STD in Førdefjorden.