- There is a difference between Mjøsa and the North Sea.
Metallic mercury is practically insoluble in fresh water. In seawater, on the other hand, metallic mercury reacts with chloride ions and forms mercuric chloride, which is relatively easily soluble and is transported by strong bottom currents out into the North Sea. Mercuric chloride is methylated in sediment and in the water column and forms methylmercury, which is a strong nerve agent. Methylmercury is lipophilic (loves fat) and is easily absorbed by plankton entering the food chain. The fish swim over large areas and are thus a poor indicator of the status of the mercury level at the submarine.
The Norwegian Coastal Administration is based on calculations carried out by NIVA which find a theoretical leaching time through the covering layer of 3,400 years. Here, only the diffusion of mercury has been taken into account. This is a very slow process. Far more important is (advection) washing out through the cover layer due to strong eroding currents. The situation is completely different from e.g. a calm fjord arm, such as the Puddefjord in Bergen, where polluted seabeds can be suitable for covering. In NIVA's U864 report 5278/2006, they admit that covering has never been done at such great depths as outside Fedje. It is admitted in the same report that calculation of transport into the cover layer gives a physically impossible result due to an artifact in the program!
Conclusion: NIVA's calculations, which the Coastal Agency is based on, have given the politicians an incorrect decision-making basis. Leaching of mercury and spreading of mercuric chloride is faster than expected due to strong bottom current.