70 tons? I guess that's at least 3 molecules.
I know it's a whole lotta mercury. As I said, it's heavier than the water, so most of it would hang around on the bottom of the North Sea. Currents would push it around some, which would cause some of it to escape, but we're talking movement like that by the molecule.
The Great Lakes have quite a bit the stuff in them, cuz of industry. I can't remember if it was from mining or if it was from the paper industry. I think a lot of inland lakes & rivers also have it. Most of it stays in the sediment on the bottom. If they're worried about it, they should test the fish in the area, cuz it stays in their fatty tissue.