The sooner the better, that means we can find a use for it in it's natural form. Too bad it doesn't sink in water as it is probably something like a water-proof pond liner once it is laid out on the bottom at a constant 1m depth. Patchin would also be as easy as finding the leak and then propping some new material there in the winter and let it sink by braking the ice, naturally or by force.
Sand is a natural filter so it could be overlaid in the shallows so cat-tails can grow and in the winter they could be harvested for whatever, the intent is to keep the birds off the water and that is why the moss is put on the deeper spots and renewed every winter, for the birds and as it dies and sinks it should trap the heavy metals in a layer that is mud and it stays as mud forever.
The Prairies have how many miles of gravel roads that get renewed how many times a year. Raw tar sands can be put on once and it is there for many seasons and reshaping takes a heavy machine. Driveways in the country same thing when concrete or asphalt is the only choice today. All the sand/salt dumped out could be tar-sand that is new sand and cleaned sand at a ratio that it doesn't bind to itself. As it wars up the tar sticks to the cars that drive over it and the sand remains on the ground as a traction device. Tar sand is also fracture sand so it sticks to each other unlike beach sand which is like a marble.