Tailings

Tailings are pollution-causing blocks that are created when smelting ores at a Bloomery and a Blast Furnace.

Stockpiles and other storage (including vehicles) containing Tailings will leak pollution into surrounding blocks. Only burying Tailings under a significant amount of material will stop them from polluting the surface ground.

Pollution Spread
If Tailings are not stored correctly, they will pollute outward from their location. The pollution will spread in all directions and over time will increase the pollution level of the affected blocks.

Once the Tailings are moved or mitigated by burying, polluted blocks which will take some time to recover back to normal.

Tailing will continue to pollute at the same rate indefinitely, with no dropoff over time. There is no difference in the pollution emitted by Tailings stored in a stockpile and Tailings placed outside a stockpile.

Preventing Tailings Pollution
There is no way to totally prevent pollution caused by tailings. The amount of pollution caused can however be mitigated by burying them underground.

The Tailings must be buried under solid blocks (not rubble, such as Stone, for example). Dirt is the most economical choice, but almost any block can be used. The hole you bury them in does not need to be lined with any specific block, and can even be open underground as long as there is material above the Tailings.

Burial is the key to mitigating Tailings pollution. The deeper you bury the tailings, the less leakage occurs. By 25-30 blocks below the surface, pollution leakage is small.

Specifics
Tailings pollute ~50% less for every 5 blocks down you bury them. As in at a depth of 5 a tailing will product 50% of its pollution; at a depth of 10 it will produce 25%; and at 15 it will produce 12.5%. Going on infinitely until it approaches zero pollution, but technically it will always release some amount of pollution.

And it counts from the top block to the tailing itself. If you placed a single block 1000 meters in the air above a tailings block, it would effectively be buried at a depth of 1000 meters. That's partially why giant towers of tailings end up not polluting as much as you think they will - that and the height differences make pollution spread slower.

Storing in vehicles
While it is possible to store Tailings in vehicles, the Tailings will emit pollution as if they were not mitigated at all, even if the vehicles are buried. The Tailings must be taken out of the vehicle inventory and buried in Stockpiles or individually.