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According to the wiki, once a source of ground pollution has been removed, the pollution will eventually clear. About how long does it take to clear, and is there any way to accelerate the process?
It's been a while since I actually did it, but trees are supposed to speed up the process. You can always try and place a ton of trees on one part and none of the other and see if it makes a difference. It's the only thing I know of which might make a difference.