personally I find devastation a really strange thing, it is supposed to be "cleaned up" over a few turns but I had the same level of devastation in a totally untouched town (untouched ever since I took over over 50 years ago), if anything, the amount of money that the devastation has gone up over the years. Now I understand that it would be calculated as a percentage of my total farmland income, so as this is improved then the amount of money lost due to this would also increase, but shouldn't the devastation have gone by now? Or is there some residue that never clears.

Maybe you have to build some settlers to go round on "pollution patrol" like in Civ. :)