I think there really should be two types of pollution. To think that the pollution from a nuclear blast is equivalent to a "local" spill of say petroleum is simply wrong.
Think about Hiroshima and, more importantly, Chernobyl and Hanford, Washington. Anyone who has been in White Russia or Ukraine lately knows about long lasting radioactive effects on the food supply and the ability to produce food. When a city gets hit with a nuke or has a meltdown, there should be a immediate pollution surrounding the city and all of these squares should be reduced to a lower form (like what happens with global warming). Thus it should take an engineer a long time (2 turns to clean up pollution and many turns to transform the terrain) and this is realistic.
However, releasing TCE (e.g.) into the water supply as a by-product of overproduction will have a (relatively) local effect and can, in fact, be cleaned up. So it makes sense that an engineer working for 2 years (later stages of game) can clean up this type of pollution.
P.S. I'm all for higher petroleum taxes for Americans (I have the pleasure of paying something like $4 per gallon here in Sweden).
