How long does radiation contamination take to clean up?

Solution 1:

"you have to wait 20+ years for all the radiation to turn into ground pollution (which trees help clean up)"

Answer by Guillaume Pierre - The Lead Gameplay Scripter and a Designer

Update

"also the lowest radiation area will take about 34 years to go away, with each band of intensity being 2x that"

Answer by Chris Schmidt - Tuning Designer at Maxis

Solution 2:

My water pump was right next to my nuclear power station that went into meltdown, I have the filtration add-on and the ground pollution is very light after a few days of pumping just below the pump station, but heavy everywhere else.

That's right, get your population to drink it!