Sounds like somebody works for the Florida Institute for Phosphate Research!
I've read quite a lot about phosphate mining here in central Florida. Yes, there is "radioactivity
" to old mine sites (Phosphate contains Uranium) but you're exposed to lots more radiation on an airplane flight than in hanging around at an old mine. I've determined, like the previous poster, that there's very very very very very little or no risk from just riding bikes around Boyette, Carter or Alafia. So ride on.
So while the radioactivity is not a problem, there are serious environmental consequences to the strip-mining of Phosphate. The most egregious environmental impact is on the water supply. The Peace River is drying up as a result of Phosphogypsum stacking and the accumulation of water in the pits left by phosphate strip mining.
I find this a very interesting subject. I wish we had more threads on this stuff. I can't help but think about why we have terrain like we do when I'm riding Ridgeline, Rollercoaster, or the fingers. The clay we find out at Alafia that becomes so evil in the wet is not naturally occuring - it is a by product of the phosphate extraction process.
The EPA banned the use of phosphogypsum of a particular level of radioactivity. So, once the phosphate is mined and the good stuff is used to make fertilizer or shipped to China to make drywall, much more bad stuff is left (5:1 ratio of bad to good). There are ways to productively use the phosphogypsum, but it's not economically profitable for the mining companies, so they tend to just stack it up. We now have enough phosphogypsum stacked up here in Florida to give more than 1 ton to each person in China.
Check out this satellite image
of the area around Fort Meade (think Jimmie Cottons). What looks like pools of water is somewhat deceiving - often Gyp stacks are topped with waste water ponds that contain a bunch of nasty stuff. In 1994 a sinkhole opened underneath one of these stacks...
You can still see evidence of the hole
OK, works over, time to go home. Sorry for all the unnecessary info.