Important to have an actual measure of radioactivity. There's a pretty wide spread between banana level and chernobyl level.

I haven't been able to find a number for this exact plant, but the same process in Idaho listed here: http://www.sciencedirect.com/science/article/pii/S0734242X05800217

This article states the highest radioactivity concentration from at 1780 Bq/kg primarily from Radon.

For reference, the potassium in Bananas makes them radioactive with a concentration of 82 Bq/Kg. So from that perspective, it's 20 times more radioactive than that same amount of Banana pulp.

I'm not sure how to directly translate, but the American standard for Radon in basements is set as being lower than 150 Bq/m^3. So your typical basement already is deemed acceptable when every 10 m^3 of basement air holds as much radioactive radon as a kg of the waste being discussed. The acceptable basement standard unquestionably takes up a much larger space, but it's mass would drastically less. I'm not an expert, but from that it almost sounds like a coin toss to whether breathing air at the highest threshold or drinking this stuff undiluted is worse for you in the long haul.

Agreed, the levels matter....that said, 10 m^2 is quite a lot, while 1kg of water is 1 liter. Multiply by 3.785, then again by over 250000000...that's what they've admitted was dumped into the drinking water. Let's say for discussions sake that it's diluted to 1/10 that strength...so every 10 liters of water you drink is equivalent to breathing 10 m^2 with unsafe levels of radon....now think about how many liters an orange tree uses per day. They're going to have to do continuous independent testing with believable results to make me feel safe eating or drinking anything from Florida from now on. It could take years for the contamination to surface...aquifers are convoluted.

Did some more digging.

We breath an average of 11,000 litres of air per day: https://www.sharecare.com/health/air-quality/oxygen-person-consume-a-day

That amounts to 11m^3 per day. So if you lived in a basement with the highest acceptable radon levels, you'd take in 11*150Bq worth per day, or 1650Bq per day. If the prior Idaho reference numbers follow, then living in a basement on the upper limits today is the same as drinking a kg of this undiluted waste water daily.

From that angle it looks like the 'radioactivity' problem is pretty tiny, given that even undiluted you have to drink a fair bit to match current standards for daily radon exposure.

So, 1 liter is a "fair bit"? I tend to drink more than that per day...not to mention cooking, cleaning, showers, watering, etc. Just watering plants with it may condense it, too, as they absorb and retain the particles. How many liters of water go into an orange? Answer...53. So, assuming if all that is retained by that orange, each one is like 53 days living exclusively in dangerous levels of radon...but condensed into 5 minutes. That's just one source of indirect contamination, and doesn't include bathing in it, drinking it directly, or breathing it.

If we were talking about whole sale replacement of the waterway with 100% pure waste water from the pond you'd be on point.

The pond in the article held 250 Mgal.

http://pubs.usgs.gov/fs/2008/3080/

The stats linked state that Florida groundwater usage in 2005 for drinking purposes alone was 4,242 Mgal per day, and another 2,626 Mgal per day was taken from surface water sources for drinking. So 250 Mgal as a one time release, of water with a very low radiation level already isn't going to hit that hard, nor linger around long enough to concentrate like in your scenarios.

Only if it spreads evenly to the entire Florida aquifer instantly.
Local users will see a far less diluted effect than those, say, 300 miles away.
Because there's absolutely no method available to test the water until it's pumped to the surface for use, prudence demands you assume maximum contamination level until proven otherwise.
There's also absolutely no measure of the aquifer itself, how it moves, mixes, flows, etc. The system is mostly unmapped. That means it could (not will) stay in the local area and not be diluted much at all, or could go directly into the main body and be diluted 1000 times per day. There's no way to know until they test the aquifer itself, something they have no way to do at this point, they can only test what they draw off at individual wells, with no knowledge of how they're connected underground.

Also, let's be clear, the 250000000 number comes from the polluter, not some independent measurement. If history is a guide, we can expect that number to rise to > 10 times that amount when independent investigators look into it. (Think BP).

Even in the best case scenario, it's exposing the already short supply of fresh water to more toxins. Just because it might be below the level that would condem your home if found there doesn't make it 'safe' by any means. Radiation exposure is cumulative, low levels over a long time can be as dangerous as high levels over a short time.
IMO, your contention is comparative to me saying 'no problem that I'm putting arsenic in your water, I put in only 1/10 the lethal dose...and arsenic is found in nature, so no harm no foul'. You would still get sick, might die, and would likely have problems and stress the rest of your life. I could still be convicted of attempted murder, and rightly so. I get that this wasn't intentional, but it was foreseeable, so more like manslaughter I suppose....of >an entire county.

EDIT: And you didn't address the orange problem. An orange uses 53 liters of water, and it takes 13-15 oranges to make a liter of juice, for a cumulative dose of 742 times the contaminants if you drink a liter of OJ (based on the assumption that an orange will trap the contaminants, a reasonable assumption). Now, at 742 times the diluted dose, are you going to continue to drink Florida OJ? I'm not....and that sucks, I like OJ. Now I'm going to have to try to grow oranges here on the N coast of Cali if I want them....an impossibility. (although I did grow a pineapple here, another impossibility, so we'll see).

@newtboy
There's also absolutely no measure of the aquifer itself, how it moves, mixes, flows, etc. The system is mostly unmapped.

Fortunately that's not entirely true. If you go check out the wiki article, it has a lot of links on a lot of mapping that has been done.
https://en.wikipedia.org/wiki/Floridan_aquifer#Hydrology_and_Geology
Most relevant to trying to analyze things, the graphic below is a mapping of the normal water flow within the aquifer based off of testing from about 1,500 different locations.
https://en.wikipedia.org/wiki/Floridan_aquifer#/media/File:Estimated_transmissivity_of_the_Floridan_aquifer_sytem.png

The Mosaic leak occurred somewhere inland from Tampa as close as I can find, if you can narrow that down it'd help. On the map that looks like good news though because that region shows upwards of 100,000 m^3 of water flow per day. So very good mixing for the quantity of leak being discussed if it falls there.

And you didn't address the orange problem.
That's because there isn't one. Radon doesn't work like lead or mercury, it's a gas and doesn't build up in irrigation or the food chain. It bleeds off very fast, irrigation systems bleed it almost instantly into the atmosphere. In animals and meat bags like us, the references I've found suggest the average time from consumption to release is about an hour so we don't hold onto Radon long. Again reason for optimism imo.

To answer your question in the description, the waste water contains phosphogypsum which is a radioactive byproduct from the production of phosphate (sulphuric acid is reacted with phosphate rock to produce phosphoric acid used for fertilizer production).

The radioactivity comes from naturally occurring uranium and radium in the phosphate ore. Central Florida phosphogypsum averages 26 pCi/g radium, and the EPA prohibites its use, but further north in Florida the phosphogypsum has an average concentration of less than 10 pCi/g radium which can be used as an agricultural amendment, but for no other use.

Also, the European news that I saw reported 980 million litres of contaminated water which is only slightly higher than the 225 million u.s. gallons reported elsewhere.

Moving this video to newtboy's personal queue. It failed to receive enough votes to get sifted up to the front page within 2 days.

I looked, and the mapping shown is incredibly idealize/generalized estimates, not actual mapping in detail that would give useful information. The only thing actually mapped is surface springs.

But this isn't radon. It's particulate contamination suspended in water. I'm pretty sure it doesn't act as you suggest, or the pond would not retain radioactivity, it would quickly release it to the environment. Since the pond was not 'capped', I'm going to go out on a limb and guess it's fairly stable, otherwise they would be contaminating the area with radioactive air constantly...which would never be allowed today.