It's less that the water itself is radioactive and more that the water is stored on phosphogypsum stacks (a byproduct of phosphate processing, consisting of the remaining rock that phosphorous was extracted from). The phosphogypsum is radioactive because phosphate rock also contains some uranium, thorium, and radium. Water itself doesn't really get radioactive - it's a great radiation shield actually, which is why spent nuclear fuel rods are often stored at the bottoms of pools. The main concern would be if a phosphogypsum slurry got mixed into the water I'd think.

Granted since there's other shit in the water I wouldn't be surprised if it's uhhhh a little more radioactive than average but probably not a ton? I don't know much about phosphate processing and byproducts though, I'm just a dumbass who knows a little chemistry, so take with a grain of salt