From that wiki page: Phosphogypsum is radioactive due to the presence of naturally occurring uranium and thorium The United States Environmental Protection Ag

From that wiki page: Phosphogypsum is radioactive due to the presence of naturally occurring uranium and thorium The United States Environmental Protection Agency has banned most applications of phosphogypsum with a radium-226 concentration of greater than 10 picocurie/gram (0.4 Bq/g). https://en.wikipedia.org/wiki/Isotopes_of_radium#Radium-226 https://en.wikipedia.org/wiki/Isotopes_of_radium#Radium-226 The longest lived, and most common, isotope of radium is 226Ra with a half-life of 1,600 years. 226Ra occurs in the decay chain of 238U (often referred to as the radium series.) With a half-life of 1600 years for Radium-226, quite a long time.