40 CFR § 61.202 - Standard.

Each person who generates phosphogypsum shall place all phosphogypsum in stacks. Phosphogypsum may be removed from a phosphogypsum stack only as expressly provided by this subpart. After a phosphogypsum stack has become an inactive stack, the owner or operator shall assure that the stack does not emit more than 20 pCi/(m 2-sec) (1.9 pCi/(ft 2-sec)) of radon-222 into the air.

[57 FR 23317, June 3, 1992, as amended at 65 FR 62158, Oct. 17, 2000]