(a) Before removing phosphogypsum from a stack for distribution in commerce pursuant to § 61.204, or § 61.206, the owner or operator of a phosphogypsum stack shall measure the average radium-226 concentration at the location in the stack from which phosphogypsum will be removed. Measurements shall be performed for each such location prior to the initial distribution in commerce of phosphogypsum removed from that location and at least once during each calendar year while distribution of phosphogypsum removed from the location continues.

(1) A minimum of 30 phosphogypsum samples shall be taken at regularly spaced intervals across the surface of the location on the stack from which the phosphogypsum will be removed. Let n1 represent the number of samples taken.

(2) Measure the radium-226 concentration of each of the n1 samples in accordance with the analytical procedures described in 40 CFR part 61, appendix B, Method 114.

(3) Calculate the mean, x 1, and the standard deviation, s1, of the n1 radium-226 concentrations:

Where x 1 and s1 are expressed in pCi/g.

(4) Calculate the 95th percentile for the distribution, x *, using the following equation:

Where x * is expressed in pCi/g.

(5) If the purpose for removing phosphogypsum from a stack is for distribution to commerce pursuant to § 61.206, the owner or operator of a phosphogypsum stack shall report the mean, standard deviation, 95th percentile and sample size. If the purpose for removing phosphogypsum from a stack is for distribution to commerce pursuant to § 61.204, the additional sampling procedures set forth in paragraphs (b) and (c) of this section shall apply.

(b) Based on the values for x 1 and x * calculated in paragraphs paragraphs (a)(3) and (4) of this section, determine which of the following conditions will be met:

(1) If x 1 <10 pCi/g and x * ≤10 pCi/g; phosphogypsum may be removed from this area of the stack for distribution in commerce pursuant to § 61.204.

(2) If x 1, <10 pCi/g and x * >10 pCi/g, the owner or operator may elect to follow the procedures for further sampling set forth in paragraph (c) of this section:

(3) If x 1 ≥10 pCi/g; phosphogypsum shall not be removed from this area of the stack for distribution in commerce pursuant to § 61.204.

(c) If the owner or operator elects to conduct further sampling to determine if phosphogypsum can be removed from this area of the stack, the following procedure shall apply. The objective of the following procedure is to demonstrate, with a 95% probability, that the phosphogypsum from this area of the stack has a radium-226 concentration no greater than 10 pCi/g. The procedure is iterative, the sample size may have to be increased more than one time; otherwise the phosphogypsum cannot be removed from this area of the stack for distribution to commerce pursuant to § 61.204.

(1)(i) Solve the following equation for the total number of samples required:

(ii) The sample size n2 shall be rounded upwards to the next whole number. The number of additional samples needed is nA = n2−n1.

(2) Obtain the necessary number of additional samples, nA, which shall also be taken at regularly spaced intervals across the surface of the location on the stack from which phosphogypsum will be removed.

(3) Measure the radium-226 concentration of each of the nA additional samples in accordance with the analytical procedures described in 40 CFR part 61, appendix B, Method 114.

(4) Recalculate the mean and standard deviation of the entire set of n2 radium-226 concentrations by joining this set of nA concentrations with the n1 concentrations previously measured. Use the formulas in paragraph (a)(3) of this section, substituting the entire set of n2 samples in place of the n1 samples called for in paragraph (a)(3) of this section, thereby determining the mean, x 2, and standard deviation, s2, for the entire set of n2 concentrations.

(5) Repeat the procedure described in paragraph (a)(4) of this section, substituting the recalculated mean, x 2, for x 1, the recalculated standard deviation, s2, for s1, and total sample size, n2, for n1.

(6) Repeat the procedure described in paragraph (b) of this section, substituting the recalculated mean, x 2 for x 1.

[64 FR 5579, Feb. 3, 1999]