Section 304.104  Averaging

a)         Except as otherwise specifically provided, proof of violation of the numerical standards of this Part must be based on one or more of the following averaging rules:

1)         A monthly average must not exceed the prescribed numerical standard.

2)         A daily composite must not exceed two times the prescribed numerical standard.

3)         A grab sample must not exceed five times the prescribed numerical standard.

b)         Terms used in subsection (a) have the following meanings:

1)         The monthly average is the numerical average of all daily composites taken during a calendar month.  A monthly average must be based on at least three daily composites.

2)         A daily composite is the numerical average of all grab samples, or the result of analysis of a single sample formed by combining all aliquots, taken during a calendar day.  A daily composite must be based on at least three grab samples or three aliquots taken at different times.

3)         A grab sample is a sample taken at a single time.  Aliquots of a daily composite are grab samples only if they are analyzed separately.

c)         Subsection (a) establishes a method for interpreting the effluent standards of this Part.  The Agency must consider the averaging rule in deciding whether an applicant has demonstrated that a facility complies with this Part for purposes of permit issuance and in writing the effluent standards into permit conditions.  Reporting and monitoring requirements are established by permit conditions under 35 Ill. Adm. Code 305.102 and 309.146.

d)         Proof of violation of effluent limitations contained in permits must be based on the language of the permit.

(Source:  Amended at 47 Ill. Reg. 4601, effective March 23, 2023)