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17.38.205    MAXIMUM TURBIDITY CONTAMINANT LEVELS

(1) The board adopts and incorporates by reference 40 CFR 141.13, 141.73, 141.173, 141.550, and 141.551, which set forth maximum contaminant levels for turbidity, except for the following changes:

(a) The terms "one turbidity unit" and "1 NTU" mean 1.0 nephelometric turbidity unit, and the terms "five turbidity units" and "5 NTU" mean 5.0 nephelometric turbidity units for the purposes of this subchapter.

(b) The following replaces 40 CFR 141.73(a) (1) : "For systems using conventional filtration or direct filtration, the turbidity level of representative samples of the system's combined filtered water must be less than or equal to 0.5 NTU in at least 95% of the measurements taken each month, and may not at any time exceed 1.0 NTU."

(c) The following replaces 40 CFR 141.73(a) (2) , and is also added at the end of 40 CFR 141.173(a) (1) and 141.551: "For systems using conventional filtration or direct filtration, the turbidity level of representative samples of a system's effluent from individual filters, measured at a point prior to mixing with effluent from other filters or other sources, may not exceed 0.5 NTU in at least 95% of the measurements taken each month, and may not at any time exceed 5.0 NTU. This requirement is not violated if the turbidity reading for the effluent from each individual filter is the first reading of the month that exceeds 0.5 NTU and the individual filter is taken off-line within 24 hours after the sample analysis that shows the exceedance."

(d) The first sentence in 40 CFR 141.551 is replaced with the following sentence: "Your system must meet three strengthened combined filter effluent turbidity limits."

(2) The department may invalidate a turbidity measurement based on documentation that demonstrates the exceedance was caused by turbidimeter performance difficulty or sample site location problems and that the measurements were not indicative of true water quality. If the department invalidates the turbidity reading on one of these bases, the turbidity reading may not be included in the 95% compliance calculations required under this rule.

History: 75-6-103, MCA; IMP, 75-6-103, MCA; NEW, 1977 MAR p. 1187, Eff. 12/24/77; AMD, 1982 MAR p. 594, Eff. 3/26/82; AMD, 1991 MAR p. 1492, Eff. 8/16/91; AMD, 1994 MAR p. 2131, Eff. 8/12/94; TRANS, from DHES, 1996 MAR p. 1499; AMD, 2000 MAR p. 3400, Eff. 12/8/00; AMD, 2005 MAR p. 257, Eff. 1/1/05.

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