Section 302.405  Dissolved Oxygen

Dissolved oxygen concentrations must not be less than the applicable values in subsections (a), (b), (c), and (d).

a)         For the South Fork of the South Branch of the Chicago River (Bubbly Creek), dissolved oxygen concentrations must not be less than 4.0 mg/L at any time.

b)         For the Upper Dresden Island Pool Aquatic Life Use waters listed in 35 Ill. Adm. Code 303.230:

1)         for March through July:

A)        6.0 mg/L as a daily mean averaged over 7 days; and

B)        5.0 mg/L at any time; and

2)         for August through February:

A)        5.5 mg/L as a daily mean averaged over 30 days;

B)        4.0 mg/L as a daily minimum averaged over 7 days; and

C)        3.5 mg/L at any time.

c)         For the Chicago Area Waterway System Aquatic Life Use A waters listed in 35 Ill. Adm. Code 303.235:

1)         for March through July, 5.0 mg/L at any time; and

2)         for August through February:

A)        4.0 mg/L as a daily minimum averaged over 7 days; and

B)        3.5 mg/L at any time.

d)         For the Chicago Area Waterway System and Brandon Pool Aquatic Life Use B waters listed in 35 Ill. Adm. Code 303.240:

1)         4.0 mg/L as a daily minimum averaged over 7 days; and

2)         3.5 mg/L at any time.

e)         Assessing Attainment of Dissolved Oxygen Mean and Minimum Values

1)         Daily mean is the arithmetic mean of dissolved oxygen concentrations in 24 consecutive hours.

2)         Daily minimum is the minimum dissolved oxygen concentration in 24 consecutive hours.

3)         The measurements of dissolved oxygen used to determine attainment or lack of attainment with any of the dissolved oxygen standards in this Section must assure daily minima and daily means that represent the true daily minima and daily means. 

4)         The dissolved oxygen concentrations used to determine a daily mean or daily minimum should not exceed the air-equilibrated concentration.

5)         "Daily minimum averaged over 7 days" means the arithmetic mean of daily minimum dissolved oxygen concentrations in 7 consecutive 24-hour periods.

6)         "Daily mean averaged over 7 days" means the arithmetic mean of daily mean dissolved oxygen concentrations in 7 consecutive 24-hour periods.

7)         "Daily mean averaged over 30 days" means the arithmetic mean of daily mean dissolved oxygen concentrations in 30 consecutive 24-hour periods.

(Source:  Amended at 47 Ill. Reg. 4437, effective March 23, 2023)