Section 302.206  Dissolved Oxygen

General use waters must maintain dissolved oxygen concentrations at or above the values contained in subsections (a), (b), and (c).

a)         General use waters at all locations must maintain sufficient dissolved oxygen concentrations to prevent offensive conditions as required in Section 302.203.  Quiescent and isolated sectors of general use waters including wetlands, sloughs, backwaters, and waters below the thermocline in lakes and reservoirs must be maintained at sufficient dissolved oxygen concentrations to support their natural ecological functions and resident aquatic communities.

b)         Except in those waters identified in Appendix D, the dissolved oxygen concentration in the main body of all streams, in the water above the thermocline of thermally stratified lakes and reservoirs, and in the entire water column of unstratified lakes and reservoirs must not be less than the following:

1)         During March through July,

A)        5.0 mg/L at any time; and

B)        6.0 mg/L as a daily mean averaged over 7 days.

2)         During August through February,

A)        3.5 mg/L at any time;

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

C)        5.5 mg/L as a daily mean averaged over 30 days.

c)         The dissolved oxygen concentration in all sectors within the main body of all streams identified in Appendix D must not be less than:

1)         During March through July,

A)        5.0 mg/L at any time; and

B)        6.25 mg/L as a daily mean averaged over 7 days.

2)         During August through February,

A)        4.0 mg/L at any time;

B)        4.5 mg/L as a daily minimum averaged over 7 days; and

C)        6.0 mg/L as a daily mean averaged over 30 days.

d)         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)