(a) Rivers and streams.

1. (1) The following dissolved oxygen criteria are applicable:

2. (2)

   1. (A) In streams with watersheds of less than then ten square miles (<10 mi²), it is assumed that insufficient water exists to support aquatic life during the critical season.
   2. (B) During this time, a dissolved oxygen criteria of two milligrams per liter (2 mg/L) will apply to prevent nuisance conditions.
   3. (C)

(i) However, field verification is required in areas suspected of having significant groundwater flows or enduring pools that may support unique aquatic biota.

1. (ii) In such waters the critical season criteria for the next size category of stream shall apply.

2. (3)

   1. (A) All streams with watersheds of less than ten square miles (<10 mi²) are expected to support aquatic life during the noncritical season when stream flows, including discharges, equal or greater than one cubic foot per second (1 cfs).
   2. (B) However, when site verification indicates that aquatic life exists at flows below one cubic foot per second (1 cfs), such aquatic biota will be protected by the noncritical season standard (refer to the State of Arkansas Continuing Planning Process for field verification requirements).
3. (4) Also in streams with watersheds of less than ten square miles (<10 mi²), where waste discharges are one cubic foot per second (1 cfs) or more, streams are assumed to provide sufficient water to support aquatic life and, therefore, must meet the dissolved oxygen criteria of the next size category of streams.

4. (5) For purposes of determining effluent discharge limits, the following conditions shall apply:

   1. (A)

      1. (i) The noncritical season dissolved oxygen standard is to be met at a water temperature of twenty-two degrees Celsius (22°C (seventy-one and six-tenths degrees Fahrenheit (71.6°F))) and at the minimum stream flow for that season.
      2. (ii) At water temperatures of ten degrees Celsius (10°C (fifty degrees Fahrenheit (50°F))), the dissolved oxygen criteria is six and five-tenths milligrams per liter (6.5 mg/L).
   2. (B) During March, April and May, when background stream flows are fifteen cubic feet per second (15 cfs) or higher, the dissolved oxygen standard is six and five-tenths milligrams per liter (6.5 mg/L) in all areas except the Mississippi Alluvial Plain Ecoregion, where the non-critical season dissolved oxygen criteria will remain at five milligrams per liter (5 mg/L).

   3. (C)

      1. (i) The critical season dissolved oxygen standard is to be met at maximum allowable water temperatures and at Q7-10 flows.
      2. (ii) However, when water temperatures exceed twenty-two degrees Celsius (22°C (seventy-one and six-tenths degrees Fahrenheit (71.6°F))), a one milligram per liter (1 mg/L) diurnal depression will be allowed below the applicable critical criteria for no more than eight (8) hours during any twenty-four-hour period.

(b) Lakes and reservoirs.

1. (1) Specific dissolved oxygen criteria for lakes and reservoirs shall be 5 mg/L.
2. (2) Effluent limits for oxygen-demanding discharges into impounded waters are promulgated in Arkansas Pollution Control and Ecology Commission’s Rules for State Administration of the National Pollutant Discharge Elimination System (NPDES), 8 CAR pt. 25.

3. (3) However, the commission may, after full satisfaction of the intergovernmental coordination and public participation provisions of the State of Arkansas Continuing Planning Process, establish alternative limits for dissolved oxygen in lakes and reservoirs where studies and other relevant information can demonstrate that predominant ecosystem conditions may be more accurately reflected by such alternate limits; provided that these limits shall be compatible with all designated beneficial uses of named lakes and reservoirs.

   1. (c) Note. Site specific dissolved oxygen criteria are located in Appendix A.