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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 260. Water Quality Standards
9/28/2020

9VAC25-260-20. General Criteria.

A. State waters, including wetlands, shall be free from substances attributable to sewage, industrial waste, or other waste in concentrations, amounts, or combinations which contravene established standards or interfere directly or indirectly with designated uses of such water or which are inimical or harmful to human, animal, plant, or aquatic life.

Specific substances to be controlled include, but are not limited to: floating debris, oil, scum, and other floating materials; toxic substances (including those which bioaccumulate); substances that produce color, tastes, turbidity, odors, or settle to form sludge deposits; and substances which nourish undesirable or nuisance aquatic plant life. Effluents which tend to raise the temperature of the receiving water will also be controlled. Conditions within mixing zones established according to 9VAC25-260-20 B do not violate the provisions of this subsection.

B. The board may use mixing zone concepts in evaluating limitations for Virginia Pollutant Discharge Elimination System permits.

1. Mixing zones evaluated or established by the board in fresh water shall not:

a. Prevent movement of or cause lethality to passing and drifting aquatic organisms through the water body in question;

b. Constitute more than one half of the width of the receiving watercourse nor constitute more than one third of the area of any cross section of the receiving watercourse;

c. Extend downstream at any time a distance more than five times the width of the receiving watercourse at the point of discharge.

2. Mixing zones evaluated or established by the board in open ocean, estuarine and transition zone waters (see 9VAC25-260-140 C) shall not:

a. Prevent movement of or cause lethality to passing and drifting aquatic organisms through the water body in question;

b. Extend more than five times in any direction the average depth along a line extending 1/3 of the way across the receiving water from the discharge point to the opposite shore.

3. A subsurface diffuser shall be required for any new or expanded freshwater discharge greater than or equal to 0.5 MGD to open ocean, estuarine and transition zone waters (see 9VAC25-260-140 C) and the acute and chronic criteria shall be met at the edge of the zone of initial mixing. The zone of initial mixing is the area where mixing of ambient water and effluent is driven by the jet effect and/or momentum of the effluent. Beyond this zone the mixing is driven by ambient turbulence.

4. Mixing zones shall not be allowed by the board for effluents discharged to wetlands, swamps, marshes, lakes or ponds.

5. An allocated impact zone may be allowed within a mixing zone. This zone is the area of initial dilution of the effluent with the receiving water where the concentration of the effluent will be its greatest in the water column. Mixing within these allocated impact zones shall be as quick as practical and shall be sized to prevent lethality to passing and drifting aquatic organisms. The acute aquatic life criteria are not required to be attained in the allocated impact zone.

6. Mixing zones shall be evaluated or established such that acute criteria are met outside the allocated impact zone and chronic criteria are met at the edge of the mixing zone.

7. No mixing zone shall be used for, or considered as, a substitute for minimum treatment technology required by the Clean Water Act and other applicable state and federal laws.

8. The board shall not approve a mixing zone that violates the federal Endangered Species Act of 1973 (16 USCA §§ 1531-1543) or the Virginia Endangered Species Act, Article 6 (§ 29.1-563 et seq.) of Chapter 5 of Title 29.1 of the Code of Virginia.

9. Mixing zones shall not be allowed for the bacteria criteria in 9VAC25-260-170.

10. The board may waive the requirements of subdivisions 1 b and c, 2 b, 3 and 4 of this subsection on a case-by-case basis if:

a. The board determines that a complete mix assumption is appropriate; or

b. A discharger provides an acceptable demonstration of:

(1) Information defining the actual boundaries of the mixing zone in question; and

(2) Information and data demonstrating no violation of subdivisions B 1 a, 2 a and B 7 of this subsection by the mixing zone in question.

11. The size of a thermal mixing zone shall be determined on a case-by-case basis. This determination shall be based upon a sound rationale and be supported by substantial biological, chemical, physical, and engineering evidence and analysis. Any such determination shall show to the board's satisfaction that no adverse changes in the protection and propagation of balanced indigenous populations of fish, aquatic life, and wildlife may reasonably be expected to occur. A satisfactory showing made in conformance with § 316(a) of the Clean Water Act shall be deemed as compliance with the requirements of this section.

12. Notwithstanding the above, no new or expanded mixing zone shall:

a. Be allowed in waters listed in 9VAC25-260-30 A 3 c;

b. Be allowed in waters defined in 9VAC25-260-30 A 2 for new or existing discharges unless the requirements outlined in 9VAC25-260-30 A 2 are satisfied.

Statutory Authority

§ 62.1-44.15 of the Code of Virginia; 33 USC § 1251 et seq. of the federal Clean Water Act; 40 CFR Part 131.

Historical Notes

Derived from VR680-21-01.2, eff. May 20, 1992; amended, Volume 14, Issue 04, eff. December 10, 1997; Errata, 14:12 VA.R. 1937 March 2, 1998; amended, Virginia Register Volume 20, Issue 09, eff. February 12, 2004; Volume 26, Issue 12, eff. February 1, 2010.

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