1105.1 Where the discharge of pollutants in quantities that prevent the attainment of, or violates, the surface water quality standards, the Director may grant a variance from a water quality standard that is the basis of a water quality-based effluent limitation included in a National Pollutant Discharge Elimination System (NPDES) permit. A water quality standard variance applies only to the permittee requesting the variance and only to the pollutant or pollutants specified in the variance. A variance does not affect, or require the Director to modify, the corresponding water quality standard for the waterbody as a whole. A variance may be granted only if the discharger can justify every three (3) years through a public hearing process that attaining the water quality standard is not feasible because at least one (1) of the following conditions exists:

1. (a) Irretrievable and irreversible conditions that prevent the attainment of the standards;
2. (b) The application of technology sufficient to attain the standards is more stringent than that required by sections 301(b) and 306 of the federal Clean Water Act, and the application of the technology would result in substantial and widespread adverse economic and social impacts; or
3. (c) One or more of the reasons specified in subsection 1101.3.

1105.2 The Director shall not grant a variance from the water quality standards if:

1. (a) The variance will result in loss of protection for an existing use, or
2. (b) The permittee fails to make the demonstrations required under subsection 1105.1.

1105.3 Variances approved by the Director shall include all permit conditions needed to implement those parts of the variance so approved. The permit conditions shall, at a minimum, require:

1. (a) Compliance with an initial effluent limitation that, at the time the variance is granted, represents the level currently achievable by the permittee, and that is no less stringent than that achieved under the previous permit;
2. (b) That reasonable progress be made toward attaining the water quality standards for the waterbody as a whole through appropriate conditions; and
3. (c) A provision that allows the permitting authority to reopen and modify the

permit based upon any triennial water quality standards revisions to the variance.

1105.4 The Director shall establish and incorporate into the water quality certification of the permittee's discharge permit, all conditions needed to implement the variance as determined pursuant to this section. A variance may be renewed, subject to the requirements of this section. As part of any renewal application, the permittee shall again demonstrate that attaining water quality standards is not feasible based on the requirements of subsection 1105.1. The permittee's application shall also contain information concerning the permittee's compliance with the conditions incorporated into its permit as part of the previous variance pursuant to this section. The Director may deny renewal of a variance if the permittee did not substantively comply with the conditions of the previous variance.

1105.5 The design flow to be used for establishing permit limitations for discharges to the District waters shall be as follows:

(a) The numerical criteria for classes A, B, and C(CCC), as delineated in subsection 1104.8, shall not apply at flows less than the average seven-day (7-day) low flow, which has a probability of occurrence of once in ten (10) years;

(b) The numerical criteria for class C(CMC), as delineated in subsection 1104.8, shall not apply at flows less than the average one-day (1-day) low flow, which has a probability of occurrence of once in ten (10) years;

(c) For carcinogenic pollutants under class D, as delineated in subsection 1104.8, the design flow shall be the harmonic mean flow, and for noncarcinogenic pollutants under class D the design flow shall be the average thirty-day (30-day) low flow, which has the probability of occurrence of once in five (5) years. The categorization of pollutants to be carcinogenic or noncarcinogenic is shown under the Class D column for Human Health Criteria;

(d) The numerical criteria for clarity shall not apply at flows greater than the long-term seasonal average flow; and

(e) For chlorophyll a, the design flow shall be the average seasonal flow for July 1 through September 30.

1105.6 High flow conditions in the District of Columbia waters are defined as follows:

(a) For the Potomac River, the following conditions shall be considered a high flow:

(1) A flow that may result due to a rainfall with an average intensity greater than two-tenths of an inch (0.2") per hour for a period of one (1) hour in the portion of the District of Columbia contributory to the Potomac River, or

(2) A flow equivalent to a three hundred percent (300%) increase in flow during a twenty-four (24) hour period.

(b) For the Anacostia River, the following conditions shall be considered a high flow:

(1) A flow that may result due to a rainfall with an average intensity greater than two-tenths of an inch (0.2") per hour for a period of one (1) hour in the portion of the District of Columbia contributory to the Anacostia River, or

(2) A flow equivalent to a three hundred percent (300%) increase in flow during a twenty-four (24) hour period.

(c) For Rock Creek and tributaries, the following conditions shall be considered a high flow:

(1) A flow that may result due to a rainfall with an average intensity greater than two-tenths of an inch (0.2") per hour for a period of one (1) hour in the portion of the District of Columbia contributory to Rock Creek, or

(2) A flow equivalent to a three hundred percent (300%) increase in flow during a twenty-four (24) hour period.

(d) For other tributaries to the Potomac and Anacostia Rivers, a flow equivalent to a five hundred percent (500%) increase in flow during a twenty-four (24) hour period, shall be considered a high flow.

1105.7

The Director may allow mixing zones for point source discharges of pollutants on a case-by-case basis, where it is demonstrated that allowing a small area impact will not adversely affect the waterbody as a whole. The following conditions shall apply:

(a) In the nontidal waters, the permissible size of the mixing zone shall be determined by the ability of organisms to pass through the mixing zone and the size of the receiving waterbody;

(b) Mixing zones shall be free from discharged substances that will settle to

form objectionable deposits; float to form unsightly masses; or produce objectionable color, odor, or turbidity;

1. (c) A mixing zone, or two (2) or more mixing zones, shall not form a barrier to the movements of aquatic life, nor cause significant adverse impact on aquatic life in shallow areas that serve as a nursery;
2. (d) The concentration of a substance in the mixing zone shall not be lethal to passing organisms, as determined by the appropriate EPA method;
3. (e) Mixing zones shall be positioned in a manner that provides the greatest protection to aquatic life and the designated uses of the water;
4. (f) Within the estuary, the cross-sectional area occupied by a mixing zone shall not exceed ten percent (10%) of the numerical value of the cross-sectional area of the waterway, and the width of the mixing zone shall not occupy more than one third (1/3) of the width of the waterway;
5. (g) Within the estuary, mixing zones may move with the prevailing hydraulic and meteorological conditions;
6. (h) The numerical standards for Criterion Continuous Concentration (CCC) in subsection 1104.8 must be met at the edge of the mixing zone and therefore the CMC criteria will be met within some portions of the mixing zone;
7. (i) The mixing zone shall be implemented in accordance with the EPA Technical Support Document for Water Quality-Based Toxics Control, EPA-505-2-90-001, March 1991; and
8. (j) The mixing zone shall be approved by the Director.

1105.8 Any permit issued pursuant to section 7 of the Water Pollution Control Act of 1984 (D.C. Official Code § 8-103.06) shall be based on the designated uses and other provisions of these water quality standards.

1105.9 When the Director requires a new water quality standard-based effluent limitation in a discharge permit, the permit may, when appropriate, specify a schedule of compliance. The schedule shall require compliance as soon as possible. The permittee shall have no more than three (3) years to achieve compliance with the limitation, unless the permittee can demonstrate, and the record reflects, that a longer compliance period is warranted.

1105.10 The numerical criteria for dissolved cadmium, trivalent chromium, copper, lead,

nickel, silver, and zinc shall be calculated by multiplying the criteria for these metals as specified in Table 2 of §1104.8 by the EPA Conversion Factors specified in Appendix A of the EPA National Recommended Water Quality Criteria: 2002, EPA-822-R-02-047, November 2002. This conversion is required to convert the metal criteria from total recoverable metals to dissolved metals.

SOURCE: Final Rulemaking published at 41 DCR 1075, 1088 (March 4, 1994); as amended by Notice of Final Rulemaking published at 47 DCR 284(January 21, 2000); as amended by Notice of Final Rulemaking published at 49 DCR 3012 (April 5, 2002); as amended by Notice of Emergency and Proposed Rulemaking published at 49 DCR 1706 (February 22, 2002); as amended by Notice of Final Rulemaking published at 49 DCR 4854(May 24, 2002); as amended by Notice of Final Rulemaking published at 52 DCR 9621(October 28, 2005); as amended by Final Rulemaking published at 57 DCR 9119, 9146-9150 (October 1, 2010); as amended by Final Rulemaking published at 72 DCR 002805 (March 14, 2025).

EDITOR'S NOTE: Prior to March 4, 1994, the Department of Consumer and Regulatory Affairs published Final Rulemaking at 32 DCR 7690 (December 27, 1985).