7 Del. Admin. Code § 7401-6.0

Current through Reigster Vol. 28, No. 6, December 1, 2024

Section 7401-6.0 - Regulatory Mixing Zones

The following requirements shall apply to regulatory mixing zones:

6.1 Applicability: In instances where the Department determines, based upon engineering calculations or field studies, that complete mix (as defined herein) of effluent with its receiving water is not expected to occur, the Department may allocate a designated portion of the receiving water to provide for mixing of the effluent and the receiving water. This area shall be defined as a regulatory mixing zone and shall be determined on a case-by-case basis taking into account critical flows, outfall configuration and receiving stream characteristics. A mixing zone will not be allocated in instances where the Department determines that complete mix of effluent and receiving water occurs at the point of discharge, in which case, the critical flows as provided in Section 7.0 shall be applied in determining if the applicable criteria are met.

6.2 Location: Regulatory mixing zones shall not impinge upon areas of special importance, including but not limited to drinking water supply intakes, nursery areas for aquatic life or waterfowl, approved or conditional shellfish areas, or heavily utilized primary contact recreation areas. Zones shall not be located in such a manner as to interfere with passage of fishes or other organisms. Shore-hugging plumes should be avoided to the maximum extent practicable. In areas where multiple discharges are located in proximity, overlapping discharge plumes may occur. In such instances, the size limitations derived under subsection 6.4 may be reduced to preclude acute toxicity in the overlap areas, or to ensure an adequate zone of passage for fish.

6.3 Outfall Design: Outfalls shall be designed to provide maximum protection for humans, aquatic life, and wildlife. Surface discharges to shallow near-shore areas shall be discouraged in preference to submerged outfalls located in deep offshore areas or other alternative discharge configurations which achieve Water Quality Standards.

6.4 Size: Size of the zone shall be no larger than is necessary to provide for mixing of effluent and receiving water. The following are the maximum size limitations that shall apply unless the discharger can demonstrate to the satisfaction of the Department that a larger mixing zone would not have an adverse impact in the receiving water:

6.4.1 Mixing zones for non-thermal pollutants shall be designed as follows:

6.4.1.1 Rivers: During critical stream flow, as detailed in Section 8.0 of these standards, the maximum distance to the edge of the mixing zone shall be described by:

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where x_m = maximum mixing zone length,

u = flow velocity for critical flow as detailed in subsection 7.2 .3 or subsection 7.3 ,

W = width of river,

H = depth of river,

g = acceleration due to gravity, and

S = slope of river surface.

6.4.1.2 Lakes: Because of the shallow depth and small size of Delaware lakes, regulatory mixing zones shall be prohibited in these waters.

6.4.1.3 Tidal waters: For mean low water slack tide conditions, the maximum horizontal distance from the edge of the outfall structure to the edge of the mixing zone shall be no greater than twenty-five percent (25%) of the width of the tidal water at the point of discharge.

6.4.2 Mixing zones for thermal (temperature) pollutants shall be defined as those waters between the point of discharge and the point at which the receiving water temperature criteria are met as defined in Section 4.0, subject to subsection 6.4 .2.1 through 6.4.2.5 below. For non-tidal freshwater, mixing zones shall be designed using the critical stream flow specified in subsection 7.1 or 7.3 .

6.4.2.1 The greatest offshore extension of the mixing zone shall not exceed 50 percent of the width of the waterbody at the point of discharge.

6.4.2.2 Thermal mixing zone cross-sectional area as measured in a vertical plane perpendicular to the receiving water flow shall not occupy more than 25 percent (25%) of the cross-sectional area of the receiving water as measured from the point of discharge to the opposite shore.

6.4.2.3 In areas where multiple discharges are located in proximity, overlapping discharges may occur. In such instances, the above size limitations shall apply to the cumulative influence of the multiple discharges.

6.4.2.4 Because of the shallow depth and small size of Delaware lakes, thermal mixing zones shall be prohibited in those waters.

6.4.2.5 As an alternative to subsections 6.4 .2.1 through 6.4.2.4 above, the size of the thermal mixing zone may be determined on a site-specific basis. This determination must be based upon a sound rationale and be supported by substantial biological, chemical, physical, and engineering evidence and analysis. Any such determination must show to the Department'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 Section 316(a) of the Water Quality Act of 1987 shall be deemed as compliance with the requirements of this paragraph.

6.5 In-Zone and Boundary of Zone Water Quality Requirements:

6.5.1 Regulatory mixing zones shall not be used for, or considered as, a substitute for minimum treatment technology required by the Clean Water Act or other applicable State and Federal laws or regulations.

6.5.2 Regulatory mixing zones shall be free of the following:

6.5.2.1 Materials which result in the accumulation of toxic substances in sediment, aquatic life, or food chains at levels that may be harmful to the health of humans or aquatic life;

6.5.2.2 Materials in concentrations that may settle to form deposits which smother benthic organisms, may exert significant dissolved oxygen demand, or may create a nuisance condition;

6.5.2.3 Floating debris, oil, scum, foam, and other matter in concentrations that may cause a nuisance condition;

6.5.2.4 Substances in concentrations that produce color, odor, taste, or turbidity that may lead to significant disruption of public water supply treatment systems, or may cause a nuisance condition; or

6.5.2.5 Substances in concentrations that may result in a dominance of nuisance species, or may affect species diversity.

6.5.3 No acute aquatic life criterion, as detailed in subsection 4.6 .3 of this document, may be exceeded at any point greater than one-tenth of the distance from the edge of the outfall structure to the boundary of the regulatory mixing zone as defined above. Substances in concentrations that may result in a dominance of nuisance species, or may affect species diversity.

6.5.4 No acute aquatic life criterion, as detailed in subsection 4.6 .3 of this document, may be exceeded at any point greater than fifty (50) times the discharge length scale in any horizontal direction from the edge of the outfall structure.

6.5.5 No acute aquatic life criterion, as detailed in subsection 4.6 .3 of this document, may be exceeded at any point greater than five (5) times the average water depth in the regulatory mixing zone in any horizontal direction from the edge of the outfall structure.

6.5.6 No chronic aquatic life criterion, as detailed in subsection 4.6 .3 of this document, may be exceeded beyond the boundary of the regulatory mixing zone as defined above.

7 Del. Admin. Code § 7401-6.0

14 DE Reg. 1392 (06/01/11)
21 DE Reg. 241 (9/1/2017) (Final)