Current through Register Vol. 28, No. 3, September 1, 2024
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 xm = 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.
