01.
Mixing
Zones for Point Source Discharges. Whether a mixing zone is authorized,
and its size, configuration and location, is determined by the Department on a
case-by-case basis. This determination is made in accordance with the
provisions of Section
060 at the time a permit is issued,
renewed, or materially modified and is in effect as long as the permit remains
in effect. Such an authorization is required before a mixing zone can be used
to determine the need for, or level of, effluent limits for a particular
pollutant. (3-31-22)
a. Mixing zones shall not
be authorized for a given pollutant when the receiving water does not meet
water quality criteria for that pollutant; provided, however, the Department
may authorize a mixing zone when the permitted discharge is consistent with an
approved TMDL allocation or other applicable plans or analyses (such as 4b
implementation plans, watershed loading analyses, or facility-specific water
quality pollutant management plans) that demonstrate that there is available
assimilative capacity and authorizing a mixing zone is consistent with
achieving compliance with water quality standards in the receiving water.
(3-31-22)
b. Water quality within
an authorized mixing zone is allowed to exceed chronic water quality criteria
for those parameters approved by the Department. If approved by the Department,
acute water quality criteria for one (1) or more parameters may be exceeded
within the zone of initial dilution inside the mixing zone. Narrative criteria
in Subsections 200.03 and
200.05 apply within the mixing
zone. All water quality criteria must be met at the boundary of any mixing zone
under its design conditions. (3-31-22)
c. The size of mixing zone(s) and the
concentration of pollutant(s) present shall be evaluated based on the permitted
design flow. The Department shall not authorize a mixing zone that is
determined to be larger than is necessary considering siting, technological,
and managerial options available to the discharger. (3-31-22)
d. Mixing zones, individually or in
combination with other mixing zones, shall not cause unreasonable interference
with, or danger to, beneficial uses. Unreasonable interference with, or danger
to, beneficial uses includes, but is not limited to, the following: (3-31-22)
i. Impairment to the integrity of the aquatic
community, including interfering with successful spawning, egg incubation,
rearing, or passage of aquatic life. (3-31-22)
ii. Heat in the discharge that causes thermal
shock, lethality, or loss of cold water refugia. (3-31-22)
iii. Bioaccumulation of pollutants (as
defined in Section 010)
resulting in tissue levels in aquatic organisms that exceed levels protective
of human health or aquatic life. (3-31-22)
iv. Lethality to aquatic life passing through
the mixing zone. (3-31-22)
v.
Concentrations of pollutants that exceed Maximum Contaminant Levels at drinking
water intake structures. (3-31-22)
vi. Conditions which impede or prohibit
recreation in or on the water body. Mixing zones shall not be authorized for
E. coli. (3-31-22)
e. Multiple nested mixing zones may be
established for a single point of discharge, each being specific for one (1) or
more pollutants contained within the discharge. (3-31-22)
f. Multiple mixing zones may be established
for a single activity with multiple points of discharge. When these individual
mixing zones overlap or merge, their combined area and volume shall not exceed
that which would be allowed if there was a single point of discharge. When
these individual mixing zones do not overlap or merge, they may be authorized
as individual mixing zones. (3-31-22)
g. Adjacent mixing zones of independent
activities shall not overlap. (3-31-22)
h. Mixing zones shall meet the following
restrictions; provided, however, that the Department may authorize mixing zones
that vary from the restrictions under the circumstances set forth in Subsection
060.01.i. below: (3-31-22)
i. For flowing waters: (3-31-22)
(1) The width of a mixing zone is not to
exceed twenty-five percent (25%) of the stream width; and (3-31-22)
(2) The mixing zone shall not include more
than twenty-five percent (25%) of the low flow design discharge conditions as
set forth in Subsection
210.03.b. of these rules.
(3-31-22)
ii. For all new
discharges to nonflowing waters authorized after July 1, 2015: (3-31-22)
(1) The size of the mixing zone is not to
exceed five percent (5%) of the total open surface area of the water body or
one hundred (100) meters from the point of discharge, whichever is smaller;
(3-31-22)
(2) Shore-hugging plumes
are not allowed; and (3-31-22)
(3)
Diffusers shall be used. (3-31-22)
iii. For all existing discharges to
nonflowing waters authorized prior to July 1, 2015, the total horizontal area
allocated to the mixing zone is not to exceed ten percent (10%) of the surface
area of the lake. (3-31-22)
iv.
Lakes and reservoirs with a mean detention time of fifteen (15) days or greater
shall be considered nonflowing waters for this purpose. Detention time will be
calculated as the mean annual storage volume divided by the mean annual flow
rate out of the reservoir for the same time period.
(3-31-22)
i. The
Department may authorize a mixing zone that varies from the limits in
Subsection 060.01.h. if it is established
that: (3-31-22)
i. A smaller mixing zone is
needed to avoid an unreasonable interference with, or danger to, beneficial
uses as described in Subsection
060.01.d., and the mixing zone
meets the other requirements set forth in Section
060; or (3-31-22)
ii. A larger mixing zone is needed by the
discharger and does not cause an unreasonable interference with, or danger to,
beneficial uses as described in Subsection
060.01.d., and the mixing zone
meets the other requirements set forth in Section
060. The discharger shall provide
to the Department an analysis that demonstrates a larger mixing zone is needed
given siting, technological, and managerial options.
(3-31-22)
j. The following
elements shall be considered when designing an outfall: (3-31-22)
i. Encourage rapid mixing to the extent
possible. This may be done through careful location and design of the outfall;
and (3-31-22)
ii. Avoid
shore-hugging plumes in those water bodies where the littoral zone is a major
supply of food and cover for migrating fish and other aquatic life or where
recreational activities are impacted by the plume.
(3-31-22)
02.
Points of Compliance as Alternatives to Mixing Zones.
Specification of mixing zones for some 404 dredge and fill activities,
stormwater, and nonpoint source discharges may not be practicable due to the
generally intermittent and diffuse nature of these discharges. Rather, the
Department may allow limited dilution of the discharge by establishing points
for monitoring compliance with ambient water quality criteria. These
alternatives to a mixing zone are still subject to requirements outlined in
Subsections 060.01.a., 060.01.d., 200.03,
and 200.05. (3-31-22)