North Dakota Administrative Code › Title 33 - State Department of Health › Article 33-16 - Control, Prevention, and Abatement of Pollution of Surface Water › Chapter 33-16-02.1 - Standards of Quality for Waters of the State › Appendix III - MIXING ZONE AND DILUTION POLICY AND IMPLEMENTATION PROCEDURE

Current through Supplement No. 393, July, 2024
PURPOSE
This policy addresses how mixing and dilution of point
				  source discharges with receiving waters will be addressed in developing
				  chemical-specific and whole effluent toxicity discharge limitations for point
				  source discharges. Depending upon site-specific mixing patterns and
				  environmental concerns, some pollutants/criteria may be allowed a mixing zone
				  or dilution while others may not. In all cases, mixing zone and dilution
				  allowances shall be limited, as necessary, to protect the integrity of the
				  receiving water's ecosystem and designated uses.
MIXING ZONES
Where dilution is
				  available and the discharge does not mix at a near instantaneous and complete
				  rate with the receiving water (incomplete mixing), an appropriate mixing zone
				  may be designated. In addition, a mixing zone may only be designated if it is
				  not possible to achieve chemical-specific standards and whole effluent toxicity
				  objectives at the end-of-pipe with no allowance for dilution. The size and
				  shape of a mixing zone will be determined on a case-by-case basis. At a
				  maximum, mixing zones for streams and rivers shall not exceed one-half the
				  cross-sectional area or a length 10 times the stream width at critical low
				  flows, whichever is more limiting. Also, at a maximum, mixing zones in lakes
				  shall not exceed 5 percent of lake surface area or 200 feet in radius,
				  whichever is more limiting. Individual mixing zones may be limited or denied in
				  consideration of designated beneficial uses or presence of the following
				  concerns in the area affected by the discharge:
1 There is the potential for bioaccumulation
				in fish tissues or wildlife.
2 The
				area is biologically important, such as fish spawning/nursery areas.
3 The pollutant of concern exhibits a low
				acute to chronic ratio.
4 There is
				a potential for human exposure to pollutants resulting from drinking water use
				or recreational activities.
5 The
				effluent and resultant mixing zone results in an attraction of aquatic life to
				the effluent plume.
6 The
				pollutant of concern is extremely toxic and persistent in the environment.
7 The mixing zone would prohibit a
				zone of passage for migrating fish or other species (including access to
				tributaries).
8 There are
				cumulative effects of multiple discharges and their mixing zones.
Within the mixing zone designated for a particular pollutant,
				  certain numeric water quality criteria for that substance may not apply.
				  However, all mixing zones shall meet the general conditions set forth in
				  Section 33-16-02-08 of the State Water Quality Standards.
While exceedences of acute chemical specific numeric
				  standards are not allowed within the entire mixing zone, a portion of the
				  mixing zone (the zone of initial dilution or ZID) may exceed acute
				  chemical-specific numeric standards established for the protection of aquatic
				  life. The ZID shall be determined on a case-by-case basis where the statement
				  of basis for the discharge permit includes a rationale for concluding that a
				  zone of initial dilution poses no unacceptable risks to aquatic life. Acute
				  whole effluent toxicity (WET) limits shall be achieved at the end-of-pipe with
				  no allowance for a ZID.
DILUTION ALLOWANCES
An appropriate dilution allowance may be provided in
				  calculating chemical-specific acute and chronic and WET discharge limitations
				  where: 1) the discharge is to a river or stream, 2) dilution is available at
				  low-flow conditions, and 3) available information is sufficient to reasonably
				  conclude that there is near instantaneous and complete mixing of the discharge
				  with the receiving water (complete mixing). The basis for concluding that such
				  near instantaneous and complete mixing is occurring shall be documented in the
				  statement of basis for the NDPDES permit. In the case of field studies, the
				  dilution allowance for continuous dischargers shall be based on the critical
				  low flow (or some portion of the critical low flow). The requirements and
				  environmental concerns identified in the paragraphs above may be considered in
				  deciding the portion of the critical low flow to provide as dilution. The
				  following critical low flows shall be used for streams and effluents:
Stream Flows
Aquatic life, chronic
4-day, 3-year flow (biologically based*)**
Aquatic life, acute
1-day, 3-year flow (biologically based)
Human health (carcinogens)
harmonic mean flow
Human health (non-carcinogens)
4-day, 3-year flow (biologically based) or 1-day,
							 3-year flow (biologically based)
Effluent Flows
Aquatic life, chronic
Mean daily flow
Aquatic life, acute
Maximum daily flow
Human health (all)
Mean daily flow
* Biologically based refers to the biologically based design
				  flow method developed by EPA. It differs from the hydrologically based design
				  flow method in that it directly uses the averaging periods and frequencies
				  specified in the aquatic life water quality criteria for individual pollutants
				  and whole effluents for determining design flows.
** A 30-day, 10-year flow (biologically based) can be used
				  for ammonia or other chronic standard with a 30-day averaging period.
For chemical-specific and chronic WET limits, an appropriate
				  dilution allowance may also be provided for certain minor publicly owned
				  treatment works (POTWs) where allowing such dilution will pose insignificant
				  environmental risks. For acute WET limits, an allowance for dilution is
				  authorized only where dilution is available and mixing is complete.
For controlled discharges, such as lagoon facilities that
				  discharge during high ambient flows, the stream flow to be used in the mixing
				  zone analysis should be the lowest statistical flow expected to occur during
				  the period of discharge.
Where a discharger has installed a diffuser in the receiving
				  water, all or a portion of the critical low stream flow may be provided as a
				  dilution allowance. The determination shall depend on the diffuser design and
				  on the requirements and potential environmental concerns identified in the
				  above paragraphs. Where a diffuser is installed across the entire river/stream
				  width (at critical low flow), it will generally be presumed that near
				  instantaneous and complete mixing is achieved and that providing the entire
				  critical low flow as dilution is appropriate.
OTHER CONSIDERATIONS
Where dilution flow is not available at critical conditions
				  (i.e., the water body is dry), the discharge limits will be based on achieving
				  applicable water quality criteria (i.e., narrative and numeric, chronic and
				  acute) at the end-of-pipe; neither a mixing zone or an allowance for dilution
				  will be provided.
All mixing zone dilution assumptions are subject to review
				  and revision as information on the nature and impacts of the discharge becomes
				  available (e.g., chemical or biological monitoring at the mixing zone
				  boundary). At a minimum, mixing zone and dilution decisions are subject to
				  review and revision, along with all other aspects of the discharge permit upon
				  expiration of the permit.
For certain pollutants (e.g., ammonia, dissolved oxygen,
				  metals) that may exhibit increased toxicity or other effects on water quality
				  after dilution and complete mixing is achieved, the waste load allocation shall
				  address such effects on water quality, as necessary, to fully protect
				  designated and existing uses. In other words, the point of compliance may be
				  something other than the mixing zone boundary or the point where complete
				  mixing is achieved.
The discharge will be consistent with the Antidegradation
				  Procedure.
IMPLEMENTATION PROCEDURE
This procedure describes how dilution and mixing of point
				  source discharges with receiving waters will be addressed in developing
				  discharge limitations for point source discharges. For the purposes of this
				  procedure, a mixing zone is defined as a designated area or volume of water
				  surrounding or downstream of a point source discharge where the discharge is
				  progressively diluted by the receiving water and numerical water quality
				  criteria may not apply. Based on site-specific considerations, such a mixing
				  zone may be designated in the context of an individual permit decision.
				  Discharges may also be provided an allowance for dilution where it is
				  determined that the discharge mixes with the receiving water in near
				  instantaneous and complete fashion. Such mixing zones and allowances for
				  dilution will be granted on a parameter-by-parameter and criterion-by-criterion
				  basis as necessary to fully protect existing and designated uses.
The procedure to be followed is composed of six individual
				  elements or steps. The relationship of the six steps and an overview of the
				  mixing zone/dilution procedure is shown in Figure 1.
Step 1 - No Dilution Available During Critical
				  Conditions
Where dilution flow is not available at critical low flow
				  conditions, discharge limitations will be based on achieving applicable
				  narrative and numeric water quality criteria at the end-of-pipe.
Step 2 - Dilution Categorically Prohibited for
				  Wetland Discharges
Permit limitations for discharges to a wetland shall be based
				  on achieving all applicable water quality criteria (i.e., narrative and
				  numeric, chronic and acute) at end-of-pipe.
Step 3 - Procedure for Certain Minor
				  POTWs
Minor POTWs that discharge to a lake or to a river/stream at
				  a dilution greater than 50:1 qualify for this procedure. Minor POTWs with
				  dilution ratios less than 50:1 may also qualify (at the discretion of the
				  permit writer) where it can be adequately demonstrated that this procedure
				  poses insignificant environmental risks. For the purposes of this procedure,
				  the river/stream dilution ratio is defined as the chronic low flow of the
				  segment upstream of the POTW discharge divided by the mean daily flow of the
				  POTW. For controlled discharges from lagoon facilities (discharging during high
				  flows), the river/stream dilution ratio is defined as the lowest upstream flow
				  expected during the period of discharge divided by the mean daily flow of the
				  discharge.
For minor POTWs that qualify for this procedure and
				  discharge to lakes, the allowance for dilution for chemical-specific and
				  chronic WET limits will be determined on a case-by-case basis. Dilution up to
				  19:1 (5 percent effluent) may be provided.
For minor POTWs that qualify for this procedure and
				  discharge to a river/stream segment, dilution up to the full chronic aquatic
				  life, acute aquatic life, and human health critical flows may be provided.
Step 4 - Site-Specific Risk
				  Considerations
Where allowing a mixing zone or a dilution allowance would
				  pose unacceptable environmental risks, the discharge limitations will be based
				  on achieving applicable narrative and numeric water quality criteria at the
				  end-of-pipe. The existence of environmental risks may also be the basis for a
				  site-specific mixing zone or dilution allowance. Such risk determinations will
				  be made on a case-by-case and parameter-by-parameter basis. These decisions
				  will take into account the designated and existing uses and all relevant
				  site-specific environmental concerns, including the following:
1. Bioaccummulation in fish tissues or
				wildlife
2. Biologically important
				areas such as fish spawning areas
3. Low acute to chronic ratio
4. Potential human exposure to pollutants
				resulting from drinking water or recreational areas
5. Attraction of aquatic life to the effluent
				plume
6. Toxicity/persistence of
				the substance discharged
7. Zone
				of passage for migrating fish or other species (including access to
				tributaries)
8. Cumulative effects
				of multiple discharges and mixing zones
Step 5 - Complete Mix
				  Procedures
For point source discharges to rivers/streams where
				  available data are adequate to support a conclusion that there is near
				  instantaneous and complete mixing of the discharge with the receiving water
				  (complete mix) the full critical low flow or a portion thereof may be provided
				  as dilution for chemical-specific and WET limitations. Such determinations of
				  complete mixing will be made on a case-by-case basis using best professional
				  judgement. Presence of an effluent diffuser that covers the entire river/stream
				  width at critical low flow will generally be assumed to provide complete
				  mixing. Also, where the mean daily flow of the discharge exceeds the chronic
				  low stream flow of the receiving water, complete mixing will generally be
				  assumed. In addition, where the mean daily flow of the discharge is less than
				  or equal to the chronic low flow of the receiving water, it will generally be
				  assumed that complete mixing does not occur unless otherwise demonstrated by
				  the permittee. Demonstrations for complete mixing should be consistent with the
				  study plan developed in cooperation with the states/tribes and EPA Region VIII.
				  Near instantaneous and complete mixing is defined as no more than a 10 percent
				  difference in bank-to-bank concentrations within a longitudinal distance not
				  greater than two river/stream widths. For controlled discharges (lagoon
				  facilities), the test of near instantaneous and complete mixing will be made
				  using the expected rate of effluent discharge and the lowest upstream flow
				  expected to occur during the period of discharge.
The following critical low flows shall be applied for streams
				  and effluents:
Stream Flows
Aquatic life, chronic
4-day, 3-year flow (biologically based*)**
Aquatic life, acute
1-day, 3-year flow (biologically based)
Human health (carcinogens)
Harmonic mean flow
Human health (non-carcinogens)
4-day, 3-year flow (biologically based) or 1-day,
							 3-year flow (biologically based)
Effluent Flows
Aquatic life, chronic
Mean daily flow
Aquatic life, acute
Maximum daily flow
Human health (all)
Mean daily flow
* Biologically based refers to the biologically based design
				  flow method developed by EPA. It differs from the hydrologically based design
				  flow method in that it directly uses the averaging periods and frequencies
				  specified in the aquatic life water quality criteria for individual pollutants
				  and whole effluents for determining design flows.
** A 30-day, 10-year flow (biologically based) can be used
				  for ammonia or other chronic standard with a 30-day averaging period.
Where complete mixing can be concluded and the environmental
				  concerns identified in step 4 do not justify denying dilution, but are
				  nevertheless significant, some portion of the critical low flows identified
				  above may be provided as dilution. Such decisions will take site-specific
				  environmental concerns into account as necessary to ensure adequate protection
				  of designated and existing uses.
Step 6 - Incomplete Mix
				  Procedures
This step addresses point source discharges that exhibit
				  incomplete mixing. Because acute WET limits are achieved at the end-of-pipe in
				  incomplete mix situations, this step provides mixing zone procedures for
				  chronic aquatic life, human health, and WET limits, and ZID procedures for
				  acute chemical-specific limits. Where a ZID is allowed for chemical limits, the
				  size of the ZID shall be limited as follows:
Lakes: The ZID volume shall not exceed 10 percent of the
				  volume of the chronic mixing zone.
Rivers and Streams:The ZID shall not exceed 10 percent of the
				  chronic mixing zone volume or flow, nor shall the ZID exceed a maximum
				  downstream length of 100 feet, whichever is more restrictive.
The following provides guidelines for determining the amount
				  of dilution available for dischargers that exhibit incomplete mixing.
Default Method
This method addresses situations where information needed for
				  modeling is not available or there are concerns about potential environmental
				  impacts of allowing a mixing zone. The default method provides a conservative
				  dilution allowance.
Stream/River Dischargers: Dilution calculation which uses up
				  to 10 percent of the critical low flow for chronic aquatic life limits or human
				  health limits. However, this allowance may be adjusted downward on a
				  case-by-case basis depending upon relevant site-specific information, designed
				  and existing uses of the segment, and especially the uses of the segment
				  portion affected by the discharge.
Lake/Reservoir Dischargers: Dilution up to 4:1 ratio (20
				  percent effluent) may be provided for chronic aquatic life analyses or human
				  health analyses. However, this allowance may be adjusted downward on a
				  case-by-case basis depending upon discharge flow, lake size, lake flushing
				  potential, designated and existing uses of the lake, and uses of the lake
				  portion affected by the discharge.
Modeling Method
An appropriate mixing zone model is used to calculate the
				  dilution flow that will allow mixing zone limits to be achieved at the critical
				  low flow. Prior to initiating modeling studies, it should be determined that
				  compliance with criteria at the end-of-pipe is not practicable.
Field Study Method
Field studies which document the actual mixing
				  characteristics in the receiving water are used to determine the dilution flow
				  that will allow mixing zone size limits to be achieved at the critical low
				  flow. For the purposes of field studies, "near instantaneous and complete
				  mixing" is operationally defined as no more than a 10 percent difference in
				  bank-to-bank concentrations within a longitudinal distance not greater than two
				  stream/river widths.
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