North Dakota Administrative Code › Title 33.1 - Department of Environmental Quality › Article 33.1-16 - Control, Prevention, and Abatement of Pollution of Surface Water › Chapter 33.1-16-02.1 - 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 ten times the
				  stream width at critical low flows, whichever is more limiting. Also, at a
				  maximum, mixing zones in lakes shall not exceed five percent of lake surface
				  area or two hundred 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 exceedances 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 North Dakota pollutant discharge elimination system
				  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 (noncarcinogens)
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 the environmental protection agency. 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 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 l ow-flow
				  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 during critical
				  low-flow conditions.
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 publicly owned treatment
				  works
Minor publicly owned treatment works that discharge to a lake
				  or to a river/stream at a dilution greater than a 50-to-1 ratio qualify for
				  this procedure. Minor publicly owned treatment works with dilution ratios less
				  than a 50-to-1 ratio 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 publicly owned treatment works discharge divided by the mean daily flow of
				  the publicly owned treatment works. 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 publicly owned treatment works 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 a 19-to-1 ratio (five percent effluent) may be
				  provided.
For minor publicly owned treatment works 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 environmental protection
				  agency region VIII. Near instantaneous and complete mixing is defined as no
				  more than a ten 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 (noncarcinogens)
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 the environmental protection agency. 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 ten percent of the
							 volume of the chronic mixing zone.
Rivers and Streams:
The ZID shall not exceed ten percent of the chronic
							 mixing zone volume or flow, nor shall the ZID exceed a maximum downstream
							 length of one hundred 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 ten 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 a 4-to-1 ratio
				  (twenty 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 ten percent difference in
				  bank-to-bank concentrations within a longitudinal distance not greater than two
				  stream/river widths.
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General Authority:
NDCC 61-28-04; S.L.2017,
					 ch.199, § 1
Law Implemented:
NDCC
					 61-28-04