Code of Colorado Regulations
1000 - Department of Public Health and Environment
1002 - Water Quality Control Commission (1002 Series)
5 CCR 1002-31 - REGULATION NO. 31 - THE BASIC STANDARDS AND METHODOLOGIES FOR SURFACE WATER
Section 5 CCR 1002-31.10 - MIXING ZONES

Universal Citation: 5 CO Code Regs 1002-31 ยง 10

Current through Register Vol. 47, No. 5, March 10, 2024

(1) Definitions

(a) Physical Mixing Zone

That portion of a waterbody, surrounding or downstream from a point source of discharge, wherein constituents of the discharge are not uniformly dispersed into the receiving waters. The physical mixing zone also can be referred to simply as the "mixing zone," except where there is possible confusion with the regulatory mixing zone, as it is defined below, which differs from the physical mixing zone

(b) Exceedance Zone

That portion of a physical mixing zone within which a numeric water quality standard for a given water quality parameter is not met during critical conditions. The size of an exceedance zone may differ from one numeric standard to another at a given location.

(c) Regulatory Mixing Zone

The maximum size allowable for an exceedance zone at a given location. An acute regulatory mixing zone limits the size of exceedance zones for acute standards, and a chronic regulatory mixing zone limits the size of exceedance zones for chronic standards. The sizes of the acute and chronic regulatory mixing zones are related to the size of the receiving water, as explained in 31.10 (3).

(d) Stream Channel Width at Bankfull Stage

The width of a stream under flow conditions when the stream just begins to enter the lowest level of the floodplain.

(e) Average Waterbody Surface Area

The average surface area for a lake shall be determined from historic data (five years or more if possible), and must be computed monthly or seasonally, as appropriate, to reflect significant monthly or seasonal changes in area.

(f) Stream, Lake, Wetland

For purposes of this regulation, streams will include Waters of the State that flow, regardless of size, and lakes will include Waters of the State that are not flowing, including reservoirs. Wetlands will be treated in the same manner as lakes.

(2) Exemptions from Restriction of Permit Limits by Mixing Zone Regulations

In the following instances, water quality-based effluent limits (permit limits) for discharges to streams will be calculated using the full chronic (30E3) and acute (1E3) low flow of the stream for dilution except where a more stringent approach is determined by the Division to be necessary to protect designated uses in the waterbody as a whole based on the factors identified in subsection 31.10 . These exemptions do not apply to lakes.

(a) Exemption tables, other procedures developed or approved by the Division, or site-specific data indicate that the chronic regulatory mixing zone is larger than the physical mixing zone;

(b) The effluent flow at maximum permitted discharge is greater than twice the chronic low flow (30E3); or

(c) The ratio of the chronic low flow (30E3) to the maximum permitted or other appropriate effluent flow is greater than or equal to 20:1 and the operation is designated by the Division as a "minor."

(3) Regulatory Mixing Zone Sizes

(a) Streams

The Division shall consider the following factors in determining the sizes of the regulatory mixing zones for streams:

(i) The size of the chronic regulatory mixing zone for any point source of discharge to a stream shall not be greater than a plan view area equal to six times the square of the stream channel width at bankfull stage.

(ii) Where the size of the physical mixing zone exceeds the size of the chronic regulatory mixing zone, the area of the acute regulatory mixing zone for a water quality parameter shall be established between 10 % and 25 % of the area of the chronic regulatory mixing zone for the same water quality parameter. The size of the acute regulatory mixing zone will be determined within this range based on a presumption that:
(A) For waters determined under subsection 31.8 to be "reviewable," the default acute regulatory mixing zone will be 10% as large as the chronic regulatory mixing zone.

(B) For waters determined under subsection 31.8 to be "use protected," the default acute regulatory mixing zone will be 25% as large as the chronic regulatory mixing zone.

An acute mixing zone may also be further reduced below default limits for reasons given in subsection 31.10 . The permittee may request that the size of the acute regulatory mixing zone be higher than recommended by the Division, but no higher than 25% of the chronic regulatory mixing zone, on the basis of arguments related to cost/benefit analysis, economic reasonableness, ecological risks, use classification, or designation. The burden is on the permittee to bring appropriate information to the Division.

(iii) The sum total of the plan view areas of all chronic regulatory mixing zones for point sources of discharge into any reach of stream for a specified water quality parameter shall not occupy more than ten percent 10% of the total plan view area of such reach of river or stream, as measured at bankfull stage. The length (approximately 10 miles) and boundaries of the stream or river reach for these purposes shall be determined by the Division. Constraints on chronic regulatory mixing zones used to determine permit limits in discharge permits resulting from the cumulative impacts of multiple point sources of discharge into a stream reach shall be shared equitably among permittees and any other sources of discharge. The distribution of the allowable loads for the pollutant of concern shall be consistent with regulations applicable to total maximum daily loads and/or upon mutual agreement amongst the permittees.

(b) Lakes

The Division shall consider the following factors in determining the size of the regulatory mixing zones for lakes:

(i) For each point source of discharge, the size of the chronic regulatory mixing zone shall not be greater than 3% of the average inter-annual seasonal or monthly surface area. The Division may apply this limit to an entire lake or to a smaller, geographically distinguishable (bay, arm, etc.), portion of a lake.

(ii) Where the physical mixing zone exceeds the chronic regulatory mixing zone, the area of the acute regulatory mixing zone for lakes, for any water quality parameter, shall be established between 10% and 25% of the area of the chronic regulatory mixing zone for the same water quality parameter. The size of the acute mixing zone will be determined within this range based on a presumption that:
(A) For waters determined under subsection 31.8 to be "reviewable" the default acute regulatory mixing zone will be 10% as large as the chronic regulatory mixing zone.

(B) For waters determined under subsection 31.8 to be "use protected" the default acute regulatory mixing zone will be 25% as large as the chronic regulatory mixing zone.

An acute mixing zone may also be further reduced below default limits for reasons given in subsection 31.10 . The permittee may request that the size of the acute regulatory mixing zone be higher than recommended by the Division, but no higher than 25% of the chronic regulatory mixing zone, on the basis of arguments related to cost/benefit analysis, economic reasonableness, ecological risks, use classification, or designation. The burden is on the permittee to bring appropriate information to the Division.

(iii) The sum total of the plan view areas of all chronic regulatory mixing zones for point sources of discharge into lakes for a specified water quality parameter shall not occupy more than ten percent 10% of the total plan view area of such lake, or a geographically distinguishable portion thereof, at any seasonally average area. Constraints on chronic regulatory mixing zones used to determine limits in discharge permits resulting from the cumulative impacts of multiple point sources of discharge into lakes shall be shared equitably among permittees and any other sources of discharge. The distribution of the allowable loads for the pollutant of concern shall be consistent with regulations applicable to total maximum daily loads and/or upon mutual agreement amongst the permittees.

(iv) For artificial lakes supplied principally with potable water, mixing zones larger than those allowed above may be designated for purposes of CDPS permits. Appropriate mixing zone size limits shall be determined by the Division on a case-by-case basis, consistent with the constraints described in subsection 31.10 . Such mixing zones shall be kept as small as practicable, on a parameter-by-parameter basis, and shall provide for protection of existing and designated uses in the waterbody as a whole.

(4) Use of Mixing Zone Regulations in Setting Permit Limits

(a) Streams

Computation of chronic or acute permit limits for point source discharges to streams shall be as follows:

(i) For discharges not exempted as explained in subsection 31.10 , the permit limit for any parameter for which there is a water quality standard shall be that resulting in acute and chronic exceedance zones equal to or smaller than the respective acute and chronic regulatory mixing zones.

(ii) Where the annual acute low flow (1E3) of the receiving stream is zero, no dilution will be provided in calculating acute permit limits. Where the chronic low flow (30E3) of the receiving stream is equal to zero, no dilution will be provided in calculating chronic permit limits.

(b) Lakes

Computation of chronic or acute permit limits for point source discharges to lakes shall be as follows:

(i) The permit limit for any parameter for which there is a water quality standard shall be that resulting in acute and chronic exceedance zones equal to or smaller than the respective acute and chronic regulatory mixing zones as shown by site-specific analysis for each regulated substance.

(5) Additional Constraints on Mixing Zones

(a) Exceedance zones from multiple point sources of discharge shall not overlap to such an extent as to harm beneficial uses.

(b) Regulatory mixing zones shall comply with the narrative basic standards included in subsection 31.11 , except that these requirements do not apply to the protection of any sessile organisms residing within acute and chronic regulatory mixing zones.

(c) Where sampling shows that the conditions described in subsection 31.10 are not attained, the mixing zone analysis will be revised as necessary to achieve compliance with subsection 31.10 (3).

(d) The Division may limit or deny regulatory mixing zones on a site-specific basis for specific regulated substances. In doing so, the Division shall consider the following:
(i) The need to provide a zone of passage for aquatic life;

(ii) The likelihood of bioaccumulation of toxins in fish or wildlife;

(iii) The special importance of certain habitat such as fish spawning or nursery areas or habitat that supports threatened or endangered species;

(iv) Potential for human exposure to pollutants through drinking water or recreation;

(v) The possibility that aquatic life will be attracted to the effluent plume;

(vi) The potential for adverse effects on groundwater; or

(vii) The toxicity or persistence of the substance discharged.

(6) Mixing Zones for Whole Effluent Toxicity-based Permit Requirements

The provisions of this section 31.10 do not apply to the determination of whole effluent toxicity-based permit requirements.

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