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 1002-31.38 - STATEMENT OF BASIS, SPECIFIC STATUTORY AUTHORITY AND PURPOSE: OCTOBER, 2000 CONTINUATION OF JULY, 2000 RULEMAKING

Current through Register Vol. 47, No. 17, September 10, 2024

The provisions of 25-8-202(1)(a), (b) and (2); 25-8-203; 25-8-204; 25-8-209 and 25-8-402 C.R.S. provide the specific statutory authority for adoption of these regulatory amendments. The Commission also adopted, in compliance with 24-4-103(4), C.R.S., the following statement of basis and purpose.

Basis and Purpose:

This statement of basis and purpose addresses the revised mixing zone provisions in section 31.10 , adopted by the Commission as the result of the October, 2000 continuation of the July, 2000 Basic Standards rulemaking.

Permit limits for point sources of discharge have been determined in Colorado based on the assumption that mixture of the discharge with the receiving water is instantaneous. While this assumption simplifies the preparation of permits, studies conducted by the Division and others have shown that the mixture of a point source discharge with a receiving water occurs over a period of time and therefore occupies a space within which full mixing has not occurred. This space, which is called the "physical mixing zone," may show concentrations of regulated substances that exceed the acute or chronic water quality standards applicable to the receiving water. The area within a physical mixing zone where a water quality standard for a given constituent is exceeded is referred to in the regulation as the "exceedence zone" for that constituent. To be fully protective of the designated uses of Waters of the State, the permit limits for point sources of discharge need to take into account not only the numeric standards that apply to the fully mixed condition, but also the appropriate maximum size for exceedence zones.

The Commission recognizes the need to limit the size of exceedence zones associated with point sources of discharge. The allowable size of the exceedence zone for a chronic water quality standard in the vicinity of a particular discharge is referred to as the "chronic regulatory mixing zone" for that particular parameter. The allowable size of the exceedence zone for an acute water quality standard for the same point source of discharge is the "acute regulatory mixing zone."

The Commission has chosen to treat mixing zones in streams differently from such zones in lakes - the rationale being that mixing in lakes is significantly more complex than mixing in streams- by limiting the use of exclusions to discharges to streams. However, a common approach, allowing the exceedence zone to occupy a limited plan view area of the water body, will be used in both cases.

The sizes of both chronic and acute regulatory mixing zones for streams in Colorado are based on an area that is a function of the "bankfull" stream width, rather than a distance from the discharge. In this way, zones of exceedence for acute and chronic standards in streams are limited to a proportionally small area of the aquatic environment in the vicinity of a discharge. The size of the mixing zone for lakes has been limited to three percent of the surface area of the lake, or a geographically identifiable aspect of the lake, so that, as with streams, the exceedence of water quality standards is limited to a relatively small area of the aquatic environment. Furthermore, the mixing zone regulation limits the cumulative area of exceedence zones resulting from multiple discharges along a reach of stream or in a lake. Finally, the regulation allows for further limitation or denial of a regulatory mixing zone where the use of such a zone, even though small, could create an unacceptable risk of impairment to beneficial uses or damage aquatic habitat of special value.

The Commission has determined antidegradation analyses conducted pursuant to subsection 31.8 are not to be conducted within mixing zones established in a CDPS permit. In addition, for purposes of determining impairment of a waterbody, the Commission will not consider ambient lake or stream data that has been collected within a mixing zone where such mixing zone has been established in a permit using site-specific in-stream measurements. Finally, the Commission has decided not to apply these mixing zone regulations to the determination of whole effluent toxicity (WET) requirements in permits as this issue is appropriately addressed in the Division's WET guidance. The Commission expects the Division to consider the application of mixing zone requirements to the determination of WET permit limits in revisions to the WET guidance that will be made as soon a practicable in conjunction with other necessary revisions to the WET guidance.

The Commission recognizes that adoption of this mixing zone regulation will add complexity to the preparation of permits and to the evaluation of future treatment requirements by dischargers. As a means of minimizing costs and delays associated with this additional complexity, the Commission has included a number of exclusions in the mixing zone regulation that it deems to be consistent with the protection of beneficial uses. As previously mentioned, the exclusions do not apply to discharges to lakes, as the simplifying assumptions that can be applied to mixing of discharges to streams are not relevant to discharges to lakes. Consequently, a mixing zone in the vicinity of a discharge to a lake must be established based on a site-specific mixing zone analysis. Exclusions will be determined based on combinations of physical characteristics of streams (discharge flow rate, stream slope, channel width, etc.) under which the rate of mixing of discharge and receiving stream is so rapid that the application of the mixing zone regulation would be highly unlikely to result in any significant modification of permit limits. For minor discharges, exclusions from the regulatory requirements for mixing zones and avoidance of costs associated with such requirements are allowed where the ratio of effluent discharged to the flow of the receiving water is low as the likelihood of a relatively large exceedence zone is small.

During the rulemaking proceeding, the Commission received testimony upon how the mixing zone provisions could prove problematic for a limited category of man-made water storage facilities utilized as urban recreation and aesthetic amenities and filled primarily with chlorinated potable water. Subsection 31.10 has been added to the rule in order to address this situation and accommodate the needs of the entities that manage these water bodies, on a case-by-case basis, such that they can continue to be filled with potable water and used as they have been historically.

The Commission expects the Division, in cooperation with a stakeholder group, to prepare guidance for the implementation of this regulation. The guidance should include detailed descriptions of procedures that are to be used to collect measurements (e.g. bankfull width) that can be used to determine the applicability of mixing zone requirements to the discharge. The guidance will be noticed for an informational hearing before the Commission. The Commission recognizes that the procedures developed to determine the applicability of exclusions may be somewhat conservative initially. As the Division and dischargers collect more data on mixing zones and the understanding of mixing in streams improves, the Commission expects the guidance to be adjusted where methodologies for determining the applicability of exclusions can be refined.

The Commission also has incorporated directly into the regulation certain assumptions and simplifications, to the extent that these are consistent with protection of beneficial uses. Most importantly, the regulation allows a single value for the size of the physical mixing zone to be used for all low-flow conditions, and directs the Division to include procedures by which this value can be estimated in the aforementioned guidance. Once the size of the physical mixing zone has been determined, it will be used, in the first instance, to determine if the size of the exceedence zone for the relevant chronic standard must be reduced. Where the size of the physical mixing zone is smaller than the chronic regulatory mixing zone, then mixing is implied to be fairly rapid, further analysis of both the chronic and acute mixing zone parameters will not be required, and the full low flow of the receiving stream will be used to calculate water quality standards based permit limits.

While use of exclusions and assumptions reduce the total burden of the mixing zone regulation on the Division and on permittees, some permits will require a full, site-specific, evaluation. A site-specific evaluation may show that a permit will not be affected by the mixing zone regulation, or may show that certain permit limits will be reduced through application of the regulation. The regulation emphasizes the importance of field data for site-specific evaluations. The guidance will allow for the direct use of field data, without the necessity for complex water quality modeling, in site-specific evaluations. Dischargers wishing to use modeling may do so, but models should be calibrated for site-specific conditions from field data. Modelling without calibration with field data will not be considered a sufficient basis for a site-specific evaluation.

The requirements prepared by the Division for site-specific evaluations will be as simple as possible and will not require a high degree of precision, but must constitute a valid estimate of true conditions upon which the adjustment of permits can be based. Although the technical and financial burden of carrying out site-specific evaluations will fall on dischargers, site-specific studies need not be repeated at every permit cycle unless there is a significant change in volume of discharge, a physical change in the receiving water, or evidence of error in the original analysis.

The mixing zone regulation for Colorado acknowledges the existence of incomplete mixing near point sources of discharge and properly limits the extent of any exceedence of standards that might occur within the mixing zone. The regulation is a means by which protection of beneficial uses of water and aquatic habitat in close proximity to point sources of discharge can be achieved without unnecessarily restricting permit limits to maintain standards in a relatively small area of the receiving water in the vicinity of the discharge.

PARTIES STATUS/MAILING LIST STATUS TO THE RULEMAKING HEARING