5 Colo. Code Regs. § 1002-31.53
The provisions of sections 25-8-202(1)(b), 25-8-204; and 25-8-402, C.R.S., provide the specific statutory authority for adoption. The Commission also adopted, in compliance with section 24-4-103(4) C.R.S., the following statement of basis and purpose.
BASIS AND PURPOSE
In this rulemaking the Commission considered revisions to criteria and revisions to implementation methodologies. The Commission adopted changes as detailed below.
In 2007, the Commission adopted temperature criteria and implementation methods for Colorado's surface waters. The criteria were derived from laboratory-based studies of individual fish species' tolerance to elevated water temperatures. The implementation methods were developed based on review of other states' methods and adaptation of methods for implementation of other water quality standards. Since that time, the Division and stakeholders have gained a great deal of experience with empirical records showing spatial and temporal patterns of temperature in surface water and effluent. Experience has shown that the adopted standards often are not attainable due to natural environmental constraints that are closely tied to elevation and may be affected by other factors as well. Consequently, revisions may be needed to incorporate those natural constraints that are an appropriate incremental improvement to the current standards. The revisions discussed in this rulemaking build on a decade of practical experience gained from massive data collection efforts and they chart a path forward to improve the basis for the standards, incorporate the effects of elevation on attainability and ensure more consistent implementation.
There are four parts to the temperature standards that were discussed in this hearing. The first, Part A, is a change to the definition of existing quality to clarify the implementation of exceedance frequency. The second, Part B, revises criteria to incorporate new information about the temperature tolerances of fish. Part C provides policy direction to address consideration of temperature standards at elevations below which the physiologically-based temperature standards are not attained routinely. Part D provides policy direction to address consideration of temperature standards in the shoulder seasons.
The Commission restructured the definition of existing quality (EQ) at 31.5(20) and modified the portion about temperature to allow one warming event above standards with a 3 year average exceedance frequency. EQ is a characteristic of the ambient condition that is used in two contexts:
The revised definition specifies that the value for EQ is the maximum DM and MWAT which allows for one warming event with a 3-year average exceedance frequency. The Commission recognizes the potential for natural systems to occasionally exceed numeric standards and that limited exceedances of the standard are expected. The Commission's intent is that thermal conditions should be sufficient for longer lived fish species to complete their lifecycles, and evidence derived from the literature suggests that 3 years is sufficient for most stream fish in Colorado. Additionally, the Commission recognizes that autocorrelation is inherent in stream temperatures, and that several days exceeding the standard may be the result of a single warming event. For standards attainment, the Commission intends that the average recurrence frequency of these warming events be limited to once every 3 years. (Table 1, footnotes 5a and 5b were edited to reflect this.) Therefore, where data records are 3 years or less, EQ will be the maximum DM or MWAT. For data records of 4 to 6 years, an allowance will be made for one warming event in either the summer or winter. For data records of 7 to 9 years two warming events are allowed. The definition of a "warming event" will be determined with statistically appropriate tests and representative data defined in the next 303(d) listing methodology process. In addition to consideration of the frequency of "warming events", the Commission would like the Division to look at the impacts of duration, multiplicity and cumulative effects.
For permitting, the Commission intends that EQ will also incorporate an allowable exceedance frequency for monthly determination. EQ will be the maximum DM or WAT with 3 or less years of representative upstream data. For data records with 4-6 years, the second highest monthly DM or WAT may be selected for one month in either winter or summer and the remaining months shall be the max DM or WAT. Allowances for each month are not appropriate because the allowable exceedance frequency (the recurrence interval) is based on the time that it takes for the aquatic community to recover from a harmful event.
The Commission retained the temperature excursions at 31.16 Table 1 - Footnote 5(c) so they could be addressed along with shoulder seasons and transition zones in a future rulemaking.
The requirement for "adequate refuge" has been awkwardly split between the temperature footnote (5(c)) and the dissolved oxygen footnote (9(c)). Footnote 5(d)(iii), the allowance for temperature exceedances in lakes where adequate dissolved oxygen is present below the mixed layer (the refuge allowance), was deleted. To maintain the requirement but simplify the regulation, in footnote 9(c), the reference to footnote 5(c)(iii) has been replaced by a clear statement that adequate refuge is required and a description of adequate refuge.
Temperature Database Updates: As part of the Division's routine review, the Colorado Temperature Database was updated using the most recent literature regarding the thermal requirements of Colorado's fishes. This effort was an initial step to support revision of the warm water winter acute values (discussed below) and also allowed for general updates of cold and warm water acute and chronic values. New acute and/or chronic thermal tolerance information was found for several species, both cold and warm water, including brook trout, brown trout, cutthroat trout, lake trout, mountain whitefish, rainbow trout, black crappie, bonytail, channel catfish, largemouth bass, mountain sucker, and stonecat. Based on this information, the Commission adopted revisions to the existing temperature standards found in Table I.
A new critical thermal maxima value for lake trout was added to the database as part of the updates. This new acute value, combined with existing chronic data, allowed for the derivation of DM and MWAT values for lake trout. Including lake trout in the Cold Lakes & Reservoirs and Cold Large Lakes & Reservoirs DM and MWAT calculations would result in MWAT values of 16.7°C for both tiers. Lake trout are currently managed in only 30 individual lakes/reservoirs, which are in a total of 17 segments; these segments comprise less than 9% of all lakes segments. Due to the relatively small number of segments containing lake trout, the Commission decided to not include the lake trout data in the derivation of statewide lakes/reservoirs temperature standards. Instead, the Commission adopted a footnote to Table I stating that where lake trout do occur and protection from thermal impacts is necessary and appropriate, the literature-based summer MWAT and DM for lake trout of 16.6°C and 22.4°C, respectively, should be applied. The Commission intends for these lake trout populations to be covered by the "adequate refuge" provision that requires concurrent attainment of the literature-based summer MWAT and DM values and dissolved oxygen standards.
A similar approach was taken for mountain whitefish. Early life stages of this species are known to be more thermally sensitive than other CS-I and CS-II species and adult mountain whitefish are known to migrate into cold tributaries to spawn. To ensure protection of sensitive early life stages, the Commission adopted a footnote to Table I stating that where and when spawning and sensitive life stages of mountain whitefish are known to occur, the literature-based summer MWAT and DM of 16.9°C and 21.2°C, respectively, should be applied.
Warm Water Winter Acute Table Values: When seasonal temperature standards were adopted in 2007, warm water winter acute and chronic standards were simply set at half the summer season values, recognizing a pattern seen in cold waters. The acute winter table values for warm water fish were revised based on lethal temperature thresholds established in laboratory experiments for fish acclimated to "winter" temperatures. This new method protects warm water fish in winter from acute effects. The Commission adopted the resulting warm tier temperature winter standards in Table I.
The physiologically-based summer temperature standards are not attainable in every year in every segment where they have been adopted. The attainability problem is not tied to specific watersheds or isolated locations, but is instead a statewide phenomenon that shows a clear spatial pattern related to elevation and could be affected by other factors. The problem arises from an unavoidable conflict between the historical distributions of fish species and the expectation that protective conditions for all life history stages can be sustained in every year throughout a segment. The environment varies naturally and fish move in response to environmental stimuli.
Temperature tiers have been adopted on the basis of the best available information concerning the fish species that have been found in the segment. The assignment of temperature tiers is logical and defensible, but an implementation problem arises if the assignment is accompanied automatically by the assumption that temperature standards are always attainable throughout the segment.
Water temperature in unimpacted streams is primarily governed by physical factors (e.g., solar radiation) that affect heat gain and loss, for which elevation is a practical surrogate. Current evidence shows that because of this natural phenomenon, maximum temperatures are expected to exceed the physiologically-based standards in some years at lower elevations for some temperature tiers.
The Division proposed a statewide elevation adjustment for the summer MWAT (the MWATelev) to define a modified expectation for maximum temperatures. The elevation range where the adjustment was proposed to be applied is called the transition zone. Several parties at the hearing disputed the sufficiency of the data presented by the Division, the extent to which anthropogenic influences were assessed and the validity of the Division's regression analysis. The Commission declined to adopt this approach in favor of a basin-by-basin consideration of attainability issues. This adjustment informs, but does not change, the narrative standard which requires maintenance of a normal pattern of increase and decrease in water temperature. The basin-by-basin approach will allow consideration of ambient -quality- based site-specific standards proposals in accordance with section 31.7 where elevation is the natural, irreversible factor. Unlike the basis for most other ambient-standards proposal, elevation occurs everywhere and has a predictable effect on water temperature. The basin-by-basin approach will provide an opportunity to consider this elevation adjustment as one of multiple lines of evidence and more specifically the basin hearings will provide for consideration of site-specific contravening evidence. The Commission intends for the experiences of this approach to inform potential changes to the Basic Standards in the future. However, the Commission does not intend that this approach is a de facto adoption of statewide standards through segment specific changes.
At this time, the Commission has not considered the same adjustment to the Daily Maxima temperature standards. Such an adjustment could be considered on a site-specific basis and future analysis may identify the same statewide attainability issues that can be addressed in future rulemaking.
Lakes
Temperature standards for lakes apply to the upper, mixed layer where water temperatures are governed by physical factors (e.g., solar radiation). Elevation may prove to be a useful surrogate for the suite of physical factors driving temperature in lakes. The Division presented evidence based on 574 lake-years of data from 116 lakes sampled over a broad range of elevations during the last 20 years. To be included in this analysis, a lake had to have been sampled during a 6-week period in mid-summer (11 July to 21 August) when maximum temperatures (MWAT) are expected. Several lakes showed evidence of anthropogenic influence in the form of "tailwater" effects from upstream reservoirs (e.g., Morrow Point) or very short retention times (e.g., Estes); these were excluded.
Regression analysis was used to define the relationship between summer MWAT and elevation. Lines for individual years were compared to assess interannual variability, which was small for the slope. The exceedance frequency was addressed by developing a regression line for the 66.7th percentile MWAT at each of the 33 lakes with at least 5 years of qualifying data. Elevation explains more than 90% of the variability in MWATs for the lakes analyzed in this hearing.
MWATelev = -0.001651 (elevation) + 32.43
At the time of the next routine review of each basin, the MWAT adjustment could be considered for lakes where the MWATelev is predicted to exceed the adopted standard. For example, the MWAT adopted for Cold Large Lakes currently is 18.3 oC, and the equation predicts that it is not routinely attainable in lakes at elevations below about 8560 ft and warm lakes below 3774 ft. This is consistent with the elevations of lakes for which site-specific temperature standards have already been adopted.
Streams
Like lakes, water temperatures in streams are governed by physical factors and elevation may be a useful surrogate for these factors. The Division presented evidence from analysis of water temperature records from 267 sites in Colorado over a broad range of elevations and throughout Colorado's varied landscape. Data from approximately 1162 site-years was used to examine the relationship between summer maximum temperatures and elevation. All sites were screened for likely anthropogenic influences from waste water treatment facilities and reservoirs (tailwaters). Of 10 different physical and geographic watershed and site attributes, site elevation most strongly predicts annual MWATs for the analyzed sites. Additionally, residuals (unexplained variance) from the relationship between each year's MWAT and elevation were analyzed to determine whether the remaining variance was related to the following attributes: slope, aspect, Strahler stream order, percent canopy cover, 30-year max air temperature, CHILI Index (an index of solar radiation, slope, latitude and aspect), watershed area, upstream active diversions count, and sum of absolute and conditional diversion rates. Regression analysis between the summer MWAT and elevation showed that over 80 percent of the variance is explained by elevation alone. Annual variability was examined by comparing the relationships for individual years; slopes were in close agreement. The exceedance frequency was addressed by developing a regression line for the 66.7th percentile MWAT at each of the 79 sites with at least 5 years of data. This value is an interpolated estimate of the once in three year exceedance value of existing quality. The resultant equation is:
MWATelev = -0.002145 (elevation) + 32.97
At the time of the next routine review of each basin, the MWAT adjustment could be considered for sites in the transition zone along with other lines of evidence. For example, for a site in a Cold Stream Tier II segment at 6800 feet elevation, the MWATelev of 18.5oC could be the operative standard instead of the 18.3oC standard for the segment.
For each temperature tier, there are summer and winter criteria, and the shift from one season to the next occurs abruptly on a single date. The rigid, first-of the-month changeover of seasons does not reflect the natural pattern of gradual, predictable change in temperature, nor does it provide flexibility to allow for inter-annual variability in the timing and rate of temperature change. These two factors reflect the natural constraints on temporal patterns of water temperature in streams and lakes, partially as a function of elevation.
The Division proposed to revise the table values for each stream and lake temperature tier to substitute the existing narrative standard for the months on either side of the transitional date (i.e., the shoulder seasons). Support for applying the narrative standard was provided by the elevation-related trend in the duration of winter (i.e., consecutive days below the adopted winter standard) and the natural variability documented for the fall and spring transition dates at individual sites. The Commission declined to adopt this approach, in favor of a basin-by-basin consideration of these issues. The Commission intends for the experiences of this approach to inform potential changes to the Basic Standards in the future. However, the Commission does not intend that this approach is a de facto adoption of statewide standards through segment specific changes.
One approach that could be considered in hearings at the basin level is revising the segment-specific standards so the numeric criteria would apply only for the core winter and summer months. The narrative standard would continue to require a normal pattern with no abrupt changes.
Attainment of the narrative standard during the fall and spring could then be assessed for 303(d) purposes by determining the direction of the general temperature trend, using the average WAT of each month. If the surface water is cooling or warming at the appropriate season, then it would not result in an exceedance of the narrative temperature standard.
For the purposes of implementation in permits, the intent would be to ensure that the natural seasonal progression is maintained. For each of the months in the shoulder seasons, simple linear interpolation could be used to establish a value for the water quality standards that could be used in the mass balance equation for setting permit limits.
To protect human health, the Commission adopted a methylmercury fish tissue basic standard at new subsection 31.11 and revised Footnote 6 to Table III (Metal Parameters) at 31.16. This water quality criterion of 0.3 milligrams (mg) methylmercury per kilogram (kg) fish tissue wet weight describes the concentration of methylmercury that protects consumers of fish and shellfish among the general population. The criterion is consistent with EPA's section 304(a) water quality criterion for methylmercury. This new standard applies to all waters of the state because fish migrate and contribute to food webs that integrate large geographic areas; therefore, it is not sufficiently protective to apply the standard only in locations where fish are expected to be caught and consumed.
Adoption of this threshold as a standard in Regulation #31 recognizes the Commission's practice in the context of Regulation #93 (Colorado's Section 303(d) List of Impaired Waters and Monitoring and Evaluation Lists). The Commission has made listing decisions using an average fish tissue criterion of 0.3 mg/kg as a numeric threshold for determining attainment of the aquatic life use.
Adoption of the 0.3 mg/kg methylmercury criterion does not represent a policy change. The current water column standard of 0.01 µg/L total mercury remains in place and is intended to be implemented alongside the fish tissue standard. The Commission expects that in some circumstances, site-specific water column standards may be developed where data are available.
After the 2010 rulemaking hearing, EPA disapproved a modification of Footnote 14 to Table III (Metal Parameters) which applies to arsenic. This footnote stated that the arsenic effluent limits would be calculated so that the arsenic concentration at the point of intake to the domestic water supply would not exceed the standard. EPA disapproved this concept because standards must protect the designated use, whether or not the use is an "actual" use. In today's action the Commission deleted Footnote 14 and renumbered the remaining footnotes and deleted the reference to Footnote 14 in Table III. The Commission found that in the majority of segments, the footnote has no effect. Most segments have a water+fish standard for arsenic that is more stringent than the water supply standard.
After the 2010 rulemaking hearing, EPA disapproved a modification of Footnote 4 to Table II (Inorganic Parameters) which applies to nitrate. As in the arsenic footnote described above, this footnote stated that the combined total of nitrate plus nitrite at the point of intake to a domestic water supply would not exceed 10 mg/L. EPA disapproved this concept because standards must protect the designated use whether or not the use is an "actual" use. In today's action the Commission repealed Footnote 4 with a delayed effective date of December 31, 2022. A delayed date allows time for stakeholders to bring forward site-specific proposals for use removal and/or resegmentation in the next round of basin hearings, and also time to obtain permit modifications before the footnote repeal date.
The Commission adopted an acute chlorine standard of 0.019 mg/L for Class 2 waters to protect aquatic life. In 2005, the chronic chlorine standard of 0.011 mg/L was adopted for Class 2 waters, and it is unclear why an acute standard was not also adopted at that time. Because chlorine is a fast-acting toxicant, both acute and chronic chlorine standards are necessary to protect the aquatic life use.
The Commission adopted revisions to 31.8(3)(c) to clarify the procedures for segments where the antidegradation designation changed from Use Protected to undesignated (i.e. Reviewable) after the previously established baseline date of September 30, 2000. The revision added the phrase "or the effective date when the Use Protected designation is removed." At the same time, subsection 31.8 was split into two sections for ease of application.
The Commission revised the regulatory language to clarify that short-term degradation associated with certain types of activities is consistent with the Outstanding Waters designation. The Commission does not intend this to change policy or procedures regarding determining the meaning of waters being "maintained and protected at their existing quality."
Examples of activities that result in long-term ecological or water-quality benefit include, among others: use of rotenone or other pesticides to remove invasive species; construction of fish barriers to prevent the spread of non-native species; construction of bridges at stream crossing to minimize damage to the stream and improve water quality; or construction of aquatic habitat improvement.
A determination that activities will result in only "short-term" degradation will occur as part of a permitting or 401 certification action by the Division. It is difficult to give an exact definition of "short-term" because of the variety of activities that might be considered. However, in broad terms, "short-term" should be weeks and months, not years. In some cases, projects may need to extend over multiple work seasons, but in all cases the impacts of a project over time must be considered. The Commission expects that in those actions the Division will ensure that conditions are imposed as necessary and appropriate to ensure that degradation occurs for the shortest amount of time possible.
Examples of "clear public interest" activities shall only be those that address public health, welfare and safety which could include in some cases: construction of public roads for the purpose of public safety, maintenance of public roads, bridges and roadways, including shoulder weed control; control of mosquitoes or other disease vectors; enhancement of significant historical and archaeological resources; and suppression of wildfires or fire pre-suppression or restoration activities.
The Commission revised section 31.8 and added two new subsections (i) and (ii) to exempt dissolved iron, dissolved manganese, and sulfate from antidegradation consideration. Federal requirements for antidegradation protection only extend to assimilative capacity for criteria that protect CWA § 101(a)(2) uses (commonly known as "fishable/swimmable"). Dissolved iron and manganese and sulfate do not fall in those categories; rather they are water supply standards which originated as secondary Safe Drinking Water Act criteria. The Colorado framework treats these secondary water supply parameters differently.
The criteria for iron, manganese and sulfate remain in place, unchanged, to protect the water supply use. These criteria do not act as surrogates for any criteria that would protect a fishable/swimmable use (e.g., chloride acts as a surrogate for an aquatic life criterion). This exemption does not negate the requirement for an antidegradation review in regards to standards that protect other classified uses.
After the 2010 rulemaking hearing, EPA disapproved a modification of section 31.8 (b)(i)(c) which allows the Commission to designate a waterbody as Use Protected if the waterbody was effluent-dominated or effluent-dependent during the period of 2000-2009. EPA disapproved this concept because federal policy is that antidegradation designations are to be made based on the quality of the water, not on the source of the water.
In today's action the Commission repealed section 31.8 with a delayed effective date of December 31, 2019. In taking this action, the Commission considered that for all reviewable waters, affected entities have an opportunity to submit an alternatives analysis (i.e., to support decisions regarding whether allowing water quality is necessary to accommodate important economic or social development). But the Commission also acknowledges stakeholder concerns regarding uncertainty about the process and criteria for alternatives analyses. Therefore, the Commission is repealing the provision with a delayed effective date to allow the Division and interested stakeholders time to work together to review alternative analyses submittals and approvals that have been done to date, and discuss whether a new alternatives analysis guidance document should be developed, and if so, to develop guidance prior to the repeal date. The delayed effective date is also intended to allow the Division and interested stakeholders time to engage in further discussions regarding an appropriate water quality test for effluent-dependant and effluent-dominated waters. The Commission may consider a proposal to amend or replace section 31.8 in a rulemaking before the repeal effective date.
The Commission added a sentence to section 31.8 to better align the Basic Standards rule with the recently-revised EPA water quality standards regulation. This modification was adopted because the Colorado antidegradation rule did not explicitly address what outcome is required in situations where, as part of a necessity of degradation determination, one or more non-degrading or less degrading alternatives are identified. It now explicitly requires selection of a non-degrading or less degrading alternative. The Commission does not intend this to change current Colorado policy or procedures.
Substantial changes were made to the portions of the Basic Standards that address the way the standards are implemented in discharge permits. Many provisions that were in 31.14 were deleted to reduce redundancy with other regulations (namely, Regulation #61, "Colorado Discharge Permit System Regulations") and to eliminate language that has outlived its useful life. Other provisions were moved to section 31.9 , to consolidate the provisions that address implementation of standards. Section 31.10 continues to contain the provisions that address Mixing Zones.
Restructuring: The title of section 31.9 was changed from "Flow Considerations" to "Implementation of Standards." Even before today's rulemaking, the section contained provisions that went beyond flow considerations. Most of the material from section 31.14 that was deemed to be still relevant was moved to section 31.9.
Results of Review of 31.14 : Section 31.14 now is blank and the section is "reserved." The history of each subsection, its origin (where known), and fate are described below:
* 31.14(1): This section pre-dates 1987 and there is no record of how or why this section was added to the Basic Standards. It appears to never have been used. The reasons behind the reference to Regulation #71 (the Dillon Control Regulation) are unclear. For these reasons, this section was deleted.
* 31.14(2): This section pre-dates 1987 and there is no record of how or why this section was added to the Basic Standards. It was deleted because it is redundant with section 61.8 , and is also in the federal rules for state programs at 40 CFR § 130.3.
* 31.14(3): This section pre-dates 1987 and there is no record of how or why this section was added to the Basic Standards. It was deleted because it is redundant with section 61.8 , and is in the federal rules at 40 CFR § 130.7.
* 31.14(4): This section pre-dates 1987 and there is no record of how or why this section was added to the Basic Standards. The portion that authorizes Compliance Schedules was moved to 31.9(2) and expanded to match the language in Regulation #61. The portion that states that effluent limits "may" be established was deleted because there was a conflict between the Regulation # 61 version ("must") and this version ("may"). The portion that describes how effluent limits shall be established was moved to Regulation #61 to replace an existing cross-reference. The statement that a rulemaking hearing can subsequently be held was moved to the statement of basis and purpose provisions of Regulation #61.
* 31.14(5): This section was added in 1988 (see 31.24.I). The "innovation" language was added to 31.3 at the same time that this provision was added to 31.14. In order to capture the concept of using innovative approaches, such as trading programs, in various water quality contexts, the language "TMDLs, Waste Load Allocations antidegradation reviews, and permits" is also being added to 31.3. Section 31.14 is generally redundant with the concepts in 31.3 and is also captured at 61.8(3)(r) of Regulation #61. A new section was also adopted during this rulemaking proceeding at 61.8(3)(u) to capture the "innovation" concept in the context of permits, and thus this section 31.14(5) was deleted.
* 31.14(6): There is no record of when this section was added. Section 61.8 addresses this concept, and thus this section 31.14(6) was deleted.
* 31.14(7): This section was added in 1987 (see 31.22 C). This section is now redundant with Regulation #61, 61.8(2)(B)(vii), and thus this section 31.14(7) was deleted.
* 31.14(8): This section was added in 1988 (see 31.24 E and F). This material is covered in sections 31.7 , 31.9 and 31.16, and thus this section 31.14(8) was deleted.
* 31.14(9): This section was added in 1989 (see 31.25 E). This section was deleted because practical quantification limits (PQLs) are now covered in a separate policy.
* 31.14(10): This section was added in 1989 (see 31.25 E). Section 61.8 of Regulation #61 addresses this concept, and thus this section 31.14(10) was deleted.
* 31.14(11): This section was added in 1989 (see 31.25 E) when organic standards were added to Regulation #31. This section was deleted because this authority is already provided to the Division. It serves no purpose substantive now, and thus was deleted
* 31.14(12): This section was added in 1989 (see 31.25 E). Section 61.8 of Regulation #61 addresses this concept, and thus this section was deleted.
* 31.14(13): This section was added in 2000. The Division is not aware of any current permits that have implemented this provision. Colorado's intake credit provisions are found at section 61.8 of Regulation #61. It is not clear how this provision is intended to be used, and thus it was deleted.
* 31.14(14): This section was moved to 31.9.
* 31.14(15) and (16): These sections were consolidated and were moved to 31.9. The Commission made revisions to these provisions to align them with the Division's practice since 2007, as expressed in various basin regulations for implementing "current condition" temporary modifications. Specifically, the Commission added references to "existing discharges" to clarify that effluent limits based upon temporary modifications only apply to existing discharges, and that effluent limits for new and expanded discharges must generally be set to the underlying standard. Additionally, the previous reference to 31.14(4) was deleted because all compliance schedules must be issued in accordance with the provisions authorizing compliance schedules.
* 31.14(17): This section was moved to 31.9. The phrase "compliance schedule" in subsection (a) was changed to "permit condition" to allow more flexibility for permitting approaches.
The Commission adopted revisions to section 31.7 that identify two types of ambient-based standards, "feasibility-based" and "natural or irreversible quality-based" standards, to recognize that in some cases water quality can be improved, but not to the level required by the table value.
Where the only sources and causes of the pollutant(s) are natural, ambient quality-based ambient standards continue to be the Commission's preference. However, where the sources and causes are to some extent anthropogenic, more clarity is needed to assure that classifications and standards are set to protect the highest water quality attainable.
The provision (the downgrading factors) that provides the authority for ambient-based standards is based on the same provisions that authorizes discharger-specific variances (DSVs) (40 CFR § 131.10(g) and 31.6(2)(b)), except that the cause is not a permitted point source, and this action would apply to the entire segment. Since it is the same regulatory foundation, it is appropriate to use the same feasibility bar for determining what improvements are appropriate. As with DSVs, this type of change to numeric standards is authorized only where a comprehensive alternatives analysis demonstrates that there are no feasible alternatives that would provide better water quality.
The Commission continues to believe that adopting ambient standards for a constituent(s) is preferable to downgrading or removing entire uses and their associated water quality standards. Adopting an ambient standard in effect creates a sub-category of the use and is a regulatory downgrade. These ambient standards protect the highest attainable use and are consistent with 31.6(1)(e), which requires that classifications should be for the highest water quality attainable. To that end, "highest attainable use" was defined and added to section 31.5.
The revisions also provide clarity regarding the analysis and documentation that is required to make the "no feasible alternatives" demonstration. The Commission encourages proponents to complete the Division's checklist to ensure that their supporting information is adequate.
The Commission revised section 31.7 to incorporate a new subsection (d) that explicitly addresses the operative value that is in place during the term of a temporary modification. These changes recognize current policy and are not meant to change that policy, only to clarify and expressly approve its use. This change authorizes the use of the narrative statement "current condition" as the operative value to preserve the status quo for the discharger and the waterbody during the term of the temporary modification. The Commission indicated that if the standards database can be adjusted to accommodate it, that future proposals for temporary modifications should include in the table the date on which the temporary modification was adopted. Temporary modifications are only appropriate where a compliance problem exists, and the adoption of the temporary modifications are intended to temporarily relax the control requirements, including direct discharge permits, indirect discharge permits, and other control mechanisms such as local limits while the uncertainty regarding the underlying standards is addressed. The Commission recognizes that during the temporary modification permitted dischargers' effluent quality may be marginally changed and that variability in effluent quality may occur. Because the status quo is to be maintained, the Commission does not intend that temporary modifications set at "current condition" apply to new or expanded discharges. Protection of existing uses means protection of the actual uses rather than protection of the full use classification. The Commission intends that the revisions to section 31.7 apply prospectively only, and do not retroactively change the basis for or implementation of previously adopted or extended temporary modifications set at "current conditions."
The Commission revised section 31.7 to clarify that the Division, not the Commission, sets the alternate effluent limits of a discharger-specific variance, and that these limits are to be expressed as a temporary hybrid standard. The hybrid approach establishes a cap on the effluent limit, but does not actually set the level of the effluent limit. The Commission added three new subsections (i), (ii) and (iii) to describe the format of the hybrid standard and how it is used by the Division to set control requirements such as discharge permit effluent limitations.
Based upon the results of a comprehensive alternatives analysis, the Commission will determine specifically which alternative(s) provide the highest degree of protection of the classified use that is feasible. The alternative effluent limit establishes conditions to be met through implementation of the selected alternative(s). The Commission expects that in most cases, the alternative effluent limit will be a numeric limit. In cases where there is a high degree of uncertainty regarding the improvement or effluent concentrations that will be achieved, the Commission may adopt an alternative effluent limit as a narrative condition that identifies specific actions to be completed through implementation of the selected alternative(s).
The Commission adopted modifications at section 31.3 to more clearly identify that water quality classifications and standards must protect downstream waters. In the past, the Commission and Division have relied on section 31.6 and Regulation #61 to provide this protection. This modification implements 40 CFR § 131.10(b) and is not intended to change Colorado's current practice that already considers and ensures the protection of downstream water quality during the development of designated uses and water quality standards.
The Commission added clarification to a number of items and corrected minor typographical errors:
* Definition of MWAT and WAT: The definitions of Maximum Weekly Average Temperature (MWAT at 31.5(26)) and Weekly Average Temperature (WAT at 31.5(50)) were clarified. The MWAT definition was shortened and does not repeat the details that are in the WAT definition. The word "mean" was inserted in the WAT definition to clarify that the WAT is calculated from daily average temperatures. This is consistent with the current implementation methods of the Permits and Assessment. The words "multiple" and "equally spaced" in the WAT definition were removed to reflect current assessment methodology.
* 31.6(4)(b): A missing parenthesis was added to this subsection.
* 31.6(2)(b)(iv): The phrase "result in attainment or the use" was to corrected to "result in attainment of the use."
* 31.7(3)(a)(ii)(C): This section was deleted as it describes a condition for granting a temporary modification that is addressed through the discharger-specific variance provisions, and was repealed effective 10/01/2013.
* 31.11(3): The content of Footnote 5 to the Table of Basic Standards for Organic Chemicals was deleted as unnecessary and replaced with the word "deleted." The Commission notes that practical quantification limits are now located in a Division policy document and not in Regulation #61.
* 31.16 Table III - Footnote 3: The word "aluminum" was added to replace the chemical abbreviation, and a space was deleted.
* 31.16 Table III - Footnote 5: The word "total" was deleted from the phrase "50 µg/L total chromium" to clarify that the sum of hexavalent and trivalent chromium is not to exceed 50 µg/L. Capitalization, spacing, and symbol use was also corrected for portions of this footnote.
PARTIES TO THE RULEMAKING
5 CCR 1002-31.53