The provisions of C.R S. 25-8-202(1)(a), (b) and (2); 25-8-203; 25-8-204; and 25-8-402; 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
The Commission revised the site-specific narrative nutrient criteria to include numeric standards for chlorophyll and total phosphorus for Bear Creek Reservoir (Bear Creek segment 1c).
Current Review: The Commission directed the Division to undertake a technical review of the scientific basis for the Bear Creek Reservoir narrative nutrient standard and Control Regulation (Regulation #74) for the following reasons:
A. There are no numeric goals for assessing water quality conditions related to excessive algal growth.B. There is no firm basis for determining what level of nutrient control is consistent with the water quality goals.C. The allowable load is not specified in the Control Regulation, making it impossible to determine the appropriateness of allocations.D. There is an implied acceptance of aeration as a permanent basis for treating the symptoms of algal productivity that is higher than the target specified in the narrative standard.The technical review showed:
A.Current Condition: Water quality has been monitored in Bear Creek Reservoir since 1987. The reservoir is more productive than allowed by the existing narrative standard, which specifies a target trophic condition between mesotrophic and eutrophic. A more productive condition has been sustained despite significant reductions in external phosphorus load. The present level of productivity would cause depletion of hypolimnetic oxygen (also contrary to the narrative standard) if aerators were not operated to destratify the reservoir.B.Characterizing Chlorophyll: Chlorophyll concentrations declined after phosphorus loads were reduced. Since 1995, typical summer average chlorophyll is about 24 µ g/L, but there are large differences among years. The differences appear to be associated with hydraulic residence time such that the highest average chlorophyll concentrations tend to be the years of longest residence time (lowest inflow).C.Role of Internal Phosphorus Load: External phosphorus loads were reduced significantly in the early 1990s largely through efforts made by domestic dischargers to control effluent phosphorus concentrations. As a result, phosphorus concentrations at the beginning of summer are relatively low. However, through the process of internal release, phosphorus concentrations increase steadily through the summer months. The net effect of internal release is more conspicuous in low-flow years, because high inflows provide more dilution. Over the long term, internal release should diminish because the external load has been reduced, but it could take 10-15 years until internal release becomes negligible.D.Characterizing Phosphorus: Phosphorus concentrations declined sharply after controls were imposed in the early 1990s. Since 1995, typical summertime concentrations of phosphorus have been about 44 µ g/L, but there are large differences among years. Differences are associated with hydraulic residence time as mentioned previously for chlorophyll. It is appropriate to set a numeric standard for phosphorus because of its importance in characterizing trophic condition, and because it is the direct link to the control regulation.E.Defining a Chlorophyll-Phosphorus Linkage: The simple ratio of chlorophyll to phosphorus defines the site-specific, net responsiveness of the resident algal community to the availability of phosphorus. It is a "net" value because it reflects the balance of growth (nutrients, light, temperature) and loss (grazing, washout, settling) processes. For the purpose of linking chlorophyll and phosphorus standards, which are summer average concentrations, the response ratio also must be a seasonal value derived from Bear Creek Reservoir.F.Allowable Frequency of Exceedance: There is no general precedent for setting an allowable frequency of exceedance for nutrient criteria, which are assessed once a year on the basis of a seasonal average, but the Division believes that one exceedance in five years is an appropriate frequency for allowable exceedances. Revised Water Quality Standards for Bear Creek Reservoir: With the benefit of the lengthy historical record now available, the Commission believes it is appropriate to set numeric chlorophyll and phosphorus standards. Both standards are considered attainable when the internal release of phosphorus becomes negligible, which is expected to occur in less than 20 years. Each standard has an allowable exceedance frequency of once in five years.
A.Chlorophyll Standard: The Commission adopted a chlorophyll standard of 10 µ g/L. The existing narrative was translated to a numeric value by defining the chlorophyll concentration at the mesotrophic-eutrophic boundary. A concentration of 8 µ g/L, which represents the boundary according to the OECD trophic classification scheme, was accepted as the typical condition expected for Bear Creek Reservoir. The typical value was translated to an 80th percentile (once-in-five year exceedance threshold) using a very strong statistical relationship developed from a set of Colorado lakes. The 80th percentile value, which is 10 µ g/L, is the chlorophyll standard.B.Phosphorus Standard: The Commission adopted a phosphorus standard of 32 µ g/L. The standard is calculated by use of a response ratio that relates the observed summer average chlorophyll concentration to the observed summer average phosphorus concentration. The median of historical distribution of response ratios (0.318) was used on the assumption that all historical values are equally likely to represent future conditions. The Commission heard testimony that a larger ratio (and thus a smaller phosphorus standard) might be preferred, but was not persuaded that a statistical argument or a mechanistic explanation would support that position.C.Assessment: Because the phosphorus and chlorophyll standards are defined as seasonal averages, some additional guidance is required concerning timing and location of samples to be used in calculating the average. Samples are to be collected at a site in deep water near the dam and should be representative of conditions in the mixed layer. Past monitoring has resulted in 5 or 6 samples during the summer months (July, August, and September); it is anticipated that the same level of effort will be applied in the future. For assessment, the average (arithmetic mean) is calculated for the summer samples in each year. At the same time that this change was adopted in Regulation #38, the Commission considered changes in the Control Regulation for Bear Creek Reservoir (Regulation #74) that would be consistent with the revised standard. The Commission decided to make no changes to Regulation #74 at this time, preferring instead to wait for TMDL development to establish new phosphorus allocations that can be implemented in the Control Regulation.
Adoption of a Temporary Modification for Chlorophyll and Phosphorus Standards in Bear Creek Reservoir: The underlying standards are not being attained in most years due to the seasonal augmentation of phosphorus concentrations from internal sources. It is uncertain how long internal release will persist, although it is expected that it will disappear within 20 years. In addition, the existing TMDL with wasteload allocations is now canceled by the new standard, and there is uncertainty about how the new standards might be translated into point source permit limits. A type iii temporary modification set at "existing conditions" to expire 12/31/2014, is adopted in order to recognize the uncertainty regarding how soon the internal load will be reduced. It will also provide certainty regarding effluent limits over the short term while a TMDL is completed which will include new wasteload allocations. During the interim, sediment monitoring will be initiated to track internal phosphorus levels over time. Progress on resolving uncertainty will be reviewed in the annual temporary modification hearings in December 2012 and 2013.
PARTIES TO THE RULEMAKING
1. Colorado Division of Wildlife2. Bear Creek Watershed Association3. U. S. Environmental Protection Agency (EPA), Region 838 CR 03, February 10, 2015, effective 6/30/201538 CR 17, September 10, 2015, effective 12/31/201539 CR 03, February 10, 2016, effective 3/1/201639 CR 03, February 10, 2016, effective 6/30/201639 CR 23, December 25, 2016, effective 12/30/201640 CR 03, February 10, 2017, effective 6/30/201740 CR 09, May 10, 2017, effective 6/30/201741 CR 01, January 10, 2018, effective 1/31/201841 CR 03, February 10, 2018, effective 6/30/201842 CR 04, February 25, 2019, effective 6/30/201943 CR 03, February 10, 2020, effective 6/30/202043 CR 17, September 10, 2020, effective 12/31/202044 CR 01, January 10, 2021, effective 2/14/202144 CR 05, March 10, 2021, effective 6/30/202144 CR 17, September 10, 2021, effective 12/31/202145 CR 17, September 10, 2022, effective 9/30/202245 CR 21, November 10, 2022, effective 11/30/202246 CR 10, May 25, 2023, effective 6/14/2023