5 Colo. Code Regs. § 1002-32.70
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
In March 2012, the commission adopted interim numerical nutrient table value standards for chlorophyll a to protect the Aquatic Life, Recreation, and Direct Use Water Supply (DUWS) uses and table value standards for total nitrogen and total phosphorus to protect the Aquatic Life and Recreation uses (31.50) in lakes, reservoirs, rivers, and streams. In its July 2016 action letter for the March 2012 rulemaking hearing, EPA approved with recommendations the numeric values for total nitrogen and total phosphorus for lakes and reservoirs. For Warm lakes, EPA recommended that the commission should apply the total nitrogen and total phosphorus table value standards only where a site-specific analysis demonstrated that uses would be protected. For Warm and Cold lakes, EPA recommended evaluation of options for developing more protective table value standards, to ensure that numeric standards for total nitrogen and total phosphorus could be assigned to individual segments with confidence that uses would be protected. EPA also suggested a classification analysis for lakes and reservoirs to account for the variability between lakes (e.g., Cold and Warm lakes), evaluating confounding factors in the stressor-response relationship between nutrients and chlorophyll a, and evaluating whether the standards are protective of lakes with a high chlorophyll a yield per unit of nutrient.
In this hearing, the commission adopted revised total nitrogen and total phosphorus table value standards for lakes and reservoirs to address EPA's 2016 recommendations and ensure protective table value standards are available for protection of lakes and reservoirs with Aquatic Life and/or Recreation uses (31.17 Table V). The numeric nutrient standards for total nitrogen and total phosphorus represent growing season (July through September) average concentrations with an allowable exceedance frequency of once in five years, and apply to lakes and reservoirs greater than 25 acres in size and with a residence time of at least 14 days. The commission adopted these standards into Regulation No. 31 and the basin regulations (Regulation Nos. 32-38) in this rulemaking; additional details about the revised total nitrogen and total phosphorus standards for lakes and reservoirs are included in 31.60.
The commission revised 32.5(4) to reflect the current status of the phased implementation framework for nutrients standards and remove information regarding implementation that concluded December 31, 2022. These revisions included removing language regarding phased implementation of chlorophyll a standards for lakes, reservoirs, rivers, and streams, as these standards now apply to all waterbodies with Aquatic Life, Recreation, and/or DUWS uses in Colorado. The information regarding the specific circumstances where nitrogen and phosphorus standards will apply before December 31, 2027 was clarified and includes additional references to 31.17. Also, to be consistent with past practice and the commission's intent in 31.55, the word "headwaters" was replaced with "waterbodies upstream of certain domestic and non-domestic wastewater treatment facilities". Finally, references to new Tables V (nutrients standards for lakes and reservoirs) and VI (nutrients standards for rivers and streams) in 31.17 were also added.
The commission revised the Table Value Standards table in 32.6(3) to include chlorophyll a, total nitrogen, and total phosphorus. Instead of replicating the numerical values for these table value standards, the table references 31.17, as 31.17 contains the numeric standards (in Tables V and VI), implementation information, and additional details regarding the phased implementation framework. As part of this change, the commission revised Footnote 1 to specify that the nitrogen and phosphorus standards are based upon the total concentration; this information was previously contained in 32.6(5)(b), which was deleted. Additionally, the commission adopted a new Footnote 6 that clarifies that, with the exception of the chlorophyll a standard to protect the DUWS sub-classification, the chlorophyll a, total nitrogen, and total phosphorus standards apply only to lakes and reservoirs larger than 25 acres surface area. The chlorophyll a standard to protect DUWS lakes and reservoirs applies to lakes and reservoirs of all sizes. This information was previously included in the segment tables in Appendix 32-1, but was moved to Footnote 6 for clarity.
Adoption of total phosphorus standards was previously limited to specific segments or portions of segments, as outlined in 31.50(IV)(A) (i.e., waterbodies above certain discharge facilities and site-specific situations where numeric standards were needed to protect uses). Prior to this rulemaking hearing, total nitrogen standards had not been adopted on any waterbodies. In this rulemaking hearing, the commission adopted total nitrogen standards on the same set of waterbodies (i.e., waterbodies above certain discharge facilities. Consistent with 31.17, the total phosphorus and total nitrogen standards apply only to lakes and reservoirs greater than 25 acres.
The phased implementation strategy developed in 2012 (31.50(IV)(A)) and revised in 2017 (31.55) also included plans for adoption of total nitrogen and total phosphorus standards on other high priority waters, including DUWS reservoirs and lakes and reservoirs with public swim beaches (defined as waterbodies with a natural swimming area per C.R.S § 25-5-801, including having a fee-based cordoned off swim area) in this rulemaking hearing. The commission previously adopted the DUWS sub-classification and notation in the Appendix 32-1 tables on several waterbodies in previous rulemaking hearings, and public swim beaches were identified in the current rulemaking (see division rebuttal revised Exhibit S). However, the commission did not adopt total nitrogen and total phosphorus standards for these waterbodies at this time, and decided to delay adoption of standards for these waterbodies until 2027 (see 31.60(III)(D)(2) and (3)).
In rivers and streams, the commission did not adopt total phosphorus standards on any additional river or stream waterbodies or total nitrogen table value standards on any waterbodies, consistent with the phased implementation timeline outlined in 31.17 and 32.5(4).
The commission changed how previously-adopted total phosphorus table value standards were presented in the segment tables in Appendix 32-1. Specifically, the table value standards were previously shown as numeric values; these were replaced with "TVS". Similarly, any total nitrogen or total phosphorus standards adopted in this rulemaking hearing were adopted into the Appendix 32-1 tables as "TVS". This approach allows the regulation to point directly to 31.17, which has a complete record of information regarding these table value standards.
The commission made no changes to the chlorophyll a table value standards to protect Aquatic Life, Recreation, and/or DUWS uses in lakes, reservoirs, rivers, or streams. In addition, the commission made no changes to existing site-specific chlorophyll a standards.
Adoption of chlorophyll a standards on individual waterbodies was previously limited to specific segments or portions of segments, as outlined in 31.50(IV)(A) (i.e., waterbodies above certain discharge facilities and site-specific situations where numeric standards were needed to protect uses). However, consistent with the phased implementation strategy developed in 2012 (31.50(IV)(A)) and 2017 (31.55), the commission expanded the adoption of chlorophyll a standards to all segments with Aquatic Life, Recreation, and/or DUWS uses.
Specifically, in this rulemaking hearing, the commission adopted the chlorophyll a table value standard of 8 µg/L for all cold water lakes or reservoirs (larger than 25 acres) with Aquatic Life or Recreation E, U, or P uses; 20 µg/L for all warm water lakes or reservoirs (larger than 25 acres) with Aquatic Life or Recreation E, U, or P uses; and 150 mg/m2 for all cold or warm water streams with a Recreation E, U, or P use. In the segment tables in Appendix 32-1, these table value standards were adopted as "TVS" and any previously-adopted table value standards shown as numeric values were changed to "TVS". This approach allows the regulation to point directly to 31.17, which has a complete record of information regarding these table value standards. In addition, the commission adopted the table value standard of 5 µg/L for all lakes or reservoirs (of any size) with DUWS; in Appendix 32-1, these table value standards are shown as "DUWS". Consistent with the approach used in 2012, the chlorophyll a table value standards for Aquatic Life and/or Recreation are only applied in lakes and reservoirs that have a residence time of at least 14 days. The chlorophyll a standard for DUWS applies to all lakes and reservoirs with a DUWS sub-classification, regardless of residence time duration. The phased implementation of the chlorophyll a standards adoption is now complete.
When determining if a site-specific chlorophyll a standard more or less stringent than the table value standard would be protective of a DUWS, the commission may consider factors such as whether disinfection byproducts (DBPs) have been or are currently being produced, the type of treatment technology in use, expected organic carbon removal efficiency during treatment, if the duration of the use is sufficient to result in chronic exposure or require management of disinfection byproducts, and any other relevant factors.
The following edits were made to Appendix 32-1 to improve clarity and correct errors:
* The Direct Use Water Supply (DUWS) references in segments in Appendix 32-1 were revised to improve clarity and consistency.
5 CCR 1002-32.70