Code of Colorado Regulations
1000 - Department of Public Health and Environment
1002 - Water Quality Control Commission (1002 Series)
5 CCR 1002-36 - REGULATION NO. 36 - CLASSIFICATIONS AND NUMERIC STANDARDS FOR RIO GRANDE BASIN
Section 5 CCR 1002-36.47 - STATEMENT OF BASIS, SPECIFIC STATUTORY AUTHORITY AND PURPOSE; JUNE 14-15, 2021 RULEMAKING; FINAL ACTION AUGUST 9, 2021; EFFECTIVE DATE DECEMBER 31, 2021

36.6(2)(b)(ii) was edited to clarify that the sulfate standard applies to dissolved sulfate concentrations. As an ion, sulfate is found in water only in the dissolved state; therefore, either unfiltered or filtered samples may be used to determine sulfate concentrations.

The following changes were made to the hardness-based table value standard equations at 36.6(3) to improve compatibility with Excel and align with corrections made to Regulation No. 31:

* Acute and chronic aluminum, chromium III, copper, lead, manganese, nickel, silver, uranium, and zinc: the first bracket was replaced with the symbol * and the second bracket was deleted from the equation.

* Acute and chronic cadmium: extra spaces were removed.

* Acute and chronic lead: the brackets were deleted and a parenthesis was moved within the conversion factor.

* Acute silver: ½ was replaced with 0.5* in the equation.

The commission revised Footnote 6 of the Table Value Standards table to improve the clarity of the footnote, which directs the implementation of the trivalent (III) and hexavalent (VI) chromium standards when data for the individual valence states are unavailable. Chromium data are infrequently reported for chromium III and chromium VI individually. Instead, data are typically reported as the total of all valence states of chromium present in the sample. This is primarily due to the difficulty of accurately measuring chromium III concentrations and the instability of chromium when the sample is acidified for analysis of the total recoverable fraction. While chromium III and chromium VI are the valence states most often found in natural waters, chromium is unstable and can convert between forms in water and in the bodies of humans and aquatic life. However, chromium VI is more water soluble and a known carcinogen. Depending on the classified use, the chromium VI standards are the same as or more stringent than the chromium III standards (Table III). Therefore, when data for individual chromium species are unavailable, the use of the chromium VI standards to assess data reported as total chromium (i.e., the total of all valence states of chromium) will ensure protection of human health and aquatic life. In addition, Footnote 6 was modified to clarify that neither the sum of the concentrations of chromium III and chromium VI (when reported individually) nor the total chromium concentration (i.e., the total of all valence states of chromium) should exceed the Water Supply standards of 50 µg/L for chromium III and chromium VI in water bodies with a Water Supply use classification.

The commission corrected the duration of all nitrite standards with a value of 0.05 or 0.5 mg/L from acute to chronic on all segments. The nitrite standards in this basin pre-date the nitrite standards in Regulation No. 31 (chloride-based equations). There has been confusion in recent years regarding the correct duration for these standards. There is no record available that explains the basis for these standards or the intended duration (acute or chronic). Based upon a comparison with the nitrite standards in Regulation No. 31, nitrite values of 0.05 and 0.5 mg/L are more consistent with the chronic values calculated using the chloride-based equations. Also, the study that the commission relied upon when adopting the nitrite standards in Regulation No. 31 indicates that these values are protective as chronic standards (1986 Nitrogen Cycle Committee of the Basic Standards Review Task Force Proposed Nitrogenous Water Quality Standards for the State of Colorado). In order to resolve the inconsistencies in the duration of the nitrite standards currently adopted in Regulation Nos. 32-38, the commission determined that these nitrite values should be consistently listed as chronic standards. Over time, the commission expects that these nitrite standards may be replaced with the more recent and well-documented chloride equation-based standards in Regulation No. 31.

The following edits were made to improve clarity, correct typographical errors, and improve consistency across the basin regulations (Regulation Nos. 32-38) and with Regulation No. 31:

* All variations of E. coli were edited to display a consistent format in the regulation and appendix tables.

* At 36.5(2) 'Table B' was added to the reference to organic standards at 31.11 to align with changes to Regulation No. 31.

* At 36.6(1), text was added to clarify that the tables in Appendix 36-1 only show the most stringent standards, and that additional, less stringent standards may be found in Regulation No. 31.

* The reference to the 'temporary modification and qualifiers' column at 36.6(2)(c)(i) was replaced with 'Other' to align with a previous change to the appendix tables.

* References to "Trec" were replaced with "total recoverable" or "T".

* Footnote 4 of the Table Value Standards table was modified to clarify that the "T" in the chronic ammonia equations stands for temperature.

* Information was added at 36.6(5) specifying that the ammonia, nitrate, and nitrate standards are to be reported as nitrogen. This is consistent with the description of the standards as they are included in Table II of Regulation No. 31. This change brings the regulation into alignment with Regulation Nos. 33, 37, and 38; the commission made this change in those regulations during triennial reviews in 2019 through 2020.

* Other minor edits were made to improve clarity and consistency.