SECTION 785:46-15-10. Nutrients

(a)   General. OAC 785:45-3-2(c) prohibits water quality degradation by nutrients which will interfere with the attainment or maintenance of any existing or designated beneficial use. OAC 785:46-13-3(a)(1) requires maintenance of any existing or designated beneficial use. This Section provides a framework which shall be used in assessing threats or impairments to beneficial uses and waterbodies and watersheds caused by nutrients, and the consequences of such assessments.

(b)   Determining whether a stream is nutrient-threatened. The dichotomous process stated in this subsection shall be used in the determination of whether a stream is nutrient-threatened.

(1)   The stream order shall be identified. If the stream order is 1, 2 or 3, then proceed to paragraph (2). If the stream order is not 1, 2 or 3, then proceed to paragraph (9).

(2)   The stream slope shall be identified. If the stream slope is greater than or equal to 17 feet per mile, then proceed to paragraph (3). If the stream slope is less than 17 feet per mile, then proceed to paragraph (4).

(3)   Subject to the application of the foregoing paragraphs of this subsection, if phosphorus concentrations in the stream are greater than 0.24 mg/L or if nitrite plus nitrate concentrations in the stream are greater than 4.95 mg/L, then proceed to paragraph (5). If such nutrient concentrations are less than the levels specified in this paragraph, then the stream is not threatened by nutrients.

(4)   Subject to the application of the foregoing paragraphs of this subsection, if phosphorus concentrations in the stream are greater than 0.15 mg/L or if nitrite plus nitrate concentrations in the stream are greater than 2.4 mg/L, then proceed to paragraph (5). If such nutrient concentrations are less than the levels specified in this paragraph, then the stream is not threatened by nutrients.

(5)   Subject to the application of the foregoing paragraphs of this subsection, if the percentage of canopy shading is greater than or equal to 80%, then the stream is not threatened by nutrients. If the percentage of canopy shading is less than 80%, then proceed to paragraph (6).

(6)   Subject to the application of the foregoing paragraphs of this subsection, if the stream's turbidity is organic, then proceed to paragraph (7). If the stream's turbidity is inorganic, then proceed to paragraph (8).

(7)   Subject to the application of the foregoing paragraphs of this subsection, if turbidity measured at seasonal base flow conditions is less than 20 NTU, then the stream is not threatened by nutrients. If turbidity measured at seasonal base flow conditions is 20 or more NTU, then the stream is threatened by nutrients.

(8)   Subject to the application of the foregoing paragraphs of this subsection, if turbidity measured at seasonal base flow conditions is less than 20 NTU, then the stream is threatened by nutrients. If turbidity measured at seasonal base flow conditions is 20 or more NTU, then the stream is not threatened by nutrients.

(9)   Subject to the application of the foregoing paragraphs of this subsection, if the stream slope is greater than or equal to 17 feet per mile, then proceed to paragraph (10). If the stream slope is less than 17 feet per mile, then proceed to paragraph (11).

(10)   Subject to the application of the foregoing paragraphs of this subsection, if phosphorus concentrations in the stream are greater than 1.00 mg/L, or if nitrite plus nitrate concentrations in the stream are greater than 4.65 mg/L, then proceed to paragraph (12). If such nutrient concentrations are less than the levels specified in this paragraph, then the stream is not threatened by nutrients.

(11)   Subject to the application of the foregoing paragraphs of this subsection, if phosphorus concentrations in the stream are greater than 0.36 mg/L, or if nitrite plus nitrate concentrations in the stream are greater than 5.0 mg/L, then proceed to paragraph (12). If such nutrient concentrations are less than the levels specified in this paragraph, then the stream is not threatened by nutrients.

(12)   Subject to the application of the foregoing paragraphs of this subsection, if the stream's inorganic turbidity measured at seasonal base flow conditions is greater than or equal to 20 NTU, then the stream is not threatened by nutrients. If the stream's inorganic turbidity measured at seasonal base flow conditions is less than 20 NTU, then the stream is threatened.

(c)   Alternative to dichotomous process for streams.

(1)   A wadable stream shall be deemed threatened by nutrients if the arithmetic mean of benthic chlorophyll-a data exceeds 100 mg per square meter under seasonal base flow conditions, or if two or more benthic chlorophyll-a measurements exceed 200 mg per square meter under seasonal base flow conditions. A non-wadable stream shall be deemed threatened by nutrients if planktonic chlorophyll-a values in the water column indicate it has a Trophic State Index of 62 or greater.

(2)   If clear and convincing evidence indicates a result for a stream different from that obtained from application of the dichotomous process in (b) of this Section, then the appropriate state environmental agency may, after completing the public participation process developed by the Secretary of Environment pursuant to 27A O.S. 1-2-101, accordingly identify the stream as threatened or not threatened by nutrients.

(d)   Demonstration that nutrients may be adversely impacting a beneficial use. If it is demonstrated by the Trophic State Index or by other relevant data as provided in 785:46-15-1(c) that nutrient loading in a waterbody may be adversely impacting a beneficial use designated for that waterbody, then the Board may determine that the waterbody and its watershed is an NLW, and shall identify the waterbody and watershed as NLW in Appendix A of OAC 785:45.

(e)   Consequence of identification as NLW. If a waterbody or its watershed is identified as NLW in Appendix A of OAC 785:45, then the Board or other appropriate state environmental agency may cause an impairment study to be performed. Provided, if an impairment study demonstrates that the uses are not threatened, then the Board shall consider deleting the NLW identification.

(f)   Consequence of assessment that use is threatened by nutrients. If it is determined that one or more beneficial uses designated for a waterbody are threatened by nutrients, then that waterbody shall be presumed to be nutrient-threatened. If it is determined or presumed, in accordance with this Section, that a waterbody is nutrient-threatened, then before the waterbody is determined to be nutrient-impaired, an impairment study must be completed by the appropriate state environmental agency.

(g)   Result of impairment study.

(1)   Impaired. If, independent of or in addition to the process set forth in this Section, an impairment study of a waterbody demonstrates that any beneficial use designated for a waterbody is impaired by nutrients, then the appropriate state environmental agency shall initiate the appropriate listing procedure in accordance with the public participation process developed by the Secretary of Environment pursuant to 27A O.S. 1-2-101 for each such beneficial use.

(2)   Not impaired. If, independent of or in addition to the process set forth in this Section, an impairment study of a waterbody demonstrates that all beneficial uses designated for that waterbody are not impaired by nutrients, then the appropriate state environmental agency shall initiate the appropriate de-listing procedure in accordance with the public participation process developed by the Secretary of Environment pursuant to 27A O.S. 1-2-101.

[Source: Added at 17 Ok Reg 1775, eff 7-1-00; Amended at 18 Ok Reg 171, eff 10-25-00 (emergency); Amended at 18 Ok Reg 3379, eff 8-13-01; Amended at 21 Ok Reg 1910, eff 7-1-04; Amended at 22 Ok Reg 1607, eff 7-1-05; Amended at 25 Ok Reg 1455, eff 7-1-08; Amended at 28 Ok Reg 1017, eff 7-1-11]