SECTION 785:46-13-8. Antidegradation review in surface waters

(a)   General. The antidegradation review process below presents the framework to be used when making decisions regarding the intentional lowering of water quality, where water quality is better than the minimum necessary to protect beneficial uses. OWRB technical guidance TRWQ2017-01 provides additional information.

(b)   Determination of assimilative capacity in Tier 2, Tier 2.5, and Tier 3 waters.

(1)   All water quality monitoring and technical analyses necessary to determine receiving waterbody assimilative capacity for all applicable numeric and narrative criteria and associated parameters protective of waterbody beneficial uses shall be conducted by the interested party.

(2)   Prior to initiating any monitoring or technical analysis to support determination of waterbody assimilative capacity, the interested party shall submit a workplan consistent with the requirements of OWRB technical guidance TRWQ2017-01 for review and approval by OWRB staff.

(3)   As part of an approved workplan, the interested party shall characterize existing water quality of the receiving waterbody for each applicable criteria and associated parameters and evaluate if there is available assimilative capacity. Consistent with OWRB technical guidance TRWQ2017-01, characterization of existing water quality shall address, at a minimum:

(A)   Measurement of load and or concentration for all applicable criteria and associated parameter(s) in the receiving water; and

(B)   The measurement of both existing and proposed point and nonpoint source discharge concentrations and or loadings, including the measurement of external and internal nutrient loading, where required by OWRB technical guidance TRWQ2017-01; and

(C)   The critical low flow or critical lake level of the receiving waterbody, including drought of record in waterbodies receiving IPR discharges; and

(D)   The limnological, hydrologic, seasonal, spatial and temporal variability and critical conditions of the waterbody; and

(E)   Volumetric determination of anoxic dissolved oxygen condition consistent with OAC 785:45 and 785:46; and

(F)   The bioaccumulative nature of a pollutant shall be considered when determining assimilative capacity; and

(G)   The 303(d) list as contained in the most recently approved Integrated Water Quality Assessment Report shall be reviewed and any difference between the water quality assessment information and the characterization of existing water quality shall be reconciled.

(4)   Assimilative capacity shall be determined by comparing existing water quality, as determined consistent with subsection (a)(3) above to the applicable narrative and numeric criteria. In Tier 2 waters, assimilative capacity shall be determined and used with a margin(s) of safety (46-13-8(c)(1)(D)), which takes into account any uncertainty between existing or proposed discharges and impacts on receiving water quality.

(5)   When existing water quality does not meet the criterion or associated parameter necessary to support beneficial use(s) or is identified as impaired on Oklahoma's 303(d) list as contained in the most recently approved Integrated Water Quality Assessment Report, no assimilative capacity shall exist for the given criterion.

(c)   Use of assimilative capacity in Tier 1 waters. Available assimilative capacity may be used in Tier 1 waters such that, water quality is maintained to fully protect all designated and existing beneficial uses.

(d)   Use of assimilative capacity in Tier 2 waters.

(1)   If it is determined that assimilative capacity is available, the consumption of assimilative capacity may be allowed in a manner consistent with the requirements in 40 CFR 131.12(a)(2) and this subchapter. In allowing the use of assimilative capacity, the state shall assure that:

(A)   Water quality shall be maintained to fully protect designated and existing beneficial uses.

(B)   Assimilative capacity shall be reserved such that all applicable narrative criteria in OAC 785:45 are attained and beneficial uses are protected.

(C)   Fifty percent (50%) of assimilative capacity shall be reserved for all applicable water quality criteria listed in OAC 785:45, Appendix G, Table 2.

(D)   In order to preserve a margin of safety; in no case shall any activity be authorized without the application of margin(s) of safety specified below:

(i)   A twenty percent (20%) margin of safety shall be applied to an applicable numeric criterion for chlorophyll-a, total phosphorus, and total nitrogen. If numeric criteria are not available, the narrative nutrient criterion (785:45-5-9(d)) shall be applied and a twenty percent (20%) margin of safety shall be applied to the parameters listed in the criterion.

(ii)   No more than forty-five percent (45%) of the lake volume shall be less than the dissolved oxygen criterion magnitude in OAC 785:45-5-12(f)(1)(C)(ii).

(iii)   If the existing value of a criterion is within the margin of safety, no assimilative capacity is available and existing water quality shall be maintained or improved.

(E)   When existing water quality does not satisfy the applicable criterion and support beneficial use(s) or has been designated as impaired in Oklahoma's 303(d) list as contained in the most recently approved Integrated Water Quality Assessment Report, the applicable criterion shall be met at the point of discharge. If a TMDL has been approved for the impairment, loading capacity for the parameter may be available if TMDL load allocations include the proposed load from the discharge.

(2)   An analysis of alternatives shall evaluate a range of practicable alternatives that would prevent or lessen the water quality degradation associated with the proposed activity. When the analysis of alternatives identifies one or more practicable alternatives, the State shall only find that a lowering is necessary if one such alternative is selected for implementation.

(3)   After an analysis of alternatives and an option that utilizes any or all of the assimilative capacity is selected, the discharger must demonstrate that the lowering of water quality is necessary to accommodate important economic or social development in the area in which the waters are located.

(e)   Use of assimilative capacity in Tier 2.5 or 3.0 waters. Consistent with 785:45-3-2(a) - (c), 785:45-5-25(a), 785:45-5-25(b), and 785:45-5-25(c)(1) - (c)(6) all available assimilative capacity shall be reserved in waterbodies classified as Tier 2.5 or 3.0 waters.

(f)   Public participation. Agencies implementing subsection 8(c), shall conduct all activities with intergovernmental coordination and according to each agency's public participation procedures, including those specified in Oklahoma's continuing planning process.

[Source: Added at 35 Ok Reg 2238, eff 9-14-18]