Section NR 119.05 - Methods for determining an SSC An SSC shall be developed using one of the following methods:
(1) SSC BASED ON AMBIENT PHOSPHORUS CONCENTRATION. For cases under s. NR 119.03 (1) when it is demonstrated that a less stringent SSC is appropriate because biological metrics are attained, the department may set the SSC at a concentration not to exceed a flowing water's median ambient concentration, or a lake or reservoir's mean ambient phosphorus concentration. If the weather-controlled total phosphorus concentration can be determined for the waterbody, this value may be used as the ambient phosphorus concentration. Note: The SSC is set at the waterbody's median or mean ambient concentration (or weather-controlled phosphorus concentration) because the ambient concentration is deemed to be protective of designated uses if the waterbody attains its phosphorus response indicators and biological assessment thresholds at current concentrations.
(2) SSC DERIVED THROUGH MODELING. (a)Modeling analysis. If a statistical or modeling analysis demonstrates that attaining a phosphorus concentration higher or lower than the statewide phosphorus criterion will attain a waterbody's suspended chlorophyll a phosphorus response indicators specified in s. NR 102.60 (2) and (3), and the department initiates rulemaking to establish an SSC, the department shall propose an SSC at the modeled phosphorus concentration expected to attain the applicable indicators. The analysis shall be conducted following requirements specified in s. NR 119.06 (4) (b). Note: Modeling demonstrations under this subsection do not require modeling of biological communities such as fish, aquatic insect, or aquatic plant communities because, while predictive modeling is well-established for chlorophyll a concentrations, there are no widely-accepted modeling approaches that predict community-scale responses of fish, aquatic insect, or aquatic plant communities to variation in phosphorus concentrations with a high degree of precision.
(b)Use of natural background phosphorus concentrations. An SSC may be set no lower than a waterbody's natural background phosphorus concentration if the background concentration can be determined. The natural background concentration may be estimated using the concentration for similar nearby waterbodies with minimal human impacts or other methods approved by the department. For natural lakes, the background concentration may be determined from a sediment core using paleolimnological methods. Note: Natural background phosphorus concentrations can be inferred from diatoms deposited in the sediment prior to significant impacts from Euro-American settlement, circa 1850s but variable across the state.
(3) ALTERNATIVE METHODS. An alternative method for setting an SSC may be used in cases that are outside of those described in ss. NR 119.03 to 119.04, when methods in subs. (1) to (2) are not appropriate or feasible, or to consider alternative or additional metrics. An SSC based on an alternative method may be approved if the department and U.S. EPA determine that the SSC is protective of the designated uses of the waterbody and downstream waters under s. NR 119.06 (6).Wis. Admin. Code Department of Natural Resources NR 119.05
Adopted by, CR 19-093: cr. Register September 2022 No. 801, eff. 10-1-22; correction in (2) made under s. 13.92 (4) (b) 7, Stats., Register September 2022 No. 801, eff. 10/1/2022