Assessing phosphorus concentration

Assessing phosphorus concentration.

(1) Data requirements.

(a) Lakes and reservoirs. The total phosphorus criteria specified in s. NR 102.06 (4) apply to samples taken near a lake or reservoir’s deepest point, within 2 meters of the surface. For assessment purposes samples shall, whenever possible, be taken at least once per month for 3 months during the sampling period of June 1 to September 15. The department shall calculate a lake or reservoir’s arithmetic mean total phosphorus concentration using at least 2 years of data from the sampling period.

(b) Flowing waters. The total phosphorus criteria specified in s. NR 102.06 (3) apply to samples taken from the main channel near the area with greatest flow. For assessment purposes samples shall, whenever possible, be taken at least once per month for 6 months during the sampling period of May 1 to October 31. The department shall calculate the median total phosphorus concentration for a stream, river, or impounded flowing water using at least one year of data from the sampling period.

(c) Assessment timeframe for lakes, reservoirs and flowing waters.

1. In this paragraph, “weather-controlled total phosphorus concentration” means a waterbody’s mean or median total phosphorus concentration during the applicable assessment period, estimated from measured data while controlling for weather variability using a method such as the department’s Phosphorus Mixed Effects Regression calculation method.

2. All representative data from the most recent 5 years shall be used for assessments, but data from the most recent 10 years may be used if representative of current conditions. If fewer than the recommended number of samples in par. (a) or (b) are available, the department may be able to make an assessment determination on a case-by-case basis. The department may calculate a site’s weather-controlled total phosphorus concentration to correct for weather variability and use this value to make an assessment determination in place of the mean or median calculated under par

If fewer than the recommended number of samples in par. (a) or (b) are available, the department may be able to make an assessment determination on a case-by-case basis. The department may calculate a site’s weather-controlled total phosphorus concentration to correct for weather variability and use this value to make an assessment determination in place of the mean or median calculated under par. (a) or (b) .

(2) Exceedance determination. The department shall compare the mean or median calculated under sub. (1) to the waterbody’s applicable total phosphorus criterion specified in s. NR 102.06 to determine whether the waterbody is exceeding the criterion. To determine whether additional data are needed to make an attainment decision for section 303 (d) listing purposes, the department shall apply the confidence interval approach in s. NR 102.52 (2) (b) to (c) . If application of those methods indicates that the waterbody is exceeding the phosphorus criterion, the department shall propose to include the waterbody on the section 303 (d) list as impaired for total phosphorus unless the department determines the waterbody is not exhibiting a biological response to phosphorus as specified in s. NR 102.60 .