The proposed rule 61.3(4) is as follows:
61.3(4) Lake criteria. So that the recreational uses listed in 61.3(1)“b”(1) through 61.3(1)“b”(3) can be supported, the following criteria are applicable to all lakes that have a mean depth of three meters or greater.
a. Transparency. The transparency of the lake as measured with a Secchi Disk shall be one meter or more in at least 75 percent of the measurements taken.
b. Chlorophyll-a. The concentration of chlorophyll-a in 75 percent of the samples analyzed shall not exceed 25 micrograms per liter (μg/l).
c. Water sampling used to determine whether a lake meets the transparency and chlorophyll-a criteria must meet the following:
(1) A minimum of nine sample results are required.
(2) At least three of the samples must be taken from the deepest part of the lake.
(3) All samples must be taken during the months of May through September.
(4) At least three sampling events must be conducted in any one summer recreation season.
(5) Samples must be taken in at least three summer seasons in a five-consecutive-year period.
d. Nitrogen and phosphorus concentrations are among several variables affecting lake transparency and chlorophyll-a levels; however, individual lake and watershed characteristics determine the precise amount of nutrients that can be tolerated within a lake while still attaining the above recreational use criteria. For this reason, nutrient reduction targets will be determined on a case-by-case basis as a result of lake-specific monitoring and data analysis. Lake nutrient response models, such as those used to establish Total Maximum Daily Loads for lakes with transparency or chlorophyll-a impairments, or other appropriate scientific methods will be utilized for this purpose.
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