The Wisconsin Department of Natural Resources (WIDNR) convened a phosphorus technical workgroup (PTW) to develop scientifically defensible phosphorus criteria for the state’s lakes and impoundments. The PTW selected historical TP data from STORET for lakes and impoundments meeting three criteria: (1) a surface area of greater than or equal to 25 acres, (2) samples collected between June 1 and September 15, and (3) samples collected at a depth of less than or equal to 4 feet. The PTW examined historical lake and impoundment TP data from STORET for the 21 subregions defined by Omernik et al. (1988) and the three regions—north, central, and south—identified by Lillie et al. (1993). WNDR concluded that data were insufficient to derive criteria for each of Omernik’s 21 subecoregions. The study by Lillie et al. (1993) had highlighted the associations between water clarity, chlorophyll a, and TP on a regional basis. WDNR examined the data using Lillie’s three phosphorus regions and determined that there were consistent differences in mean TP concentrations for each region.
Data were grouped into north, central, and south regions of the state and combined for each region by drainage type and potential for thermal stratification. Lower quartiles were generated for individual data points using SAS univariate procedures, and those values were rounded down to the nearest multiple of 5. The PTW agreed that lower quartiles of the data would provide a conservative estimate of background total TP concentrations in lakes and impoundments. Although the PTW agreed that lower quartiles were the best available method to estimate ambient WQS that would lead to sufficient protection, they suggested that final criteria language should allow for development of site-specific criteria when sufficient data are available. The PTW also performed a trip analysis on mean TP concentrations for lakes and impoundments to determine the relative proportion of water bodies that would likely exceed the lower quartile estimate.
The PTW performed similar analyses on a random lakes dataset in an attempt to determine whether water quality conditions had been maintained over time. However, data from the same lakes in the two datasets were insufficient to make the comparison. The PTW determined that the random lakes data should be used for subsequent criteria development. For natural lakes, criteria values were further differentiated by drainage and thermal stratification, and values for impoundment waters were differentiated only by thermal stratification. The PTW concluded that the values developed would not be used as statewide phosphorus criteria, but rather as triggers that, when exceeded, would result in further action (USEPA 2000a).
References:
Lillie, R.A., S. Graham, and P. Rasmussen. 1993. Trophic State Index Equations and Regional Predictive Equations for Wisconsin Lakes. Research Management Findings Number 35. Wisconsin Department of Natural Resources. Madison, Wisconsin.
Omernik, J.M. 1986. Ecoregions of the United States. U.S. Environmental Protection Agency, Corvalis Environmental Research Laboratory, 1 p.
USEPA. 2000a. Nutrient Criteria Technical Guidance Manual: Lakes and Reservoirs. EPA-822-B-00-001. U.S. Environmental Protection Agency, Office of Water, Office of Science and Technology, Washington, DC. Accessed October 2016. https://nepis.epa.gov/Exe/ZyPDF.cgi/20003COV.PDF?Dockey=20003COV.PDF.