In 1992, Tennessee Division of Water Pollution Control (DWPC) initiated a new method for establishing reference conditions based on ecoregions. In the first phase of a 3-phase effort, DWPC arranged for EPA scientists James Omernik and Glenn Griffith to update the state ecoregions and delineate subecoregions from the original eight Tennessee ecoregions identified by Omernik in 1986 (Omernik 1986). Geographic data were gathered for the development of the draft subecoregions, and the subecoregions were ground-truthed, with a resulting delineation of 14 ecoregions. As part of that effort, scientists developed new maps and created digitized geographical information system (GIS) coverages for the DWPC.

In the second phase, EPA and DWPC identified potential reference streams from relatively unimpacted watersheds for each ecoregion, including watersheds within federally or state-protected areas, where possible. DWPC conducted site visits of those potential reference streams and selected a minimum of three per subecoregion. Phase three began in August 1996, when DWPC began collecting chemical, physical, and biological data from reference streams and compiling the information into a database. The data in the database represent least impacted conditions in the state and can be used in discussions about setting nutrient criteria. For reference streams in each ecoregion, DWPC has documented TP and nitrate-nitrate concentrations and calculated median concentrations and ranges for each. To finalize the nutrient database, DWPC plans to incorporate data from other states once data are collected from within shared ecoregions and sampled in a manner similar to DWPC’s monitoring plan. DWPC also will perform quality checks on the database to ensure data quality and check outliers.

In 2000, DWPC will propose ecoregion-specific interpretations of narrative TP and nitrate-nitrite criteria. Those interpretations will be based on the 90th percentile of reference stream data, unless the 75th percentile is more appropriate. DWPC plans to conduct statistical analyses of nutrient concentrations and their ranges in each subecoregion. Where significant differences exist between subecoregions, DWPC will develop nutrient criteria at the subecoregion level. Streams with nutrient concentrations less than the 90th percentile (or the 75th percentile) of reference conditions will be considered to be in compliance with the narrative standard, and streams with higher concentrations will be considered to be in violation of the standard (USEPA 2000b).

References:

Omernik, J.M. 1986. Ecoregions of the United States. U.S. Environmental Protection Agency, Corvalis Environmental Research Laboratory, 1 p.

USEPA. 2000b. Nutrient Criteria Technical Guidance Manual: Rivers and Streams. EPA-822-B-00-002. 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/20003CVP.PDF?Dockey=20003CVP.PDF.