The U.S. Environmental Protection Agency (EPA) used a stream classification scheme and the reference condition approach to derive total nitrogen (TN) and total phosphorus (TP) numeric nutrient criteria (NNC) for Florida streams. EPA developed a regional nutrient classification system to represent natural variations in nutrient concentrations resulting from a region’s underlying geology and differences in watersheds. The resulting nutrient watershed regions (NWRs) were used to inform stream classifications and subsequent NNC thresholds.
Researchers determined the reference condition using both a stream condition index population of biologically healthy conditions and a benchmark population of least disturbed sites based on land cover, aerial surveys, site visits, nitrate concentration, Clean Water Act (CWA) section 303(d) impairments, and biological condition measures. The Florida Department of Environmental Protection (FDEP) used all available nutrient data that met data quality standards from Florida’s STORET and FDEP’s Status and Trends databases. A multistep evaluation used to further identify reference sites included six screening and elimination steps based on standards such as Florida’s Landscape Development Intensity Index, Florida’s CWA section 303(d) list of impaired waters, nitrate concentration thresholds, and macroinvertebrate indices. Final selection was based on the 90th percentile of annual geometric mean concentrations in water bodies from each NWR, except for the West Central NWR criterion, which was based on the 75th percentile because of uncertainty in the distribution for that region (USEPA 2010b).
Reference:
USEPA. 2010b. Technical Support Document for U.S. EPA’s Final Rule for Numeric Criteria for Nitrogen/Phosphorus Pollution in Florida’s Inland Surface Fresh Waters. U.S. Environmental Protection Agency. Accessed October 2016.