EPA's Ecoregional Reference Approach

Phosphorus criterion for the whole of the Plains region (Ecoregion 9) would be 20 ug/L based on the 75th percentile of data from reference waterbodies in the region. If this level of phosphorus was used to set criterion for the Plains Region, 77 of 82 reservoirs would be listed as impaired.

Note: There are sub-regions within each Ecoregion, and each of these sub-regions have different phosphorus concentrations associated with them. Northern and western Missouri could be divided into three sub-regions that would have phosphorus criterion ranging from 30 ug/L along the eastern edge of the state to 55 ug/L in the far north-west corner. The majority of northern and western Missouri would end up with criterion of 40 ug/L. Estimates suggest that 26 of 82 Plains reservoirs would be listed as impaired based on sub-region criteria.

The Regional Technical Assistance Group (RTAG) Approach

Phosphorus criterion for most of the four-state region would be set at 35 ug/L (an exception made for the phosphorus-rich lakes of the Sand Hill lakes in Nebraska). Any lake or reservoir with a total phosphorus concentration higher than 35 ug/L would be listed as impaired. This value was selected after a group of scientists looked at data from the region, with emphasis on the 75th percentile of reference waterbodies and the 25th percentile of all available data. If Missouri used this criterion, 61 of 82 reservoirs would be listed as impaired.