NUMBER OF IMPAIRED WATERBODIES VS. PERCENT OF ASSESSED WATERBODIES
Data on the number of impaired waterbodies in a particular geographic area are derived from an accounting submitted to EPA by each state (the TMDL Tracking System). Data on the percentage of particular types of waterbodies assessed by the state are also derived from the states own accounting (as complied by EPA in its 1998 National Water Quality Inventory).
However, in some cases these two statistics can be inconsistent - even though the same state provides both pieces of information. The reader should not assume that the percentage of assessed waterbodies is always a limit on the number of impaired waterbodies, or vice versa. For example, a state can report that 100% of its rivers and streams were unassessed, but still have provided data on impaired waterbodies to the TMDL tracking system. This is a weakness of the existing data as reported by U.S. EPA. No other data are available to the public that would make it possible to eliminate these possible inconsistencies.