Other states also discussed the need to revise criteria that are difficult to  use in identifying impairments. These five  pollutants have been found to be among the leading causes of impairment  nationwide. EPA and the states have identified numerous inconsistencies of this kind. States Use a Range of Quality Assurance Procedures

States use a range of quality assurance procedures to ensure that the data  they use to assess their waters are valid. Reliability of EPA’s Impaired Waters Database Limited by Inconsistent Data

Owing, in part, to the inconsistencies in states’ approaches to identifying  impaired waters, the information in EPA’s database of impaired waters is  of questionable reliability. Because states identify  the size of impaired waters differently, EPA’s tally of both the total  number of impaired waters nationwide and the number of TMDLs that  must be established is not reliable. Indeed, some flexibility in key listing-related  functions, such as the adoption of water quality standards and water  quality monitoring, is provided under both the Clean Water Act and EPA  regulations. Recommendations for	 To provide greater consistency in the way states list their impaired waters,  Executive Action  we recommend that the Administrator, EPA, provide additional guidance to the states on carrying out the key functions (including standard-setting, water quality monitoring, and data assessment) that influence how states identify the waters for their section 303(d) lists; work with the agency’s regional offices to ensure a more consistent interpretation of the agency’s policies on the criteria that states must meet to remove waters from their section 303(d) lists; provide clear guidance to the states on the information they should use to describe their methodologies for developing their section 303(d) lists; and work with the states to help resolve discrepancies that arise in the listing of interstate waters.
What GAO Found

The Environmental Protection Agency (EPA) believes that more than 20,000 bodies of water throughout the country are too polluted to meet water quality standards. States use different approaches to identify impaired waters. This variation has led not only to inconsistencies in the listing of impaired waters but also to difficulties in identifying the total number of impaired waters nationwide and the total number of total maximum daily loads (TMDL) needed to bring such waters up to standards. Under the Clean Water Act and its regulations, EPA has given the states some flexibility to develop listing approaches that are tailored to their circumstances. However, some of the approaches have no appropriate scientific basis. States apply a range of quality assurance procedures to ensure the quality of data used to make impairment decisions. Although states have long used quality assurance procedures for the data they collect directly, they have become increasingly vigilant about applying such procedures to data from other sources. Because of inconsistencies in states' approaches to identifying impaired waters, the information in EPA's database of impaired waters is of questionable reliability. The number of impaired waters cannot be compared from one state to the next, and EPA cannot reliably tally the number of TMDLs that must be completed nationwide. EPA's database also distorts the size of some of the states' impaired waters when they are mapped on EPA's website.