The official EPA sources that Scorecard uses for its water reports have important limitations and flaws. Most importantly:
1) not all water bodies or watersheds are included. In fact, EPA and the states have assessed only 1/3 of the nation's waterways. For example, if a report on Clean Water Act assessment indicates that "18% of reported surface waters with reported problems," that statistic covers only the surface waters that the state itself has chosen to include in its report to EPA.
2) The criteria for including water bodies are not uniform, so state-to-state comparisons can be misleading. For example, a state that has left out a major category of water bodies included by another state might report a comparatively low level of Clean Water Act problems, but might in fact show a comparatively high level of problems if the same types of water bodies had been assessed for both states.
3)The criteria for determining whether a problem exists also are not uniform, so what counts as an impaired or threatened use in one state may not count in another state.

These serious limitations, which are inherent in the official data, mean that there is a constant risk of apples-to-oranges comparisons in comparing one state's reported Clean Water Act problems with another's. A high policy priority for EPA should be to require more uniform reporting from the states under the Clean Water Act reporting requirements. The user of this information should always keep in mind what percentage of a state's water bodies are actually included in that state's report to EPA (i.e., how much is left out), and what percentage of the water bodies being reported were actually assessed for the particular type of water quality impairment the reader is interested in (i.e., how thorough was the assessment).