Scorecard's profiles of Clean Water Act status and watershed health are derived from three U.S. EPA sources: the 1998 TMDL Tracking System, the 1999 Index of Watershed Indicators, and the 1998 National Water Quality Inventory.
The TMDL Tracking System database contains information on the waters listed under section 303(d) of the Clean Water Act. The CWA requires states to identify all waters where required pollution controls are not sufficient to attain or maintain applicable water quality standards and rank the waters taking into account the uses of the water and severity of the pollution problem. Fifty-six states, territories, and tribes submitted 1998 303(d) lists to EPA for review and approval. Scorecard utilizes the database version of these lists as of December 10, 1999. States submitted updated information on Clean Water Act compliance status to EPA in 2000.
EPA believes that the CWA list provides a comprehensive public accounting of all impaired or threatened waterbodies, but several data quality problems effect comparisons across geographic areas. The l998 figures reflect the states' assessment of only one third of the nation's waterways. States assessed almost 25% of the nation's total river and stream miles; 40% of its lake, pond and reservoir acres; and 30% of its estuarine square miles. Not all states and territories report the same types of information on their lists (e.g., some report the potential sources of the impairment while others do not). The information in the database for Idaho and Lousiana are based on lists that are not complete and fully approved. Also, data in the TMDL tracking system may change as states have an opportunity to review the interpretation of their list submission as compared to the Tracking system.
The Index of
Watershed Indicators (IWI) characterizes the condition and vulnerability of
aquatic systems in each of the 2,262 watersheds in the 50 states and Puerto
Rico. The Index is based on the June 1996 Indicators of Water Quality in the United States, developed
by EPA in partnership with states, tribes, private organizations, and
other federal agencies. Scorecard utilizes version 1.4 of the IWI database (September, 1999). Note that the data used to construct different indicators was compliled over the period 1990-1999.
EPA created the IWI to focus the attention of the public and decision makers on watershed health, but there are important limitations to the data. For each watershed, the Index evaluates condition (water quality), vulnerability, and data sufficiency. Extensive professional judgement is used to integrate water quality indicators and watershed vulnerability indicators into an overall watershed health indicator. EPA's Science Advisory Board has noted that the data available to EPA for constructing its indicators do not provide good measures of ecological health, and warns that differences in IWI indicator scores may not correspond to meaningful differences in environmental quality.
The Index of Watershed Indicators makes use of data collected by a wide variety of regulatory and scientific agencies at specific sites across the country. More data are available in some watersheds than others. In order to ensure that sufficient data exist to make a valid judgment of aquatic resource health, U.S. EPA set threshold levels for the number of indicators needed to categorize watersheds. For the condition assessment, watersheds must have information for at least four of the seven indicators. For the vulnerability assessment, at least six of the eight indicators are required. In watersheds which fall short of these threshold requirements, the IWI categorizes them as having "insufficient data." Note that this categorization does not necessarily mean that there are no data available for the watershed. Information for those indicators with data is reported in Scorecard's county reports.
The National Water Quality Inventory: 1998 Report to Congress is a biennial report to Congress prepared under Section 305(b) of the Clean Water Act. It contains information from each state on the quality of its rivers, lakes, wetlands, estuaries, coastal waters, and ground water, along with information on public health and aquatic life concerns. It serves as a snapshot of water quality conditions across the country. Scorecard uses NWQI as its source of information about the percent of waterbodies in an area that have been asssessed by state agencies and can therefore be evaluated for Clean Water Act compliance.
To produce the NWQI, EPA combines information reported by states under two Clean Water Act reporting requirements. To assess water quality, states compare their monitoring results to the water quality standards they have set for their waters. Under Section 303(d) of the Clean Water Act, states provide lists of all of their impaired waters to support the development of Total Maximum Daily Loads (TMDLs). These different sources of state water quality data are generally consistent, although the 303(d) lists often include specific information from more targeted monitoring activities.
Scorecard's data on the percentage of particular types of waterbodies assessed by each state are derived from the states own accounting as complied in the appendices of the 1998 NWQI. Data on the number of impaired waterbodies in a particular geographic area are derived from EPA's TMDL Tracking System.
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. 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.