This paper summarizes a major EPA initiative to locate and map impaired waters across the country using innovative GIS techniques. Results for waters listed by the states in 1998 under Section 303(d) of the Clean Water Act are also presented. Over 18,000 listed waterbodies were located
and georeferenced to the EPA Reach File (RF3) hydrography database, which contains locational data for over 3.2 million traces representing surface water features in the continental United States (Dewald et al., 1996). Prior to this initiative, many of the impaired waters listed by the 50
states had never been mapped and could not be displayed on demand or analyzed spatially. Knowing the exact locations and attributes of these listed waters is crucial to developing Total Maximum Daily Loads (TMDLs) and to assessing water quality improvement after TMDLs are implemented. Using
any information each state could provide (hard-copy maps, GIS coverages, Section 305(b) water quality assessment databases, etc.), special GIS coverages were created for the 303(d) listed waters. To achieve national consistency and efficiency, the approach for these coverages involved the
creation of an “event” database, which functions as a GIS coverage when used with software such as ESRI's ArcView. A customized program, the Reach Indexing Tool (RIT), was developed as an extension to ESRI's ArcView desktop mapping software. The RIT was made available
to the states through training courses and via the Web to enable states to do future reach indexing themselves. The products of EPA's initiative include detailed maps and GIS coverages, event tables that provide locational data linked to RF3, and conventional relational database files
containing information on Section 303(d) pollutants and source categories. Examples of these products, including Web-based materials available to the public, are provided in this paper. The above technical approaches are now being applied to other major EPA data systems and upgraded to
the National Hydrography Dataset (NHD). The newly released NHD replaces RF3 and offers many improved features and capabilities geared to GIS applications. EPA is working with states to georeference their 303(d) lists to NHD; the new NHD version of the RIT is described in this paper. Also described
are recent efforts to map Section 305(b) assessment data, which allows the spatial analysis of healthy as well as impaired waters and state designated uses and criteria, which are crucial to state monitoring, assessment, and management initiatives. The ultimate goal is to create data systems
at the state level that can be easily compiled into uniform national data systems. To this end, EPA also funded development of the Assessment Database, a user-friendly relational database system that states are using to store and analyze their water quality assessment data. The linkage between
the GIS coverages and the Assessment Database provides powerful querying and mapping capabilities, as illustrated in this paper. Ultimately, EPA plans to make available these and other types of water quality information through interactive Web-based applications that will allow citizens to
select data layers, run database queries, and generate on-screen maps of the results at the high level of spatial resolution offered by the NHD.
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