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georeferencing wqi to the nhd
Georeferencing Water Quality
Information to the National Hydrography Dataset
This paper describes the application of innovative GIS
techniques to georeference or link water quality related
information for the country to the National Hydrography
Dataset (NHD). One major effort by the U.S. Environmental
Protection Agency has resulted in the georeferencing of
over 18,000 impaired waterbodies listed by the states in
1998 under Section 303(d) of the Clean Water Act. 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. To achieve
national consistency and efficiency, the technical approach
involved the creation of an "event" database, which functions
as a GIS coverage. A customized 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. Based on this
experience, the states' official "designated uses" for tens
of thousands of individual waterbodies are also being georeferenced
to the NHD, as well as the locations of each state's fish
consumption advisories and bans. The products of EPA's initiatives
include detailed maps and GIS coverages, event tables that
provide locational data linked to the NHD, and relational
database files containing water quality information. The
ultimate goal is to create data systems at the state level
that can be easily compiled into uniform national data systems.
Example products, including Web-based materials available
to the public, are provided in this paper.
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