Percent of Impaired Waters, 1998
Source: United States Environmental Protection Agency (EPA)
Map Description:
Section 303(d) waters, of the Clean Water Act listing, show water quality standards impairments or threats to the attainment of beneficial uses or anti-degradation provisions. This map is a representation of threatened and impaired streams, rivers, coastlines, estuaries, lakes and wetlands within an 8 -digit Hydrologic Unit Code (HUC), divided by the total number of water miles within the HUC. All 303(d) listing information available and mapped as of July 1999 was included.

Cautions for this Product:
Many waterbody types such as estuaries, wetlands and lakes are typically measured in square miles or acres. For the sake of comparison, this analysis measured all of these waterbodies in miles. The impaired and threatened water mileage for lakes and estuaries was calculated using the distance for a line running through the middle of the waterbody. The exceptions to this method include the Great Lakes, Long Island Sound, Delaware Bay, and Chesapeake Bay. For these waters their coastline mileage was used. Breakpoints for the scores were determined using modified quartiles.

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