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Percentage of Surface Drinking Water from Intermittent, Ephemeral, and Headwater Streams in the Continental United States

A joint analysis by EPA’s Office of Wetlands, Oceans, and Watersheds and the Office of Groundwater and Drinking Water

Summary

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The Clean Water Act not only protects the biology and water quality of the nation's streams, rivers, lakes and wetlands, it also ensures that these vital aquatic resources continue to safely provide much of the nation's drinking water supply. The results of this analysis reveal the extent to which intermittent, ephemeral, and headwater streams provide important public health benefits by supplying water to public surface drinking water systems. These benefits are at risk due to recent Supreme Court decisions that cast doubt on whether these types of streams remain protected under the Clean Water Act.

This analysis was performed using GIS (Geographic Information System) to overlay national stream data with public drinking water system data provided by states. This analysis compared the combined stream length of intermittent, ephemeral, and headwater streams to total stream miles contained in all mapped Source Protection Areas (SPAs). A SPA is the area upstream from a drinking water intake that provides water to that public drinking water system during a 24-hour period. Each public drinking water system serves a specific population based on the primary county served. Populations dependent on intermittent, ephemeral or headwater streams include only populations served by drinking water systems fed by SPAs containing those stream types.

Conclusions

In the continental United States, approximately 117 million people, or over one third of the total U.S. population, get some or all of their drinking water from public drinking water systems that rely at least in part on intermittent, ephemeral or headwater streams. This represents 94% of the approximately 124 million people who are served by public drinking water systems included in this analysis. In the continental U.S., 357,404 total miles of streams provide water for public drinking water systems using surface water. Over half (58%) of that total are intermittent, ephemeral or headwater streams.

Darkest orange indicates counties most dependent on these streams for drinking water.

Data

Intermittent streams are streams containing water only seasonally. Ephemeral streams flow only in response to precipitation events. First-order streams have been used to represent headwater streams. USGS map data used in this analysis generally does not include streams less than one mile in length. Only public drinking water systems that identify a primary county were included in the analysis, although secondary counties may also be served. Consequently, counties labeled as “no data” may actually be receiving their drinking water from a system located in a different county. Counties labeled as “no data” alternatively may depend heavily on private surface drinking water systems or rely on groundwater. Data is from the National Hydrography Dataset at medium resolution and the Federal Safe Drinking Water Information System (4th Quarter 2006).

Methods

Over one third of the U.S. population is dependent on these waters for drinking water – the intermittent, ephemeral, and headwater streams that are harder to protect after the Supreme Court rulings. – Lisa P. Jackson, EPA Administrator

Darkest orange indicates counties most dependent on vulnerable stream types for drinking water

http://www.epa.gov/owow/wetlands/science/surface_drinking_water

Source Protection Area (SPA)

Intake

A Public Water System Supplied by Streams