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Causes of Impairment

IMPORTANT NOTE: The precise numbers presented in these tables should not be assigned a great deal of significance. Even the exact order in which the different stressors are listed should not be considered definitive. What can be said with considerable confidence is that the three most frequently encountered causes of impaired uses are nutrients, pathogens and sediments. By contrast “toxic chemicals” such as metals, pesticides, synthetic organics, and ammonia are not as frequently encountered. (This is not to say that toxics need not be addressed in those water bodies where they are a problem.) Data years for the information presented in the table are 2002–2010 and vary by state. For more information, please see EPA’s WATERS Web page.

The table to the left shows that the most commonly cited causes of impaired water body uses vary from one major water body type to another. Of course, this does not mean that the key pollutants for a particular river, lake, or estuary would reflect the national picture shown here. The data presented in the table are drawn from years 2002–2010 and vary by state. See EPA’s WATERS National Summary of State Information Web page for details.

Because of the implementation of CWA regulatory programs controlling point sources of pollution over the last four decades, industrial facilities and municipal sewage treatment plants no longer are the major cause of use impairment of most of the nation’s surface waters. On the other hand, diffuse sources of precipitation-induced runoff are the sole cause of impairment of nearly half of the waters that states, territories, and authorized tribes list in their 303(d) reports. Also likely is that in many of that 50 percent of the water quality-limited waters in which point and nonpoint sources are significant contributors, nonpoint sources contribute considerably more pollutant loads than do point sources