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Water: CWA Methods

Toxic and Priority Pollutants

Two lists have special significance to water quality regulatory programs in the Clean Water Act (CWA):

List of Toxic Pollutants

Key Features

  1. The Clean Water Act references the list of toxic pollutants at §307(a)(1) (also labelled §1317(a)(1)).
  2. The list appears in the Code of Federal Regulations at 40 CFR 401.15
  3. The list is an important starting point for EPA to consider, for example, in developing national discharge standards (such as effluent guidelines) or in national permitting programs (such as NPDES).
  4. The list contains 65 entries. Many of the entries, such as "haloethers," are for groups of pollutants.

Connection between CWA §307(a)(1) and the List of Toxic Pollutants

  1. Section 307(a)(1) says, "…the list of toxic pollutants or combination of pollutants subject to this Act shall consist of those toxic pollutants listed in table 1 of Committee Print Numbered 95-30 of the Committee on Public Works of the House of Representatives…"
  2. Committee Print 95-30 (November 1977) is titled "Data Relating to H.R. 3199 (Clean Water Act of 1977)."
  3. Table 1 is titled "Section 307—Toxic Pollutants." EPA incorporated Table 1 into the Code of Federal Regulations at §401.15.

History of the List of Toxic Pollutants

  1. Source of the list: The list was negotiated among parties to a settlement agreement (NRDC et al. vs Train, 6 ELR 20588, D.D.C. June 9, 1976).
  2. That settlement agreement is sometimes referred to as the Toxics Consent Decree, or the Flannery Decision (for presiding U.S. District Court Judge Thomas A. Flannery).
  3. Congress subsequently ratified the Settlement Agreement and the list of toxic pollutants when they amended the CWA (Public Law 95-217) in 1977.
  4. Note to readers: The Congressional Research Service prepared a paper in 1993 on Toxic Pollutants and Clean Water Act Exit EPA Disclaimer .
  5. The list was first published on January 31, 1978 in the Federal Register (43 FR 4108).
  6. In a final rule on July 31, 1979 (44 FR 44501), EPA published the list again and added the list to the CFR at 40 CFR 401.15.


  1. EPA removed three pollutants from the list in 1981, after determining that their chemical properties did not justify their inclusion:
    • Dichlorodifluoromethane and trichlorofluoromethane were de-listed on January 8, 1981 (46 FR 2266) at the request of E.I. duPont de Nemours and Co. because of low solubility in water and high volatility combined with low human and mammalian toxicity. Bis(chloromethyl) ether was de-listed on February 4, 1981 (46 FR 10723) based on data that indicated a half-life in water of 38 seconds at 20°C.
  2. De-listing the three pollutants did not change the 65 entries because the three de-listed pollutants were specific compounds within entries for the groups Halomethanes (list entry 38) and Haloethers (list entry 37).

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Priority Pollutants

Key Features

Key features of the list of priority pollutants and its relationship to the list of toxic pollutants:

  1. The Priority Pollutants are a set of chemical pollutants EPA regulates, and for which EPA has published analytical test methods.
  2. The Priority Pollutant list makes the list of toxic pollutants more usable, in a practical way, for the purposes assigned to EPA by the Clean Water Act. For example, the Priority Pollutant list is more practical for testing and for regulation in that chemicals are described by their individual chemical names. The list of toxic pollutants, in contrast, contains open-ended groups of pollutants, such as "chlorinated benzenes." That group contains hundreds of compounds; there is no test for the group as a whole, nor is it practical to regulate or test for all of these compounds.


Starting with the list of toxic pollutants, EPA used four criteria to select and prioritize specific pollutants:

  1. We included all pollutants specifically named on the list of toxic pollutants;
  2. There had to be a chemical standard available for the pollutant, so that testing for the pollutant could be performed;
  3. The pollutant had to have been reported as found in water with a frequency of occurrence of at least 2.5%, and
  4. The pollutant had to have been produced in significant quantities, as reported in Stanford Research Institute's 1976 Directory of Chemical Producers, USA.

Number of Entries

Originally, there were 129. When three pollutants were removed from the list of toxic pollutants in 1981 (see above), they were also removed from the Priority Pollutant list.

  1. Entry numbers 17, 49, and 50 were removed.
  2. The last number on the list is still 129, although there are 126 entries.


Why is the Priority Pollutant list published at 40 CFR 423, Appendix A, rather than at section 401, or some other, more general section?

  1. One of the first industrial categories for which EPA developed effluent regulations was the Steam Electric Power Generating Point Source Category. The Priority Pollutant list was included to support regulations for that category.
  2. Although the other sections within part 423 apply only to Steam Electric Power Generating, the Priority Pollutant list in Appendix A is not limited in terms of its relevance to that one industrial category.
  3. Some users find it helpful to think of Appendix A to Part 423 as a convenient storage place for the list, or as a matter of convenience for citation.
  4. The list of Priority Pollutants can be found here.

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