303(d): Threatened and Impaired Waters List

If monitoring and assessment indicate that for some uses and/or parameters, a waterbody or segment is not meeting WQS, then that water is considered "impaired" and goes on a special list called the "303(d) list," named after the section of the CWA that calls upon states, approved tribes, and territories to create such lists.

The 303(d) list should include not only currently impaired waterbodies but also waters believed to be threatened that are likely to become impaired (i.e., not meet WQS) by the time the next 303(d) list is due.

Current EPA regulations call for 303(d) lists to include only waters impaired by "pollutants," not those impaired by other types of "pollution" (altered flow and/or channel modification). If it is certain that a waterbody's impairment is not caused by a "pollutant" but is due to another type of "pollution" such as flow, the waterbody does not need to be on the 303(d) list. If, however, biological monitoring indicates there is impairment of aquatic life uses, but it is not clear whether a pollutant is at least one of the reasons, the water should be on the 303(d) list, and further analysis to identify the causes are needed. Waters impaired by "non-pollutant pollution" should be identified in 305(b) reports.

EPA guidance documents mention a number of different types of data and information that are considered "exiting and readily available." EPA has stated that such data include: (1) evidence of exceedance of a numeric WQC, (2) direct evidence of beneficial use impairment, (3) evidence that narrative standards are not being met, and (4) results of computer modeling of the waterbodies. EPA also requires that data from sources other than the state agency itself -- federal agencies, universities, volunteer monitoring groups -- must be considered if they meet the state's requirements for data quality.

Some of the above actions may initially seem obvious, such as evidence of numeric WQC exceedances. But even this can be subject to debate. For instance, suppose you are dealing with a WQC expressed as a 30-day average concentration of pollutant "x," and you have only two data points for the relevant 30-day period, each representing just one "grab sample." Suppose both were higher (more polluted) than the WQC. Should this water be listed as "impaired," or should more data be collected before putting the water on 303(d) list?

How would you measure impairment of a designated use directly? Use of a biological assessment of aquatic life uses could be one method. Epidemiological studies showing a correlation between people swimming in the water and incidence of waterborne disease could be a direct measure of impairment of contact and recreation uses.

How should narrative WQC be interpreted? For example, how much "scum or floating debris" would constitute an exceedance? Would algal mats floating on a surface of the lake represent an exceedance of this narrative WQC, or perhaps of an "undesirable or nuisance aquatic life" narrative?

What if water quality computer modeling studies indicated that WQC would be exceeded at critical low flows, but actual monitoring data available from numerous samples from more typical flow conditions showed no exceedances of criteria. Should the waterbody be listed?

What level of training for volunteer monitors and what extent of quality assurance/quality control (QA/QC) measures should be required before data collected via volunteer monitoring efforts could be used as the basis of putting a waterbody on the 303(d) list?

The Atlas of America's Polluted Waters reflects a national picture of waterbody impairment as reported in the 1998 303(d) list.