Click on “Download PDF” for the PDF version or on the title for the HTML version.

If you are not an ASABE member or if your employer has not arranged for access to the full-text, Click here for options.

Federal regulations require that determinations of TMDLs take into account critical conditions for stream flow, loadings, and water quality parameters. Although determination of critical conditions is straightforward for some TMDLs, it is often difficult or confusing for others.

In order to identify what kind of additional guidance is needed on selecting critical conditions, this research included a review of national and state guidance on critical conditions for TMDLs, a review of 176 approved TMDLs, and a survey of selected state TMDL programs. These reviews and the survey showed, in general, that there is a lack of guidance on how to select critical conditions. These also showed a wide variation of methods being used to address critical conditions, even for a single pollutant. Therefore, some TMDLs that are being developed may be too conservative and some may not be conservative enough to protect designated uses.

From this research, it is recommended that critical conditions be selected based on the following items:

- factors that affect the pollutant of interest (e.g., temperature and pH for ammonia toxicity

TMDLs or hardness and TSS for metals TMDLs). - Sources of the pollutant (e.g., low flow may be critical for pollutants from groundwater or continuous point sources, but high flow may be critical for pollutants from surface runoff).

- Water quality standards and designated uses (e.g., critical conditions to protect for long term exposure to humans will be different than to protect for short term toxicity exposure to aquatic life).

- Low recurrence interval (i.e., the frequency of occurrence for critical conditions should be low enough that the designated use will be maintained).

- Antecedent history where appropriate (e.g., for conditions where sequences of events are important such as overflows from ponds whose available storage at the beginning of a storm is highly variable).