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Selecting Critical Conditions for TMDLs
Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org
Citation: Pp. 215-215 in Total Maximum Daily Load (TMDL) Environmental Regulations–II Proceedings of the 8-12 November 2003 Conference (Albuquerque, New Mexico USA), Publication Date 8 November 2003. .(doi:10.13031/2013.15560)
Authors: Philip Massirer, Wendy Larson, Dennis Ford, Kent Thornton, Thomas Soerens
Keywords: Selecting Critical Conditions for TMDLs
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
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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
- 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
- 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