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TMDLs, continued

TMDLs are required for “pollutants,” but not for all forms of “pollution.” The Clean Water Act regulates pollutants that are discharged to water, while “pollution” is a broader term that can be caused by actions involving discharges as well as those not involving discharges, such as removing stream cover or flow modification. “Pollutants” include substances such as clean sediments, nutrients (nitrogen and phosphorus), pathogens, acids/bases, heat, metals, cyanide, and synthetic organic chemicals. As noted previously, pollution includes all pollutants but also includes flow alterations and physical habitat modifications.

At least one TMDL must be done for every water body or segment impaired by one or more pollutants. TMDLs are done pollutant-by-pollutant. Although if a water body or segment were affected by two or more pollutants, the TMDLs for each pollutant could be done simultaneously.

EPA is encouraging states, tribes, and territories to do TMDLs on a “watershed basis” (e.g., to “bundle” TMDLs together) to realize program efficiencies and foster more holistic analysis. Ideally, TMDLs would be incorporated into comprehensive watershed strategies. Such strategies would address protection of high quality waters (antidegradation) as well as restoration of impaired segments (TMDLs). They would address the full array of activities affecting the water body. Finally, such strategies would be the product of collaborative efforts between a wide variety of stakeholders.

TMDLs must be submitted to EPA for review and approval/disapproval. If EPA ultimately decides that it cannot approve a TMDL that has been submitted, the Agency would need to develop and promulgate what it considers to be an acceptable TMDL. Doing so requires going through the formal federal rulemaking process.

Elements of a TMDL
The first element of a TMDL is “the acceptable load,” also referred to as the pollutant “cap.” It is basically a budget for a particular pollutant in a particular body of water, or an expression of the “carrying capacity.” This is the loading rate that would be consistent with meeting the WQC for the pollutant in question. The cap is usually derived by using mathematical models, which can be derived via simple calculations or through computer modeling.

The CWA requires that all TMDLs include a safety factor as an extra measure of environmental protection, taking into account uncertainties associated with estimating the acceptable cap or load. This is referred to as the margin of safety (MOS).

Once the cap has been set (with the MOS factored in), the next step is to allocate that total pollutant load among various sources of the pollutant for which the TMDL has been done. This is, essentially, the “slicing of the pie.”