Restoring impaired waters two

"Daily" pollutant loads versus other expressions

One of the problems encountered when evaluating TMDLs is that the cap and the allocations in the Total Maximum Daily Load are often not specified as maximum daily amounts. They are frequently expressed as monthly or annual loads. The TMDLs are required to account for daily loads to assure that standards are met at ALL times, despite daily or seasonal changes in conditions. It is possible to meet a monthly or an annual load cap even if critically poor water quality conditions occurred during part of that month or year.

Realistically, it is difficult to express some TMDLs, such as those dealing with habitat or flow problems, as a daily pollutant load. If the target of a TMDL is presented as something other than a daily load, insist that the agency explain how the water quality criteria will be met and the uses will be protected every day throughout the year.

The Margin of Safety

TMDLs must include a Margin of Safety (MOS) to account for the uncertainty about the relationship between pollutant limits and water quality targets (CWA, Section 303(d)(1)(c)), which might be due to data gaps, missing sources, modeling assumptions, etc. The MOS is intended to provide a cushion in the TMDL because it is difficult, if not impossible, to determine exactly what is needed to restore or protect the water body on the first try.

A MOS can be introduced either as an explicit, set-aside part of the total allowable load, or through implicit conservative assumptions used in determining the TMDL. The implicit approach is more commonly used by agencies, but it is less informative to the public. An explicit MOS set-aside more clearly accounts for uncertainties in the assumptions used to establish a pollutant cap and allocate pollutant loads among sources.

Holding all pollutant sources accountable

The TMDL can allocate the pollutant “pie” in several ways. For point sources, the situation is straightforward. In most cases, each NPDES permittee will be allocated a certain amount of the pollutant load. In some TMDLs, smaller point sources are grouped together for a collective pollutant allocation. This approach can make it difficult to track an individual permittee's compliance with the TMDL, and it can result in “hot spots” where violations are greater if the sources are in close proximity. Regulators need to include every point source of the targeted pollutant as a part of a Waste Load Allocation (WLA) and assign specific pollutant loads to each source that can be translated into numeric, enforceable permit limits.

For nonpoint sources, pollutant loads are often allocated to broad categories of activities in a water body. For example, pollutant loads may be divided into agricultural and silvicultural sources. Because nonpoint sources of pollution are typically unregulated, allocation of a portion of the “pie” to specific nonpoint pollutant sources in the Load Allocation (LA) may provide the best opportunity to secure local support and funding for necessary controls.

Regulators may overlook a pollutant source. Identification of pollutant sources can be a fruitful contribution of interested watershed residents to the TMDL process.