What is a TMDL?

A Total Maximum Daily Load (TMDL) is a pollution reduction plan that accounts for all pollutant sources to the water and determines how much each source is allowed to contribute. The basic premise is that if existing pollutant inputs (loads) from all sources are reduced to a specified level (the maximum daily load), and a margin of safety is added, then water quality goals will be achieved. Following is the basic TMDL formula:

TMDL =
Load Allocations (nonpoint sources and natural background)
+ Waste Load Allocations (point sources)
+ Margin of Safety

The EPA requires that TMDLs be developed for all watersheds not meeting water quality standards. These watersheds are compiled on a 303(d) list, which is submitted to EPA for approval. Once the 303(d) list is approved, the process of developing TMDLs begins. The 303(d) list and the TMDLs are based on the designated uses established for a watershed. The designated uses are the quality goals.

TMDLs must be developed for every pollutant that causes a watershed to exceed clean water limits. It is important to identify the point and nonpoint sources as closely as possible by using water quality data and computer modeling. The greater the level of uncertainty about the source of a pollutant, the greater the margin of safety must be for that pollutant. A larger margin of safety means that load allocations and waste load allocations must be smaller.

As a fictional example, assume that the TMDL for tennis balls in a community swimming pool is 10. Some tennis balls are added by Johnny, who is playing in the pool with his friends; this is a point source. Other balls are added to the pool by crazy shots from the tennis courts near the pool; this is a nonpoint source. The lifeguard conducts a study and finds that anywhere from 4 to 7 tennis balls are added by the courts. Because of the uncertainty, the pool owner requires a margin of safety of at least 2. Therefore, Johnny would only be allowed to bring 1 tennis ball to the pool (TMDL 10 = 1 point source + 7 nonpoint source + 2 margin of safety). If, on the other hand, the lifeguard is able to do a more focused study and finds that only 6 balls are added to the pool per day from crazy tennis players, then Johnny would be able to bring 3 tennis balls every day. Since these numbers are well-defined, a margin of safety of 1 tennis ball is acceptable (TMDL 10 = 3 point sources + 6 nonpoint sources + 1 margin of safety).

EPA allows states to determine how the pollutant loads will be allocated among point and nonpoint sources. To continue the example, the lifeguard can ask Johnny to bring fewer balls, or require the tennis courts to construct higher fences. Point source contributions are usually well defined from NPDES permits. Estimates of nonpoint sources and background are usually determined by current land uses in the watershed. The TMDL allocation is based on projected contributions from these sources. Once the TMDL allocations are established, the TMDLs are submitted to EPA for approval. TMDLs can be implemented to allow for pollutant trading among sources contributing to the watershed addressed by the TMDL. In fact, the lower Boise River watershed has been selected by the EPA as a demonstration project for pollutant trading, which brings additional funding to our TMDL efforts and will set the national standard for the pollutant trading process.

More information about the TMDL process in Idaho is provided on Idaho DEQ's TMDL page.