A TMDL (Total Maximum Daily Load) is defined in federal regulations as “the sum of the individual waste load allocations for point sources and load allocations for nonpoint sources and natural background such that the capacity of the waterbody to assimilate pollutants (the Loading Capacity) is not exceeded.”

In layman’s terms, a TMDL sets a limit for the total loading of a particular pollutant, such that the pollutant loads from all sources will not impair the beneficial uses designated for the waterbody. The limit identified by a TMDL is allocated among different point sources (wasteload allocations) and nonpoint sources (load allocations). TMDL targets also incorporate a margin of safety to account for uncertainty. Therefore a TMDL target equals the wasteload allocations + load allocations + margin of safety. The timeframe for compliance with TMDL targets varies, but may take many years. Therefore the TMDL will often include a compliance schedule, identifying interim and final targets.

A TMDL is developed when a waterbody has been identified as impaired, meaning that a pollutant, or multiple pollutants, are exceeding the water quality standards designated by the State, Territory, or Tribe. Water quality standards protect a waterbody’s designated beneficial uses, such as contact recreation (swimming), drinking water supply, and wildlife habitat.

When a waterbody is determined to be impaired, it is placed on what is known as the 303(d) list (named after a section of the Clean Water Act). Section 303(d) requires states to establish a listing of all impaired waterbodies and to rank those waterbodies according to priority for TMDL development. The US Environmental Protection Agency has oversight authority for the 303(d) program and is required to review and either approve or disapprove the TMDLs submitted by states. If EPA disapproves a TMDL, then the EPA is required to establish a TMDL for that waterbody.

The TMDLs that have been developed for Newport Bay* are:

Sediment TMDL
Nutrient TMDL
Toxics TMDL
Fecal Coliform TMDL

*Note: The Sediment, Nutrient, and Toxics TMDLs have been developed jointly for both San Diego Creek and Newport Bay. The Fecal Coliform TMDL applies only to Newport Bay.