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Total Maximum Daily Loads

What is a TMDL?

A Total Maximum Daily Load (TMDL) is a "pollution budget" for a water body (such as a stream, creek, bay or lagoon). That is, it sets limits on the amount of pollution that a stream or other water body can tolerate and still maintain water quality standards. Through the TMDL process, states establish water-quality based controls to reduce pollution and meet water quality standards. TMDLs are issued on a watershed basis and do not recognize political boundaries as multiple government agencies and other groups may be involved.

TMDLs adopted in the San Diego Bay Watershed

Chollas Creek

• Diazinon TMDL: Diazinon in Chollas Creek Watershed San Diego County (adopted 2003)
• Dissolved Metals TMDL: Dissolved Copper, Lead, and Zinc in Chollas Creek, Tributary to San Diego Bay (adopted 2008)
• Indicator Bacteria TMDL: Revised Project I - Twenty Beaches and Creeks in San Diego Region, including Tecolote Creek (adopted 2010)
• Addressing pollution in Chollas Creek

Shelter Island

• Dissolved Copper: Shelter Island Yacht Basin Dissolved Copper TMDL (Adopted 2005)
• Indicator Bacteria TMDL: Project II - Baby Beach in Dana Point Harbor and Shelter Island Shoreline Park in San Diego Bay (adopted 2008)
• Addressing copper pollution through the Port of San Diego’s Copper Reduction Program

When is a TMDL required?

When a water body fails to meet state or federal water quality standards, the specified area may be placed on the 303(d) Impaired Waters List. According to the Clean Water Act, each state must develop TMDLs for all the waterbodies on the 303(d) Impaired Waters List. The US Environmental Protection Agency’s (EPA) Water Quality Management and Planning Regulation (40 CFR Part 130) requires states to develop TMDLs for each pollutant. Each state may prioritize waters on the 303(d) Impaired Waters List, in order to maximize resources.

TMDLs account for different sources of pollution for the specified area. Sources of pollution are characterized as two types: point source and non-point source. Point sources are easily recognizable and are discharges from a specific location, such as pipes, outfalls, and stormwater conveyance channels. Nonpoint sources include many different sources, such as polluted runoff from fields, pastures, and construction sites, and are usually difficult to track. In order to develop a TMDL, natural background concentrations of the pollutant and seasonal variations in water quality are also considered.

Once a TMDL is developed and approved by EPA, steps must be taken to reduce pollution levels for the area identified in the TMDL. These steps, which can include the use of better treatment technology and the installation of best management practices (BMPs), are implemented in a staged process that is described along with specific BMPs in an Implementation Plan.

Stakeholder Involvement

Stakeholders are individuals who live or have land management responsibilities in the watershed, including government agencies, businesses, private individuals and special interest groups. Stakeholder participation and support is essential for achieving the goals of any TMDL effort (i.e., improving water quality and removing identified areas from the impaired waters list).