Total Maximum Daily Load (TMDL)


Welcome to the TMDL Website for the Central Coast Regional Water Quality Control Board. You can:

What is TMDL?

TMDL stands for a Total Maximum Daily Load. This is the amount of a particular material that a waterbody can assimilate on a regular basis and still remain at levels that protect beneficial uses designated for that waterbody.

A TMDL is approved by the Regional Water Quality Control Board, the State Water Resources Control Board and the US Environmental Protection Agency. Once approved, it establishes 1) an allowable amount of a pollutant to a waterbody, 2) proportional responsibility for controlling the pollutant, 3) numeric indicators of water quality, and 4) implementation to achieve the allowable amount of pollutant loading. The process to derive a TMDL and prepare the report that documents this process and recommends implementation typically takes at least one year, and may take several years, to complete.

How do TMDL's Work?

TMDLs are developed by analyzing data and information provided by existing or commissioned studies, and/or by stakeholders interested in the waterbody or conditions being investigated. Development results in a clear definition of water quality problems in a waterbody or watershed, a numeric value for the TMDL, and an implementation plan that identifies how the problems will be solved and the TMDL achieved. The implementation plans identify new requirements, based on existing regulations, in conjunction with other existing water quality management activities. The implementation plans identify which requirements or activities (via voluntary or regulatory programs) apply to which agencies, landowners, resource managers, and/or the public. Typically, TMDLs and their implementation plans will be approved by adoption into the Regional Board’s Basin Plan.

General Information and Background

Section 303(d) of the Clean Water Act requires States to identify waters not attaining applicable water quality standards. The Section states:

Each State shall establish for the waters identified in paragraph (1)(A) of this subsection, and in accordance with the priority ranking, the total maximum daily load, for those pollutants which the Administrator identifies under section 1314(a)(2) of this title as suitable for such calculation. Such load shall be established at a level necessary to implement the applicable water quality standards with seasonal variations and a margin of safety which takes into account any lack of knowledge concerning the relationship between effluent limitations and water quality.

The State complies with this requirement by periodically assessing the conditions of the rivers, lakes and bays and identifying them as "impaired" if they do not meet water quality standards. These waters, and the pollutant or condition causing the impairment, are placed on the 303(d) List of Impaired Waters. In addition to creating this list of waterbodies not meeting water quality standards, the Clean Water Act mandates each state to rank each waterbody by factors such as the severity of the problem, potential to restore beneficial uses, availability of data, etc., and develop TMDLs for each waterbody listed. TMDLs will be developed based on a schedule that accounts for priority ranking, availability of resources, and other considerations.

Water quality standards are comprised of beneficial uses and water quality objectives. Beneficial uses include drinking, aquatic habitat, irrigation, and several other types of use. Water quality objectives consist of narrative protections and numeric protective values. Water quality standards are set forth in the Water Quality Control Plan (Basin Plan). An electronic version of the Basin Plan is available on this Website.

The Central Coast Regional Water Quality Control Board is the agency responsible for protecting water quality consistent with the Basin Plan, including developing TMDLs for waterbodies identified as not meeting water quality objectives.

The State of California has developed guidance called, "A Process for Addressing Impaired Waterbodies in California." The goal of this guidance document is to assist the California State Water Resources Control Board and Regional Water Quality Control Boards in addressing impaired waters. This document is also intended to provide the public with a better understanding of the process and products associated with the assessment of impaired waters and development of implementation plans to improve them.

303(d) Process

Updates to 303(d) List