TMDL Definitions

Beneficial Use

Streams, lakes, rivers, and other water bodies, have uses to humans and other life. These uses, or beneficial uses, are outlined in a Water Quality Control Plan, also called the Basin Plan of the Central Coast Region. There are 24 categories of beneficial uses, including water contact recreation, non-water contact recreation, municipal water supply, cold fresh water habitat, and more. Each body of water in the state has a set of beneficial uses it supports that may or may not include all 24. Different beneficial uses require different water quality control. Therefore, each beneficial use has a set of water quality objectives designed to protect that beneficial use. Below is a list of some of the beneficial uses described in the Water Quality Control Plan.

Water used for the following purposes: domestic (homes, human consumption, etc.), irrigation (crops, lawns), power (hydroelectric), municipal (water supply of a city or town), mining (hydraulic conveyance, drilling), industrial (commerce, trade, industry), fish and wildlife preservation, aquaculture (raising fish etc. for commercial purposes), recreational (boating, swimming), stockwatering (for commercial livestock), water quality, frost protection (misting or spraying crops to prevent frost damage), heat control (water crops to prevent heat damage), ground water recharge, agriculture, etc.

Impaired Waterbody

A body of water is said to be impaired when a water quality objective(s) or standard is not met. For example, if an objective for nitrate is set at a maximum of 45 milligrams per liter, and a stream has nitrate occurring at greater than 45 milligrams per liter, then that body of water is said to be impaired. Water quality objectives are established to protect Beneficial Uses.

Listed Body of Water

A listed waterbody is one that is designated as impaired pursuant to Section 303(d) of the Clean Water Act . As such the quality of its water is not supporting a beneficial use(s) or violates a water quality standard. Bodies of water that are ‘listed' are identified on the 303(d) list.

Loading Allocation

A loading allocation is a portion of the total amount of pollutant entering a waterbody that is attributed to a particular source. In terms of a TMDL it is a portion of the total maximum load allowed (or the loading capacity). For example, if a TMDL states that 1000 lbs/day of a pollutant can be discharged into a stream from all sources, and discharger A is allowed to discharge 100 lbs. of the 1000 lbs., then discharger A is said to have an allocation of 100 lbs.

Loading Capacity

The greatest amount of loading, of a pollutant, that a water body can receive without violating water quality objectives; the total maximum daily load.

Priority Ranking

Section 303(d) of the Clean Water Act mandates that states prioritize listed water bodies.  Typically, those bodies of water having highest priority will be considered to receive first attention.


The initials used for ‘Total Maximum Daily Load.' The initials ‘TMDL' are used to denote the quantity of a pollutant that can be assimilated by a waterbody and still meet water quality objectives. TMDLs are also referred to as the loading capacity or assimilative capacity of the waterbody. TMDLs are not always identified as daily loads, but rather monthly or annual loads, but the term TMDL is commonly still used for familiarity. Similarly, TMDLs are commonly, but not always, expressed as “loads.” They can also be expressed as concentrations or other appropriate measure.

Water Quality Control Plan

The State Water Resources Control Board carries out its water quality protection authority through the adoption of specific Water Quality Control Plans (also referred to as the Basin Plan). These plans establish water quality standards for particular bodies of water. Water Quality Control Plans also describe programs of implementation designed to ensure that water quality standards are met

Water Quality Objective

Water quality objectives are the limits or levels of water quality constituents or the characteristics of a waterbody that are established for the reasonable protection of beneficial uses of water. Water quality objectives are numeric limits and narrative objectives designed to ensure that bodies of water in the state can support their designated beneficial uses. At concentrations equal to or greater than numeric objectives, constituents (or pollutants) are considered to have impaired the beneficial uses of the state's water. Sometimes, the objectives are narrative, which are qualitative objectives. A narrative objective in the Basin Plan might state, “Waters shall not contain biostimulatory substances in concentrations that promote aquatic growths…” With this narrative objective, the actual numeric limit for the concentration is not articulated.

Water Quality Standard

(a) A state adopted ambient standard for a surface or ground water body. The standard covers the beneficial use of the water and the water quality criteria that must be met to protect the designated use or uses.

(b) Pursuant to the federal Clean Water Act, a combination of the designated beneficial uses of water and criteria or water quality objectives to protect those uses.

Provisions of State or Federal law consist of a beneficial use or uses for the waters of the United States and water quality criteria for such waters based upon such uses. Water quality standards are to protect the public health or welfare, enhance the quality of water, and serve the purposes of the Clean Water Act (Federal Law). California water quality standards are composed of four parts: the designation of beneficial uses of water, water quality objectives designed to protect those uses, implementation programs designed to achieve or maintain compliance with the water quality objectives, and an anti-degradation policy.