Total Maximum Daily Load
Encyclopedia
A Total Maximum Daily Load (TMDL) is a regulatory term in the U.S. Clean Water Act
, describing a value of the maximum amount of a pollutant
that a body of water
can receive while still meeting water quality standards. Alternatively, TMDL is an allocation of that water pollutant deemed acceptable to the subject receiving water
s.
TMDLs have been used extensively by the United States Environmental Protection Agency
(EPA) and state environmental agencies in implementing the Clean Water Act by establishing maximum pollution limits for industrial wastewater
dischargers. EPA published regulations in 1992 establishing TMDL procedures. Application of TMDL has broadened significantly in the last decade to include many watershed-scale efforts. This process incorporates both point source and nonpoint source
pollutants within a watershed
.
s such as aquatic life, fisheries, drinking water
, recreation, industry, or agriculture
. These inventories are known as 303(d) Lists and characterize waters as fully supporting, impaired, or in some cases threatened for beneficial uses.
data for TMDL planning.
Throughout the U.S., data are often lacking adequate spatial or temporal coverage to reliably establish the sources and magnitude of water quality degradation.
TMDL planning in large watersheds is a process that typically involves the following steps:
Necessary values for each pollutant target to restore fisheries can be uncertain. The potential for a water body to support a fishery even in a pristine state can be uncertain.
The natural background load for a pollutant may be imprecisely understood. Industrial dischargers, farmers, land developers
, municipalities
, natural resource agencies, and other watershed stakeholders each have a vested interest in the outcome.
(e.g. agriculture) are generally in a voluntary compliance scenario. The TMDL implementation plan is intended to help bridge this divide and insure that watershed beneficial uses are restored and maintained. Local watershed groups play a critical role in educating stakeholders, generating funding, and implementing projects to reduce nonpoint sources of pollution.
Clean Water Act
The Clean Water Act is the primary federal law in the United States governing water pollution. Commonly abbreviated as the CWA, the act established the goals of eliminating releases of high amounts of toxic substances into water, eliminating additional water pollution by 1985, and ensuring that...
, describing a value of the maximum amount of a pollutant
Pollutant
A pollutant is a waste material that pollutes air, water or soil, and is the cause of pollution.Three factors determine the severity of a pollutant: its chemical nature, its concentration and its persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the...
that a body of water
Body of water
A body of water or waterbody is any significant accumulation of water, usually covering the Earth or another planet. The term body of water most often refers to large accumulations of water, such as oceans, seas, and lakes, but it may also include smaller pools of water such as ponds, puddles or...
can receive while still meeting water quality standards. Alternatively, TMDL is an allocation of that water pollutant deemed acceptable to the subject receiving water
Surface water
Surface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....
s.
TMDLs have been used extensively by the United States Environmental Protection Agency
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
(EPA) and state environmental agencies in implementing the Clean Water Act by establishing maximum pollution limits for industrial wastewater
Industrial wastewater treatment
Industrial wastewater treatment covers the mechanisms and processes used to treat waters that have been contaminated in some way by anthropogenic industrial or commercial activities prior to its release into the environment or its re-use....
dischargers. EPA published regulations in 1992 establishing TMDL procedures. Application of TMDL has broadened significantly in the last decade to include many watershed-scale efforts. This process incorporates both point source and nonpoint source
Nonpoint source pollution
Nonpoint source pollution refers to both water and air pollution from diffuse sources. Nonpoint source water pollution affects a water body from sources such as polluted runoff from agricultural areas draining into a river, or wind-borne debris blowing out to sea. Nonpoint source air pollution...
pollutants within a watershed
Drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...
.
State Inventories
The Clean Water Act requires states to compile lists of water bodies that do not fully support beneficial useBeneficial use
Beneficial use is a legal term describing a person's right to enjoy the benefits of specific property, especially a view or access to light, air, or water, even though title to that property is held by another person. This may also be termed "beneficial enjoyment". Black's Law Dictionary...
s such as aquatic life, fisheries, drinking water
Drinking water
Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...
, recreation, industry, or agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
. These inventories are known as 303(d) Lists and characterize waters as fully supporting, impaired, or in some cases threatened for beneficial uses.
TMDL Planning Process
Beneficial use determinations must have sufficient credible water qualityWater quality
Water quality is the physical, chemical and biological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of standards against which...
data for TMDL planning.
Throughout the U.S., data are often lacking adequate spatial or temporal coverage to reliably establish the sources and magnitude of water quality degradation.
TMDL planning in large watersheds is a process that typically involves the following steps:
- Watershed Characterization—understanding the basic physical, environmental, and human elements of the watershed.
- Impairment Status—analyzing existing data to determine if waters fully support beneficial uses
- Data Gaps and Monitoring Report—identification of any additional data needs and monitoring recommendations
- Source Assessment—identification of sources of pollutants, and magnitude of sources.
- Load Allocation—determination of natural pollutant load, and load from human activities (i.e. diffuse nonpoint sources and point discharges).
- Set Targets—establishment of water quality targets intended to restore or maintain beneficial uses.
- TMDL Implementation Plan—a watershed management strategy to attain established targets.
Water Quality Targets
The purpose of water quality targets is to protect or restore beneficial uses and protect human health. These targets may include state/federal numerical water quality standards or narrative standards, i.e. within the range of "natural" conditions. Establishing targets to restore beneficial uses is challenging and sometimes controversial. For example, the restoration of a fishery may require reducing temperatures, nutrients, sediments, and improving habitat.Necessary values for each pollutant target to restore fisheries can be uncertain. The potential for a water body to support a fishery even in a pristine state can be uncertain.
Background
Calculating the TMDL for any given body of water involves the combination of factors that contribute to the problem of nutrient concentrated runoff. Bodies of water are tested for contaminants based on their intended use. Each body of water is tested similarly but designated with a different TMDL. Drinking water reservoirs are designated differently than areas for public swimming and water bodies intended for fishing are designated differently than water located in wildlife conservation areas. The size of the water body also is taken into consideration when TMDL calculating is undertaken. The larger the body of water, the greater the amounts of contaminants can be present and still maintain a Margin of Safety. The Margin of Safety (MOS) is the nutrients allowed to be present in a body of water and have it still meet state water quality requirements for its intended use. TMDL is the end product of all point and non-point source pollutants of a single contaminant. Pollutants that originate from a point source are given allowable levels of contaminants to be discharged; this is the Waste Load Allocation (WLA). Nonpoint source pollutants are also calculated into the TMDL equation with Load Allocation (LA).Load Allocations
Load allocations are equally challenging as setting targets. Load allocations provide a framework for determining the relative share of natural sources and human sources of pollution.The natural background load for a pollutant may be imprecisely understood. Industrial dischargers, farmers, land developers
Land development
Land development refers to altering the landscape in any number of ways such as:* changing landforms from a natural or semi-natural state for a purpose such as agriculture or housing...
, municipalities
Local government in the United States
Local government in the United States is generally structured in accordance with the laws of the various individual states. Typically each state has at least two separate tiers: counties and municipalities. Some states have their counties divided into townships...
, natural resource agencies, and other watershed stakeholders each have a vested interest in the outcome.
Implementation of TMDLs
To enforce TMDLs with point sources, water quality-based effluent limitations (WQBELs) must be developed and incorporated into discharge permits for these sources. The permits are issued by EPA and state agencies under the National Pollutant Discharge Elimination System (NPDES). Nonpoint source dischargesNonpoint source pollution
Nonpoint source pollution refers to both water and air pollution from diffuse sources. Nonpoint source water pollution affects a water body from sources such as polluted runoff from agricultural areas draining into a river, or wind-borne debris blowing out to sea. Nonpoint source air pollution...
(e.g. agriculture) are generally in a voluntary compliance scenario. The TMDL implementation plan is intended to help bridge this divide and insure that watershed beneficial uses are restored and maintained. Local watershed groups play a critical role in educating stakeholders, generating funding, and implementing projects to reduce nonpoint sources of pollution.
See also
- Clean Water ActClean Water ActThe Clean Water Act is the primary federal law in the United States governing water pollution. Commonly abbreviated as the CWA, the act established the goals of eliminating releases of high amounts of toxic substances into water, eliminating additional water pollution by 1985, and ensuring that...
- Wastewater quality indicatorsWastewater quality indicatorsWastewater quality indicators are laboratory tests to assess suitability of wastewater for disposal or re-use. Tests selected and desired test results vary with the intended use or discharge location. Tests measure physical, chemical, and biological characteristics of the...
- Water pollutionWater pollutionWater pollution is the contamination of water bodies . Water pollution occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful compounds....
- Water qualityWater qualityWater quality is the physical, chemical and biological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of standards against which...
- Water quality modellingWater quality modellingWater quality modeling involves the prediction of water pollution using mathematical simulation techniques. A typical water quality model consists of a collection of formulations representing physical mechanisms that determine position and momentum of pollutants in a water body...