A total maximum daily load (TMDL) is a regulatory term in the U.S.

Clean Water Act The Clean Water Act (CWA) is the primary federal law in the United States governing water pollution. Its objective is to restore and maintain the chemical, physical, and biological integrity of the nation's waters; recognizing the responsibiliti ...

, describing a plan for restoring impaired waters that identifies the maximum amount of a

pollutant A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...

that a

body of water A body of water or waterbody (often spelled water body) is any significant accumulation of water on the surface of Earth or another planet. The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such as ...

can receive while still meeting water quality standards.

State and federal agency responsibilities

The Clean Water Act requires that state environmental agencies complete TMDLs for impaired waters and that the United States Environmental Protection Agency (EPA) review and approve / disapprove those TMDLs. Because both state and federal governments are involved in completing TMDLs, the TMDL program is an example of

cooperative federalism Cooperative federalism, also known as marble-cake federalism, is defined as a flexible relationship between the federal and state governments in which both work together on a variety of issues and programs. In the United States In the American f ...

. If a state doesn't take action to develop TMDLs, or if EPA disapproves state-developed TMDLs, the EPA is responsible for issuing TMDLs. EPA published regulations in 1992 establishing TMDL procedures. Application of TMDLs has broadened significantly in the last decade to include many watershed-scale efforts, including the

Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the Eastern Shore of Maryland / E ...

TMDL. TMDLs identify all point source and

nonpoint source Nonpoint source, or non-point source, or NPS, is a source that does not come from a single point. * Point source, contrasts with nonpoint source * Nonpoint source pollution, water pollution * Nonpoint source water pollution regulations, water pollu ...

pollutants within a

watershed Watershed is a hydrological term, which has been adopted in other fields in a more or less figurative sense. It may refer to: Hydrology * Drainage divide, the line that separates neighbouring drainage basins * Drainage basin, called a "watershe ...

State inventories

The Clean Water Act requires states to compile lists of water bodies that do not fully support

beneficial 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. It is also referred to as " ...

s such as

aquatic life An aquatic ecosystem is an ecosystem formed by surrounding a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms that are dependent on each other and on their environment. The tw ...

fisheries Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life; or more commonly, the site where such enterprise takes place ( a.k.a. fishing ground). Commercial fisheries include wild fisheries and fish farms, bot ...

, drinking water, recreation, industry, or

agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...

; and to prioritize those water bodies for TMDL development. These inventories are known as "303(d) lists" and characterize waters as ''fully supporting'', ''impaired'', or in some cases ''threatened'' for beneficial uses.

Planning process

Beneficial use determinations must have sufficient credible water quality 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 Monitoring may refer to: Science and technology Biology and healthcare * Monitoring (medicine), the observation of a disease, condition or one or several medical parameters over time * Baby monitoring * Biomonitoring, of toxic chemical compounds, ...

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 from areas for public swimming and water bodies intended for fishing are designated differently from 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 numeric estimate included in the TMDL calculation, sometimes 10% of the TMDL, intended to allow a safety buffer between the calculated TMDL and the actual load that will allow the water body to meet its beneficial use (since the natural world is complex and several variables may alter future conditions). 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).

Calculation

The calculation of a TMDL is as follows:
:

TMDL = WLA + LA + MOS

where WLA is the waste load allocation for point sources, LA is the load allocation for nonpoint sources, and MOS is the margin of safety.

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,

municipalities A municipality is usually a single administrative division having corporate status and powers of self-government or jurisdiction as granted by national and regional laws to which it is subordinate. The term ''municipality'' may also mean the go ...

, natural resource agencies, and other watershed stakeholders each have a vested interest in the outcome.

Implementation

To implement TMDLs with point sources, wasteload allocations are incorporated into discharge permits for these sources. The permits are issued by EPA or delegated state agencies under the National Pollutant Discharge Elimination System ( NPDES). Nonpoint source discharges (e.g. agriculture) are generally in a voluntary compliance scenario. The TMDL implementation plan is intended to help bridge this divide and ensure 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.

References

External links

EPA TMDL ProgramEPA's AskWATERS Expert Query Tool
- data on TMDLs
EPA Water Quality Standards ProgramStochastic Empirical Loading and Dilution Model Documentation Page
- US Geological Survey
The Center for TMDL and Watershed Studies at Virginia Tech
{{DEFAULTSORT:Total Maximum Daily Load Hydrology Waste management concepts Water pollution in the United States Environmental law in the United States