Total Maximum Daily Load

A Total Maximum Daily Load (TMDL) is 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 pollutant deemed acceptable to the subject receiving waters. TMDL was used extensively by the U.S. EPA in implementing the Clean Water Act by establishing maximum pollution limits for industrial waste dischargers. Application of TMDL has broadened significantly in the last decade to include many watershed scale efforts. This process incorporates both point and non-point source pollutants within a watershed.

Additional recommended knowledge

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States are required to compile a list of waterbodies that do not fully support beneficial uses such as aquatic life, fisheries, drinking water, recreation, industry, or agriculture. This inventory is known as a 303(d) list and characterizes waters as fully supporting, impaired, or in some cases threatened for beneficial uses.

Beneficial use determinations must have sufficient credible water quality data for TMDL planning. Throughout the US, 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:

1. Watershed Characterization Report- understanding the basic physical, environmental, and human elements of the watershed.
2. Impairment Status Report-analyzing existing data to determine if waters fully support beneficial uses
3. Data Gaps and Monitoring Report-identification of any additional data needs and monitoring recommendations
4. Source Assessment-identification of sources of pollutants, and magnitude of sources.
5. Load Allocation-determination of natural pollutant load, and load from human activities (i.e. diffuse nonpoint sources and point discharges).
6. Set Targets-establishment of water quality targets intended to restore or maintain beneficial uses.
7. TMDL Implementation Plan-a watershed management strategy to attain established 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 waterbody to support a fishery even in a pristine state can be uncertain.

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 discharges, agricultural operations, development, municipalities, natural resources agencies, and other watershed stakeholders each have a vested interest in the outcome.

Point source dischargers of a pollutant are regulated by state or federal discharge permits. 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 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.