Skip to main navigation Skip to main content

The following selection of terms will be helpful in understanding the TMDL program.

Section 303(d) CWA — Section of the Clean Water Act that requires states periodically to identify waters that do not or are not expected to meet applicable water quality standards. These waters are identified on the Sec. 303(d) Impaired Waters List. A TMDL must be developed for each waterbody on the Sec. 303(d) list. If a listed waterbody has multiple impairments, a TMDL must be developed for each impairment.

Section 305(b) CWA — Section of the Clean Water Act that requires states to submit a biennial report in even-numbered years to the U.S. Environmental Protection Agency describing the quality of the state's waters. The Sec. 305(b) report describes the overall water quality conditions and trends in the state.

303(d) and 305(b) Integrated Report — EPA-recommended approach to integrating water quality conditions data submitted by states under Clean Water Act sections 303(d) and 305(b). EPA guidance provides recommended organization for states' Integrated Report submittals.

Ambient water quality — Natural concentration of water quality constituents prior to mixing of either point or nonpoint source load of contaminants. Reference ambient concentration is used to indicate the concentration of a chemical that will not cause adverse impact on human health.

Antidegradation policies — Policies that are part of each state's water quality standards. These policies are designed to protect water quality and provide a method of assessing activities that might affect the integrity of water bodies.

Aquatic life — Fish, invertebrates and other organisms that live in the water.

Assimilative capacity —The amount of contaminant load that can be discharged to a specific water body without exceeding water quality standards or criteria. Assimilative capacity is used to define the ability of a water body to naturally absorb and use a discharged substance without impairing water quality or harming aquatic life. This is also referred to as loading capacity.

Background levels — Levels representing the chemical, physical, and biological conditions that would result from natural geomorphological processes such as weathering or dissolution.

Bacteria — Single-celled microorganisms. Bacteria of the coliform group are considered the primary indicators of fecal contamination and are often used to assess water quality.

Benthic — Refers to material, especially sediment, at the bottom of an aquatic ecosystem.

Benthic organisms — Organisms living in, or on, bottom substrates in aquatic ecosystems.

Best management practices (BMPs) — Methods, measures, or practices determined to be reasonable and cost-effective means for a landowner to meet certain, generally nonpoint source, pollution control needs. BMPs include structural and nonstructural controls and operation and maintenance procedures.

Clean Water Act —The series of legislative acts that form the foundation for protection of U.S. water resources, including the Water Quality Act of 1965, Federal Water Pollution Control Act of 1972, Clean Water Act of 1977, and Water Quality Act of 1987. CWA Secs. 305(b) and 303(d) deal specifically with water quality assessment and TMDL development.

Critical  (flow) condition —The critical condition can be thought of as the 'worst case' scenario of environmental conditions in the water body in which the loading expressed in the TMDL for the pollutant of concern will continue to meet water quality standards. Critical conditions are the combination of environmental factors (e.g., flow, temperature, etc.) that results in attaining and maintaining the water quality criterion and has an acceptably low frequency of occurrence.

Designated Use — Those uses specified in water quality standards for each waterbody or segment (e.g. Primary Contact Recreation = swimming)

Discharge — Flow of surface water in a stream or canal, or the outflow of groundwater from a flowing artesian well, ditch, or spring. Can also apply to discharge of liquid effluent from a facility.

Drainage basin — A part of a land area enclosed by a topographic divide from which direct surface runoff from precipitation normally drains by gravity into a receiving water. Also referred to as a watershed, river basin, or hydrologic unit.

Effluent — Municipal sewage or industrial liquid waste (untreated, partially treated, or completely treated) that flows out of a treatment plant, septic system, pipe, etc.

Existing use — Use actually attained in the water body on or after November 28, 1975, whether or not it is included in the water

Geometric mean — A measure of the central tendency of a data set that minimizes the effects of extreme values.

Impaired waterbody — A waterbody (i.e., stream, lake) that does not meet water quality standards and designated uses because of pollutant(s), pollution, or unknown causes of impairment.

Instantaneous Flow value — Flow that is contributed by a source or the stream flow itself, in cfs

KPDES Permit — Kentucky Pollution Discharge Elimination System permit issued under the statel program for issuing, modifying, revoking and reissuing, terminating, monitoring, and enforcing permits, and imposing and enforcing pretreatment requirements, under Section 307, 402, 318, and 405 of the Clean Water Act.

Load or Loading —The total amount of pollutants entering a waterbody from one or multiple sources, measured as a rate, as in weight per unit time or per unit area.

Load allocation — The allowable load of a pollutant into a stream from nonpoint sources of pollution (sources that do not require a KPDES permit) and from the natural background.

Margin of Safety (MOS) — An accounting of uncertainty about the relationship between pollutant loads and receiving water quality. The margin of safety can be provided implicitly through analytical assumptions or explicitly by reserving a portion of loading capacity.

National Pollutant Discharge Elimination System (KPDES) —The national program for issuing, modifying, revoking and reissuing, terminating, monitoring, and enforcing permits, and imposing and enforcing pretreatment requirements, under Section 307, 402, 318, and 405 of the Clean Water Act. Facilities subjected to NPDES permitting regulations include operations such as municipal wastewater treatment plants and industrial waste treatment facilities.

Natural Background levels — Chemical, physical, and biological levels representing conditions that would result from natural processes, such as weathering and dissolution.

Nonpoint source pollution — Pollution that is not released through pipes but rather originates from multiple sources over a relatively large area. Nonpoint sources can be divided into source activities related either to land or water use including failing septic tanks, improper animal-keeping practices, forestry practices, and urban and rural runoff.

Point source pollution — Pollutant loads discharged at a specific location from pipes, outfalls, and conveyance channels from either municipal wastewater treatment plants or industrial waste treatment facilities. 

Pollutant — As defined in Clean Water Act Sec. 502(6), a pollutant means dredged spoil, solid waste, incinerator residue, sewage, garbage, sewage sludge, munitions, chemical wastes, biological materials, radioactive materials, heat, wrecked or discarded equipment, rock, sand, cellar dirt, and industrial, municipal, and agricultural waste discharged into water.

Pollutant-waterbody combination (PWC) - Each pollutant causing a waterbody to be impaired or threatened. For example, each entry on the 303(d) list is referred to as a pollutant-waterbody combination. If the same impaired waterbody was 303(d)-listed for two pollutants (E.coli and fecal coliform), the waterbody would appear twice on the 303(d) list with an entry for E.coli and an entry for fecal coliform. All pollutant-waterbody combinations on the 303(d) list require the development of a TMDL.

Pollution —The man-made or man-induced alteration of the chemical, physical, biological, and radiological integrity of water.

Priority Ranking — The "priority ranking" of an impaired water is indicated by the assignment of high, medium,  or low for each impairment on the current impaired waters list. The higher the priority the sooner a TMDL report will be completed for that waterbody.

Septic system — An on-site system designed to treat and dispose of domestic sewage. A typical septic system consists of a tank that receives waste from a residence or business and a drain field or subsurface absorption system consisting of a series of percolation lines for the disposal of the liquid effluent. Solids (sludge) that remain after decomposition by bacteria in the tank must be pumped out periodically.

Sewer — A channel or conduit that carries wastewater and storm water runoff from the source to a treatment plant or receiving stream. Sanitary sewers carry household, industrial, and commercial waste. Storm sewers carry runoff from rain or snow. Combined sewers handle both.

Stakeholder — Any person or organization with vested interest in TMDL development and implementation in a specific watershed

Streamflow — Discharge that occurs in a natural channel. Although the term 'discharge' can be applied to the flow of a canal, the word 'streamflow' uniquely describes the discharge in a surface stream course. The term 'streamflow' is more general than 'runoff' since streamflow may be applied to discharge whether or not it is affected by diversion or regulation.

Surface runoff — Precipitation, snowmelt, or irrigation water in excess of what can infiltrate the soil surface and be stored in small surface depressions; a major transporter of nonpoint source pollutants.

Surface water — All water naturally open to the atmosphere (rivers, lakes, reservoirs, ponds, streams, impoundments, seas, estuaries, etc.) and all springs, wells, or other collectors directly influenced by surface water

Total Maximum Daily Load — The sum of the individual wasteload allocations (WLAs) for point sources, load allocations (LAs) for nonpoint sources and natural background, and a margin of safety (MOS). TMDLs can be expressed in terms of mass per time, toxicity, or other appropriate measures that relate to a state's water quality standard.

TMDL alternative plan — A near-term water quality restoration plan with a schedule of actions and milestones more immediately beneficial or practicable to achieving water quality standards than a TMDL Report.  The impaired waters in a TMDL alternative plan will remain on the 303(d) list, but will be assigned a lower priority for TMDL development. If water quality standards are not achieved, a TMDL report is still required.

TMDL Implementation plan — A water quality restoration plan guided by an approved TMDL that provides details of the actions needed to achieve load reductions, outlines a schedule of those actions, and specifies monitoring needed to document action and progress toward meeting water quality standards.

Tributary — A lower order-stream compared to a receiving water body. 'Tributary to' indicates the largest stream into which the reported stream or tributary flows

Wasteload allocation — The allowable load of a pollutant into a stream from an existing or future point source of pollution (sources required to have KPDES permits)

Wastewater treatment — Chemical, biological, and mechanical procedures applied to an industrial or municipal discharge or to any other sources of contaminated water to remove, reduce, or neutralize contaminants.

Water quality — The biological, chemical, and physical conditions of a waterbody. It is a measure of a waterbody's ability to support beneficial uses.

Water quality criteria — Elements of state water quality standards expressed as constituent concentrations, levels, or narrative statement, representing a quality of water that supports a particular use. When criteria are met, water quality will generally protect the designated use.

Water Quality Standards — State or federal law or regulation consisting of a designated use or uses for the waters of the United States, water quality criteria for such waters based upon such uses, and an antidegradation policy and implementation procedures. Water quality standards protect the public health or welfare, enhance the quality of water and serve the purposes of the Clean Water Act.

Waterbody — A geographically defined portion of navigable waters, waters of the contiguous zone, and ocean waters under the jurisdiction of the United States, including segments of rivers, streams, lakes, wetlands, coastal waters and ocean waters.

Watershed — Area that drains or contributes water to a particular point, stream, river, lake, or ocean. They can be few acres for a small stream or cover large areas of the country.

Watershed Management plan — A strategy that provides assessment and management information for a geographically-defined watershed, including the analyses, actions, participants, and resources for developing and implementing the plan. Also refered to as a Nine Element Watershed Management Plan.


​Stay Connected

Subscribe to the TMDL mailing list 

 Email Us:

 Call Us: 502-564-3410

 Fax: 502-564-4245

TMDL and Program Support Section
Division of Water
300 Sower Blvd, 3rd floor
Frankfort, KY 40601