TMDL Glossary

For more detailed description of key TMDL pieces, see the Network’s Compilation of EPA’s Regulations and Guidance on Key TMDL Issues.

Antidegradation:  Policies that are designed to protect water quality for a particular water body where the water quality exceeds levels necessary to protect aquatic life and recreational uses.  In a nutshell, keeping clean waters clean. Antidegradation also includes special protections for waters designated as "Outstanding Natural Resource Waters."

Background levels.  The level of a pollutant the represents the chemical, physical, and biological conditions that result from natural processes like weathering.

Best Management Practices (BMPs):  Permit condition(s) used to prevent or control the discharge of pollution.  BMPs may include a schedule of activities, prohibition of practices, maintenance procedures, or other management practices.  BMPs are sometimes used in NPDES permits in place of or in conjunction with effluent limitations.  BMPs are also used in nonpoint source programs for voluntary control of polluted runoff.

Best Professional Judgment (BPJ): A method used by permit writers to develop technology-based NPDES permits limits and conditions on a case-by-case basis.  BPJ is also used to list waterbodies on the 303(d) list in some cases. 

Critical conditions: Basically the worst casescenario of conditions for a waterbody.  The loading capacity (the number) set by a TMDL must take into account "critical conditions" for the particular waterbody.  These critical conditions must take into account "critical conditions for stream flow, loading, and water quality parameters."  See 40 C.F.R. 130.7(c)(1)

Concentration: The strength of a pollutant in a unit volume of water such as milligrams per litter or parts per million.

Designated use:  These are the uses, designated in the state water quality standards, which we desire to protect. These uses must be protected under state water quality standards.  However, they may or may not be attained at any given point in time (e.g. the waterbody may not be safe for the designated use and so may be identified as impaired under the Total Maximum Daily Load Program).

Dynamic model:  A mathematical system that describes and simulates the physical behavior of a waterbody over time.

Effluent limitations: Restrictions imposed on the quantities, discharge rates, and concentrations of pollutants that can be discharged from point sources of pollution into waterways.  These restriction are incorporated into each polluter’s NPDES permit under the Clean Water Act.

Existing use:  The uses (ex.’s swimming, fishing, or support of aquatic life) that the waterway was clean enough to support on or after November 28, 1975.  These uses must be protected by the state’s water quality standards.

Load allocation: The portion of a receiving water’s loading capacity that is attributed either to one of its existing or future nonpoint sources of pollution or to natural background sources. The load allocations can be "best estimates of loading" or allocations to specific nonpoint sources or broad allocations to a class of sources.  See 40 C.F.R. 130.2 (g)

Loading capacity:  The greatest amount of loading that a waterbody can receive without violating water quality standards. The loading capacity must take into account "critical conditions for stream flow, loading, and water quality parameters."  The loading capacity expresses loadings as either mass-per-time, toxicity or other appropriate measure.  See 40 C.F.R. 130.2(f)

Margin of safety:  A required element of a TMDL that is meant to account for any lack of knowledge concerning the relationship between load and wasteload allocations and water quality.  EPA allows the margin of safety to be either implicit or explicit in the TMDL.

Mass balance:  An equation which basically establishes that the mass of the pollution going into an area and the mass leaving that area must balance each other.

National Pollutant Discharge Elimination System (NPDES): The national system for issuing, modifying, revoking, monitoring and enforcing permits. NPDES permits regulate point sources of pollution.    The system also imposes and enforces pretreatment requirements.  See Sections 307, 318, 402 and 405 of the Clean Water Act.

Nonpoint source:  Nonpoint source pollution, also known as polluted runoff, is the single largest source of water pollution nationwide. Polluted runoff is the result of rain or melting snow carrying pollutants or sediments from the land to the water. Polluted runoff results in water pollution from land-disturbing activities like agriculture, forestry, mining and urban development.  Congress added Section 319 to the Clean Water Act in 1987, which directs states to assess their waters for runoff damages and create watershed-based programs to repair damages and prevent further pollution.

Point source: Any discernible, confined, and discrete conveyance, including but not limited to any pipe, ditch, channel, tunnel, conduit, well, discrete fixture, container, rolling stock, concentrated animal feeding operation, landfill leachate collection system, vessel, or other floating craft from which pollutants are or may be discharged.  See Section 502 of the Clean Water Act.

Pollutant: As defined by the Clean Water Act in Section 502(6):  "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."

Reference site: A waterbody that represents the characteristics of the region (i.e. geology, etc.) and has been subject to minimal disturbance (i.e. pollution, etc.) by humans.

Technology-based  limitations:  Effluent limitations set for individual industries that apply when the discharge will not cause a violation of water quality standards, even at low stream flows.

Total Maximum Daily Load:  In short, a TMDL is a calculation and a plan: a calculation of the maximum amount of a pollutant that a river, lake or coastal water can receive before becoming unsafe and a plan to lower pollution to that identified safe level.  In the legal sense, a TMDL is "[t]he sum of the individual WLAs for point sources and LAs for nonpoint sources and natural background"  See 40 C.F.R. 130.2(i)

Use Attainability Analysis:  An assessment of the factors -- physical, chemical, and economic -- that are used to decide if a waterbody can in fact meet water quality standards.  See Section 131.10(g) of the Clean Water Act and 40 CFR 131.3. 

Variance:  Allows a modification or waiver of the applicable effluent limitations requirements or timelines of the Clean Water Act.  The provisions for variances are found under Sections 301 or 316 of the Act, in 40 CFR 125,  or in the applicable effluent limitation guidelines.

Wasteload allocation (WLA): The amount of the total loading capacity allocated to an individual point source of pollution.  Also used to describe the total amount of the loading capacity allocated to all point sources in a TMDL (e.g. the sum of individual wasteload allocations).  See 40 C.F.R. 130.2 (h)

Water Quality Based Effluent Limit (WQBEL):  Used when the reasonable potential analysis shows that a technology based effluent limit has a reasonable potential to cause or contribute to a violation of water quality standards.  The WQBEL is a value set by selecting the most stringent of effluent limits calculated using all applicable criteria for a specific point source to a specific receiving water for a specific pollutant.

Water Quality Criteria: May be either numeric or narrative.  Numeric criteria are scientifically derived limits set for a specific pollutant in order to protect human health or aquatic life (ex. Dissolved oxygen levels must be 5.0 ppm or higher).  Narrative criteria are statements that describe the desired water quality goal (ex. "Waters shall be free from floating debris")

Water Quality-Limited Segments: Those segments of waterbodies that do not or are not expected to meet water quality standards even after application of technology-based effluent limitations under the Clean Water Act. 

Water Quality Standard: Made up of the designated use of the waterbody, the numeric or narrative water quality criteria necessary to protect that use, and the antidegradation policy used to keep clean waters clean.  WQS take the form of laws or regulations, usually promulgated by the states.

Waters of the United States:  “All waters that are currently used, were used in the past, or may be susceptible to use in interstate or foreign commerce, including all waters subject to the ebb and flow of the tide.  Waters of the United States include but are not limited to all interstate waters and intrastate lakes, rivers, streams (including intermittent streams), mudflats, sand flats, wetlands, sloughs, prairie potholes, wet meadows, playa lakes, or natural ponds.”

[1] See 40 CFR 122.2 for the regulatory definition.
 

[1] NPDES Permit Writer Training Manual, U.S. EPA, December 1996, page G-13.
MF -- 2002.