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Indiana Department of Environmental Management


TMDL Glossary

# A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


  • 303(d) Clean Water Act (CWA)
    • Requires states to identify waters that do not or are not expected to meet applicable water quality standards. These waters are identified on the Section 303(d) Impaired Waters List. A TMDL must be developed for each waterbody on the Section 303(d) list. If a listed waterbody has multiple impairments, a TMDL must be developed for each.
  • 305(b) Clean Water Act (CWA)
    • Requires states to submit a biannual report in even-numbered years to the U.S. Environmental Protection Agency (U.S. EPA) describing the quality of the state's waters. The Section 305(b) report describes the overall water quality conditions and trends in the state.
  • 303(d) Impaired Waters List
    • Developed every two years, as mandated by Section 303(d) of the CWA. The state identifies all waters where required pollution controls are not sufficient to attain or maintain applicable water quality standards. States are required to establish priorities for development of TMDLs for waters on the 303(d) List (40C.F.R. §130.7(b)(4)).
  • 303(d) and 305(b) Integrated Report
    • The U.S. EPA recommended approach to integrating water quality conditions data submitted by states under the CWA Sections 303(d) and 305(b). U.S. EPA guidance provides recommended organization for states' integrated report submittals.


  • Allocations
    • That portion of a receiving water's loading capacity attributed to one of its existing or future pollution sources (nonpoint or point) or to natural background sources. A wasteload allocation (WLA) is that portion of the loading capacity allocated to an existing or future point source, and a load allocation (LA) is that portion allocated to an existing or future nonpoint source or to natural background levels. Load allocations are best estimates of the loading, which can range from reasonably accurate estimates to gross allotments, depending on the availability of data and appropriate techniques for predicting loading.
  • 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 Policy
    • A required process for protecting all existing uses, keeping healthy waters healthy and giving strict protection to outstanding waters.
  • Aquatic Ecosystem
    • Complex of biotic and abiotic components of natural waters. The aquatic ecosystem is an ecological unit that includes the physical characteristics (such as flow or velocity and depth), the biological community of the water column and benthos, and the chemical characteristics such as dissolved solids, dissolved oxygen, and nutrients. Both living and nonliving components of the aquatic ecosystem interact and influence the properties and status of each component.
  • 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.


  • 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.
  • Benthic
    • Refers to material, especially sediment, at the bottom of an aquatic ecosystem. It can be used to describe the organisms that live on, or in, the bottom of a water body.
  • Benthic Organisms
    • Organisms living in, or on, bottom substrates in aquatic ecosystems.
  • Best Management Practices (BMP's)
    • 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.
  • Bioassessment
    • An evaluation of the biological condition of a water body that uses biological surveys and other direct measurements of resident biota in surface waters. Most often evaluated is the fish population, the bottom dwelling insects and other invertebrates, plants or attached algae. Data collected from bioassessments can be used to determine whether the biological health of the water body is what would be expected if pollution and other water quality stressors were not causing an effect. Bioassessment data are the foundation for developing biocriteria.
  • Biological Criteria (Biocriteria)
    • Numeric values or narrative expressions that are used to describe the reference biological condition of aquatic communities inhabiting waters of a given designated aquatic life use. These values are developed by biologists and other natural resource scientists who use bioassessments to characterize the ecoregion reference conditions for a state or tribe's water bodies. Biocriteria can be used in a variety of ways by water quality managers to determine if waters are affected by chemical pollution or other factors.
  • Biological Integrity
    • The ability to support and maintain a balanced, integrated and adaptive community of organisms having a species composition, diversity and functional organization comparable to those of natural habitats within a region. Biological integrity is equated with pristine conditions or those conditions with no or minimal disturbance, and it is used as the baseline for the index of biotic integrity (IBI).
  • Biotic Impairment
    • A divergence from the expected biological condition of a lake, stream or wetland, commonly in a fish and/or macroinvertebrate community.


  • Clean Water Act (CWA)
    • 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 Sections 305(b) and 303(d) deal specifically with water quality assessment and TMDL development.
  • Conventional Pollutants
    • As specified under the Clean Water Act, conventional contaminants include suspended solids, E. coli, biological oxygen demand (BOD), pH, and oil and grease.
  • Critical Condition
    • 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. Recreational uses; the propagation and growth of a balanced, indigenous population of aquatic life; wildlife; and, the production of edible and marketable natural resources are generally stated as "fishable and swimmable" uses. Other uses may be industrial water supply, irrigation and navigation.
  • Discharge
    • Flow of surface water in a stream or canal, or the outflow of ground water from a flowing artesian well, ditch, or spring. Can also apply to discharge of liquid effluent from a facility.
  • Dissolved Oxygen (DO)
    • The amount of oxygen dissolved in water. This term also refers to a measure of the amount of oxygen available for biochemical activity in a water body, an indicator of the quality of that water.
  • Drainage Basin
    • A part of a land area enclosed by a topographic divide from which direct surface run-off from precipitation normally drains by gravity into a receiving water. Also referred to as a watershed, river basin or hydrologic unit.


  • Escherichia coli bacteria (E. coli)
    • Bacterial indicator organisms (organisms indicating presence of pathogens) associated with the digestive tract of warm-blooded animals.
  • Ecosystem
    • An interactive system that includes the organisms of a natural community association together with their abiotic physical, chemical and geochemical environment.
  • Effluent
    • Municipal sewage or industrial liquid waste (untreated, partially treated or completely treated) that flows out of a treatment plant, septic system, pipe, etc.
  • Endpoint
    • Also known as an indicator or target - a characteristic of an ecosystem that may be affected by exposure to a stressor. Assessment endpoints and measurement endpoints are two distinct types of endpoints commonly used by resource managers. An assessment endpoint is the formal expression of a valued environmental characteristic and should have societal relevance (an indicator). A measurement endpoint is the expression of an observed or measured response to a stress or disturbance. It is a measurable environmental characteristic that is related to the valued environmental characteristic chosen as the assessment endpoint. The numeric criteria that are part of traditional water quality standards are good examples of measurement endpoints (targets).
  • Eutrophic
    • A water system high in nutrients with high organic production. Eutrophic lakes contain more phytoplankton (algae) than other lakes and are common among more naturally fertile lowland regions in which human activity provides an increased supply of nutrients.
  • Eutrophication
    • The aging process by which lakes are fertilized with nutrients. Natural eutrophication will very gradually change the character of a lake. Cultural eutrophication is the accelerated aging of a lake as a result of human activities.
  • Eutrophic Lake
    • A nutrient-rich lake that isusually shallow, green in color and with limited oxygen in the bottom layer of water.


  • Great Lakes Water Quality Initiative (GLWQI)
    • Established uniform water quality standards, implementation procedures,and nondegradation procedures for toxic chemicals in the Great Lakes basin. Particular emphasis was placed on persistent, bioaccumulative toxic chemicals.


  • Hydrologic Cycle
    • The circuit of water movement from the atmosphere to the earth and its return to the atmosphere through various stages or processes, such as precipitation, interception, run-off, infiltration, storage, evaporation and transpiration.
  • Hydrology
    • The study of the distribution, properties and effects of water on the earth's surface, in the soil and underlying rocks and in the atmosphere.


  • Index of Biotic Integrity (IBI)
    • A regionally based index used to measure the integrity of rivers and streams, and to determine the level of their biotic impairment. The IBI relies on multiple parameters (termed "metrics") based on fish community structure and function to evaluate a complex biotic system. In order to implement biological criteria, a formal method for sampling the biota of streams, evaluating the resulting data and clearly describing the condition of sampled stream reaches is needed. The IBI incorporates professional judgment with quantitative criteria that enables determination of a continuum between very poor and excellent conditions.
  • Impaired Waterbody
    • A waterbody (i.e., stream reaches, lakes, waterbody segments, etc.) with chronic or recurring monitored violations of the applicable numeric and/or narrative water quality criteria.
  • Implementation Plan
    • The document or section of a document detailing the suite of corrective actions needed to reduce pollution and remediate an impaired waterbody. Once fully implemented, the plan should result in the waterbody achieving a "fully supporting" status. Current 303(d) regulations do not require implementation plans, though some state regulations do require an implementation plan for a TMDL.
  • Indicator
    • A measurable quantity that can be used to evaluate the relationship between pollutant sources and their impact on water quality.
  • Indicator Organism
    • An organism used to indicate the potential presence of other (usually pathogenic) organisms. Indicator organisms are usually associated with the other organisms, but are usually more easily sampled and measured.


  • Load, Loading, Loading Rate
    • The total amount of material (pollutants) entering the system from one or multiple sources; measured as a rate in weight per unit time.
  • Load Allocation (LA)
    • The portion of the loading capacity attributed to the existing or future nonpoint sources of pollution and natural background sources. Wherever possible, nonpoint source loads and natural loads should be distinguished.
  • Loading Capacity (LC)
    • The greatest amount of loading a water body can receive without violating water quality standards.


  • Margin of Safety (MOS)
    • A required component of the TMDL that accounts for the uncertainty in the response of the waterbody to loading reductions.
  • Mean
    • The sum of the values in a data set divided by the number of values in the data set.
  • Mg/L
    • A scientific measurement unit (milligrams per liter) that is used for water quality standards.


  • Narrative Criteria
    • Nonquantitative guidelines that describe a desired water quality goal or goals.
  • National Pollutant Discharge Elimination System (NPDES)
    • The national program for issuing, modifying, revoking, reissuing, terminating, monitoring and enforcing permits, and imposing and enforcing pretreatment requirements, under Sections 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.
  • Natural Waters
    • Flowing water within a physical system that has developed without human intervention and where natural processes continue to take place.
  • 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 run-off.
  • Nonpoint Numeric Targets
    • A measurable value determined for the pollutant of concern which, if achieved, is expected to result in the attainment of water quality standards in the listed water body.


  • 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. Point sources can also include pollutant loads contributed by tributaries to the main receiving water stream or river.
  • Pollutant
    • As defined in Clean Water Act Section 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, or industrial, municipal or agricultural waste discharged into water.
  • Pollution
    • The presence of matter or energy whose nature, location or quantity produces undesired environmental effects. Under the Clean Water Act, for example, the term is defined as the man-made or man-induced alteration of the physical, biological, chemical and radiological integrity of water.
  • Public Comment Period
    • The time allowed for the public to express views and concerns regarding action by U.S. EPA or states


  • Receiving Water
    • Creeks, streams, rivers, lakes, estuaries, ground water formations or other bodies of water into which surface water, treated waste or untreated waste are discharged.
  • Riparian, Riparian Area, Riparian Buffer, Riparian Zone
    • Of, on or relating to the banks of a natural course of water. The landscape areas adjacent to a stream or river that have vegetation, soil and hydrologic mosaics that are distinct from the predominant landscape surface types. In a broad sense, the riparian zone is both a transition and interface between riverine and upland systems. Functionally and structurally, riparian areas are different from surrounding uplands because of proximity to a watercourse. Riparian areas have unique features that provide desirable habitat for a variety of species. The same features that make these ecosystems relatively rare and important also make them relatively sensitive. Hydrologic changes to the water body also alter the associated riparian ecosystem. Riparian ecosystems generally occupy relatively small areas, and their occurrence along waterways makes them vulnerable to severe alteration caused by a variety of development activities.


  • 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 run-off from the source to a treatment plant or receiving stream. Sanitary sewers carry household, industrial, and commercial waste. Storm sewers carry run-off from rain or snow. Combined sewers handle both.
  • Stakeholder
    • Any person or organization with vested interest in an action or project.
  • Substrate
    • Bottom sediment material in a natural water system.
  • Surface Run-off
    • 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.


  • Technology-Based Standards
    • Effluent limitations applicable to direct and indirect sources that are developed on a category by category basis using statutory factors, not including water quality effects.
  • Threatened Waterbody
    • Any waterbody of the United States that currently attains water quality standards, but for which existing and readily available data and information on adverse declining trends indicate that water quality standards will likely be exceeded by the time the next list of impaired or threatened waterbodies is required to be submitted to U.S. EPA.
  • Total Maximum Daily Load (TMDL)
    • The sum of the individual wasteload allocations (WLA) for point sources, load allocations (LAs) for nonpoint sources and natural background levels, plus 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.
  • 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.
  • Trophic Status
    • The level of growth or productivity of a lake as measured by phosphorus content, algae abundance and depth of light penetration.
  • Turbidity
    • Measures particles in the water, such as sediment and algae. Related to the depth sunlight can penetrate into the water. Higher turbidities reduce the penetration of sunlight in the water and can affect species of aquatic life that survive in the water body.



  • Wasteload Allocation (WLA)
    • The portion of a receiving water's loading capacity that is allocated to one of its existing or future point sources of pollution (e.g., permitted waste treatment facilities).
  • 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.
  • 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.
  • Water Quality
    • The biological, chemical and physical conditions of a water body. It is a measure of a water body's ability to support beneficial uses.
  • Water Quality Criteria
    • Levels of water quality expected to render a body of water suitable for its designated use, composed of numeric and narrative criteria. Numeric criteria are scientifically derived ambient concentrations developed by U.S. EPA or states for various pollutants of concern to protect human health and aquatic life. Narrative criteria are statements that describe the desired water quality goal. Criteria are based on specific levels of pollutants that would make the water harmful if used for drinking, swimming, farming, fish production or industrial processes.
  • Water Quality Modeling
    • A system of mathematical expressions that describe both hydrologic and water quality processes. When used for the development of TMDLs, models can estimate the load of a specific pollutant to a waterbody and make predictions about how the load would change as corrective actions are implemented.
  • 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.
  • Watershed
    • An area that drains or contributes water to a particular point, stream, river, lake or ocean. Larger watersheds are also referred to as basins. Watersheds range in size from a few acres for a small stream to large areas of the country.

Water Quality