United States
Environmental Protection
Agency
Office of Water
(4503F)
EPA841-K-94-005b
September 1994
&EPA
Background
What is an
endpoint?
Watershed Protection:
TMDL Note #3
TMDL Endpoints
A Total Maximum Daily Load (TMDL) is established to reduce an environmental
stressor to a level that will not harm aquatic ecosystems and to ensure that level will
not be exceeded. An endpoint is an expression of that desired level.
Since TMDLs are quantitative, TMDL endpoints must also be quantitative (USEPA
1991). A TMDL analysis is likely to fail if its endpoint lacks a measurable,
unambiguous operational definition. While phrases like "fishable" and
"swimmable" adequately express decision makers' desire for a good natural
environment, they are not suitable endpoints for a TMDL analysis because they
cannot be estimated, measured, or modeled.
Assessment endpoints and measurement endpoints are two distinct types of
endpoints commonly used by resource managers. An assessment endpoint is the
formal expression of an environmental value that is to be protected, such as the
diversity of an aquatic community, a population of salmon, or the clarity of a
lake (Suter, 1990). It should have societal relevance; that is, the assessment
endpoint should be understood and valued by the public and by decision makers.
Ideally, it should also have ecological relevance. Ecological relevance is a
function of the endpoint's implications for the next higher level of ecological
organization. (For example, phytoplankton, which are the primary producers of
many aquatic food webs, are ecologically significant to the fish that feed on
them.)
The beneficial uses that are part of traditional water quality standards are,
essentially, assessment endpoints. They are intended to define a healthy, usable
aquatic resource. Beneficial uses can include primary said secondary recreation,
a warm- and/or cold- water fishery, residential water supply, irrigation, and
industrial water supply.
While assessment endpoints are important within the TMDL process for
determining whether a problem exists and for communicating to the public how
a particular waterbody is impaired, they cannot be useful in a technical TMDL
analysis unless they are quantitative. An endpoint that is quantitative and
measurable is needed to represent the environmental goal or goals reflected in
the assessment endpoint.
A measurement endpoint is the expression of an observed or measured response
to a stress or disturbance (Suter, 1990). It is a measurable environmental
characteristic (e.g., percent reduction by weight of bottom fines, dissolved
oxygen concentration, total phosphorus concentration, density of trout) that is
related to the valued characteristic chosen as the assessment endpoint. The
numeric criteria that are part of traditional water quality standards are good
examples of measurement endpoints. While it is necessary to meet criteria to
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How can a
TMDL
endpoint be
determined?
maintain ecological health, the criteria themselves do not define what ecological
health is.
While the specific format of a particular TMDL endpoint depends on the type of
stress that is impairing a given waterbody, any complete operational endpoint
requires a subject (e.g., salmon, dissolved oxygen, sediment) and a measurable
characteristic of the subject (e.g., number returning to spawn, a minimum of
6.0 mg/L, maximum percentage of fines by weight). TMDL endpoints may be
expressed as a percent reduction in the level of stress (e.g., current loading), as a
maximum or minimum allowable value, as an acceptable range of values, or
some other appropriate measure.
It is important to select or to develop the endpoint for a TMDL very carefully.
The methodologies chosen for the TMDL analysis stage of TMDL development
vary depending on the TMDL endpoint that is selected. The ultimate success or
failure of the TMDL in restoring or maintaining a viable aquatic ecosystem will
be judged by its attainment. The TMDL endpoint chosen might also impact
stakeholder support and enthusiasm.
The more complex the stresses in a watershed and the more degraded the
environment, the more numerous are the possible TMDL endpoints. It is
therefore best to begin the selection process by recalling that a single TMDL
requires only one measurement endpoint. If more than one TMDL is needed to
address waterbody impairment, the water resource manager must decide the order
in which they will be developed. The manager must then select an endpoint for
each TMDL as needed.
Sometimes endpoints are readily available in an appropriate quantitative format
(e.g., numeric water quality criteria). Sometimes they are not (e.g., narrative
criteria) and must be quantified on a site-specific basis. In any case, the
procedure below can be useful for determining appropriate TMDL endpoints for
individual stressors:
1. The designated uses of the receiving water should be examined to
determine whether the protection of aquatic life, human health, or both is
of concern. Appropriate criteria protective of aquatic life or human
health should then be selected in the steps below for those stressors
present or suspected of being present in the waterbody.
2. The applicable state water quality standards for the receiving water
should be examined to determine whether a numerical criterion value
exists for the parameter of concern and whether it appropriately reflects
the aquatic life or human health protection needs of the waterbody. If so,
then this criterion should be applied.
3. If no appropriate criteria appear in the state's water quality standards,
then the EPA national criteria for protection of aquatic life and human
health (USEPA, 1981, 1976, 1972; FWPCA, 1968) may be consulted for
the pollutant parameters of concern. If a discharge is to a receiving
water designated as a domestic water supply, then the finished drinking
water health advisories (adjusted for treatment capabilities) should also be
consulted. Where a pollutant has both EPA human health water quality
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References
criteria and drinking water advisories, the more stringent of the criteria
should normally be applied.
4. For those stressors which have no EPA water quality criteria or drinking
water advisories, or for which the criteria or advisories are inapplicable
to the waterbody of concern, site-specific endpoints or criteria that are
protective of the waterbody's designated uses should be developed based
on the state's general narrative standards. The site-specific endpoints
should be developed using scientifically defensible procedures.
Suter, G.W. II. 1990. Endpoints for regional ecological risk assessments.
Environmental Management 14:9-23.
USEPA. 1991. Guidance for water quality-based decisions: The TMDL process.
United States Environmental Protection Agency, Office of Water,
Washington, DC.
USEPA. 1981. Water quality criteria documents. 45 FR 79318, November 28,
1980, 46 FR 40919, August 13, 1981. United States Environmental
Protection Agency, Washington, DC.
USEPA. 1976. Quality criteria for water. GPO Stock No. 005-001-01049-4.
United States Environmental Protection Agency, Washington, DC.
USEPA. 1972. Water quality criteria. EPA-R3-73-033. United States
Environmental Protection Agency, Washington, DC.
FWPCA. 1968. Water quality criteria. Federal Water Pollution Control Act.
Any Questions or Comments? Please, contact Theresa Tuano, Watershed
Branch, Office of Water, U.S. Environmental Protection Agency, 401 M
Street SW, Washington, DC 20460, phone 202/260-7079, fax 202/260-7024.
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