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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