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MODELS FOR TMDL APPLICATION
IN TEXAS WATERCOURSES:
SCREENING AND MODEL REVIEW
By George H. Ward, Jr. & Jennifer Benaman
ABSTRACT
The concept of Total Maximum Daily Load
(TMDL), as expressed in Section 303(d) of the
Clean Water Act, is a generalization and formalization of the older
concept of wasteload
assimilative capacity, viz. the upper limit on the discharge of
a wasteload into a receiving
watercourse so that the resulting concentration of some indicator
parameter carried by the waste stream remains within a predetermined
limit. Generally, the predetermined limit was a stream standard,
and the assimilative capacity was established under a set of critical
conditions, typically extreme low flows and high temperatures. An
associated concept was that of wasteload allocation, in which the
assimilative capacity, once determined, was apportioned among several
waste dischargers. The TMDL includes not only point source discharges,
but also natural sources of the pollutant and so-called nonpoint
sources that arise from the watershed (EPA, 1991) or environs of
the watercourse.
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