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Although the TMDL programme originated in the 1970s, it was largely overlooked
until the early 1990s, when the EPA was forced to develop guidance in response
to increasing litigation concerning the status of many water bodies. Under new
regulations issued by the EPA in 1992, all states and tribes were tasked with
assessing the condition of their water bodies every two years, establishing priorities
for pollutant load reductions and implementing improvements. As a result, over
21,000 water bodies have since been identified as being impaired, which will
necessitate the development of more than 40,000 TMDLs over the next 10–15 years.
To address this problem, the EPA published proposed changes to the TMDL rules in
1999, which were intended to strengthen the programme’s ability to achieve clean
water goals, by offering clearer guidance, new tools and improved stakeholder
engagement via knowledge transfer. However, while the proposed rules were seen
by some as a welcome step towards a more holistic approach to managing water
quality, it was also clear that they would significantly alter the politics, economics and
implementation of water quality regulation. As a result, the proposed rules attracted
significant criticism, particularly in terms of the timetable for implementation and
the adequacy of the science underpinning the control of diffuse pollution. The new
rules were finally approved in 2000, but Congress delayed their implementation,
and the EPA has since withdrawn them, stating that their implementation would be
untenable. For now, the 1992 regulations remain in effect.
Under the 1992 regulations, all states and tribes must assess the condition of their
water bodies in relation to numeric and narrative water quality standards set for
designated uses (e.g. aquatic life protection, drinking water supply, recreation).
It is recognised that such standards may not always be met, due to background
variability and measurement errors, and as a result a 10% violation rate is allowed
(up to 10 % of samples collected may exceed set standards). However, in instances
where the violation rate exceeds 10%, the affected water body must be classed as
impaired, and a TMDL developed for the pollutant causing the impairment (EPA,
1999). The first step in the process of TMDL development is the identification of the
key factors and background information that describe the nature of the impairment
and the context for the TMDL. The second step is the identification of numeric or
measurable indicators and target values that can be used to evaluate attainment of
water quality standards in the impaired water body. Often the TMDL target will be
the numeric standard for the pollutant of concern, but in some cases TMDLs must
be developed for parameters that do not have numeric standards. In these situations
impairment is determined using narrative standards or identifiable impairment of
designated uses (e.g. no fish), which are then interpreted to develop quantifiable
target values to measure attainment or maintenance of the desired level of water
quality.
The third step in the process is a source assessment, in which the sources of
pollutant loading to the impaired water body are identified and characterised by type,
magnitude and location. In the fourth step of the process, a linkage must be defined
between the selected indicator(s) or target(s) and the identified sources. This linkage
establishes the cause and effect relationship between the pollutant of concern
and the pollutant sources, and enables the total loading capacity to be estimated.
Acceptable pollutant loadings that will not exceed the total loading capacity and will
lead to attainment of the desired level of water quality are then calculated in step
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