| Section 303(d) of the federal Clean Water Act requires states
to identify waters that do not meet applicable water quality
standards or do not fully support their designated uses. States
are required to submit a prioritized list of impaired waters,
known as the 303(d) List, to the U.S. Environmental Protection
Agency for review and approval. The CWA also requires that a
TMDL be developed for each pollutant of an impaired water body.
Illinois EPA is responsible for carrying out the mandates of
the Clean Water Act for the state of Illinois.
The establishment of a Total Maximum Daily Load sets the
pollutant reduction goal necessary to improve impaired waters.
It determines the load, or quantity, of any given pollutant
that can be allowed in a particular water body. A TMDL must
consider all potential sources of pollutants, whether point
or nonpoint. It also takes into account a margin of safety,
which reflects scientific uncertainty, as well as the effects
of seasonal variation.
Developing TMDLs in a watershed begins with the collection
of vast amounts of data on factors including water quality,
point source discharge, precipitation, soils, geology, topography,
and land use (construction, agriculture, mining, etc.) within
that specific watershed. All impaired water-body segments
within the watershed are identified, along with the potential
pollutants causing the impairments.
Next, Illinois EPA determines the tools necessary to develop
the TMDL. In most cases, computer models are used to calculate
pollutant loads. The appropriate model or models are selected
based on the pollutants of concern, the amount of data available,
and the type of water body. Once the model is selected, the
data collected for the watershed are entered, and the model
is calibrated and verified so that the computed values match
those of known field data. The model can then be used to develop
different scenarios, by first determining the amount of specific
pollutants each source contributes, then calculating the amount
each pollutant needs to be reduced, and finally specifying
how the reduced pollutant load would be allocated among the
different sources.
After the reduced pollutant loads have been determined, an
implementation plan is developed for the watershed spelling
out the actions necessary to achieve the goals. The plan specifies
limits for point source discharges and recommends best management
practices (BMPs) for nonpoint sources. It also estimates associated
costs and lays out a schedule for implementation. Commitment
to the implementation plan by the citizens who live and work
in the watershed is essential to success in reducing the pollutant
loads and improving water quality. |
