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Objectives of TMDL Monitoring
Water quality monitoring is a critical component throughout the
entire TMDL process. The three
overall objectives of TMDL monitoring are as follows:
1. Determine compliance with regulations. How much higher are the
actual loads than the
target loads (sometimes referred to as tolerable loads). Do they vary
with time?
In most cases, TMDLs will not be set unless the actual loads are
higher than the target loads.
Target loads can be determined using existing field data or new
experimentation.
Existing field data can be collected from research publications,
monitoring reports and other
sources such as discharge permits to come up with a best estimate of
what the water body can
tolerate without degradation or loss of beneficial uses: the target
load.
Experiments can be conducted in the laboratory to investigate the
effects of contaminants. For
instance, to determine a target load for phosphorus, water from the
water body in question can be
subjected to various concentrations of P, and the growth rates of
algae recorded to determine the
threshold above which point algae flourish.
Once a threshold for degradation is determined the next step is
deciding how to express the target
load. To inhibit algal blooms in a stream, a threshold for a certain
concentration of total phosphorus
might be in order, multiplied by stream flow to determine load; to
ensure safe drinking
water, a focus on total suspended solids could be more appropriate.
The maximum allowable
total load - the product of concentration and flow – is then
allocated among all sources.
In Oregon’s Tualatin River – one of the first TMDLs written in
the U.S. – total phosphorus
concentrations exceeded 1 mg/L before a TMDL was written for
phosphorus. Regulators determined
that the river could tolerate just 0.07 mg/L total phosphorus based
on studies that related
algal growth to phosphorus concentrations. The timing of the
pollutant inputs is also important.
For example, nutrients entering a stream during the period of rapid
algal growth will have the
most pronounced effect on algal growth. Therefore, summer might be
considered the sensitive
time and winter less sensitive. In the case of the Tualatin River,
for example, the effective dates
for TMDLs were set to coincide with peak algal growth – from about
May 1, the beginning of the
warm, relatively dry season, to October 31, the beginning of the cool
rainy season.
2. If not in compliance, what are the sources of major loadings?
Typically upstream/downstream monitoring approaches will indicate
which reaches of a stream contribute the most pollutant. Changes in water quality between two
points are used to determine where pollution originates. In lakes, monitoring surface water inputs
into the lake will indicate which watersheds are contributing most, and monitoring within those
watersheds will indicate which sections of the watershed are responsible for the loadings.
If major increases in load are determined between two monitoring
points, inputs must be coming from land areas between those points that contribute to surface water
flow.
Some lakes receive subsurface inputs directly from groundwater. In
those cases, groundwater inputs need to be monitored and understood. It is also important to
keep in mind, that in some cases, lakes will discharge water into groundwater. If the lake is
polluted with nitrates, for instance, the lake groundwater recharge from the lake could pollute
the groundwater.
3. How well are the Best Management Practices (BMPs) working to bring
the water body back
into compliance?
Where TMDLs are not met, stakeholders must develop watershed
management plans that include
BMPs designed to improve water quality. Monitoring is required to
determine which BMPs are
actually effective in improving water quality and to assess their
degree of effectiveness. Although
this can require a long-term commitment to high-quality discrete and
continuous monitoring,
the alternative is not knowing if implemented BMPs are actually
working as intended to
improve water quality. Without this information on BMP effectiveness,
time and money may be
wasted with little or no benefit to the water body.
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