|
TMDLs: What are they?
"The primary mission of the TMDL program is to protect public
health and the health of impaired
aquatic ecosystems by ensuring attainment of water quality standards,
including beneficial uses."
(US EPA, 1998)
The Clean Water Act (CWA), passed in 1972, addressed both point
(permitted discharges such as
wastewater treatment plants, industrial effluent) and nonpoint
(runoff and polluted groundwater
from urban, agricultural, and forestry) sources of pollution and
water quality in the United States.
Initial efforts were oriented toward implementing Best Available
Technology. Municipal waste-water
treatment plants, for example, were required to install secondary and
tertiary treatment but,
for most of the first two decades of CWA enforcement, water quality
in the receiving water itself
was not incorporated into the criteria. In essence, for the first 20
years of the CWA, nonpoint
source was rarely evaluated.
TMDLs increase the scope of water quality management to include
nonpoint source pollution by
requiring that the actual quality of the water body itself be
considered. A flurry of lawsuits,
begun in the late 1980s, pushed US EPA and state environmental
agencies into the world of TMDLs, shifting focus from cleaning up pipe discharges to addressing
both point and nonpoint
source pollution on a watershed basis.
The need for TMDLs is clear. Despite tremendous improvements in point
source pollution
reduction, serious water quality problems persist. The National Water
Quality Inventory: 1996
Report to Congress Executive Summary cites nutrients (nitrogen and
phosphorus) as one of the
leading causes of water quality impairment in our rivers, lakes and
estuaries. In order to direct
scarce resources to areas that pose the greatest risk, the water
bodies suspected to be the most
polluted were surveyed. Nineteen percent of the nation’s total
river and stream miles were
surveyed. EPA and the states believe that about 40% percent of those
river miles were impaired
(not meeting designated uses such as fishing or swimming). Forty
percent of the total acres of
the nation’s lakes, ponds and reservoirs were surveyed: 51% of that
area did not meet designated
uses. Seventy-two percent of the total square miles of the nation’s
estuaries were surveyed, of
which 57% of that area was adversely affected. The impacted water
bodies are excellent candidates
for TMDLs.
In a typical TMDL application, the beneficial uses of the water body
are first established. These
could include drinking water, recreation, aesthetics, irrigation,
fishing and swimming. The
TMDL is set at a level that will allow the water body to achieve
water quality standards for
beneficial uses.
One section of the CWA, Section 303(d), requires that states report
streams and water bodies that
do not meet ambient water quality standards – streams still
requiring TMDLs. The resulting
inventory of impaired streams and water bodies – called the 303(d)
list – is updated every two
years by each state and tribe, and provides a basis for decisions
related to restoring water quality.
Listing rivers under Section 303(d) is the responsibility of state
and tribal environmental agencies.
They must report their findings to the U.S. Environmental Protection
Agency (US EPA).
Impaired water bodies on the 303(d) list are slated for TMDLs, which
take into account all the sources of pollution from nonpoint as well as point sources. The
analyses are done on a watershed-wide basis rather than being based on the quality of water coming out
of a point source effluent pipe.
To develop a TMDL, the TMDL manager must first establish the amount
of pollutant a waterbody can take in, or assimilate, without degradation in beneficial
uses. Pollutant loads are calculated in mass/unit time entering a water body, with typical
units of pounds/day.
These loads are called Total Maximum Daily Loads, or
TMDLs. TMDLs
represent the sum of waste load allocations (WLA, for point sources), load allocations
(LA, for nonpoint sources), background loads (BL) and the margin of safety (MOS).
Load allocations – for nonpoint sources – are often allocated
based on land use types, including urban runoff (e.g., stormwater runoff from streets, yards, etc.);
agricultural runoff (erosion of sediments, fertilizers, manure and pesticides); forestry (for
instance, soil erosion from roads or tractor logging). Because much of the nonpoint source pollution is
tied to runoff, its timing tends to be closely related to weather events, especially periods of
intense rainfall or snowmelt.
The very focus on loads presents technical and policy challenges
because sometimes a "load" is not a load at all. While loads (in pounds/day) are generally easy to
calculate for point sources, nonpoint source pollution is usually measured by concentration.
Concentration is measured in units of mass/volume, such as mg/L, and must be multiplied by stream
flow rates (e.g., L/day) to determine load.
As a result, measuring water quality and
complying with TMDLs often requires TMDL managers to convert data from concentration to load, or at least to agree on
which approach to measurement will be acceptable in discussing target loads for the
stream in question. Sometimes, surrogate measurements must be used. For instance,
habitat alterations to the shape of the stream, the riparian zone, etc., often affect
temperature, erosion of the stream banks, sediment transport, and nutrient loads, and
these can be regulated as loads that reflect habitat alterations. BMPs to control these
loads often include practices that improve habitat quality. Further complications arise
when runoff flow from a particular portion of the landscape is difficult to measure,
such as runoff flow from fields, forested lands, housing developments and urban
development. Assigning a load, for instance, to each acre of farm and forested
land would be impractical, very expensive and very time-consuming. To be effective,
water quality managers must look at the whole section of a watershed where inputs
are increasing pollution in a stream or lake and ask, "What is going on in this watershed? Are there
feedlots, high fertilizer rates applied to crops, or heavy pesticide use?" Once these questions have been
answered with supporting water quality data, best management practices (BMPs) can be established
that will bring the biggest bang for the buck. Good planning and communication can help secure
cost-share monies to help fund the BMPs where they will do the most good.
We will discuss concentration and load in greater detail later in
this document. In the meantime, keep in mind that TMDLs bridge many, often disparate, concepts. In
the absence of very specific guidance documents from US EPA, the key components of TMDL
development and monitoring hinge upon generating valid data and communicating this data and
associated results among all the stakeholders.
|
