West Metro Water Alliance
A Path to Clean Water – Understanding TMDLs and Watershed Planning
September 21, 2011
Diane Spector
Wenck Associates, Inc.
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What is a TMDL?
What are the most common impairments?
How are TMDLs calculated?
What is the difference between a wasteload
and load and why should I care?
Biotic impairments: fish and bugs matter
1972 Federal Water Pollution Control Act
was intended:
“… to restore and maintain the
chemical, physical, and biological
integrity of the Nation’s waters.”
As of 2008
 2,575 total impairments
 Restored 14 impairments (2 lakes and 12 river
segments)
 680 TMDLs in development (excl toxics)
 220 approved TMDLs in implementation
In the Upper Mississippi River Basin
 69 rivers and 263 lakes are impaired
 468 TMDL studies needed
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1,774
impairments
on 388 rivers
and 647 lakes.
3,049 total
impaired
waters
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A Total Maximum Daily Load is a
calculation of the maximum amount of a
pollutant that a water body can receive and
still meet water quality standards, and an
allocation of that amount to the pollutant
sources.
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WLA = Wasteload Allocation (attributed to
existing or future permitted sources)
LA – Load Allocation (attributed to existing or
future non-permitted sources)
MOS – Margin of Safety
RC – Reserve Capacity (Future Capacity)
Identify and measure or
quantify sources of pollutant
load
Watershed runoff
Point sources
Septic systems
Lake bottom sediments
Stream sediment oxygen
demand
 Atmospheric deposition
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Model watershed hydrology
and pollutant loading
Hydrologic and water
quality model: P8, XPSWMM, SWAT, HSPF
 Predicted runoff volume
based on actual precip
 Pollutant load
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Total Phosphorus Load
Judicial Ditch 1
25%
Internal Load1
44%
County Ditch 11
8%
Atmosphere
6%
Upstream Lakes
3%
Septic Systems
0%
Bald Eagle Direct
14%
Model lake or stream
response
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Lake response model:
BATHTUB, MNLEAP
Streams: QUAL2K
Inputs: annual runoff
volume, load, lake volume,
depth, stream
morphometry, fitting
factors
Adjust factors until model
predictions match actual
observed data
Using calibrated model, reverse calculate
maximum load
Existing TP Load
Allocation
Wasteload
Load
Source
(lbs/year)
Watershed
Runoff
(lbs/day)
TP TMDL Allocations
(lbs/year)
(lbs/day)
2,132
5.8
1,323
3.6
Upstream Lakes
135
0.4
133
0.4
Atmosphere
254
0.7
254
0.7
Internal Load
1,991
5.5
181
0.5
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Implicit
Implicit
4,512
12.4
1,890
5.2
MOS
TOTAL
Load Reduction
(lbs/year)
809
(38%)
2
(<1%)
0
1,809
(91%)
-2,620
(58%)
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A Wasteload is a permitted source
 Industrial point source
 Construction permit
 MS4s: stormwater
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Wasteload allocations are regulated in your
NPDES Phase I and II permits
Load allocations are not regulated by permit:
internal load, natural processes, atmospheric
deposition, rural runoff
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Individual allocations
 One or more permitted sources are allocated their
own wasteload and reduction
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Categorical allocations
 Groups of permittees are allocated wasteloads
and reductions
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Measure with an Index of Biotic Integrity
IBI reflects composition and health of the biotic
community in comparison to a reference
 Summary of metrics, e.g.:
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# target species
% community composition by target species
% or # pollution intolerant or tolerant species
# individuals or species
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First step is Stressor Identification study
Often identify stressors other than
conventional pollutants
May be a numeric TMDL or surrogate such as
flow or impervious cover
May also include non-numeric targets
Numeric:
 Sediment oxygen demand load allocations,
channel reshaping standards
 Chloride wasteload allocations
Non-numeric:
 Specific habitat enhancements
 Removal of fish barriers
 Desirable flow duration curve
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The purpose of a TMDL is to identify the load
reductions necessary to restore water quality
and biotic integrity
Also includes general implementation actions
that will likely help meet those targets
Not expected to “solve the problem”
Implementation Plan contains more detailed
actions