Helpful Practices for Addressing Point Sources and
Implementing TMDLs in NPDES Permits
Prepared by EPA Region 9 June 2015
Forward
Based on "lessons learned" over the past 25 years in developing and implementing Total Maximum Daily
Loads (TMDLs), this document describes approaches and practices for developing and interpreting
TMDLs in ways that facilitate incorporation of appropriate requirements in National Pollutant Discharge
Elimination System (NPDES) permits. We hope the suggestions in this document help practitioners
avoid unintended mistakes that are often repeated and can be easily avoided. TMDL writers should
confer with NPDES permit writers early and often in the TMDL development process to ensure they can
be properly implemented through permits. Also, permit writers should confer early and often with TMDL
developers when interpreting existing TMDLs to better understand wasteload allocations and underlying
assumptions. Better communication will help reduce misunderstandings, disconnected information
transfer, and most importantly, improve the effectiveness with which TMDLs guide implementation of
permits. Clear expression of point source control requirements will help restore impaired waters.
This is not a U.S. Environmental Protection Agency (EPA) guidance document; nor does it represent
official EPA policy. This document discusses practices and ideas that, in the experience of staff in EPA
Pacific Southwest Region (Region), can prove effective in integrating TMDLs and permits. The Clean
Water Act (CWA) and associated implementing regulations contain the legally binding requirements
associated with the development of TMDLs and NPDES permits. This document does not substitute for
the CWA or associated implementing regulations. Approaches and practices identified in this document
are not binding; the TMDL and permitting authorities may consider other approaches consistent with the
CWA and associated implementing regulations. When EPA makes a TMDL or permitting decision, it
will make each decision on a case-by-case basis and will be guided by the applicable requirements of the
CWA and associated implementing regulations and information applicable to individual cases. This
document is intended to be consistent with but does not modify existing EPA policy or guidance.
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Table of Contents
INTRODUCTION	4
A.	NPDES PERMIT-FRIENDLY TMDL DEVELOPMENT	5
1.	Address All Point Sources in the Watershed	5
2.	Disaggregate WLAs As Much As Possible	9
3.	Clarify Where WLAs Apply	11
4.	Clarify When WLAs Apply	13
5.	Clarify Whether Mass and/or Concentration-based WQBELs Will Be Needed	16
6.	Setting WLAs Based on Formulas	18
7.	Special Considerations for Stormwater WLAs and WQBELs	18
8.	Implementation Provisions in TMDLs	19
B.	INTERPRETING EXISTING TMDLs WHEN DEVELOPING NPDES PERMITS	20
1.	Check for and review TMDLs before writing the NPDES permit	20
2.	Addressing TMDLs That Do Not Include a WLA For Your NPDES Permit	20
3.	Translating WLAs into Wastewater NPDES Permit WQBELs	22
4.	Considerations in Permitting Stormwater Sources	23
5.	Providing Time to Implement Needed Controls	25
C.	CONCLUSION	27
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INTRODUCTION
The Total Maximum Daily Load (TMDL) process provides a critical opportunity to assess the relative
importance of pollutant sources and apportion pollution control burdens among various sources. NPDES
permits are the main vehicles through which point source pollution control responsibilities identified in
permits are implemented and enforced. As the TMDL and NPDES programs developed, they often
operated independent of each other. Operating guidance and customary practices in each program have
not always been well coordinated. As a result, we have observed that many TMDLs are written in ways
that make them difficult to implement through permits and, conversely, many NPDES permits contain
effluent limitations that do not accurately implement the intent of associated TMDLs.
This document describes approaches for improving effectiveness in developing TMDL wasteload
allocations (WLAs) and implementing WLAs in NPDES permits. Based on "lessons learned" through
our experience in Region 9 developing and reviewing TMDLs and NPDES permits, we identified
practices that TMDL and NPDES permit writers may want to use and common mistakes to avoid. In
addition, while the focus of this document is on point sources and the translation of TMDLs into NPDES
permits, many "helpful practices" are also relevant to the development and implementation of TMDLs
focusing on nonpoint sources that receive load allocations (LA). We've included appropriate TMDL and
permit examples that illustrate these practices.
Our key "lesson learned" is that WLAs can be difficult to translate accurately and effectively into
numeric water quality based effluent limitations (WQBELs) in NPDES permits unless the WLAs are
accompanied by interpretive information that specifies where, when, and how the WLAs should be
incorporated into NPDES permits. This document is roughly divided into two sections. Section A
discusses practices for effectively addressing point sources when developing TMDLs and associated
implementation plans. Section B discusses issues that arise in interpreting existing TMDLs when
developing NPDES permits for discharges to waters for which there is an approved, applicable TMDL.
In 2013, EPA issued "A Long-Term Vision for Assessment. Restoration, and Protection under the Clean
Water Act Section 303(d) Program." The new Program Vision details enhancements made to the CWA
303(d) Program informed by the experience gained over the past two decades in assessing and reporting
on water quality and in developing approximately 65,000 TMDLs. It is designed to enhance overall
efficiency of the CWA 303(d) Program, and in particular encourages focusing attention on priority waters
and provides States flexibility in using other available clean water program tools beyond TMDLs to attain
water quality restoration and protection. EPA recognizes it may be appropriate in some cases to use
alternative restoration approaches to evaluate pollution problems in watersheds and devise solutions to
those problems. These alternative restoration approaches are not "in lieu" of a TMDL. However, EPA
recognizes that under certain circumstances there are alternative restoration approaches that may be more
immediately beneficial or practicable to achieve water quality standards than pursuing the development of
TMDLs in the near future. While these alternative restoration approaches may not incorporate all
elements of formal TMDLs, they will often have similar characteristics. We believe the suggestions in
this document will help ensure that such alternative restoration approaches effectively account for point
source discharges and that the provisions of such alternative approaches are properly incorporated in
NPDES permits. In considering whether alternative restoration approaches are appropriate to address
situations in which point source discharges contribute to water quality threats or impairments, we
recommend that practitioners carefully consider whether these alternatives will yield the legal and
technical rigor necessary to properly inform NPDES permitting. If alternative approaches are used that
do not specifically articulate point source control approaches necessary to protect and restore water
quality, it will be necessary during the permits process to fully evaluate whether each NPDES permitted
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source has reasonable potential to contribute to water quality standards exceedences, and how permit
requirements should be structured to ensure that applicable standards are met.
Here are several important terminology reminders to help avoid future problems:
a.	We refer to TMDL "attainment" or "achievement" and NPDES permit "compliance" since
permits, unlike TMDLs, are directly enforceable. TMDLs guide implementation actions but are
not directly enforceable.
b.	TMDLs and associated implementation provisions cannot directly incorporate compliance
schedules. While TMDLs can identify appropriate implementation timeframes, "compliance
schedules" can be implemented only through NPDES permitting decisions. The permitting
decision record must demonstrate that compliance schedule regulatory provisions are met for that
permit, regardless of what the TMDL might say about implementation timeframes. Before
compliance schedules may be implemented in NPDES permits, their use must be authorized
pursuant to CWA section 303(c) and approved by US EPA. Moreover, compliance schedules
cannot be used to provide time to develop or revise a TMDL, a water quality standard, or a
mixing zone analysis.
A. NPDES PERMIT-FRIENDLY TMDL DEVELOPMENT
Over the past 25 years, both TMDL development and NPDES permitting have evolved and become more
complex. TMDLs are addressing increasingly complex watershed settings and pollutant or stressor
problems, and the challenge of addressing nonpoint sources through TMDLs has become a high priority.
The range of discharges requiring NPDES permits has expanded beyond traditional wastewater facilities,
and some discharges are increasingly being addressed through general permitting approaches. Both
TMDL developers and NPDES permit writers have learned valuable lessons on how to meet each others'
needs while improving the use of TMDLs to strengthen control of discharges from point sources. Section
A discusses lessons learned and recommends practices to help ensure that TMDLs provide the critical
information that is needed in order to support the issuance of effective permits.
1. Address All Point Sources in the Watershed
Some TMDLs do not include wasteload allocations (WLAs) for all NPDES permitted discharges in the
area covered by the TMDL. In some cases, the TMDLs mention a facility or permit but do not provide a
WLA. It is very difficult for a permit writer to address a facility discharging to the TMDL water if the
TMDL does not include a clear and source-specific WLA. Moreover, in situations where a facility does
not discharge the pollutant identified in the TMDL or discharges an insignificant amount of the pollutant,
many TMDLs are unclear about how the NPDES permit should account for this situation. Here are some
suggestions for details to include in the TMDL to address these situations.
a. Name All Permitted Discharges.
The TMDL should name all NPDES permitted discharges in the TMDL watershed. This includes all
major and minor NPDES discharges, including discharges covered by individual and general NPDES
wastewater, stormwater, construction, industrial, and other permits. For example, we have found many
TMDLs ignore state and federal highways, which are often regulated under municipal separate storm
sewer system (MS4) NPDES permits. Watershed scale TMDLs should generally account for all point
sources that discharge to tributaries or upstream from the water segments targeted by the TMDL. If the
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TMDL does not specifically address all point sources that discharge to tributaries or upstream of the
TMDL segment or analysis area, the TMDL can incorporate an allocation or assumption regarding
contributions of pollutant loads from upstream sources and discuss how their NPDES permits should
address these pollutants.
b.	Include NPDES permit number and the facility name.
The TMDL WLA should specifically include the NPDES permit number and facility name as they appear
in the permit itself to ensure accurate incorporation of the TMDL's WLA when the associated permit is
reissued or revised. If feasible, the TMDL can discuss specific WLAs for specific outfalls if the facility
has more than one outfall, or suggest an approach to applying an aggregate WLA to a facility with
multiple outfalls.
c.	Include a specific WLA for each permitted facility.
In the case of facilities permitted by a general NPDES permit, express the WLAs such that they can be
effectively implemented on a facility-by-facility basis. See Section A.2 below regarding disaggregated
WLAs. For example, concentration-based WLAs are probably easiest to implement in situations where
multiple facilities are covered by the same WLA and it is difficult to disaggregate WLAs by discharger.
Another example would be to clearly explain how the load-based WLA was calculated (e.g., based upon
XX mg/L TSS and YY discharge flow rates).
d.	Account for permitted facilities that do not discharge the pollutant of concern or that discharge
insignificant amounts of this pollutant in the TMDL.
A TMDL writer has several options for addressing a facility that does not discharge or is an insignificant
discharger of a pollutant in comparison with other sources. The TMDL source analysis should account
for all known point source dischargers, noting that some may not discharge the pollutant of concern or
discharge insignificant amounts that would not need to be limited with a specific wasteload allocation in
order to achieve applicable standards. Care should be taken in evaluating insignificant or "de minimus"
discharges to ensure that they are really unimportant at all geographic scales and need not be limited. In
cases where individual permitted facilities do not discharge or discharge insignificant amounts of the
pollutants of concern, it greatly assists permit development if the TMDL specifies how the NPDES
permits should account for these discharges. Potential options for addressing this situation include:
1.	The TMDL can specify that a particular point source need not be addressed by a WLA or permit
limitation (likely including monitoring requirements to help ensure the facility does not discharge
the pollutant at significant levels in the future). In this case the TMDL would explain why no
allocation is necessary for this facility.
2.	The TMDL can specify that the permit for a facility should incorporate performance-based
limitations to ensure its loading of the pollutant of concern does not increase in the future.
3.	The TMDL can incorporate an explicit margin of safety (MOS) to account for all insignificant
sources along with discussion of how this MOS may be available for use in calculating future
permit limits (e.g. performance based limits).
4.	The TMDL can incorporate a WLA of zero for facilities that do not discharge the pollutant of
concern, in which case the associated permit would generally prohibit discharge of the pollutant.
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Example TMDLs:
•	Glen Annie/Los Carneros (CA) Nitrate TMDL (2014) - The TMDL source analysis explains that
non-point sources are responsible for almost all nitrate loading and that point sources contribute
insignificant loads. The TMDL narrative identifies these very small/insignificant point sources
and provides a rationale for not including numeric WLAs, while acknowledging these point
sources can continue to discharge nitrate at existing very low levels.
•	Spokane River (WA) DO TMDL (2011) - This TMDL includes a WLA appendix to explain how
the WLAs were developed and are intended to apply to permits, especially upstream waste water
treatment plants (WWTPs) in Idaho. Also, seasonal WLAs were clarified on a permit-specific
basis.
•	Bear Creek (MO) Nutrients TMDL (2011) - The TMDL determined that one of four WWTPs in
the watershed was the most significant (3.12 MGD) source of TN and TP and a WLA with
loading reductions was assigned to this facility. The other WWTPs received WLAs equivalent to
their existing permit limits as additional load reductions from these facilities were not needed to
achieve the allowable TMDL.
•	Shenandoah River (VA) Mercury TMDL (2011) - Minor municipal facilities and facilities
discharging under general permits were considered insignificant sources of mercury and were not
assigned wasteload allocations. WLAs were assigned to industrial and major municipal facilities.
The TMDL states that the Virginia DEQ will reevaluate those NPDES permits with assigned
WLAs to ensure compliance, and that NPDES permits should include the following provisions:
o Additional monitoring requirements using low-level detection techniques; and
o If such monitoring results show exceedences of the applicable standard, the permittee is
required to submit for review and approval a Pollutant Minimization Plan (PMP).
•	Kiskiminetas-Conemaugh River (PA) Metals TMDLs (2010) - The TMDL provides gross WLAs
for "negligible" point source discharges that were assumed to be discharging below the applicable
water quality standard. The gross WLAs were calculated based on application of water quality
critieria and available information about facility flows. The TMDL lists specific permits covered
under the aggregate WLA for negligible sources.
•	Dumps Creek (VA) Sediment (2004; 2010) - The TMDL and approval was first issued in 2004.
Several years later Virginia DEQ realized it had omitted 3 point sources and so it re-opened the
TMDL to make a narrow revision to include these three small sources and their WLAs. The re-
approval of TMDL occurred in 2010, just prior to re-issuance of the permits for those 3 point
sources. The LAs were decreased to accommodate for the increased loading within the WLAs.
•	Upper Moncacv (MP) Sediment TMDL (2009) - The TMDL addresses each of the 34 permitted
facilities, including individual industrial facilities, POTWs, mining operations permitted under
the state's industrial stormwater general NPDES permit and MS4s. Permits were grouped into
process wastewater and stormwater sources of sediment. Based on an analysis of the permit
information, the TMDL writer determined that the total permitted load from these process
wastewater sources equaled 0.2% of the total watershed load. Because these sources were so
small relative to other sediment loads in the watershed, the TMDL established WLAs for this
group of permitted facilities based on existing loads.
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•	Guvandotte River (WV) Metals. Fecal Coliform and pH TMDLs (2004) - Based on the types of
activities and the nature of their discharges, permitted non-mining sources as shown in Table 3-3
of the TMDL are believed to be negligible. In this TMDL, these minor facility discharges are
assigned WLAs that allow them to operate under their current permit limits.
e.	Account for NPDES discharges to tributaries.
Preferably, TMDLs will clarify how they apply to permitted discharges from facilities located in
tributaries upstream from the TMDL study area. The Region recommends setting WLAs for all permitted
facilities discharging to tributaries upstream from the study area unless available information indicates
those facilities do not discharge the pollutant of concern at levels that could reach the downstream waters
for which TMDLs are being established.
Example TMDLs:
•	Chesapeake Bay TMDL (2011) - EPA Region 3, in collaboration with the Chesapeake Bay
jurisdictions, established this federal TMDL which allocates wasteloads and loads to sources in
the states and the District of Columbia, many of which are upstream from the Bay.
•	Columbia River Basin (OR) Dioxin TMDL (1991) - The TMDL established WLAs for the
chlorine bleaching pulp mills in the basin. This TMDL addressed pulp mills discharging to
Columbia River as well as those on its tributaries; e.g., on the Willamette and Snake Rivers and
those further north within Washington. Recent state-developed TMDLs have defined pollutant
load allocations at the upstream stateline. While this newer approach reduces potential concern
about a downstream state setting WLAs for facilities located in an upstream state, it reduces
clarity about how upstream sources need to be specifically controlled to ensure the TMDL is met.
f.	ClarifV how to calculate a performance-based limit if a WLA is set at current performance.
Many TMDLs include numeric WLAs for less significant sources based on their "current performance."
As there are many possible approaches to calculating "current performance," it is preferable for TMDL
documents to specify how WLAs based on this concept should be translated into permit limitations.
Many permits interpret "current performance" in terms of an extreme upper range statistical value
estimate of that performance(e.g., the 95th or 99th percentile performance level) to help ensure the limit is
set at a level that is highly unlikely to be exceeded. While this approach reduces the likelihood that a
facility will violate the "performance-based" limit, it may not be consistent with the method used in the
TMDL to determine that current performance is an adequate limitation for the facility given its
pollutant(s) contribution within the watershed in the first place.
TMDLs should provide clear direction on how to translate performance-based WLAs into permit limits in
a manner consistent with how the TMDL load assessment was conducted. It may be clearest for the
TMDL to set a specific mass and/or concentration-based value or set of values for a specific facility that
reflects the specific load analysis conducted for the TMDL, with instruction to the permit writer to set the
WQBELs to ensure these current performance levels are not exceeded in the future. Alternatively, the
TMDL could set a particular percentile representation of current performance to serve as the basis for the
permit limit. The percentile value should be set at a reasonable upper value (e.g. 90th percentile) but not at
a value so extreme that it results in a limitation far less stringent than contemplated in the TMDL.
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g. Consider setting WLAs at current performance if current loadings are lower than available loading
capacity.
While it may be legally permissible under federal regulations to set WLAs and associated permit
WQBELs at a level that is less stringent than current performance, it may be preferable for TMDL
developers to maintain current performance levels if they are more stringent than might otherwise be
established under a TMDL. This approach helps to ensure that antidegradation and antibacksliding
requirements are met, and is consistent with overall CWA goals to reduce point source discharges. This
may not apply in situations where a facility is experiencing growth in effluent flows or pollutant loads, in
which case the TMDL may provide the appropriate mechanism for planning for discharge growth.
Example TMDLs:
•	Rio Hondo (NM) Sediment/Nutrients (2005) - The TMDL cited the state's antidegredation policy
and maintained existing loads in setting the WLA for the Village of Taos Ski Valley even though
loading capacity calculation would have allowed an increase in phosphorus loads from the
facility. It appears the extra loading capacity was put into an explicit margin of safety.
•	San Diego Creek/Newport Bay (C'A) Toxics TMDLs (2002) - Hydrodynamic and water quality
models were used to estimate the existing loads vs. the calculated allowable loads using numeric
targets. WLAs were set at the lower or more protective of the two values. See TMDL Appx. G.
2. Disaggregate WLAs As Much As Possible
a. Provide separate WLAs for each permitted discharge.
WLAs that apply to multiple dischargers can be difficult to implement unless they are scalable to
individual discharge situations. Separate WLAs should be set for each permited discharger if at all
possible. In the case of a general permit which covers a large number of facilities or sites that discharge
to the impaired waterbody, the WLAs should be designed to clearly delineate responsibilities for different
owners or operators and in different areas of watersheds addressed by TMDLs. Options for
disaggregating WLAs at the individual discharge level include (in rough order of preference):
•	By discharge facility, which can be outfall by outfall, or using representative outfalls;
•	By jurisdiction;
•	By sub-watershed;
•	By land use type.
It may be efficient to express disaggregated WLAs based on an analysis of discharges from representative
outfalls as surrogates for discharges from the entire jurisdiction, sub-watershed, or land use type
represented by the representative outfall. For a concentration-based WLA, pollutant analysis results from
the representative outfall would be used to set the WLAs applicable to a larger jurisdiction, subwatershed,
or land use type represented by that outfall. For mass-based WLAs, pollutant loading analysis for a
representative outfall can be used in a similar manner after weighting the loading results for the
representative outfall based on the proportion of the jurisdiction, subwatershed, or land use type drained
by that outfall. The benefit of the representative outfall approach to the discharger is that fewer outfalls
would need to be monitored for compliance. The benefit to the TMDL and permitting authorities is that
more specific WLAs would be simpler to calculate, individual permit limits easier to establish and permit
compliance easier to determine.
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b. Provide guidance on how to divide up grouped WLAs in permits.
If the TMDL writer cannot disaggregate the WLAs on a geographical or jurisdictional basis, it is still
important to explain how the grouped WLA was calculated so it can be implemented accurately in
individual permits. If the WLAs are expressed on a concentration basis, it may be more straightforward
to implement them as numeric limits for each permitted discharger covered by the WLA. If WLAs are
expressed on a mass basis, the TMDL document should explain how the available loading capacity in the
WLA was calculated and divided among different discharging point sources. For example, the Ballona
Creek, CA metals TMDL and San Gabriel River, CA metals and selenium TMDLs divided available mass
loading capacity in a grouped urban point source WLA based on the percentage of land area in the TMDL
watershed occupied by the permitted facility. In this scenario, for example, if an industrial facility
regulated under an industrial general permit occupied 1% of the TMDL watershed area, it could be
assigned 1% of the available load in the grouped WLA.
c. Provide for future point source discharges.
As it can be difficult to revise an approved TMDL, it may be advisable to include a growth WLA when
developing the TMDL to account for new and/or expanded permitted source(s) that arise after the TMDL
is approved. Fully allocating all available loading capacity to existing sources leaves little or no flexibility
to address new sources. Some existing TMDLs have taken the following approaches to providing for
future sources. First, a part of the available loading capacity is reserved for potential future allocation to
point or nonpoint sources as an explicit margin of safety, assumption, or unspecified allocation. Second,
some TMDLs establish a "future growth WLA" to be used specifically for future permitted sources. In
either case, the TMDL writers should explain how this reserved loading capacity can be used when
permitting point sources that do not have WLAs under the TMDL For example, Minnesota requires a
"reserve capacity" to be included in its TMDLs and describes that all or a portion of the reserve capacity
would be available through future permitting action(s).
Example TMDLs:
•	Chest Creek (PA) Sediments (2011) - Pennsylvania DEP identified one point source discharge
within Chest Creek watershed; however, an additional allocation of 1% of the TMDL was
incorporated into the WLA as a bulk reserve to take into account future permit activity. It also
contained a 10% explicit MOS and the remaining load was allocated to nonpoint sources.
•	Cedar River (IA) E. coli (2010) - EPA Region 7 wrote this TMDL and at the request of Iowa
DNR, they included "reserve WLA" for unsewered communities that were likely to become
connected to sewage treatment plants in the future. This was applied to certain river segments and
not to others.
•	Lake Houston (TX) Watershed Bacteria (2009) - TMDL developers analyzed Houston-Galveston
area future population projections to adjust TMDL allocations based on projected loads per day.
Growth estimates per sub-watershed ranged from 79% to 182% between 2008 and 2035. WLAs
for wastewater treatment plants were adjusted based on these projections.
•	Rio Hondo (NM) Nutrients (2005) - This TMDL set aside 2% of the loading capacity for a
growth allocation to account for unknown or future nitrogen discharge sources, whether point
source or non-point source.
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• Long Island Sound (C'T) Nitrogen (2001) - Connecticut established a total watershed POTW
WLA based on current plant flows and a 5.5 mg/L TN numeric target. The TMDL establishes a a
trading program that facilitates trading of loads among sources. Future increases in TN
discharges require improved treatment or purchase of excess nitrogen credits from POTWs to
ensure overall attainment of the TMDL WLA by the POTWs as a group.
d. Allocations for discharges that may become subject to NPDES.
In situations where it is unclear whether a discharge source is subject to NPDES permitting requirements,
the TMDL can include language indicating that if a source receiving a load allocation is later found to be
subject to NPDES requirements, its LA is to be interpreted as a WLA for NPDES permitting purposes.
For example, areas near urban centers may not now be subject to requirements of municipal stormwater
permits but may later become subject to NPDES requirements following adjustment of urban boundaries
resulting from population census updates. A TMDL could include language such as: "If any sources
currently assigned load allocations are later determined to be point sources requiring NPDES permits, the
portion of the load allocations applied to those sources are to be treated as wasteload allocations for
purposes of determining appropriate water quality based effluent limitations pursuant to 40 CFR
122.44(d)(1)." This interpretation of a TMDL would not require resubmittal or reapproval of a TMDL.
3. Clarify Where WLAs Apply
TMDLs are sometimes unclear about where WLAs apply because they only identify a facility but do not
clearly specify the actual point of compliance where a WQBEL based on the WLA should apply. While
the location for the point of compliance is obvious for some traditional facility discharges, in more
complex discharge situations, such as large industrial facilities with multiple stormwater outfalls, it can be
difficult to define the correct point of compliance when applying a WLA during permit preparation.
WLAs associated with MS4 stormwater permits may be notably challenging since there can be many
outfalls and often several jurisdictions whose discharges are authorized by the same MS4 permit that are
assigned one numeric WLA value.
a. Specify Location where the WLA was calculated.
In most situations, the WQBEL based on a WLA would be applied as an end of pipe limit. If the WLA is
mass-based and there are multiple outfalls, the available load may need to be divided among several
facility outfalls. Concentration-based WLAs generally can be applied to all outfalls. Some states (e.g.,
California) sometimes express WLAs for receiving water locations that are downstream of outfalls to
receiving waters. There are advantages and disadvantages to this approach. Expressing WLAs and
associated WQBELs as receiving water limitations is usually simpler to calculate as it would not require
outfall-by-outfall analysis, may result in reduced monitoring costs, and can be protective of water quality
if it is reasonably clear how responsibility for any violations will be apportioned among discharges
upstream of the receving water point of compliance. On the other hand, it may be difficult to apportion
responsibility for violations if multiple entities discharge upstream from a receiving water point of
compliance, and similarly, it may be difficult to detect and address causes of violations. In situations
where receiving water-based WLAs are developed, the TMDL should include a specific monitoring plan
and specify a mix of receiving water and outfall monitoring that is sufficient to: (i) detect violations under
different flow and discharge scenarios and (ii) support apportionment of responsibility among different
responsible permittees discharging upstream from the receiving water point of compliance. It may be
necessary to incorporate a monitoring design that requires monitoring at points upstream and downstream
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of changes in jurisdictional status to help distinguish pollutant loads associated with different
jurisdictions. In watersheds for which individual WLAs cover multiple dischargers or jurisdictions, the
monitoring plan should be designed to to accurately "bracket" different responsible entities and facilitate
distinguishing among the multiple contributions to overall pollutant loadings.
b.	Special considerations for MS4 stormwater WLAs.
In the case of MS4 permits, which may regulate dozens or even hundreds of separate outfall locations,
specifying the point of attainment requires more detailed analysis and description. The TMDL should
account for multiple outfall situations by describing how WLAs apply. It may be possible to develop
WLAs in multiple outfall situations that focus on a subset of representative outfalls. We have seen three
approaches to stratifying WLAs within larger stormwater-permitted settings. For example, the land uses
within a stormwater-permitted jurisdiction can be stratified consistent with the TMDL's pollutant source
analysis to distinguish among the different levels of pollutant discharges associated with different land
uses. Subwatersheds within a larger stormwater-permitted jurisdiction can also be similarly stratified to
distinguish how WLAs apply to smaller management areas. Finally, for MS4 permit situations in which
there are multiple co-permittees, it is possible to stratify the WLAs based on jurisdiction to distinguish the
requirements applicable to individual co-permittees. If WLA stratification methods are used, it may be
appropriate to identify representative outfalls for each stratified land use, sub-watershed, or sub-
jurisdiction to ensure requirements are clear and monitoring can be conducted that will enable detection
and clear apportionment of responsibility for violations of WQBELs based on WLAs.
Example TMDLs:
•	Rock Creek (MP) Sediment TMDL (2011) - The TMDL included a five-page technical
memorandum regarding WLAs for regulated point sources within the watershed. Permits were
placed into two groups: process water and stormwater. Maryland Dept. of Planning applied land
use classifications to establish individual and aggregate WLAs for two Phase IMS4 permits, one
Phase IIMS4 permit and "other NPDES regulated stormwater" sources. While the TMDL
technical memo does not supply an exhaustive analysis, it describes information the TMDL writer
relied on and cites existing land use methodology previously described in another MDP document
to develop WLAs.
•	Lower St. Johns River (FL) Nutrients TMDL (2009) - Some facilities addressed by this TMDL
asked the State to combine their WLAs into an aggregate WLA to allow flexibility so that
reductions from one facility can be shifted to another as long as the net reduction reaches the
aggregate WLA. For these aggregate allocations, Florida DEP plans to issue watershed permits
that will require compliance with the aggregate WLA.
•	Black River (MI) Bacteria TMDL (2010) - Both individual and aggregate WLAs were provided.
The Port Huron WWTP received an individual WLA that applies only during high flow
conditions and equals 0.1% of the total annual discharge from this facility. Twelve municipal and
industrial stormwater permits were given an aggregate WLA with acknowledgement that land use
analysis could be used to break this down further at the time of permit development.
c.	Account for Mixing Zones.
If the existing permit allows for a mixing zone, it is helpful for the TMDL WLA to specify whether the
WLA is to be met before discharge to the mixing zone or at the edge of the mixing zone. Both the TMDL
and permit writer should carefully consider whether a past mixing zone analysis remains appropriate in
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light of the pollutant loading and effects analysis, and associated allocation decisions, contained in a
TMDL
Example TMDL:
Indian River (DE) Temperature (2004) -The WLA is established as an end-of-pipe allocation and
must be met at the Indian River Generating Station (IRGS) discharge outfall on Island Creek.
While applicable water quality standards allow for a mixing zone for thermal pollutants, Island
Creek does not provide enough dilution capacity to allow a mixing zone.
4. Clarify When WLAs Apply
Many, but not all water quality standards clearly specify the duration and frequency of allowable
excursions in addition to their magnitude. The duration and frequency elements vary substantially
depending upon the kinds of effects different pollutants have on human or ecological health. TMDL
writers should be particularly attentive to the duration and frequency elements of standards when
calculating TMDLs, WLAs, and LAs.
Generaly, EPA permit guidance provides that permit writers should calculate limits differently when
implementing standards that focus on aquatic life protection as opposed to those that focus on human
health protection. Where both acute and chronic aquatic life criteria are applicable, permit writers
generally calculate both short term (i.e., daily or weekly) and long term (i.e., monthly limits). Where
criteria focusing on human health protection are used, permit writers generally calculate a long term (i.e.,
annual or monthly) limit from which a short term (i.e., daily) limit is derived. In cases where pollutants
are of concern both for aquatic and human health protection, the most limiting set of effluent limitations
are generally included as WQBELs in the permit. TMDL allocations delineated in daily time increments
can pose problems for permit writers when translating a daily load into the weekly, monthly or annual
limits under the permit regulations. Although TMDLs generally establish allowable daily loads, this does
not preclude establishing supplemental wasteload information expressed on different timescales (e.g.,
weekly, monthly, seasonal, or annual loads or concentrations, or daily allocations that vary by month or
season). These additional calculations should be linked back to the daily loads to assist the permit writer
in demonstrating the additional loads are consistent with the WLAs. The challenge for TMDL writers is
to be keenly aware of these required permit elements and ensure the TMDL document and specific WLAs
provide sufficient guidance on how they should be implemented in permits consistent with these long-
standing permitting principles. In these cases, close and frequent coordination and communication
between the permit writer and TMDL writer is critical.
a. Set clear averaging periods for WLAs.
For point sources, NPDES permitting regulations at 40 CFR 122.45(d) specify how WQBELs should
address the duration component of standards that apply for different types of discharges. EPA's
Technical Support Document (TSD) guidance provides detailed procedures for appropriately converting
standards and associated WLAs into appropriate permit WQBELs. Since TMDL writers generally
express allocations as daily loads and it is often useful to include WLAs expressed in other timeframes
and durations, we encourage early discussions between TMDL and permit writers to determine the most
appropriate time components that are consistent with the applicable regulations and guidance. Here are
some general approaches used in developing permit limits that should be considered in developing WLAs
for consideration:
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•	For continuous POTW discharges, EPA regulations provide that effluent limitations should be set
as average weekly and average monthly WQBELs unless it is impracticable to do so. In our
experience, we have observed that permitting authorities have generally found that weekly
average limits for POTWs are impracticable and that daily maximum limits and average montly
limits are appropriate discharges.
•	For continuous non-POTW discharges, WQBELs are generally set as maximum daily and
average monthly WQBELs; and
•	For non-continuous discharges, WQBELs should specify the frequency of allowed discharges,
total mass allowed per discharge event, maximum discharge rate, and concentration-based or
other limitations as necessary.
TMDL writers should incorporate such duration components in WLAs, or at least indicate the underlying
temporal assumptions of each WLA. TMDLs should distinguish the time periods of concern for different
pollutants addressed by TMDLs. For pollutants that are of principal concern due to short term
environmental exposures (e.g., ammonia and other acute toxicants), WLAs should include a shorter
averaging period based on the applicable standard. For pollutants associated principally with longer term
environmental effects (e.g., bioaccumulative toxins), WLAs should include a longer averaging period
based on the applicable standard. As many pollutants are problematic both in the short term and the
longer term (e.g., nutrients that can cause short term dissolved oxygen swings and long term problems
with nuisance plant growth), it may be appropriate to set both short and longer term averaging periods and
associated WLAs to be consistent with how the underlying water quality standard is expressed. The
TMDL document should specify the translation methods to convert the WLAs into WQBELs. The TSD
provides sound methods for translating WLAs based on acute standards into short term permit limits,
such as daily or weekly, and WLAs based on chronic standards into long term limits, usually monthly.
Some TMDLs focus on the effect period of principal concern (e.g., chronic effects are of much greater
concern than acute effects) and do not include information on averaging periods based on other effect
periods. We often see mass-based TMDLs with very long averaging periods (monthly, seasonal, or
annual). We also see concentration-based TMDLs that are intended principally to address short term
loading and effect periods. WLAs set for these TMDLs should clearly discuss: (i) how permit limits
should be expressed both for the effect period of principal concern and (ii) whether and how to set permit
limits for other effect periods of lesser concern. If TMDLs do not explicitly describe the most sensitive
exposure durations, then permit writers are likely to be uncertain how to make valid assumptions that are
consistent with the applicable WLAs.
b. Document any allowable exceedance assumptions in WLAs.
Some water quality standards are written such that they can be exceeded a certain amount of the time
without resulting in adverse impacts to protected designated uses (e.g. 1 excursion in 3 years for many
toxic pollutant criteria). Where possible, WLAs should reflect allowable exceedence frequencies,
consistent with applicable water quality standards, in order to avoid a need to set permit limits more
stringent than necessary. Following either State NPDES implementation procedures or EPA's TSD and
WLA guidance methods, it is possible to calculate WLAs that account for magnitude, duration, and
frequency components of water quality standards and can be converted easily into permit WQBELs.
Load duration curves can also be very useful in identifying the allocable pollutant load that corresponds to
an allowable exceedance frequency within the water quality standard.
For some pollutants, applicable water quality standards are set as values never to be exceeded (e.g. some
bacterial indicator standards). In these cases, associated TMDLs and permit WQBELs generally do not
incorporate an allowable exceedance frequency. We have found that WLAs and associated permit
WQBELs expressed as values never to be exceeded can be difficult to implement and meet in practice,
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especially for sporadic, flashy discharges such as many stormwater discharges. Note that EPA's current
criteria for bacterial indicators are no longer expressed as values never to be exceeded.
In cases where the applicable standards do not specify an allowable exceedance frequency, it may be
possible to evaluate the standard during the TMDL process to identify an appropriate exceedance
frequency. For example, TMDLs for bacterial indicators for discharges to Southern California coastal
waters often are based on the concept of exceedance-days. Relatively undeveloped reference watersheds
were monitored to determine the frequency with which applicable standards are exceeded (i.e., average
days per year). In this case the State formally revised the water quality standard to authorize this
approach concurrent with the TMDL adoption action. The WLAs were then expressed as the number of
days per year or percent of days that exceed the frequency of exceedance in the reference watersheds.
WLAs developed through this type of approach may be more realistic to attain than WLAs expressed as
never to exceed values. Again it is critical that the allowable exceedances within WLA be consistent with
those explicitly expressed in the applicable water quality standard.
c.	Describe the seasonal, flow-based, or wet weather/dry weather WLAs.
As pollutant loadings and effects often vary substantially based on seasonal or rainfall-runoff conditions,
TMDLs often set different requirements for different seasons, flow conditions, or rainfall-runoff
conditions. The TMDL writer usually has the best knowledge with regards to the applicable flow and
duration components in the TMDL. It is therefore very helpful for permit writers when TMDLs can
feasibly discuss how permits should be written to reflect seasonal, flow-based, or wet weather/dry
weather-based WLAs.
d.	Clarify how WLAs work under critical and non-critical conditions.
As many TMDLs are set based on critical low flow or high flow conditions, it is vital for the TMDL
document to explain in detail the types of permit limits that need to be developed in order to address the
full range of flow and discharge conditions likely to occur. Traditionally, low-flow based TMDLs are
designed to support a single set of acute and chronic NPDES permit WQBELs that, if implemented,
ensure the water quality standard will be met throughout the year. If a TMDL is designed to authorize
different loading levels at different flow levels (to account for different levels of available dilution), it is
helpful for the TMDL document to clearly explain how the WQBEL should be expressed to account for
different allowable discharges at different flow levels. A single set of concentration-based WQBELs
could work across a wide range of flows; however, different mass-based WQBELs may be required for
different flow tiers to ensure the WLA is met under all loading scenarios.
Example TMDLs:
• Ventura River (CA) Algae/Nutrients TMDL (2012) - The TMDL contains seasonal allocations to
address the critical condition of low stream flows that are dominated by Ojai WWTP effluent
discharges during the dry season (331 days). The TMDL includes dry season WLAs for total
phosphorus and identifies two separate total nitrogen WLAs for summer and winter dry weather
conditions, recognizing that the WWTP has lower nitrogen removal efficiency during cooler
winter months. Other point sources, such as Caltrans and the local MS4, get similar dry and wet
season mass-based nutrient WLAs to make it easier to evaluate attainment. Wet weather WLAs
are concentration-based; the Ojai WWTP received a WLA based on "current performance"
interpreted as the 90th percentile of the 12 year data record.
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•	Potlatch River (ID) E. coli TMDL (2009) - The TMDL contains clear language on translating
WLAs into WQBEL: "Wasteload allocations apply as instantaneous maximum WQBEL and to
any 30-day/calendar month period when effluent discharge occurs."
•	Santa Monica Bay (C'A) Beaches Wet Weather Bacteria TMDLs (2003) - The TMDL uniquely
addressed pathogen impaired beaches by applying the reference beach approach to define WLAs
as "allowable exceedance days" relative to reference/natural beach conditions. Corresponding
permits have successfully implemented this approach within receiving waters.
5. Clarify Whether Mass and/or Concentration-based WOBELs Will Be Needed
To set mass or concentration-based WLAs and WQBELs for multiple water quality standards and
pollutant types, the appropriate way to measure the pollutant of concern is not obvious. While we
recognize that TMDLs are generally set in terms of mass per unit of time, it makes sense in many
situations to complement mass-based TMDLs with other indicators and measures that are more sensitive
to the type of discharges and receiving water situations and implementation activities addressed in the
TMDL. For example, a mass-based TMDL can be set for official purposes but accompanied by a
concentration-based expression that can be incorporated more effectively into an associated NPDES
permit WQBEL or other implementation actions. Similarly, NPDES regulations and associated guidance
generally specify that mass-based WQBELs are required in permits except when the standard is expressed
in a way that does not make sense to take a mass-based approach or it is otherwise infeasible to do so.
See 40 CFR 122.45(f). Permitting regulations provide that concentration-based WQBELs can be used to
supplement mass-based WQBELs where appropriate, which may frequently arise when implementing
WLAs. If WLAs suggest use of a particular approach or mix of approaches for expressing permit
WQBELs that are different from conventional permitting practices, it is much easier for the permit writer
to justify use of such non-traditional approaches in the affected permits.
a.	Focus on timeframes in which pollutants do their work.
For pollutants for which short term concentration is the key concern in water quality protection, it makes
most sense to base the TMDL and associated WLAs on concentration-based indicators (e.g. ammonia,
bacteria or currently used pesticide levels downstream from stormwater outfalls). Where longer term
mass loading is the key problem to be solved, the WLA and WQBELs should focus on mass (e.g. long
term nutrient load to a lake or bioaccumlative pollutants like PCBs or mercury).
b.	Consider discharge characteristics and ease of monitoring.
Facility discharge characteristics and ability to monitor efficiently are equally important to consider. It is
easier to measure and regulate irregular, non-continuous discharges such as industrial discharges on a
concentration basis, particularly in settings in which facility and receiving water flows are not routinely
monitored. For example, a concentration-based allocation may make the most sense for a pollutant
source such as dry weather "illicit discharges" of non-stormwater from MS4s', which are typified by non-
continuous discharges that are difficult to predict or measure. It is generally easier to monitor on
aconcentration basis. On the other hand, it is easier to evaluate mass-based loads from continuous
discharges such as municipal wastewater treatment plants with predictable flow rates and volumes.
c.	Clarify whether mass load WLAs are based on actual facility flows or design flows.
Permit regulations for WWTPs require that WQBELs be calculated based on facility design flows, which
are normally greater than actual flows. See 40 CFR 122.45(b). Some TMDL WLAs may have been
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calculated based on actual flows or loads, which presents a challenge for permit writers (particularly when
discharge flows are substantially lower than permit design flows). The TMDL document should clarify
whether flow-based WLAs should be translated directly into WQBELs or whether a correction factor
needs to be included to accurately convey the intent of the WLAs in WQBELs based on WWTP design
flows. If this is not done, WWTP permit limits could be set at higher loads than intended in the WLA.
d.	Be careful with percent reduction allocations.
Many TMDLs express allocations as needed percentage reductions in pollutant loading. Unless these
WLAs are accompanied by a very clear baseline load from which the percent reduction is to be
calculated, it is very difficult to set enforceable permit limits for this type of WLA. WLAs expressed as
mass-based and/or concentration-based WLAs are preferred, in addition to expressing them as percent
reduction from a given baseline, since this makes it easier to evaluate whether associated permit limits are
achieved. It is also important to be clear about the averaging periods in which the required percentage
reductions are supposed to be accomplished.
e.	Consider concentration-based WLAs for individual facilities permitted under general NPDES permits.
Many point sources regulated under general permits (e.g., industrial and construction stormwater sources)
may discharge pollutants addressed by TMDLs but do not have available discharge flow measurements.
In many cases, these discharges are relatively short term in duration, are sporadic, or vary substantially
during storm periods. For these reasons, it may be difficult to set mass-based WLAs. In cases where
flow rates are not available, it may be best to assign concentration-based WLAs for facilities authorized
by a general NPDES permit to assist in setting practicable WQBELs for these discharges. This may also
be the case for bioaccumulative pollutants. The lack of flow discharge rates makes it difficult to calculate
mass-based WLAs, and concentration-based WLAs applicable at end-of-pipe may be more practical.
Example TMDLs:
•	Los Angeles-Long Beach Harbor (CA) Toxics TMDLs (2012) - Concentration-based WLAs
were identified for such sources as: construction stormwater, industrial stormwater, power
generating stations, minor permits and irregular dischargers. Any future minor NPDES permits
or enrollees under a general non-stormwater NPDES permit will also be subject to the
concentration-based wasteload allocations. The TMDL provides both acute and chronic WLAs
which are then translated to WQBELs by following state implementation procedures (similar to
EPA TSD guidelines).
•	Charles River (MA) Bacteria TMDL (2007) - The TMDL expresses the wasteload allocations in
three ways: (i) concentration-based (equal to WQS); (ii) percent reduction for implementation
purposes and (iii) mass-based in colonies/day. This multi-allocation approach was also useful for
defining pollution prevention TMDLs for waters that were not currently impaired but might be
found to be impaired in future assessments within the watershed.
•	Bavou Cocodrie (LA) Copper TMDL (2007) - The TMDL for dissolved copper in Bayou
Cocodrie has been set to the water quality criteria concentration for all sources at all effluent flow
rates (criteria is met at the end-of-pipe). In the event future revisions to the criteria are made, the
TMDL would remain set at the criteria at the end-of-pipe. The TMDL is set as a daily maximum
value.
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f. Mass-based WLAs may be more appropriate for pollutants that accumulate over time in water,
sediment, or tissue.
Adverse effects of certain pollutants may occur over longer exposure timeframes or appear over longer
residency within the waterbody. For example, aquatic or wildlife or humans may bioaccumulate and/or
biomagnify pollutants via food web dynamics and are at risk due to long-term exposures. As nutrients
can also build up in sediments and later re-solubilize to create eutrophic conditions, their long-term
loading is important and mass-based WLAs are more appropriate.
6.	Setting WLAs Based on Formulas
For pollutants whose environmental effects vary based on water quality factors like hardness,
temperature, and pH, TMDLs and WLAs are sometimes set as a formula that makes the allocation a
function of an independent water quality factor. In contrast, it is preferable for NPDES permit limits be
set as firm, not floating, values since it can be very difficult to monitor and evaluate compliance with
floating limits. If a WLA is set based on such a formula, the TMDL document should provide clear
direction about what critical value or values should be assumed when calculating permit limits for that
pollutant (e.g., default values for temperature, hardness, or pH). If no single critical value is set, the
document should specify that a particular percentile value from the actual receiving water data set must be
used to result in a protective permit limit. There are examples where the critical percentile value was set
in the 90-99% range. Where appropriate, different critical values can be set to correspond to different
seasonal requirements or variability in seasonal conditions as long as the seasonal variations are clearly
articulated in the TMDL document and associated permit. Also, it is important to base a formula-based
limit on data collected from the receiving water, not on data for the effluent waste stream itself.
Example TMDL:
• LA River (C'A) Metals TMDLs (2005) - The TMDL applied median hardness to determine
chronic WLA during dry weather conditions and 90th percentile hardness for acute WLA during
wet weather.
7.	Special Considerations for Stormwater WLAs and WOBELs
In setting WLAs for stormwater sources, it is important to account for the unique characteristics of these
discharges and associated impairments.
a. For extremely short-duration stormwater events, acute-based WLAs may be appropriate.
If it can be demonstrated in a particular TMDL analysis that stormwater discharges and associated
receiving water flows never or very rarely last for extended periods of time (e.g., 4 days or longer), it may
be appropriate to base the TMDL and stormwater WLAs solely on the acute standards for pollutants that
have both acute and chronic standards. Conversely, if the observed duration of storm flows exceeds
critical short term thresholds (e.g., 4 days) with a frequency greater than the allowable exceedance
frequencies embodied in the underlying standards (e.g. once every 3 years on average), the TMDL and
WLAs should generally be based on more protective chronic standards. For example, if TMDL analysis
of a long term stream flow data set showed that storm-related high flows exceeding 4 days in duration
occur only once every 50 years on average in that stream, then one could consider basing the WLAs on
the acute standard.
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b. Concentration-based TMDLs may be easier to implement for stormwater sources.
As flows and pollutant loads vary substantially during and across different storm events, it can be difficult
to develop workable mass-based TMDLs and WLAs to address certain stormwater sources. In
conjunction with a load-based WLA, concentration-based WLAs are often easier to implement both from
the standpoint of monitoring ease and the ability to associate changes in concentration with BMP
effectiveness. It is important to collect samples at different times during the storm period and for a
representative range of storms during the year in order to provide the data needed to accurately
characterize stormwater-related pollutant loadings. For MS4s, in which sufficient data are collected to
yield event-mean loading and concentration data, it may be desirable to express WLAs in terms of event
mean concentration values. This approach allows the permit writer to assess and possibly express the
WQBEL in terms of event mean concentrations.
8. Implementation Provisions in TMDLs
Actions to address the causes of stubborn water quality impairments may take many years to implement.
It is helpful for a TMDL to include implementation recommendations, either within the TMDL itself or in
a separate implementation plan, about the appropriate implementation timeframes and mechanisms. This
information is extremely useful during the permitting process to help ensure the permit incorporates
requirements necessary to implement WLAs.
a. TMDL implementation recommendations can assist in permit development and other control actions.
It is very helpful for TMDL documents to describe the state's expectations for implementation, including
actions needed to implement WLAs and LAs, appropriate timeframes for implementation, responsible
parties, and necessary monitoring provisions. See Section B.5 below. TMDL and permit writers should
keep in mind that including time schedules in a TMDL does not mean that individual permit compliance
schedules, enforcement orders, or variances have been legally established. TMDLs are not self-
implementing; they are implemented only through subsequent permitting or other implementation actions
which are then evaluated for compliance. When EPA acts on a TMDL submittal, EPA is only approving
the TMDL calculations, not the State's implementation provisions incorporated in the TMDL document
or a separate implementation plan. The information provided in the implementation provision portion of a
TMDL or accompanying documentation should be clear that its purpose is to assist the permit writer by
providing context and further information.
If a state intends to authorize compliance schedules to provide time to implement WLAs, it must establish
compliance schedule authorizations pursuant to CWA 303(c) and 40 CFR 122.2 and 122.47. After EPA
approves a compliance schedule authorization request that is specifically applicable to the permit(s)
addressed by a TMDL, the permitting authority must then incorporate specific compliance schedule
provisions within individual permits (or, if applicable, general permits) as part of an NPDES permit
proceeding. As part of that proceeding, once the permittee submits a request and supporting
documentation for a compliance schedule, the permitting authority would then need to establish that the
compliance schedule requirements at 40 CFR 122.2 and 122.47 have been satisfied on a permit-by-permit
basis.
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B. INTERPRETING EXISTING TMDLs WHEN DEVELOPING NPDES PERMITS
The challenges a permit writer faces in interpreting existing TMDLs are probably as daunting as the
challenges a TMDL writer faces in writing a permit-friendly TMDL. Section B offers a few basic
considerations that may improve the TMDL to permit translation process. One overarching observation is
that permit writers need to thoughtfully evaluate and interpret TMDLs, and avoid simply importing
verbatim the WLAs into the permit unless it is clear that the WLA itself is suitable for use as a permit
WQBEL. It is critical for permit writers to discuss with TMDL writers point source permitting and
control issues that forthcoming TMDLs need to address. Early coordination often helps avoid many of
the interpretation problems described throughout this document.
1.	Check for and review TMDLs before writing the NPDES permit
In many regulatory agencies, there is no clear mechanism through which permit writers can learn about
the existence of TMDLs that affect the permits they are writing. TMDL and permitting authorities should
establish regular methods for keeping permit writers up-to-date on new or revised TMDLs, and make
TMDL and permit writers responsible for keeping their counterparts up to date on new TMDL actions.
Ideally, permit writers should become aware of and assist in the development of the TMDL well before
the TMDL is completed to help ensure that the final TMDL includes needed WLAs that are written in a
"permit-friendly" way. At times permit writers do not take this simple step, and miss approved,
applicable TMDLs and relevant WLAs therein. Permitting specialists should ensure they are fully aware
of how TMDLs apply to specific individual and general permit situations. The permit writer should
carefully review the TMDL problem statement, targets, source analysis, linkage analysis, margin of
safety, and TMDL assumptions in addition to the WLA section in order to develop a clear understanding
of how the TMDL relates to a particular NPDES facility. Additionally, the permit writer may gain insight
by reviewing the TMDL approval document which may contain language on specific items that EPA did
not act on or did not approve, such as interim limits, timeframes, etc.
2.	Addressing TMDLs That Do Not Include a WLA For Your NPDES Permit
Many already approved TMDLs do not include a WLA for NPDES permitted facilities or new NPDES
point sources discharging to a waterbody addressed by a TMDL. How should these permits be addressed
in the permitting process? One (albeit "strict") reading of this situation is that the permit should prohibit
any discharge of the pollutant of concern since the TMDL made no provision for such discharges.
However, in some circumstances, there may be a reasonable approach to ensure that the permit
incorporates WQBELs and provisions as necessary to be consistent with the assumptions of the TMDL
and ensure that applicable water quality standards are met. Doing so in the absence of a specific WLA
presents some permitting challenges. In an ideal world, the TMDL would be revised to account for the
unaccounted NPDES dischargers, but in the absence of that ideal, permit writers should work with TMDL
developers to weigh these factors and decide an appropriate, protective course of action. Permit writers
should not automatically assume that the absence of a WLA means the TMDL is not relevant for the
NPDES permit. If the facility at issue discharges the pollutant of concern to a waterbody for which there
is an approved and applicable TMDL for that pollutant, the permit must contain WQBELs consistent with
the assumptions of "any available wasteload allocation for the discharge." See 40 CFR
122.44(d)(l)(B)(vii). As discussed in Section A. 1., some TMDLs do not include WLAs for point sources
discharging to a waterbody for which there is an approved and applicable TMDL. This seems to occur
for three main reasons, discussed below.
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a.	TMDL does not recognize existing NPDES discharges.
Many TMDLs do not address all NPDES discharges that occur in the TMDL watershed. This is
particularly common with respect to discharges permitted by general permits, temporary discharges, and
intermittent discharges. The Region has also encountered TMDLs that mention a permitted facility
within the source analysis and then neglect to assign a WLA for the facility. One interpretation of this
situation when writing or renewing a permit is that no assimilative capacity is available and the NPDES
discharge without a WLA must be prohibited from discharging the pollutant of concern or receive a zero
permit limit. Depending upon the type of TMDL, it may also be appropriate to set a WQBEL equal to
water quality criteria end-of-pipe; e.g., if the TMDL applies concentration-based WLAs for all sources, it
may be possible to find that WQBEL equal to water quality criteria is consistent with the TMDL's terms
and assumptions.
In rare circumstances, if the TMDL includes an unallocated explicit margin of safety created in part to
account for unidentified point source discharges, it may be possible to create a numeric WQBEL
consistent with the TMDL that, in essence, allocates a portion of the explicit margin of safety. It might be
possible to take this approach in settings where the TMDL explicitly discusses this as a valid
implementation approach, and the new discharges covered under this approach are not significant
dischargers of the pollutant of concern. The preferred approach is for the TMDL to include future growth
WLAs or reserve capacity as described in Section A.2.c.
b.	TMDL analysis views NPDES discharge as insignificant.
In some cases, the TMDL writer may not set a WLA for an NPDES because the discharge is considered
to be very small/insignificant in comparison to other pollutant sources. Presumably, the TMDL contains
additional information to make clear that the decision was made on this basis. If the TMDL document
does not describe this situation, it is the functional equivalent of ignoring an NPDES facility. See Section
B.2.a immediately above. If the TMDL document includes an analysis of why a specific discharge is
insignificant and explains how any associated permit may want to address the pollutant identified in the
TMDL, the permit writer will likely have the guidance needed to develop appropriate permit provisions
consistent with the assumptions in the TMDL.
Such permit provisions may include:
•	Limit the discharge to current performance. See Section A. 1 .e.; or
•	Limit the discharge based on a criteria end-of-pipe WQBEL and including monitoring
requirements for the pollutant of concern to support review of the facility contribution of a
pollutant of concern in the future.
c.	TMDL does not account for new NPDES sources.
Some TMDLs do not incorporate WLAs for potential new NPDES sources that do not exist at the time of
TMDL development. The approaches discussed in Sections B.2.a and B.2.b above may be appropriate in
this situation. Other options to address this situation include:
•	Revise the TMDL to include WLAs for new and/or expanded point sources and then incorporate
appropriate WQBELs in the permits; or
•	Set WQBELs based on criteria end-of-pipe, although this may be problematic for fully allocated
mass-based TMDLs.
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3. Translating WLAs into Wastewater NPDES Permit WOBELs
In the best case scenario, the TMDL tells the permit writer all he or she needs to know in order to
translate the WLA into a permit. If not, the permit writer can consider following the permit derivation
methods recommended in the TSD and EPA's NPDES Permit Writers' Manual (2010) to develop
appropriate WQBELs based on WLAs. As discussed above in Section A, it may not be appropriate to
simply copy the WLA and paste it into the permit as it may be difficult to conduct appropriate compliance
monitoring and assess compliance with limits expressed in unconventional ways.
a.	A separate reasonable potential analysis is generally unnecessary.
In many circumstances, where TMDL WLAs indicate the need for a corresponding WQBEL, a separate
reasonable potential analysis should not be conducted if a WLA exists for a particular facility, absent any
new information indicating changes in the facility's discharges of the pollutant of concern.
b.	SpecifV duration and frequency elements consistent with WLA.
As discussed above, it may be necessary to translate WLAs so that they "fit" with how WQBELs are
expressed consistent with permitting regulatory requirements. This may require some translation of WLA
averaging periods to fit how short and long term permit WQBELs are expressed. The TSD provides
useful guidance in how to perform these translations. For WLAs that are set on a seasonal basis or under
different flow scenarios, the permit should specify the periods in which different seasonal limits or flow-
based limits apply, consistent with the TMDL provisions.
c.	Take care in setting mass and concentration-based WOBELs.
As discussed above in section A.5, it may be difficult to translate some WLAs into a form that makes
sense for the permit (e.g., the WQBEL is clear, able to be monitored, and susceptible to compliance
determination). For example, the use of concentration-based permit WQBELs may be most appropriate
to address non-continuous discharges (e.g. stormwater discharges without associated flow measurements)
where the underlying water quality standards and associated TMDLs are expressed in terms of
concentration. Also, take care in setting mass-based WQBELs based on design flow so that the limit is as
stringent as necessary to ensure an associated mass-based WLA will be met. Limits based on design
flows could be inconsistent with TMDL WLAs based on actual flow or some flow assumption lower than
design flows used in calculating prior permit limits.
d.	Clarify where WQBELs apply.
Even in cases where WLAs do not clearly specify where allocations apply (e.g. end of pipe, in receiving
water), permit limits should include clear explanation of where they apply and where monitoring data
should be collected to evaluate compliance.
e.	Remember technology-based limits.
Many facilities require both technology-based and water quality-based effluent limitations. Permit
writers should include technology-based effluent limitations in permits if required, even if they do not
address water quality impairment that TMDLs and WLAs are designed to address. Where technology-
based and water quality-based requirements apply for a particular pollutant, the more stringent of the two
approaches should generally form the basis for the effluent limit.
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f. Respect TMDL recommendations on permitting approaches.
Particularly in cases in which TMDL developers provide specific guidance or assumptions on how permit
limits should be expressed to implement WLAs, permit writers should give that guidance/assumptions
careful consideration. This is particularly important in cases where the WLA is expressed in terms of
multiple expression indicators (e.g., both mass and concentration-based), unless it can be shown within
the NPDES permit factsheet that one permit WQBEL form serves as an effective surrogate for other
indicators used in the WLA.
4. Considerations in Permitting Stormwater Sources
As discussed in Sections A.5 and A.7, developing stormwater permit provisions to implement WLAs can
be challenging. Prior EPA guidance concerning TMDLs and stormwater permits provides valuable
recommendations that permit writers should consider in developing appropriate permit language to
address stormwater WLAs. See EPA Office of Water memo (Nov. 26, 2014) "Revisions to the
November 22. 2002 Memorandum "Establishing Total Maximum Daily Load (TMDL) Wasteload
Allocations (WLAs) for Storm Water Sources and NPDES Permit Requirements Based on Those WLAs".
and EPA Office of Water document (June 2014) "Municipal Separate Storm Sewer System Permits- Post
Construction Performance Standards and Water Quality Based Requirements- A Compendium of
Permitting Practices. EPA 833-R-14-003. The following considerations are particularly relevant in
developing stormwater permits.
a.	Not all stormwater permits are subject to the same requirements.
In cases where WLAs are established for industrial or construction stormwater discharges, numeric
WQBELs are generally practicable and appropriate for such stormwater permits. As discussed in the 2014
memo cited above, numeric limitations are often appropriate for inclusion in municipal stormwater
permits as well. We have found that inclusion of numeric effluent limitations in stormwater permits
improves their clarity and enforceability.
b.	BMP-based approaches to implementing WLAs can work but should be supported by robust analysis.
In permits where BMP-based limitations are used in stormwater permits, it is important for the permitting
authority to provide a strong basis to support the assertion that a BMP-based approach to WLA
implementation will be sufficient to result in timely attainment of the WLA. This can be done if the
permit is supported by detailed identification of the BMP systems to be implemented and a strong
analytical documentation showing how implementation of the BMP system will result in attainment of
specific WLAs in specific compliance locations. Model-based approaches to demonstrating that a BMP
based approach can work may provide the necessary framework for supporting a prospective conclusion
that a particular BMP-based implementation plan will provide reasonable assurance that the WLA will be
implemented. Other approaches may be appropriate, but we have found it is difficult to document a
robust connection between specific implementation practices and associated water quality outcomes
based solely on analysis of monitoring data and BMP effectiveness studies.
As modeling and the monitoring tools used to evaluate the effectiveness of stormwater management
BMPs improve, it should become increasingly plausible to establish BMP-based approaches to
implementing numeric WLAs. For example, when connected with watershed scale water quality models,
BMP siting and optimization models that are capable of associating BMPs with the levels of expected
water quality improvements at different downstream locations can be used to quantitatively link BMP
implementation plans to the attainment of specific WLAs. That is, the various BMPs would serve as
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functional equivalents to the interim numeric WQBELs in the permit, provided the permittees include a
robust analysis showing the BMPs 'add up' to reducing the pollutant load outlined in the TMDL. To be
effective, these models need to be capable of breaking down BMP planning to a fairly small scale and
then integrating BMP plans across TMDL jurisdictions to establish reasonable assurance that a selected
BMP system will result in WLA attainment in a reasonable period of time. BMP-based approaches based
on such model-based planning systems may be advantageous to jurisdictions as they provide a long-term
framework for planning investments in stormwater controls and obtaining funding needed for BMP
implementation and ongoing maintenance. This framework can also provide a basis for reviewing and, if
necessary, revising TMDL implementation timeframes as the modeling results can help indicate how
quickly WLA results can be attained through implementation of affordable plans for stormwater control
and green infrastructure capital investments.
Example Stormwater Permits:
•	Lake Tahoe MS4 Permits (2011). applicable to urban areas of two counties and one city in the
Tahoe basin, implements the Lake Tahoe Sediment and Nutrient TMDL. The TMDL incorporates
a model-based system for setting sediment reduction requirements and providing pollution
control credits for specific BMP implementation actions necessary to meet sediment reduction
needs. The credits awarded are determined based on the practices implemented, the locations of
the implementation sites in the watershed, and the projected sediment reductions associated with
these actions. The permit contains the WLAs based on a crediting system as numeric WQBELs
and provides the framework for determining specific implementation requirements and evaluating
attainment with WLAs.
•	San Diego (C'A) Regional MS4 permit (2013) - The MS4 permit incorporates final TMDL WLAs
as WQBELs for several pollutants including nutrients, metals, and bacterial indicators, expressed
as Receiving Water Limits, and Effluent Limits that are measured at outfalls. It also establishes
interim WQBELs, for which compliance may be demonstrated via monitoring of receiving
waters, effluent, or by implementation of BMPs in an approved Water Quality Improvement Plan
(WQIP) providing reasonable assurance that interim milestones will be achieved consistent with
established schedules. WQIPs are prepared for all major watersheds, and include adaptive
planning processes that identify and address the highest priority water quality conditions. Permit
compliance deadlines extend until 2031 for some TMDLs, and it is recognized that the state
permitting authority may revise these schedules based on WQIP implementation, either via the
permit's re-opener provisions, or through future renewal of the permit.
•	Arlington County (VA) MS4 permit (2013) - This permit implements the Chesapeake Bay
TMDL that includes mass-based WLAs for TN, TP and TSS. (The TMDL also provides
underlying information about pollutant specific loading rates (lbs./acre) for sub-watersheds within
each State.) The MS4 permit includes many different measureable requirements to reduce
nutrient and sediment loads, including:
¦	Reduce by 5 percent the loadings from existing developed lands within the first permit
term. The permit includes tables with required pollutant-specific loading rates to calculate
required loading reductions from existing sources.
¦	Offset (by 5 percent) increased loads from new construction projects disturbing one or more
acres and from grandfathered projects.
¦	Identify and submit to the state at least 7 retrofit projects within its watershed retrofit plans
that will be implemented within County rights-of-way or on County property within 60
months of permit issuance.
¦	Plant a minimum of 2,000 trees on County lands and develop a program to distribute a
minimum of 2,000 trees to private property owners.
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¦	Fund a minimum of 200 participants in the StormwaterWise Landscape program which
provides cost-sharing and technical assistance for the installation of small scale best
management practices to reduce stormwater runoff from private properties.
•	Washington State Department of Transportation (WSDOT) MS4 permit (2014) - The permit
includes specific mandated action items for WSDOT that are associated with individual
TMDLs. For example, to implement the Hangman's Creek Fecal Coliform and TSS/Turbidity
TMDLs, WSDOT is required to reduce fecal coliform loads by 72% and sediment loads by 80%.
Here are a few action items outlined in Appendix 3, Applicable TMDL Requirements:
¦	Within 6 months of permit issuance, WSDOT will update the initial inventory findings report
with updates on potential TMDL concerns, and follow-up actions taken and/or notification to
others where a concerns has been identified but occurred outside WSDOT's right-of-way and
control.
¦	If stormwater discharges that transport bacteria over natural background levels to listed
receiving waters are found from sources within WDOT's right-of-way and control, WSDOT
will apply BMPs from their SWMPP or perform remediation to correct bacteria discharges.
To address TSS/turbidity, WDOT will work to prevent sediment from entering area
waterways along SR 27 (in upper watershed) and SR 195 right-of-ways. WSDOT will
prioritize problem areas and work with individual property owners to prevent sediment from
entering area waterways via WSDOT's MS4.
•	Western Washington Phase II Municipal Stormwater Permit (2012) includes tables that establish
additional watershed-specific actions that are required of each named MS4. The actions are
differentiated based on the applicable TMDL and pollutant of concern. See Appendix 2.
(Note: More stormwater permits are presented in EPA Office of Water document (June 2014) titled,
Post-Construction Performance Standards and Water Quality-Based Requirements in Municipal Separate
Storm Sewer Systems Permits: A Compendium of Permitting Approaches.)
c. Numeric WOBELs can be easier to measure and enforce.
The Region has conducted several dozen stormwater permit audits over the past 5 years, and we have
concluded that inclusion of non-numeric permit requirements, especially those based on relatively vague
iterative BMP requirements, are difficult to evaluate for enforcement purposes and have not demonstrated
significant water quality improvements. This is particularly true in cases where insufficient monitoring
was required in permits to support evaluation for compliance with permit requirements. Numeric
WQBELs can be much more straightforward to implement and monitor as long as permit writers are
sensitive to the variability of stormwater discharges in determining how they should be expressed and
monitored. As discussed above in Section A.7.b, concentration-based WQBELs are often easier and more
sensible to implement than mass-based WQBELs for stormwater discharges. If, however discharge flow
measurements exist and long-term pollutant exposure is of concern, then mass-based WQBELs can be
applied if sufficient data are available to evaluate long term loads.
5. Providing Time to Implement Needed Controls
Some TMDLs can take decades to implement as many sources will need to implement changes in facility
operations and treatment technology. There are several mechanisms for providing time needed to
implement new permit requirements based on TMDLs. The key point to remember is that permit writers
must follow the permit regulations. While implementation timeframes can be identified during the
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TMDL process, compliance schedules can be established only through permit proceedings following
establishment of compliance schedule authorization in applicable water quality standards. Similarly,
variances can be established only through water quality standards proceedings. The TMDL document can
provide much of the needed information on why extra time may be needed and the interim steps that
should be taken. The three main mechanisms used to provide extra time for TMDL implementation in the
NPDES permit are variances, compliance schedules, and enforcement orders.
a.	Variances provide a time-limited designated use and criterion for the purpose of NPDES permitting
that are less stringent than the underlying water quality standards.
Variances can be used to delay the effective date of water quality standards applicable to some NPDES
discharges. Variances are intended to provide short term relief from immediate responsibility to comply
with a water quality standard that may be unattainable in the short term, but are not intended to be
permanent. Variances must be authorized under applicable water quality standards and approved by EPA.
Although specific variance procedures vary and are specified in applicable water quality standards and
associated implementation procedures, and their supporting documentation, variances generally:
•	Are pollutant-specific;
•	Specify the highest level of water quality that can be attained during the time period the variance
is in effect;
•	Explain why the variance is needed;
•	Provide appropriate justification for temporary use attainability changes per WQS regulations;
and
•	Specify interim controls and studies that will be conducted to improve water quality and/or
determine whether underlying designated beneficial uses are attainable.
b.	Compliance schedules provide extra time but must contain interim steps to attain limits ASAP
While NPDES permits can contain compliance schedules if additional time is needed to implement new
water quality based control requirements, NPDES regulations require that findings must be made at the
time of permit issuance that demonstrate that the specific compliance schedule requirements are met for
the permitted facility. See 40 CFR 122.47. The permit record must demonstrate that: (i) the compliance
schedule is needed, (ii) the compliance schedule will result in compliance with the limit as soon as
possible, and (iii) interim compliance schedules and action-based milestones are incorporated to ensure
reasonable progress is being made toward completing control actions needed to meet the new WQBEL.
The permit included the final WQBEL and its deadline which may be beyond the five year permit cycle.
See EPA Office of Water memo (May 10, 2007) from Jim Hanlon, EPA OWM to Region 9, titled
"Compliance Schedules for Water Quality Based Effluent Limits in NPDES permits." Compliance
schedules cannot be used to provide time to develop or revise a TMDL, a water quality standard, or a
mixing zone analysis.
c.	Enforcement Orders provide greater flexibility but must be outside permit.
In some cases where variances and/or compliance schedules are not authorized under state water quality
standards or are inappropriate for a specific case where a new WLA is considered when a permit is
issued or re-issued, it is often possible to issue a companion enforcement order with an NPDES permit.
These orders can provide additional time for a facility to come into compliance with a permit requirement
and specify interim actions that are needed to result in eventual compliance.
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C. CONCLUSION
EPA and state TMDL and NPDES permitting authorities are still learning how to best link the TMDL to
NPDES permits process to effectively guide implementation of controls on point source discharges as
needed to help restore impaired waters. The Region is seeing significant improvements in comparison to
the "early days" of TMDL development and implementation. This document provides some initial
lessons learned and examples to illustrate these concepts. The Region anticipates that as NPDES permits
are reissued in the coming years, the information in this document will need to be updated to include new
examples or modified information. The Region has an interest in improving and ensuring the accuracy of
the information contained in this document and therefore welcomes any comments, corrections and
examples on any aspect of this document at any time. The Region may update this document as needed
based on the comments received as well as any TMDL or permit examples to strengthen it. Please
provide any feedback to David Smith (smith.davidw@epa. gov) and Peter Kozelka
(kozc 1 ka. pctc x'a ,c pa. go v).
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