E.14 PEAIP Item:
Methodology to Quantify Pollutant Load Reductions Achieved by the Program as a Whole
1. Purpose
This document presents (insert municipality name) intended approach to comply with the Phase II General
Permit Section E.14.a.ii.a.6 “Quantification of pollutant loads and pollutant load reductions achieved by the
program as a whole,” which is one element of our Program Effectiveness Assessment and Improvement
Plan (PEAIP). Specifically, this document outlines our intended approach for quantification of pollutant
loads and reductions for those stormwater program elements that can be reasonably and credibly
quantified using science-based estimates (Permit Sections E.14.a.ii.b.4 and E.14.a.ii.d.2) including
treatment Best Management Practices (BMPs) (e.g., LID facilities, infiltration ponds, water quality
treatment filtration units, detention/retention basins) and certain Pollution Prevention / Good
Housekeeping BMPs (e.g., street sweeping, catch basin cleaning, erosion control, etc.). Several of our
stormwater program elements are not conducive to quantitative estimations of pollutant load reductions
(e.g., IDDE, outreach/education, public involvement and participation, construction site runoff control
programs). Our effectiveness assessment for these programs will be addressed using other methods (insert
reference for the companion portion of the Municipal Separate Storm Sewer System (MS4) PEAIP).
The intended approach to quantify pollutant loads and reductions, as outlined in this document, will serve
two functions in assessing the effectiveness of our stormwater program:
1. Estimate pollutant load reductions based on our program actions that can be quantified in
terms of pollutant load
2. Estimate relative pollutant loading by catchment, in our MS4, which will inform our stormwater
program priorities
A depiction of our pollutant load estimation and influence of BMPs is shown in Figure 1. For purpose of
brevity, our intended pollutant load reduction approach is referred to as the Tool for Estimating Load
Reductions (TELR).
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Figure 1. The TELR approach quantifies pollutant loading on an urban (MS4)
catchment basis, ranks all MS4 catchments by pollutant loading rates to the
receiving waters. Subsequent re-quantification of pollutant loading resulting from
recent stormwater program actions it repeated annually.
2. Background
Relationship to Phase II Stormwater NPDES Permit Requirements
The intended TELR approach supports compliance with the Phase II MS4 Municipal Permit’s PEAIP
requirements (E.14). Specifically, the TELR methodology will address the following requirements:
Identify assessment methods to quantitatively assess BMP performance at reducing pollutant loads
Identify a strategy to gauge the effectiveness of prioritized BMPs and program implementation as a
whole (E.14.a.i.)
Prepare a PEAIP that includes:
o quantification of pollutant loads and pollutant load reduction achieved by the program as a
whole (E.14.a.ii.a.6);
o assessment of pollutant source reductions achieved by individual BMPs (E.14.a.ii.a.5);
o assessment of BMP and program effectiveness in terms of Outcome level 4 – pollutant load
reductions (E.14.a.ii.b.4);
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2.2 Use of TELR results
TELR will output estimates of average annual pollutant loads and load reductions associated with
stormwater improvement actions using a spatial, catchment-scale analysis. The TELR methodology will
build upon similar work developed in the Lake Tahoe Region to create a user-friendly, transparent and
scientifically-credible tool to estimate pollutant loads. The pollutant load estimations, catchment
delineations and other analytical components of TELR are not ground-breaking or complex. Mapping
products and associated pollutant load information from TELR can be used iteratively to guide priorities
and quantify the load reduction achieved by the program over time. Rather than attempting to model,
track and evaluate multiple pollutants, the TELR methodology will use credible and effective proxies (i.e.,
stormwater volumes and total suspended solids (TSS) to create a ranking of MS4 catchments in terms of
relative potential risk to the receiving water.
TELR will be used to generate two pollutant load scenarios for an urban catchment; a “Land Use Load” and
a “Current Load”.
Land Use Load: A catchment land use load is the baseline estimated average annual load without
any stormwater program actions or improvements. The land use load is based solely on land use
distribution and unmodified hydrologic connectivity of the catchment to the receiving waters.
Current Load: The current load is the estimate of the average annual pollutant load with the
implementation and assumed effectiveness of stormwater program actions including source control
and structural BMP implementation in the catchment for the respective year evaluated.
The difference between the Land Use Load and Current Load is the estimated Load Reduction achieved as a
result of the stormwater program for the year and catchment of interest.
Each year the current load value for each catchment area can be normalized (e.g. acre feet per acre per
year; termed Current Loading Rate) and integrated and ranked for all catchments within the municipality.
The catchments with the highest Current Loading Rate per unit area are the catchments with the potential
greatest risk to receiving water quality for the current year. The results of this prioritization are mapped to
simplify communication of priority catchments to managers, funders, regulators, etc. Figure 2 illustrates
example catchments shaded purple and red where stormwater improvements and associated maintenance
should be prioritized to achieve and sustain reductions in stormwater pollutant loading to the receiving
waters over time.
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Figure 2. MS4 catchment pollutant loading results of ranking are mapped to clearly
illustrate the catchments with the greatest potential risk per unit area to the receiving
water quality (i.e., purple and red catchments) and are therefore priorities for
stormwater program BMPs.
The TELR approach will be developed and implemented using readily available data, including the results of
BMP performance assessments. Using performance assessment information, the annual estimated
pollutant load reductions are revised and the information used to reevaluate our stormwater program
priorities each year. Stormwater managers can then aim to implement stormwater program BMPs that will
result in changing a high risk catchment to a lower risk catchment. The implementation of this standardized
and consistent process can be used to adaptively manage the annual priorities of a stormwater program
over time.
The continued estimation and reporting of Land Use and Current Loads annually allows tracking of the
water quality benefits of the entire MS4 program over time. Figure 3 provides an example of how we
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intend to use graphic summaries to track and communicate program load reduction progress (i.e. program
effectiveness) over time.
Figure 3. Example MS4 program reporting formats. Using TELR results, the aggregation of
the load reductions estimated for each catchment is used to document the estimated
MS4 pollutant load reduction each year.
The development of the TELR approach will be developed with involvement from the Central Coast
Regional Water Quality Control Board (Central Coast Water Board), the Low Impact Development Initiative
(LIDI), 2NDNATURE, LLC and regional MS4 stakeholders.
2.3 Relationship of TELR to Long-term Receiving Water Quality Data
Phase II Stormwater NPDES Permittees must manage their MS4 to protect and improve receiving water
quality. Ultimately, receiving water quality data can provide information to assess whether water quality
standards and other receiving water beneficial uses have been achieved. One of the greatest difficulties in
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stormwater management is our collective inability to confidently link the municipal BMPs to water quality
improvements in the receiving water. This is due to the fact that detecting the benefits of MS4 stormwater
quality BMPs can be masked due to (1) natural variability, (2) error associated with sampling and/or,
potentially, the (3) signal in the water quality variability due to other sources beyond the MS4
contributions. Permittees need an ability to assess their own actions (i.e., within the MS4) in terms of water
quality improvements and they need to be able to do so on a temporal scale that aligns with their needs to
identify and prioritize stormwater quality actions, verify the relative effectiveness of their actions, provide
some standardized accounting of the benefits of their actions, and adaptively manage their program on
yearly time scales. TELR will be an easy to use and standardized tool that can provide this information in
useable formats to meet these needs. Stormwater Program Managers cannot make appropriate
programmatic decisions based solely on receiving water quality data on annual time intervals. Rather, a
long-term, consistent and well executed water quality monitoring program in strategic locations could be
used to verify the hypothesis that effective stormwater BMPs will result in measurable decreasing trends in
receiving water pollutant concentrations and loads, beyond natural variability and sampling error.
Relationship of TELR to the California Stormwater Quality Association’s (CASQA) Program
Effectiveness Assessment and Improvement Plan (PEAIP) Framework
CASQA recently developed a PEAIP Framework to support development of MS4 PEAIPs. In many respects,
the intended TELR approach is consistent and supports the CASQA framework. For example, the use of
information concerning pollutant source contributions and MS4 contributions is consistent between both
approaches to estimate pollutant loading. However, there is a key difference between the two approaches:
The CASQA PEAIP Framework includes identification of the highest priority “Pollutants of
Concern” (POCs) based on available knowledge, data, etc. (e.g., 303(d) listings, TMDLs) and
subsequent identification and prioritization of BMPs based on these POCs. In comparison, the
TELR approach is based on the premise that stormwater volume and TSS are reliable proxies for
the majority of pollutant types and that knowledge of the greatest potential risk due to pollutant
loading is sufficient to direct the majority of stormwater program actions and priorities. Focus on
water quality improvement actions in catchments with the current highest pollutant loading rates
(i.e., highest volume or TSS loading rates) will result in program management actions that reduce
all related urban derived pollutant generation and delivery from stormwater to receiving waters,
including those of interest due to 303(d) listing or TMDLs.
TELR assumes that urban pollutant fate and transport for nearly all urban derived pollutants can be
reasonably represented by stormwater volumes and sediment loads. If stormwater runoff volumes are
effectively treated at the source, stormwater pollutant loading to receiving waters will be minimized.
Pervious or impervious surface with relatively higher accumulations of sediment are also likely to contain
relatively higher mass loads of other urban pollutants (e.g., the typical pollutants of concern (POCs): metals,
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oils/greases, bacteria, pesticides/herbicides, nutrients). While all catchment prioritization ranking may not
exactly match the accurate prioritization for every individual POC; our hypothesis is that in the majority of
instances, the TELR approach will credibly inform program priorities and action. This decreases the
complexity of the modelling effort as well as the cost while still providing high-value information to inform
short- and long-term program priorities. A detailed description of the intended TELR approach is provided
in Section 3 below.
3. Tool for Estimating Load Reductions (TELR) Approach
The TELR approach includes use the MS4 map products and BMP inventory to support estimation of MS4
catchment volume and pollutant loads. Section 3 provides detail related to the TELR approach:
Section 3.1 describes the desired functions (i.e. product output) for estimating pollutant
loads and associated load reductions,
Section 3.2 describes the process to evaluate existing hydrologic and pollutant load models
to meet our desired functions and the decision to develop a separate approach and tool (i.e.
TELR), and
Section 3.3 provides detail regarding the TELR approach including how our MS4 mapping will
be integrated with the pollutant load/reduction approach to result in desired products and
3. 1 Desired Function for Estimating Pollutant Loads and Reductions
The approach to estimate our MS4 pollutant loading and reductions is guided by six fundamental
1. Create credible pollutant load estimations: The approach must generate scientifically defensible
and relatively accurate estimations of average annual loading for the MS4 by catchment(s).
2. Focus modelling on pollutants that are representative of urban impacts: The inclusion of several
different pollutant types will not improve the ability of stormwater managers to select and
implement effective actions to protect receiving water quality. In fact, we argue more pollutants
over complicates the process of modelling, thereby distracting resources and focus from
implementation of effective actions. Because all urban pollutants, with perhaps the exception of
trash, are associated with stormwater runoff volumes and/or TSS, using pollutant loading estimates
of these two parameters significantly reduces cost and complexity while providing reliable
information to guide the implementation and management of our stormwater quality improvement
program. Use of urban pollutant proxies should not negate source control BMPs that target specific
pollutants but rather help identify where those actions might be most cost-effective.
3. Focus analysis on factors that most influence pollutant loading: The approach should focus on the
key drivers (e.g., land use, hydrologic connectivity, BMP presence and sizing) that influence average
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annual urban pollutant loading. Additionally, these model parameters should be readily available to
municipal stormwater managers (e.g., public data such as GIS layers, precipitation data, BMP
inventory etc.).
4. Sensitivity to water quality improvement actions: The approach must be sensitive to pollutant
loading reductions from stormwater Best Management Practices (BMPs) and isolate the variability
in urban pollutant loading from natural climatic variability.
5. Appropriate to the entire MS4: The method should be applicable and the results scalable to the
entire MS4. The results should (1) inform meaningful spatial comparisons to identify catchments
with the greatest current pollutant loading to the receiving waters and (2) allow program wide
documentation of pollutant load reduction estimates over time.
6. Ease of initial set-up and on-going use: The approach must be easy to use by the Permittee and
include clear user guidance, a complete and functional data management structure, and
automatically generate results in directly usable formats. The approach should be publically
available (i.e., non-proprietary) for use. Results must be reported in useable and simple formats that
inform program decisions; satisfy annual reporting requirements to the Central Coast Water Board;
and, track water quality improvement progress of the MS4 stormwater program.
Evaluation of Existing Models to Meet Desired Functions
A review of a number of available public and proprietary urban hydrology and pollutant load model(s)
(HSPF, SWMM, STEPL, SWC, PLRM, HEC HMS, Watershed Treatment Model, etc.) indicated that no existing
available models achieve all of the fundamental objectives stated in 3.1. Therefore, a customized approach
and model is being created to achieve the stated objectives above. The customized approach and desired
objectives will be achieved by the Tool for Estimating Load Reductions (TELR). While no one model provides
the necessary framework to achieve the desired objectives, review and testing of three specific model
platforms will inform our hydrology and pollutant loading as appropriate. (i.e. Spreadsheet Tool for
Estimating Pollutant Loads (STEPL: TetraTech 2011, version 4.1), Pollutant Load Reduction Model (PLRM v2;
nhc et al. 2014) and National Stormwater Calculator (SWC), (EPA/600/R-13/085c, 2014i)
The Tool for Estimating Load Reductions (TELR) Approach
The information below summarizes how TELR will achieve the desired functionality to estimate urban
catchment pollutant loads, utilize the results to prioritize catchments for stormwater improvement actions,
and estimate the annual load reduction benefits of the actions implemented under a municipal stormwater
3.3.1 Credible Pollutant Load Estimation
The model will integrate the most sensitive parameters driving existing urban water quality models in a
manner that minimizes user complexity yet preserves the relative technical accuracy and rigor necessary to
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generate relatively reliable stormwater pollutant loading estimates to inform management decisions and
account for progress using average annual pollutant load reductions as the unit of measure. TELR will run
on regionally representative precipitation using a probability-based integration approach of daily rainfall
and runoff intensities. Precipitation inputs will be held constant between Land Use and Current Load
scenarios to document the estimate load reductions due to BMP implementation and performance. The
water quality benefits of source control and small scale structural BMPs have a level of performance that
aligns with the density of implementation by land use, literature values regarding performance, PCR design
requirements, and/or annual effectiveness assessments. Larger scale centralized BMP performance
includes an event volume accounting and treatment performance also informed by existing literature and
annual effectiveness assessments. While the absolute estimates of all models will vary, TELR will be
developed to generate catchment results that will generally correlate with estimates using more complex
modelling platforms. The intent is to have confidence that the priorities and/or relative load reductions
identified by TELR are relatively accurate, thereby providing the correct information to guide effective
stormwater management. A transparent documentation and supporting template of the fundamental
assumptions and how user inputs are integrated TELR to generate average annual loads will be created and
readily available.
3.3.2 Modelled Pollutants
The pollutants selected for modeling are stormwater volumes and sediment loads. These pollutants are
assumed to serve as reliable collective proxies to guide prioritization of catchments and implementation of
improvements for the majority of urban pollutants known to impact receiving water quality. The
catchments identified with the highest loading rates are, in most instances, the greatest potential threat of
most urban derived pollutants delivered to receiving waters.
Stormwater Volume: If stormwater program actions are effective, stormwater volumes discharging
from the MS4 to receiving waters will be substantially reduced or eliminated, even during the
highest intensity storms. Eliminating stormwater discharge equates to removal of urban pollutants
associated with these wet weather discharges and concomitant reduction of pollutant loading to
the receiving waters.
Sediment: Sediment, reported as total suspended solids (TSS), is a common and well researched
stormwater quality parameter. There is extensive literature and national datasets documenting the
ranges of TSS concentrations in land use and mixed urban runoff. While TSS loading from urban
catchments may or may not exceed regulatory thresholds, there is a well-documented range of
concentrations emanating from urban lands that are maintained from good to poor condition. In
addition, stormwater improvement actions implemented to reduce sources of TSS or treat TSS loads
entrained in stormwater are, in many instances, also effective at reducing the concentrations and
loads of other common urban pollutants. TSS has been documented to correlate to pathogens, trace
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metals, hydrocarbons, and phosphorus in urban runoff (Chen and Chang 2014; National Stormwater
Quality Database v1.1). Processes commonly used to treat urban stormwater via structural BMPs infiltration, particle capture and media filtration - are equally effective at treating most hydrophobic
pollutants. Hydrophilic pollutant load reductions (i.e., nitrate) are assumed to occur concurrently
with effective stormwater volume reductions.
3.3.3 Use of Model Inputs that Influence Pollutant Loading
TELR will use regionally relevant precipitation data, local soils, slope, and land use to inform a probability
distribution approach to estimate average annual catchment volumes. All of the required inputs will be
readily available to stormwater managers through a variety of information sources including MS4 mapping
products and online source. Pollutant generation by land use will be further refined by the defined relative
condition of the individual catchment’s land use. The MS4 catchment/outfall mapping products include a
complete delineation of the MS4 into unique drainage catchments and a number of associated catchment
attributes (i.e. catchment area, impervious area, land use distribution, hydrologic connectivity, etc.). These
spatial attributes are important to conduct to pollutant load estimates for each catchment within our MS4.
MS4 mapping conducted as part of the IDDE requirements (Section E.9) will be used to support our TELR
approach including:
• Location of outfalls
• Catchment drainage area delineations
• Land use contributions and imperviousness
• Location of water bodies receiving direct discharges from outfalls
• Hydrologic connectivity of each catchment to the receiving waters
The pollutant load reductions associated with the presence and condition of water quality improvement
BMPs will be directly integrated into catchment load estimates. Clear user guidance, automated functions
and an innovative data management system will simplify the process required to generate representative
model inputs.
3.3.4 Sensitivity to water quality improvement actions
Precipitation inputs will be held constant for all load estimation scenarios to eliminate any result variability
due to climatic differences, and therefore isolate the load reduction estimates attributed to stormwater
quality improvement actions. Land Use Loads will be calculated to estimate catchment loading without any
BMPs in place. Catchment Current Loads for each water year will include the estimated benefits of source
control programs (e.g., street sweeping, erosion control), LID implementation on urban land uses,
dispersed and centralized structural BMPs (e.g., infiltration features, dry basins, bioretention facilities).
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Each year, Current Loads are subtracted from Land Use Loads to estimate the load reduction associated
with the implementation and maintenance of the stormwater program actions within each catchment.
3.3.5 Appropriateness to the MS4
The municipality’s MS4 consists of many individual urban catchments. The customized model will simplify
user workflow and provide clear user guidance for stormwater managers so they can utilize readily
available data to populate necessary model inputs. A customized user input template coupled with
automated data reporting formats will be created to simply translate inputs into pollutant load results for
all catchments within the MS4. The results will presented be in graphical formats to immediately inform
program decisions and track progress over time. A comprehensive data management structure and annual
process will minimize data entry year after year, allowing stormwater managers to focus on estimating
current pollutant loads where improvements were implemented. The difference between the pollutant
loads estimated without the implementation of stormwater program actions (i.e. Land Use Load) and the
Current Load as a result of the stormwater program provides a quantitative measure to track annual MS4
program progress over time (see Figure 3).
3.3.6 Ease of initial set up and on-going use
We have leveraged Phase II Permit MS4 mapping requirements to develop specific catchment attributes
that inform the pollutant load model estimates. For example, catchment delineation, hydrologic
connectivity and land use distribution are all direct inputs to TELR.
The initial set up will require the Permitee to populate TELR with the required data for all MS4 catchments.
However, once TELR is populated with existing mapping and water quality BMP data, on-going use will be
simple and relatively low maintenance. The only changes year after year will be mainly focused on
catchments where water quality improvement actions are prioritized and implemented.
Spreadsheet Tool for Estimating Pollutant Loads (STEPL) is a macro-enabled MS Excel spreadsheet tool developed by EPA Region
5 and TetraTech (TetraTech 2011, version 4.1). The STEPL hydrology model is similar to the desired approach necessary to meet
the custom model objectives, though some modifications to incorporate a probabilistic approach are necessary. STEPL uses an
empirical approach to estimate runoff based on surface type and static land use runoff pollutant concentrations to estimate
loads. Average annual results include volume and load estimates by catchment and land use, and the benefits of a variety of BMP
types, but does not incorporate BMP condition or larger scale structural BMPs. A large amount of complexity within STEPL is not
necessary to achieve objectives and will be removed. STEPL graphically summarizes load reduction estimates between scenarios.
Multiple catchments with multiple scenarios can be saved within a single MS Excel file; though the data management
functionality is limited when estimates are generated in Excel files.
The Pollutant Load Reduction Model (PLRM) is a customized platform (PLRM v2; nhc et al. 2014) that is used by the Lake Tahoe
TMDL to estimate average annual volume and pollutant load reductions as a result of stormwater improvements. PLRM
integrates the SWMM (EPA, 2010) continuous hydrology model with a customized water quality module that estimates land use
pollutant concentrations as a function of land use condition. Large scale structural BMPs are explicitly incorporated and modelled
as water quality treatment structures at the outfall of a catchment. The customized model will incorporate the land use
condition and structural modelling approach of PLRM. In addition, PLRM is used to conduct a number of sensitivity tests to
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isolate the most sensitive parameters driving urban pollutant loads to guide custom model development. Model calibration will
include comparisons to PLRM volume and sediment estimates. Data management is limited to scenario evaluations for a single
catchment, a feature that will be greatly improved by the new model.
National Stormwater Calculator (SWC), developed by the EPA (EPA/600/R-13/085c, 2014) is an online tool used to estimate
runoff via the continuous simulation SWMM (EPA, 2010) platform. The National SWC has a user-friendly interface to input
precipitation, land cover, climate change and the implementation of LID improvements on a catchment scale. The user friendly
formats will be leveraged for the custom tool. SWC lacks a number of critical features to isolate the effectiveness of the
stormwater program actions. In addition, data management within the platform is limited to two load reduction estimates,
required an external data management process to achieve custom tool objectives.
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