Overview of models used to meet MS4 TMDL permit requirements Overview of models used to meet MS4 TMDL permit requirements

The MS4 (Municipal Separate Storm Sewer System) permit requires permittees to demonstrate progress toward meeting applicable total maximum daily load (TMDL) wasteload allocations (WLAs). Permittees claiming to meet a WLA must document how they achieved the WLA. Whether a permittee is making progress toward or has achieved a WLA, they must quantify pollutant loads or load reductions.

Models are frequently used to calculate pollutant loading or load reductions associated with stormwater management practices. Many water quality models can be used to demonstrate permit compliance. Based on a survey of permittees, the following four models are commonly used for assessing or demonstrating TMDL permit compliance.

• Program for Predicting Polluting Particle Passage thru Pits, Puddles, & Ponds (P8; Walker, 1990)
• Minimal Impact Design Standards Calculator (MIDS Calculator; MPCA, 2017)
• Minnesota Pollution Control Agency Simple Estimator (MPCA Simple Estimator; MPCA, 2015)
• Source Loading and Management Model for Windows (WINSLAMM)

We developed guidance for using these four models to meet permit requirements. Model guidance for each of the four models is provided on separate pages (P8; WinSLAMM; MIDS calculator; MPCA Simple Estimator), or can be viewed as a single document containing guidance for all four models.

Individual MS4 permittees can use the summary information in the following tables to determine which model is most-suitable for demonstrating permit compliance based on circumstances unique to the MS4 and given WLA requirements.

• TMDL model description and overview: A brief description of each model and its applicability for demonstrating permit compliance
• TMDL model comparison matrix: A matrix comparing general model capabilities and relative complexity
• Water quality BMP comparison matrix for TMDL models: A list of water quality best management practices (BMPs) which can be modeled in each program and a description of BMP types which are specifically included as a unique BMP type with unique BMP parameterization (i.e., “specific”) and which BMPs are not specifically included but can be modeled using general or user-defined model inputs (i.e., “general”).

Costs to develop the models should generally range between $5,000 and $20,000 for evaluating compliance with TSS or TP TMDL WLAs that involve approximately ten BMPs in ten subwatersheds for five or fewer outfalls draining to an impaired water body.

TMDL model description and overview
Link to this table

Model	Model description	Applicability for Permit Compliance and Reporting
P8	P8 is a physically-based water quality model which simulates the generation and transport of sediment and associated pollutants from urban watersheds. The model is capable of predicting sediment particulate removal of five (5) particle sizes (including one soluble fraction) and associated pollutants at a variety of BMP types.	P8 is an acceptable model for demonstrating compliance with TSS and TP WLAs.
MIDS calculator	The MIDS Calculator is an Excel-based stormwater quality tool used to estimate runoff and pollutant removal at a variety of stormwater BMPs. The model was originally developed by the MPCA to assist designers and regulators evaluate conformance to MIDS performance goals for development-scale models. The MIDS Calculator is an empirical model which predicts pollutant removal based on correlation to P8 results and design-standard BMP removal rates from literature.	MIDS calculator is an acceptable model for demonstrating compliance with annualized TSS and TP WLAs for simplified study areas. Specifically, the tool is limited in its ability to evaluate bypass from undersized BMPs and predict pollutant removal through non-volume reduction BMPs in series.
MPCA Simple Estimator	The MPCA Simple Estimator is a spreadsheet-based tool that utilizes the Simple Method to estimate land use based pollutant loading from urban watersheds. The empirically-based model estimates pollutant removal from nine (9) BMP types based on design-standard BMP removal rates from literature.	The MPCA Simple Estimator is an acceptable model for demonstrating compliance with annualized TSS and TP WLAs for simplified study areas. The tool is not capable of evaluating bypass from undersized BMPs or pollutant removal through BMPs in series.
WinSLAMM	WinSLAMM is a water quality model originally developed for the USGS to evaluate nonpoint pollution in urban areas. The model predicts pollutant loading from a variety of land use and impervious area types and calculates pollutant reduction at a variety of control devices (BMPs). Pollutant reduction at control devices is based both on experimental field results (empirical) and tracking of particulate settling and filtration (physically-based).	WinSLAMM is an acceptable model for demonstrating compliance with TSS and TP WLAs.

TMDL model comparison matrix
Link to this table

Comparison categories	Water quality model
		P8	MIDS calculator	MPCA Simple Estimator	WinSLAMM
Relative input complexity	High	Medium	Low	High
Public domain	Yes	Yes	Yes	No
TSS modeled	Yes	Yes	Yes	Yes
TP modeled	Yes	Yes	Yes	Yes
Volume reduction modeled	Yes	Yes	Yes	Yes
Model time step	Event/continuous	Annual	Annual	Event
Pollutant Loading Methodology	EMC & Buildup/ Wash-off	Simple method	Simple method	EMC & Buildup/ Wash-off
Pollutant removal methodology	Physically based	Empirical	Empirical	Physically-based / Empirical
Pollutant Removal Mechanisms	Filtration, Sedimentation, Infiltration	Empirically based	Empirically based	Filtration, Sedimentation, Infiltration
Capable of evaluating bypass from undersized BMPs?	Yes	Only for volume-reduction BMPs	No	Yes
Capable of modeling removal from BMPs in series?	Yes	Only for volume-reduction BMPs	No	Yes
GIS compatibility	Low	Low	Low	Medium
Model used for TMDL Development and Modeling	Yes	No	No	Yes
Costs - software	None	None	None	medium
Costs - model development	Medium	Low	Low	High

Water quality BMP comparison matrix for TMDL models
Link to this table

Water quality BMP	BMP included in model1
		P8	MIDS calculator	MPCA Simple Estimator	WinSLAMM
Bioretention, Infiltration / Filtration	General	Specific	Specific	Specific
Bioswale, Infiltration / Filtration	Specific	Specific	Specific	Specific
Cistern (e.g., Rain Barrel)	General	Specific		Specific
Constructed Wetland	General	Specific	Specific	General
Detention Basin, Dry	Specific			Specific
Detention Basin, Wet	Specific	Specific	Specific	Specific
Grass Buffer / Filter Strip	Specific	Specific	Specific	Specific
Green roof	General	Specific	Specific	Specific
Hydrodynamic Separator / Grit Chamber2	General			Specific
Infiltration Basin	Specific	Specific	Specific	Specific
Other (i.e., user-defined BMP)	General	Specific	Specific	Specific
Permeable Pavement	General	Specific	Specific	Specific
Rain garden	Specific	Specific	Specific	Specific
Sand filter	General	Specific	Specific	Specific
Sand Filter, Iron-Enhanced	General	Specific	General	General
Street cleaning	Specific			Specific
Tree Trench / Planter	General	Specific		General
Underground Infiltration / Filtration	General	Specific	General	General

¹"Specific" = BMP type distinctly included in model; "General" = BMP can be modeled but is not distinctly included as a unique BMP type; and "--" = BMP cannot be modeled.
²Typically used as a pretreatment practice.