Summary of TMDL requirements in stormwater permits

Information: This page was last updated following issuance of the 2013 MS4 permit. It will be updated just prior to issuance of the 2020 MS4 permit.

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Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs) approved by the U.S. Environmental Protection Agency (EPA) prior to the November 16, 2020 issuance date of the MS4 (Municipal Separate Storm Sewer System) General Permit (permit) must be addressed by Permittees in their Stormwater Pollution Prevention Program (SWPPP) Document, which includes the TMDL Application Form. The Clean Water Act requires the Minnesota Pollution Control Agency (MPCA) to include, in all National Pollutant Discharge Elimination System (NPDES) permits, discharge requirements that are consistent with requirements established in TMDLs. The permit requires applicants to submit information at the time of application on applicable WLAs and document how they will make progress on performance-based WLAs (bacteria, chloride, temperature), demonstrate they are currently meeting their numerical WLAs (oxygen demand, nitrate, total phosphorus or total suspended solids), or develop a compliance schedule for those numerical WLAs that are not being met.

Understanding Wasteload Allocations

Wasteload allocations are allowable loads from regulated or future regulated sources in TMDLs. They are expressed as a load (i.e. pounds per day, kilograms per year, etc.) in TMDLs. They may also be expressed as a percent reduction in pollutant loading from existing conditions. The MPCA’s Stormwater Program believes the percent reduction, when provided in the TMDL report, is a useful expression of the information for implementation and planning because it provides an estimate on the magnitude of pollutant loading reduction needed for regulated entities to meet their assigned WLA(s) from the baseline year.

WLAs for MS4 sources are either individual or categorical. Individual WLAs provide a single load for each Permittee, whereas categorical WLAs group multiple Permittees into one load that collectively must be achieved by all contributing sources. Regardless of the type of WLA, the information required to be submitted at the time of application is the same.

It is important to note watershed boundaries define TMDL study areas, while MS4 permits apply to a political boundary or jurisdiction. Further, WLAs apply only to the area that drains to the receiving water defined in the TMDL report. Therefore, it is possible that only a portion of a regulated entity’s area will be within the TMDL study area. Credit for pollutant removal from best management practices (BMPs) can only be applied to the treatment applied to stormwater runoff generated within the TMDL study area for a particular WLA; however, BMPs often treat water generated both inside and outside the study area.

A single TMDL project may contain multiple wasteload allocations for a single MS4. This can occur in the following situations:

• A project that addresses nutrient impairments in multiple lakes within a watershed. For example, the Crystal, Keller, and Lee Lakes Nutrient Impairment TMDL includes WLAs for nutrient impairments in all three lakes, and MS4s have WLAs for each lake based on their drainage area and imperviousness in each watershed.
• A stream or river TMDL in which multiple WLAs for the same Assessment Unit Identification/Water Body Identification (AUID/WID) are developed over multiple flow regimes. For example, in the Brown’s Creek Lack of Coldwater Assemblage and Impaired Biota TMDL, only one creek segment, or AUID, is the subject of the study; however, each of the regulated MS4s in the study received ten WLAs – one for each of five flow regimes (high, moist, mid-range, dry, and low) for each pollutant addressed in the TMDL.
• A single AUID/WID that has impairments for multiple pollutants.
• A combination of these.

The term “applicable WLA” is used in the MS4 General Permit and is considered to be any wasteload allocation assigned to a regulated MS4 that is

1. in an EPA-approved TMDL,
2. requiring more than a zero percent reduction, and
3. for a waterbody that is listed as impaired on the Minnesota's 303(d) Impaired Waters List.

The permit conditions apply only to those TMDLs approved prior to the effective date of the General Permit and are the only WLAs that must be addressed in the SWPPP Document. MPCA created the 2020 Municipal stormwater permit TMDL WLAs list which is a spreadsheet that contains WLA information for all TMDLs approved prior to the effective date of the General Permit. Each line on the spreadsheet represents a different WLA. This spreadsheet also has a tab which lists the TMDL project webpage for each WLA. The information found on the TMDL project webpages and linked reports can provide helpful information to MS4s.

In addition to the 2020 Municipal stormwater permit TMDL WLAs list, the MPCA created custom TMDL Application forms for each permittee, which need to be completed in order to comply with MS4 general permit. The link to the 2020 General MS4 Permit TMDL Application forms is found here. Guidance for completing the forms is located here.

Stormwater Pollution Prevention Program (SWPPP) development

Bacteria: For each applicable WLA where a reduction in pollutant loading is required for bacteria,

• the permittee must maintain a written or mapped inventory of potential source areas for bacteria,
• maintain a written plan to prioritize reduction activities that address the inventoried sources and areas, and
• identify who is responsible for implementing those items.

The MPCA and partners have developed a form that can be used to fulfill this requirement. Permittees are not required to use it, but they must have a method to inventory and prioritize reduction activities. Guidance for meeting bacteria requirements is located here.

Chloride: If the permittee has an applicable waste load allocation (WLA) where a reduction in pollutant loading is required for chloride,

• they must document the amount of deicer applied each winter maintenance season to all permittee owned/operated surfaces, and
• conduct an assessment of the permittee's winter maintenance operations to reduce the amount of deicing salt applied to permittee owned/operated surfaces and determine current and future opportunities to improve BMPs.

The permittee may use the Agency's Smart Salting Assessment Tool or other available resources and methods to complete this assessment. The permittee must document the assessment. Guidance for meeting chloride requirements is located here.

Temperature: If the permittee has an applicable waste load allocation (WLA) where a reduction in pollutant loading is required for temperature,

• they must maintain a written plan that identifies specific activities that will be implemented to reduce thermal loading during the permit term; and
• identify who will be responsible for implementing those activities.

Guidance for meeting temperature requirements is located here.

Oxygen demand, nitrate, TSS, TP: If the applicant is claiming to meet an applicable WLA where a reduction in pollutant loading is required for oxygen demand, nitrate, TSS, or TP, the applicant must provide documentation to demonstrate the applicable WLA is being met. At a minimum, the applicant must provide the following information:

• a list of all structural stormwater BMPs implemented to achieve the applicable WLA, including the BMP type (e.g., constructed basin, infiltrator, filter, swale or strip, etc.), location in geographic coordinates, owner, and year implemented; and
• documentation using an Agency-approved method, which demonstrates the estimated reductions of oxygen demand (or its surrogate pollutants), nitrate, TSS, or TP from BMPs meet the MS4 WLA reductions included in the TMDL report, if that information is available (e.g., percent reduction or pounds reduced); or
• documentation using an Agency-approved method, which demonstrates the applicant's existing load meets the WLA.

This requirement is met by completing the TMDL application form.

Compliance schedules

For each applicable WLA not met for oxygen demand, nitrate, total suspended solids (TSS), and total phosphorus (TP), at the time of application, a compliance schedule is required. Compliance schedules can be developed to include multiple WLAs and shall include

• proposed BMPs or progress toward implementation of BMPs to be achieved during the permit term,
• the year each BMP is expected to be implemented,
• a target year the applicable WLA(s) will be achieved, and
• if the applicant has an applicable WLA for TSS or TP, a cumulative estimate of TSS and TP load reductions (in pounds) to be achieved during the permit term and the Agency-approved method used to determine the estimate.

This requirement is met by completing the TMDL application form.

The compliance schedule must demonstrate that progress toward reducing pollutant loads will be achieved within the permit cycle. Per federal rule, dates and reporting on implementation of the interim milestones must be at least annual.

Reductions in pollutant loading are achieved by implementing Best Management Practices (BMPs). When selecting BMPs to include in a compliance schedule, the applicant should consider the following:

• What is the magnitude and nature of pollutant reduction needed to achieve the WLA?
• What BMPs will be effective in reducing pollutant loads and can these BMPs be reasonably implemented within the permit cycle?
• If the WLA is categorical, what portion of the WLA is the Permittee’s responsibility?
• If a categorical WLA is expressed as a percent reduction, the reduction applies equally to all areas and MS4s included in the WLA.
• If a categorical WLA is not expressed in terms of a percent reduction, the Permittee may determine their portion of the load based on their MS4’s land area in the TMDL study area. For example, if an MS4 constitutes 25 percent of the land area in a TMDL study area, the Permittee may be responsible for 25 percent of the categorical WLA. NOTE: Although it is not necessary for the applicant to identify an individual numeric load from a categorical WLA, it may be useful to consider during implementation planning and BMP selection.
• Are there opportunities to incorporate pollution prevention practices?
• Are there upcoming projects planned and what pollutant reduction will these projects achieve?
• Are there existing practices, both structural and non-structural, that may be modified or adapted to recognize additional pollutant removal?
• Are there policy or ordinance updates that may provide a mechanism for the Permittee to require pollutant reductions from others (e. g. volume control ordinance)?
• Are there opportunities to partner with other entities to achieve pollutant reductions to address WLAs?

The applicant will include BMPs they plan to implement over the five-year permit term in the Compliance Schedule section of the TMDL Application form associated with the SWPPP Document. BMPs are often applicable to multiple WLAs and can reduce the contribution of multiple pollutants. For example, volume control practices are likely applicable to both nutrient and sediment WLAs. MPCA recommends selecting BMPs that address multiple WLAs when possible.

It should be noted target dates that extend beyond the five-year permit term are expected for WLAs requiring a significant reduction in pollutant loading from an MS4.

Meeting Wasteload Allocations

For permittees with best management practices in place, they may be meeting their applicable WLAs. This can be demonstrated through modeling exercises. Refer to Overview of models used to meet MS4 TMDL permit requirements for more information.

TMDL Annual Reporting Requirements for the 2013 permit

Please see Guidance for completing the TMDL reporting form.