Summary of TMDL requirements in stormwater permits

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Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs) approved by the U.S. Environmental Protection Agency (EPA) prior to the effective date of the MS4 General Permit (permit) must be addressed by Permittees in their Stormwater Pollution Prevention Program (SWPPP) Document. 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 indicate how they will make progress toward achieving those WLAs over the current five-year permit term. Applicants must include long term strategies for meeting WLAs that will not be fully achieved in the current permit term and target dates for fully achieving all WLAs.

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 from existing conditions. 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 information on the effectiveness of Best Management Practices (BMPs) is often expressed as a percent of pollutant removed.

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 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.

Compliance schedules

For each WLA not being met at the time of application, a compliance schedule must be included in the SWPPP Document. The compliance schedule must demonstrate that progress in reducing pollutant loads will be achieved within the permit cycle. Multiple WLAs can be combined and referenced by TMDL project name to consolidate the compliance schedule. Compliance schedules must contain annual interim milestones, dates for implementation of each milestone, a strategy for ongoing implementation, and a target date for achieving each WLA. Interim milestones may be expressed in the form of BMPs, implementation of BMPs, or progress in the implementation of BMPs. Per federal rule, dates and reporting on implementation of the interim milestones must be at least annual. MPCA recommends applicants align reporting on compliance schedules with the MS4 General Permit Annual Report.

Reductions in pollutant loading are achieved by implementing Best Management Practices (BMPs). When selecting BMPs to include as interim milestones, 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 current five-year permit term in the Compliance Schedule section of 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 turbidity WLAs. MPCA recommends WLAs included in the same TMDL Project be grouped together and BMPs selected that address multiple WLAs when possible. MPCA also recommends BMP identification numbers be included at the time of application as they are a required component of pond inventories and annual reporting. BMP IDs should be unique and can be developed using any numeric naming convention useful to the Permitte (e.g. POND-0362). A long term strategy and target date must also be included in the compliance schedule that describes the approach the Permittee will take to continue to recognize reductions in loading from their MS4 until all WLAs are being met. Incorporating pollution prevention strategies that reduce or eliminate pollution at its source can be an effective approach when developing long term strategies. This may include a general timeframe developed as part of the TMDL report or TMDL Implementation Plan, or a schedule the Permittee finds more appropriate. To incorporate this into the SWPPP Document, include a short narrative statement describing a general approach the Permittee will take to continue implementation of BMPs, making progress toward the ultimate achievement of all WLAs. It should be noted target dates that extend beyond the current five-year permit term are expected for WLAs requiring a significant reduction in pollutant loading from an MS4. In subsequent permitting cycles, the target date can be refined to more accurately reflect full implementation of BMPs to achieve the WLAs.