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.
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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.