The following guidance, developed by MPCA’s Stormwater Program in conjunction with MPCA’s TMDL Program, discusses recommended procedures for addressing wasteload allocations for current or future municipal stormwater discharges regulated under a National Pollutant Discharge Elimination System (NPDES) permit. The guidance clarifies previous policy for setting wasteload allocations for stormwater and will be modified to address new issues as they arise in TMDLs. The guidance provides clarity about existing stormwater discharges that are covered under a NPDES permit and is specific to municipal stormwater discharges (not industrial or construction).
The Wasteload Allocation (WLA) is the portion of a receiving water's loading capacity that is allocated to one of its existing or future point sources of pollution. WLAs constitute a type of water quality-based effluent limitation (40 CFR 130.2). NPDES-permitted stormwater discharges must be included in the WLA.
To understand the basis for making decisions about what should or should not go into the WLA, it is necessary to understand the definition of an MS4.
The figure below illustrates the eight Urban Areas that occur in Minnesota based on the 2010 U.S. Census. They include the Twin Cities, St. Cloud, Duluth, Rochester, Mankato, Fargo-Moorhead, Grand Forks-East Grand Forks, and La Crosse-La Crescent metropolitan areas.
Minnesota’s MS4 General Permit defines MS4 as a “conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm drains)”. This term may lead to confusion, however, since some of these features may also be classified as waters of the state and therefore protected for compliance with water quality standards. Minnesota Rules 7050.0130 Subp. 2 states “… disposal systems or treatment works operated under permit or certificate of compliance of the agency are not "waters of the state." Therefore, an MS4 cannot also be a water of the state. For example, some ditches are assessed as 2B waters and therefore cannot be MS4s. Any waterbody that could be considered a water of the state should be treated as such and therefore be included in the LA unless the MPCA’s stormwater program has made a determination that they are MS4s.
The issuance of a NPDES permit provides reasonable assurance that the WLAs contained in a TMDL will be achieved. This is because 40 C.F.R. 122.44(d)(1)(vii)(B) requires that effluent limits in permits be consistent with "the assumptions and requirements of any available waste load allocation" in an approved TMDL. The MPCA’s Phase 1 permit requires Minneapolis and St. Paul to estimate pollutant loadings and compare those to any applicable WLA. The Phase 2 General Permit requires permittees to demonstrate annual progress toward meeting any applicable WLA to the Maximum Extent Practicable.
MS4 permits only cover water in a publicly-owned or -operated conveyance. Discharges to conveyances are not covered under permit. However, the conveyance must be in compliance with requirements of the permit, which means discharges to a public conveyance must also be managed. Municipalities and townships have necessary regulatory tools to control discharges to their system, although these authorities vary widely from location to location. Factors influencing local regulatory authority include local zoning ordinances, county regulations, and ditch law. Other MS4 permittees typically do not have these regulatory authorities.
MPCA policy on setting WLAsprovides a general framework for addressing WLAs, but experience with TMDLs indicated the policy needed updating. The following guidance provides clarification for the policy. The guidance will need to be updated as new TMDLs are developed and new issues arise.
When assessing which pollutant loads should be placed into the WLA, it is necessary to clearly define what is covered under a NPDES permit. Minnesota’s Phase 2 General Permit states “Only your [the permittee’s] system and the portions of the storm sewer system that are under your operational control are authorized by your permit”(). MPCA interprets this to mean an MS4 is responsible for ensuring its discharge is in compliance with water quality based effluent limits (WQBELs), since the discharge is within the publicly owned or operated conveyance system. The permittee must take appropriate measures to bring its MS4 into compliance. All drainage discharging from a regulated MS4 or to a regulated MS4 from property owned or operated by the regulated entity are put into the WLA. Other drainage to a regulated MS4 are also put into the WLA when the permittee has regulatory authority to control the discharges.
TMDLs may narrowly define the sources that are part of the WLA. For example, individual pipes can receive a WLA. This typically will not be the case because there is rarely adequate information to assign WLAs this narrowly. WLAs, however, should be defined as narrowly as data allow. The table below summarizes the above discussion for the general case where contributing sources are not narrowly defined. For a list of MS4s covered by permit, see .