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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).
Summary of guidance
- The Wasteload Allocation (WLA) must include all drainage from within a regulated municipality or township that discharges from a Municipal Separate Storm Sewer System (MS4) owned or operated by the municipality or township. The WLA may include all drainage from within a regulated municipality or township that is likely to be served by a regulated Municipal Separate Storm Sewer System (MS4) in the future and will be owned or operated by the municipality or township.
- For regulated counties, watershed districts, and non-traditional MS4s, the WLA should only include the area of the regulated MS4 (i.e. road right-of-ways) and property owned or operated by the regulated entity.
- Current or future drainage fromoutside a regulated municipality or township must be included in the Load Allocation (LA).
- A separate Reserve Capacity cannot be developed for regulated stormwater. Future discharges may be included in the WLA or the Load Allocation (LA).
- Future discharges from MS4s that are currently permitted can be determined using land use plans or similar documents.
- MS4s not currently under permit coverage but that will be under NPDES permit coverage, based on one or more criteria described later in this document, may be accounted for in the WLA. These MS4s should receive individual WLAs.
- Load may be transferred from either the LA or WLA to a WLA. The TMDL should state that the process for transferring load will be consistent with the method used to establish the TMDL.
- Direct discharges from nonpoint sources to an impaired water cannot be regulated under a NPDES permit. However, for purposes of practicality, loads associated with these discharges may be placed into the WLA if they are not easily quantifiable and constitute a small percentage of the total WLA.
- The WLA for stormwater may include discharges from municipal, construction, and industrial stormwater if all industrial and construction stormwater discharges occur within the current or future boundaries of a regulated municipality or township.
Definition of wasteload allocation
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.
Definition of regulated MS4s
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.
- An MS4 is any publicly owned or operated separate storm sewer system. An MS4 is not a municipality, township, county, etc. – it is the conveyance system owned or operated by one of those public entities.
- Regulated MS4s are those stormwater systems covered under a NPDES permit. MS4s include the following:
- MS4s owned or operated by municipalities – the permit covers any MS4 within the entire jurisdictional area of a regulated municipality;
- MS4s owned or operated by townships – the permit covers any MS4 within the entire jurisdictional area of a regulated township;
- MS4s owned or operated by counties – only MS4s owned or operated by a county within a U.S. Census Bureau Urban Area are covered under permit;
- MS4s owned or operated by a watershed district – only MS4s owned or operated by a watershed district within a U.S. Census Bureau Urban Area are covered under permit;
- MS4s owned or operated by MnDOT (Minnesota Department of Transportation) – only MS4s owned or operated by MnDOT within a U.S. Census Bureau Urban Area are covered under permit; and
- MS4s owned or operated by a nontraditional entity such as a hospital or university – only MS4s owned or operated by a nontraditional entity within a U.S. Census Bureau Urban Area are covered under permit.
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.
- It is only the conveyance system owned or operated by the regulated entity that is covered under permit.
Location of Minnesota's eight urban areas, based on the 2010 U.S. Census.
Definition of conveyance
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.
Discussion of guidance
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.
Which loads should go into the WLA?
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 .
The table reveals several complexities. These are discussed below. The flowchart in Figure 1 illustrates a decision process. Examples are provided in Appendix A.
- Any discharge from a regulated stormwater conveyance must go into the WLA. This includes publicly owned or operated pipes and includes drainage from publicly-owned or –operated land (e.g. a city street, sidewalk, park or state college) that enter a regulated conveyance. Ditches should not be considered stormwater conveyances unless the MPCA stormwater program has made a determination that a particular ditch is a conveyance. Nonpoint discharges to ditches should therefore be in the LA.
- Point and nonpoint discharges that receive drainage from land area within the jurisdiction of a regulated municipality or township and that enter the regulated conveyance system must be in the WLA. This includes stormwater runoff from permitted construction projects, permitted industrial facilities, private lawns, agricultural land use, parkland that is not publicly owned, etc. Construction or industrial discharges covered under a NPDES permit may receive a separate WLA or be incorporated into an overall WLA that includes municipal discharges if the construction and industrial discharges are to a regulated MS4.
- If drainage from land outside a stormwater conveyance is to a watershed district, county, or nontraditional MS4 (including MnDOT), and that drainage is not from property owned by one of these entities, the discharge should go into the LA unless the discharge ultimately is to a regulated conveyance owned by a municipality or township and the discharge is comprised of drainage received from land areawithin the jurisdiction of the township or municipality. In this case, the township or municipality receives the WLA, except for the area of the conveyance(s) owned by a watershed district, county, or nontraditional MS4.
- Nonpoint discharges that are not to a regulated conveyance system must be in the LA unless the discharge will, in the future, be to a regulated conveyance owned by a municipality or township. If a future discharge is comprised of drainage from within the jurisdiction of a municipality or township, the discharge may be part of the WLA for the municipality or township. This future discharge may also be part of the LA provided the TMDL report describes a process for transferring pollutant load from the LA to the WLA. The next section discusses future discharges.
- Discharges comprised of drainage fromoutside the jurisdiction of a municipality or township will be in the LA unless the area will eventually be under the jurisdiction of the municipality or township and the discharge will be to a regulated municipality or township conveyance, in which case the discharge may be part of the WLA. An example would be a regulated city that annexes adjacent land that is not currently covered under a NPDES permit. This load may also be placed in the LA provided the TMDL report describes a process for transferring pollutant load from the LA to the WLA. The next section discusses future discharges.
Flowchart for determining whether a discharge goes into the WLA or the LA.
Addressing future regulated discharges in the WLA
Stormwater discharges (pollutant loads) that typically go to the LA can be included in the WLA if the discharge will eventually be regulated under a NPDES permit. Accounting for future growth in this manner potentially provides incentives to MS4s to implement Low Impact Design (LID) Best Management Practices (BMPs) as development occurs. Incorporating future loads into the WLA also reduces the likelihood that pollutant load will have to be transferred from the Load Allocation (LA) to the WLA which can be problematic if the TMDL report does not clearly establish the procedure for LA to WLA transfers. NOTE: Per EPA’s recent memorandum discussing WLAs for regulated stormwater, LA to WLA transfers are now acceptable without having to re-notice the TMDL, provided the TMDL report explains how the transfer will occur.