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 .
The table reveals several complexities. These are discussed below. The flowchart in Figure 1 illustrates a decision process. Examples are provided in Appendix A.
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.
MPCA does not advocate individual terms for Reserve Capacity. Reserve Capacity should be built into the WLA or LA. Generally, the WLA will be a lumped total that may include future loads. However, the TMDL can break the WLA down into current and future loads for existing permitted entities. This is not recommended because it makes accounting of loads difficult.
This guidance on future loads applies to nonpoint discharges that currently do not end up in a regulated conveyance system. There are six cases where loads normally put into the LA could be put into the WLA (see below). Note that in all cases, the TMDL report must either assign a WLA to all entities that will come under permit coverage or describe how load transfers will occur once growth occurs. For example, a regulated municipality that will annex a non-regulated township may be given additional WLA to account for growth. If the municipality includes a state highway and a state college, the highway and college should either be given WLAs or the TMDL must describe the mechanism by which WLA will be transferred from the municipality to the other MS4s (see the section on Transfer of Loads).
WLAs assigned for situations 3 through 5 should be individual. WLAs for situation 6 must be individual. Assigning loads for these situations represents an over-allocation based on current land use. The MS4s essentially grow into the allocation. For situations where discharges to impaired waters trigger designation for permit coverage, MPCA’s stormwater program will not designate new MS4s unless they are given a WLA in a U.S. EPA-approved TMDL.
Appendix B contains a list of MS4s that meet designation criteria but have not yet been designated for permit coverage and a list of MS4s that are likely to become mandatory MS4s as a result of a change in an Urban Area. Appendix B is based on 2010 census data.
A memo by the U.S. EPA, dated November 26, 2014, indicates that transfers from the LA to the WLA are acceptable (). This represents a change in U.S. EPA guidance. When LA to WLA transfers are allowed, there is no need to identify future growth areas and account for them in the TMDL. LA to WLA transfers are appropriate for addressing any future growth situation. However, the TMDL report must clearly define the process of such a transfer.
Both LA to WLA and WLA to WLA transfers are acceptable. In no case can the overall TMDL change unless the TMDL is resubmitted to the U.S. EPA.
We strongly recommend all TMDL reports describe a process for transferring either LA or WLA to WLA regardless of the likelihood that such transfers will occur. Example language is provided below. Note item 5, which also addresses future point sources that are not MS4s but discharge stormwater.
Future transfer of watershed runoff loads in this TMDL may be necessary if any of the following scenarios occur within the project watershed boundaries:
Load transfers will be based on methods consistent with those used in setting the allocations in this TMDL [Specify method, if needed, e.g., “Loads will be transferred on a simple land-area basis.”]. In cases where WLA is transferred from or to a regulated MS4, the permittees will be notified of the transfer and have an opportunity to comment.
The last paragraph in this language indicates the method(s) of transfer will be consistent with the method(s) used to establish the allocations in the TMDL. It is preferred that this methodology be included in the above language. For example, the paragraph may state the following:
Load transfers will be based on the method used in setting allocations in the TMDL. Load transfers will therefore be based on the land area requiring WLA and an assumption that loading rates are equivalent for all land uses in the watershed. For example, conversion of 10 percent of the watershed from LA to WLA will result in a 10 percent decrease in the LA and a 10 percent increase in the WLA. In cases where WLA is transferred from or to a regulated MS4, the permittee(s) will be notified of the transfer.
When feasible, it is best to accommodate future loads in the WLA rather than transfer load from the LA. Having growth accounted for in the WLA does not require re-visiting a TMDL every time growth occurs. Placing future loads into the WLA also may provide a better estimate of ultimate loading for an MS4, which in turn may allow permittees to better plan for future loads and future growth.
When determining WLAs using the above guidance, TMDL authors should use the best available data and professional judgment in making determinations about what goes into a WLA. When appropriate, TMDL authors should consult with Stormwater program staff to ensure consistency across all TMDLs. Some factors that may influence the amount of rigor involved in determining the WLA include the following.
In general, we want to avoid a situation where there is a gross over- or under-allocation. An under-allocation occurs when insufficient load is included to accommodate future growth. This forces a permittee to reduce loading from the MS4 within its’ existing area and limits flexibility in choosing BMPs. An over-allocation occurs when too much load is included in the WLA. In this situation, permittees can easily demonstrate they meet their WLA, which results in the permittee not having to implement any BMPs even though their current discharge may contribute to a water quality violation.
Case studies can be found at MPCA’s stormwater webpage ().
Permittees are required to include compliance schedules in their permits when a TMDL WLA is not being achieved at the time of permit application. The TMDL may define the time frame for achieving the TMDL, considering projections of future loads. However, permittees determine a target date they believe each applicable WLA can reasonably be acheived. This date is not enforceable through the MS4 permit.
It is typically not practical to base future loads on knowledge of where stormwater runoff flows. Future loads should therefore be estimated using land use information. In general, areas projected to be urbanized (medium and high density residential, commercial, industrial) can be placed into the WLA because they will likely be served by a regulated MS4. Other land uses, such as agriculture, can be placed into the WLA if there is adequate information to indicate these discharges will be to a regulated MS4 owned or operated by a municipality or township. Some land uses are difficult to assign to the WLA or LA. Appendix C provides a guide for determining how to assign loads based on projected land use.
Conveyance - a system designed for transporting stormwater runoff and owned or operated by a state, city, town, borough, county, parish, district, association, or other public body (created by or pursuant to state law) having jurisdiction over disposal of sewage, industrial wastes, or stormwater. Often referred to as an MS4.