Technical guidance used by MPCA to develop guidelines for setting TMDL WLAs for regulated stormwater

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Successful implementation of stormwater requirements in a Total Maximum Daily Load (TMDL) requires integration of three components:

• The permit must contain language that links the TMDL language to Stormwater Pollution Prevention Program (SWPPP) requirements.
• There must be a procedure for incorporating TMDL requirements into the SWPPP.
• A TMDL must contain clear language that can be linked to the permit and SWPPP.

MPCA is preparing a series of modules designed to address a variety of stormwater topics. Modules 4.1, 4.2 and 4.3 address the 3rd requirement stated above.

Modules 4.1, 4.2 and 4.3 are for stormwater staff, TMDL staff, or stakeholders involved in development of TMDLs that involve a permitted stormwater component. The modules present information on a variety of policy issues that impact development of a TMDL and subsequent activities by regulated stormwater entities in response to the TMDL. Prior to viewing the material in this module, material in Stormwater module 1, Stormwater Overview, and TMDL module 1, TMDL Overview, should be understood.

The learning objectives for Modules 4.1, 4.2 and 4.3 include the following:

• Understand the basics of MPCA’s Stormwater Program;
• Understand language in stormwater permits; and
• Understand TMDL-stormwater issues, including potential solutions and problems.

The following document provides a discussion of issues related to setting wasteload allocations for permitted stormwater and to implementing activities to address a stormwater wasteload allocation. Several recommendations are presented. In some cases, these recommendations are developed as policy to be implemented in developing TMDLs that include a stormwater component.

Stormwater program

The Federal National Pollutant Discharge Elimination System (NPDES) was mandated by Congress under the Clean Water Act. Many activities are regulated under the NPDES Program, including confined animal feeding operations (CAFO), combined sewer overflows (CSO), sanitary sewer overflows (SSO), and stormwater. Stormwater can further be divided into three permit areas – construction activities, industrial activities, and municipal activities. Minnesota regulates the disposal of stormwater through State Disposal System (SDS) permits. The MPCA issues combined NPDES/SDS permits for construction sites, industrial facilities and municipal separate storm sewer systems (MS4s).

Each of the three stormwater permitting programs has a general permit. Individual permits may also be issued within each program.

Under the Phase I construction permit, operators of large construction activity resulting in the disturbance of five or more acres of land are required to obtain general permit coverage. Phase II includes small construction activity that results in the disturbance of equal to or greater than one acre, or less than one acre if that activity is part of a "larger common plan of development or sale". Owners and operators of projects meeting the above criteria must obtain permit coverage and implement practices to minimize pollutant runoff from construction sites. Permits may also be required for activity disturbing less than one acre but deemed by MPCA to represent a risk to water resources. The current general construction permit was issued August 1, 2003. The construction permit is applied statewide, except for Tribal areas. For example, some feedlot activities require permit coverage. For more information, see [1].

Public and private operators of industrial facilities included in one of the 11 categories of industrial activity defined in the federal regulations by an industry's Standard Industrial Classification (SIC) code or a narrative description of the activity found at the industrial site are required to apply for a permit. A facility may be eligible for a conditional no-exposure exclusion from permitting provided their industrial materials and activities are entirely sheltered from storm water. The federal regulations can be found at 40 CFR 122.26 (b)(14)(i)-(xi). For more information, see [2].

A municipal separate storm sewer system is a conveyance or system of conveyances (roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, storm drains):

• 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, stormwater, or other wastes, including special districts under State law such as a sewer district, flood control district or drainage districts, or similar entity, or an Indian tribe or an authorized Indian tribal organization, or a designated and approved management agency under section 208 of the federal Clean Water Act, United States Code, title 33, section 1288, that discharges to waters of the United States;
• Designed or used for collecting or conveying stormwater;
• Which is not a combined sewer; and
• That is not part of a publicly owned treatment works.

Not all MS4s require permit coverage. The cities of Minneapolis and St. Paul are Phase 1 permittees and require individual permits. The remaining regulated MS4s are Phase 2 permittees and are either mandatory or designated. Mandatory MS4s occur in urbanized areas as defined by the 2010 Census. An "urbanized area" is a land area comprising one or more places (“central places”) and the adjacent densely settled surrounding area (“urban fringe”) that together have a residential population of at least 50,000 and a density of at least 1,000 people per square mile. The definition also includes any other public storm sewer system located fully or partially within an urbanized area. For example, the University of Minnesota Twin City campus is a mandatory MS4 because it operates a conveyance system and is part of an urban area. There are 212 mandatory MS4s in eight urban areas in Minnesota.

MS4s outside of urbanized areas that have been designated by the MPCA for permit coverage include cities and townships with a population of at least 10,000 and cities and townships with a population of at least 5,000 and discharging or the potential to discharge to valuable or polluted waters. These designated MS4s are required to obtain permit coverage by February 15, 2007. For more information see [3].

Permit language for impaired waters

Although this document focuses on TMDL language, it is important to understand permit language that pertains to impaired waters and TMDLs. TMDL language must be written in a manner that is consistent with permit language and requirements. This section provides a summary of permit language. Permits can be found on MPCA’s Stormwater website ([4]).

Construction stormwater

The 2013 construction general permit contains language that addresses impaired waters for which TMDLs have or have not been completed and approved by the United States Environmental Protection Agency (US EPA). The permit can be found at [5].

WLAs for construction stormwater should be determined when the pollutant or stressor for the impairment is phosphorus (nutrient eutrophication biological indicators), turbidity, dissolved oxygen, or biotic impairment (fish bioassessment, aquatic plant bioassessment and aquatic macroinvertebrate bioassessment). Construction activities that occur within one mile of an impaired water must comply with the additional BMPs described in Appendix A of the permit. This requirement exists for impaired waters with or without a US EPA-approved WLA for construction stormwater. If a US EPA-approved TMDL contains a WLA for construction stormwater and the TMDL describes additional BMPs needed to meet the TMDL, then the permittee must comply with these additional BMPs. The additional requirements can be extended beyond the one mile distance.

Municipal stormwater

The 2013 issuance of the municipal (MS4) permit provides language for discharges to impaired waters with a USEPA-Approved TMDL that includes an applicable WLA. The MS4 permit requires a Stormwater Pollution Prevention Program (SWPPP). As a part of the SWPPP document, permittees are required to address all WLAs in TMDLs approved prior to the effective date of the permit (August 1, 2013). In doing so, they must determine if they are currently meeting their WLA(s). If the WLA is not being achieved at the time of application, a compliance schedule is required that includes interim milestones, expressed as best management practices (BMPs), that will be implemented over the current five-year permit term to reduce loading of the pollutant of concern in the TMDL. Additionally, a long-term implementation strategy and target date for fully meeting the WLA must be included.

The permit also contains language requiring permittees to demonstrate continuing progress toward meeting each applicable WLA approved prior to the effective date of the permit. This will come in the form of annual reporting on the interim milestones described in the compliance schedule of the SWPPP application. The report will be completed on a form provided by the commissioner and include the following:

• A list of all BMPs being applied to achieve each applicable WLA. For each structural stormwater BMP, the permittee shall provide a unique identification number and geographic coordinate. If the structural stormwater BMP is also included as a part of the pond inventory, the same ID number shall be used.
• A list of all BMPs the submitted at the time of application and the stage of implementation for each BMP.
• An updated estimate of the cumulative reductions in loading achieved for each pollutant of concern associated with each applicable WLA.
• An updated narrative describing any adaptive management strategies used (including project dates) for making progress toward achieving each applicable WLA.

MPCA’s Stormwater Program has developed maps to assist MS4s with identifying impaired waters to which the MS4 discharges. The same may eventually be done for approved TMDLs. One concern is the search criteria are restricted to selected water and downstream waters are difficult to identify. For example, a city such as Minnetonka discharges to Minnehaha Creek, which discharges to the Mississippi River, which could have an impact on Lake Pepin, which is impaired for sediment and nutrients (phosphorus).

MPCA has developed guidance for MS4s to use when interpreting the permit ([6])(I believe this document is outdated). Although the guidance covers the entire permit, there are sections that focus on impaired waters and TMDLs.

TMDL stormwater issues

It is important to provide language in a TMDL that can be supported with the permit. The potential issues can be roughly divided into setting and achieving wasteload allocations. Prior to discussing these, it is important to understand certain aspects of TMDLs.

General information about TMDLs

The TMDL equation

A total maximum daily load (TMDL) is the amount of pollutant loading that can occur and have a water body meet water quality standards. A TMDL may be written as an equation which allocates pollutant loading to four separate categories,

TMDL = WLA + LA + MOS + RC

where WLA is wasteload allocation, LA is load allocation, MOS is margin of safety, and RC is reserve capacity. WLA includes pollutant loading from sources covered by a NPDES permit (often called point sources), LA includes sources not covered by a NPDES permit (often called nonpoint sources), MOS accounts for uncertainty in the estimates of WLA and LA, and RC allows for future growth.

Contents of a TMDL

In addition to pollutant loads, a TMDL must include additional information. Items of greatest interest for permitted stormwater are methods for calculating WLA, reasonable assurances, monitoring, and implementation (which includes timelines). Generally, models are used to calculate pollutant loads. Reasonable assurance language in the TMDL includes guarantees that the pollutant loads are achievable. Language on monitoring provides a general overview of how the TMDL will be tracked. Detailed language on monitoring is often deferred to the TMDL Implementation Plan. Implementation language provides a general overview of how the TMDL loads will be achieved and what the timelines are for meeting the TMDL requirements. The Implementation Plan contains more specific information on implementation. Requirements of a TMDL are enforceable, while the Implementation Plan is not.

Types of TMDLs

TMDLs may be grouped into one of three categories based on an August 2, 2006 EPA memo ([7]). These categories are somewhat arbitrary but identifying the appropriate type of TMDL early in the process should help develop the TMDL language. In particular, identifying the TMDL category should help identify long term data needs and help frame the Implementation Plan.

The first type is a Phased TMDL. Phased TMDLs may be preferable when there is significant uncertainty about how the WLA will be met. This uncertainty results from one of two conditions. First, when there is a significant nonpoint component included in the WLA, there may be concerns with reasonably assuring how the WLA will be met. In developing a TMDL, it may be desirable to include nonpoint sources within the WLA if they occur within a municipalities boundaries and if there are reasonable assurances that the municipality can develop the legal tools to achieve the WLA. This is one mechanism to account for future growth (see Section 4b.viii.). EPA recommends that some additional provision in the TMDL, such as a schedule and description of the implementation mechanisms for nonpoint source control measures, be included to provide reasonable assurance that the nonpoint source measures will achieve the expected load reductions. For example, in the Potash Creek TMDL in Vermont, the WLA included several nonpoint sources of pollutant. These nonpoint sources were associated with impervious surfaces greater than one acre in size but not covered by a NPDES permit. Vermont developed a statewide general permit to cover these sources. Consequently, these nonpoint sources were included in the WLA ([8]). Another example is the Lake Independence TMDL, in which the WLA included nonpoint sources within the city boundaries of Independence, Loretto, and Medina. The TMDL states “In the event that voluntary implementation of manure management plans does not occur on the majority of feedlot, Medina and Independence will revise existing Conditional Use Permits or Zoning Ordinances to require compliance.” In the case of a Phased TMDL where the WLA includes nonpoint sources, the TMDL should contain language about the tools that might be available to achieve the WLA.

A second type of Phased TMDL involves a situation where available data only allow for "estimates" of necessary load reductions or for "non-traditional problems" where predictive tools may not be adequate to characterize the problem with a sufficient level of certainty. For this type of TMDL, there is an assumption that the TMDL will need to be revised once adequate data are available to estimate WLAs. The TMDL therefore contains language describing how the data will be collected, rather than focusing on implementing activities to achieve a WLA.

Phased TMDLs must include all elements of a regular TMDL, including load allocations, wasteload allocations and a margin of safety. However, phased TMDLs are assumed to require revision at each phase of the TMDL. Each phase must be established to attain and maintain the applicable water quality standard. In addition, EPA recommends that a phased TMDL document or its implementation plan include a monitoring plan and a scheduled timeframe for revision of the TMDL. Since phased TMDLs will in all likelihood need to be revised and therefore require more overall effort, States should carefully consider the necessity of such TMDLs, for example to meet consent decree deadlines or other mandatory schedules. Upon revision of the loading capacity, wasteload, or load allocations, the TMDL would require re-approval by EPA. Although no examples of Phased TMDLs were found in the literature, the State of Montana has had lengthy discussions on the subject of Phased TMDLs ([9])(This is a dead link).

A second type of TMDL is based on adaptive management and trading provisions ([10]). Adaptive implementation is an iterative implementation process that makes progress toward achieving water quality goals while using any new data and information to reduce uncertainty and adjust implementation activities. Adaptive implementation includes immediate actions, an array of possible long-term actions, effectiveness monitoring, and experimentation for model refinement. An important component of the adaptive management approach is monitoring, which is required to adjust implementation activities. Using adaptive implementation, new information from monitoring is used to appropriately target the next suite of implementation activities. The TMDL should contain language about monitoring requirements needed to implement an adaptive management approach. If monitoring supports significant modification to the TMDL, the TMDL may need to be reopened. This requires EPA approval.

The third type of TMDL, called a staged TMDL, anticipates implementation in several distinct stages. It differs from the adaptive implementation scenario because it is anticipated that the load and wasteload allocations will not require any significant adjustments. Instead, implementation actions will be staged over a period of time. For example, EPA has approved mercury TMDLs where the wasteload allocation to point sources (which would be implemented within five years through the NPDES process) was predicated on long-term reductions in atmospheric mercury deposition.

It is not necessary to fit each TMDL into one of these three categories. It is important to understand the distinctions between these types of TMDLs. That understanding will help guide TMDL language, particularly on the issues of monitoring, information gathering, and implementation. The figure below provides a schematic for selecting a TMDL approach.

Categories of TMDLs and general requirements for language in the TMDL

Setting wasteload allocations (WLAs)

A November 22, 2002 memo drafted by Robert Wayland and James Hanlon of the US EPA provides some clarification on the issue of setting WLAs for stormwater. Some key points of this memo are summarized below:

• NPDES-regulated stormwater discharges must be addressed by the WLA component of a TMDL;
• NPDES-regulated stormwater discharges may not be addressed by the Load Allocation (LA) component of a TMDL;
• Stormwater discharges not currently covered by a NPDES permit may be addressed by the LA component of a TMDL;

This language makes it clear that construction, industrial, and municipal (MS4) activities covered by a NPDES permit must be addressed by the WLA.

In cases where a TMDL lumps more than one category of regulated stormwater into a single WLA, assumptions must be made to determine what part of the WLA is assigned to each category of stormwater. For example, the Lower Minnesota River Dissolved Oxygen TMDL states “Permitted Stormwater Sites: Municipal Separate Storm Sewer Systems (MS4), Construction Stormwater Sites, Industrial Stormwater Sites: 1,863 pounds of phosphorus over the two critical low flow months studied or 30.5 pounds of phosphorus per day.” In this case, all permitted stormwater was given a lumped WLA. To address these situations, the following guidance is provided.

Construction stormwater

In most states, construction stormwater general permits contain language that the permit must be consistent with the requirements and assumptions of the TMDL. This places a burden on the TMDL to clarify WLAs for construction stormwater.

There are examples of TMDLs where construction stormwater is given a separate WLA (see Appendix B for examples of specific TMDL language). In the Caney Fork River Watershed, Tennessee, construction stormwater was given the same percent pollutant reduction as MS4s ([11]). The reductions are to be implemented as BMPs that are described in the state general permit ([12]). However, the permit does not provide specific BMPs, but instead requires the permittee to address the TMDL in their permit. In Idaho, the permitting authority requires incorporation of a gross wasteload allocation for anticipated construction storm water activities into the stream's water quality improvement plan. This WLA is a categorical value which accounts for allowable construction activity in the TMDL for any given point in time. For the San Gabriel Metals TMDL, construction stormwater is given a WLA based on the percent of land area in construction at any one time. Permittees are given a WLA on a per unit area basis ([13]).

MPCA favors a permit-driven process rather than have TMDLs set water quality goals for construction stormwater. Many states assume that meeting the conditions of the construction permit satisfies the TMDL requirement. There are several reasons for employing a permit-driven process.

• Monitoring data from construction sites is lacking. This makes it difficult to accurately calculate a WLA.
• The nature of construction activity is transient. Since the WLA is a static number, loads from construction activity in any given year may be less than or greater than the WLA.
• Reviewing individual permits to determine if they are compliant with the conditions of a TMDL can be burdensome. In some cases, the burden of meeting the TMDL will be passed on to local authorities, who will have to develop specific BMPs for construction activities within their jurisdiction and then ensure compliance. This can be particularly burdensome each time a new TMDL is completed and it places additional requirements on construction stormwater.
• Construction activity constitutes a very small percentage of loading in most watersheds, probably less than 1 percent of the total load. For example, the Rock River and Groundhouse River TMDL studies indicate that land under permitted construction activity constitutes 0.14 and 0.16 percent of the total land area in the watersheds.
• Research indicates that sediment loads from construction sites are small when BMPs are properly installed and maintained (Owens, D.W., P. Jopke, D.W. Hall, J. Balousek and A. Roa 1999. Soil Erosion from Small Construction Sites. Draft USGS Fact Sheet. USGS and Dane County Land Conservation Department, WI).
• MPCA’s proposed construction stormwater general permit contains additional BMPs when construction activity occurs within one mile of an impaired water.

MPCA therefore assumes that permittees in compliance with the requirements of a construction stormwater permit are achieving their WLA.

However, the TMDL process requires a balanced equation. The WLA therefore must be a number. Individual sectors that contribute to loading must be included in the TMDL equation, regardless of whether that loading is considered to be negligible. The following conditions must be considered when setting a WLA for construction stormwater activity.

• Failure to include construction stormwater in a WLA can be interpreted to mean zero allowable discharge from construction activity;
• Use of an approximate load, such as “de minimus” or “insignificant” can be interpreted as zero.

Discharges of zero are not appropriate for construction stormwater. It is therefore not acceptable to include these general terms when setting a WLA for construction stormwater.

Because construction activity must receive a numerical WLA the following approach is recommended.

• A categorical WLA can be established for stormwater. This WLA would include municipal, construction, and industrial stormwater. The TMDL would therefore assign a WLA to stormwater without differentiating how that WLA is distributed among the permitted entities. This should only be done when all construction and industrial stormwater discharges are to a regulated MS4.
• Individual WLAs can be assigned for the different stormwater sectors, including construction stormwater. The WLA would lump all construction stormwater into an individual construction stormwater WLA. If monitoring data is available, it can be used to set the construction WLA. Monitoring data is generally not available. Consequently, the recommended approach for construction stormwater is to estimate the area of the watershed under permitted construction activity. A five year median could be used for this calculation to account for variation in levels of construction activity. The WLA would then be estimated as the median percent of land area under construction times the sum of the overall LA and WLA (i.e. the Margin of Safety and Reserve Capacity are not included in the calculation). MPCA’s Stormwater Program can provide the information needed to make this calculation.
• In some cases a Reserve Capacity may be established to account for increases in and uncertainty in loading from construction activity. This Reserve Capacity can be sector specific. Thus, construction activity may receive a separate Reserve Capacity. This may be a preferred approach when construction activity will greatly increase in the future. For most situations, this is not a favored approach because Reserve Capacity removes load from other sources, there are challenges to distributing Reserve Capacity equitably, it is difficult to estimate what the Reserve Capacity should be, and Reserve Capacity ties up load once construction activity decreases. In addition, MPCA’s TMDL program recommends against using reserve capacity.

These options should be utilized in any TMDL where the pollutant or stressor of impairment is phosphorus (nutrient eutrophication biological indicators), turbidity, dissolved oxygen, or biotic impairment (fish bioassessment, aquatic plant bioassessment and aquatic macroinvertebrate bioassessment).

It is important for TMDL developers to remember that the MPCA favors a permit-driven process for construction stormwater. Appendix A of the 2013 Construction General Permit includes additional BMPs for construction activities that occur within one mile of an impaired water. In addition to or in place of these BMPs, a TMDL can prescribe BMPs for construction stormwater. Permittees must comply with these BMPs. In the case where a TMDL provides a specific WLA for construction stormwater, it will be important for TMDL developers to receive input from Construction Stormwater personnel.

Because MPCA believes that following the conditions of the Construction Stormwater general Permit meets the conditions of a TMDL, TMDLs should contain the following language in the load calculation section of the TMDL.

• For categorical stormwater WLAs:The stormwater wla includes loads from construction stormwater. Loads from construction stormwater are considered to be a small percent of the total WLA and are difficult to quantify. The WLA for stormwater discharges from sites where there are construction activities reflects the number of construction sites one or more acres expected to be active in the watershed at any one time, and the best management practices (BMPs) and other stormwater control measures that should be implemented at the sites to limit the discharge of pollutants of concern. The BMPs and other stormwater control measures that should be implemented at construction sites are defined in the State's NPDES/SDS General Stormwater Permit for Construction Activity (MNR100001). If a construction site owner/operator obtains coverage under the NPDES/State Disposal System (SDS) [Permit] General Stormwater Permit and properly selects, installs and maintains all BMPs required under the permit, including those related to impaired waters discharges and any applicable additional requirements found in Appendix A of the Construction General Permit, the stormwater discharges would be expected to be consistent with the WLA in this TMDL. It should be noted that all local construction stormwater requirements must also be met.
• For an individual WLA for construction stormwater: The WLA for stormwater discharges from sites where there are construction activities reflects the number of construction sites one or more acres expected to be active in the watershed at any one time, and the best management practices (BMPs) and other stormwater control measures that should be implemented at the sites to limit the discharge of pollutants of concern. The BMPs and other stormwater control measures that should be implemented at construction sites are defined in the State's NPDES/SDS General Stormwater Permit for Construction Activity (MNR100001). If a construction site owner/operator obtains coverage under the NPDES/State Disposal System (SDS) [Permit] General Stormwater Permit and properly selects, installs and maintains all BMPs required under the permit, including those related to impaired waters discharges and any applicable additional requirements found in Appendix A of the Construction General Permit, the stormwater discharges would be expected to be consistent with the WLA in this TMDL. It should be noted that all local construction stormwater requirements must also be met.

Municipal stormwater

Publicly owned conveyance systems partly or fully within one of Minnesota’s eight urban areas (Metro, Duluth, Rochester, St. Cloud, Winona-LaCrosse, East Grand Forks-Grand Forks, Moorhead-Fargo, Mankato) are considered Mandatory MS4s. This includes conveyance systems owned by cities, townships, counties, watershed districts, MN DOT, and other public entities such as the University of Minnesota, Minnesota state colleges and technical institutes, and state-owned correctional facilities. The list of mandatory MS4s in Minnesota can be found at http://www.pca.state.mn.us/publications/wq-strm4-74.pdf. Minneapolis and St. Paul are Phase I cities and require individual permits. The figure below illustrates the location of Minnesota’s urban areas, as defined by the U.S. Census Bureau.

Location of Minnesota's eight urban areas, as defined by the U.S. Census Bureau

There are also several designated MS4s in Minnesota. These include cities with populations greater than 10,000, and cities with populations of 5,000 to 10,000 that have the potential to discharge to an impaired or Outstanding Resource Value water. The figure below illustrates the location of designated MS4s in Minnesota.

Location of designated MS4s in Minnesota

Permit coverage exists only for those parts of the stormwater conveyance system that are owned and operated by the permittee. For example:

• MN DOT requires permit coverage for only that part of the conveyance system associated with a state highway that occurs within one of the seven urban areas. Permit coverage is not required for portions of the same highway that exist outside an urban area.
• Counties and watershed districts require permit coverage only for those conveyances it owns (e.g. county ditches, roads, etc.) within an urban area. Permit coverage is not required for portions of the same conveyance structure that exist outside an urban area.
• Portions of state owned conveyances, such as state colleges, universities, and correctional facilities, require permit coverage if they exist within an urban area. Portions outside an urban area do not require coverage (e.g. University of Minnesota at Morris)
• Privately-owned conveyances, including conveyances owned by industrial facilities, commercial facilities, and private residences, are not covered under the MS4 general permit even if they discharge to a permitted conveyance. Note that most industrial facilities and some commercial facilities require individual permits or are covered under one of the industrial general permits.

The above information raises three concerns. First, it can be difficult to identify all permitted MS4s for a particular TMDL. Second, calculating WLAs can be difficult for some of the non-traditional MS4s, such as MN DOT, counties, watershed districts, and state-owned facilities. Finally, the stormwater permit only requires the permittee to address its own operations and new construction (post construction requirements). Consequently, the permit does not cover runoff from private property that does not have post construction requirements. However, a regulated MS4 community is responsible for all discharges from conveyances they own or operate, regardless of how those discharges reach their system.

Despite these difficulties, TMDL language must address each of these issues. A TMDL must list all permitted MS4s within the TMDL study area. Preferably, the TMDL would include the MS4 Permit Number and individual IDs for each MS4 receiving a WLA. For counties, highways, or watershed districts that require permit coverage only within an urban area, the TMDL should identify those areas that require permit coverage.

Each of these MS4s must be given a WLA. The WLA may be categorical or individual (individual allocations should be distributed if at all possible). An example of a categorical WLA is the 30 percent phosphorus reduction for ten MS4s named in the Lower Minnesota River Low Flow Dissolved Oxygen TMDL ([14]). An example of individual WLAs is the Lake Independence nutrient TMDL, where individual phosphorus WLAs were given to each of the three municipalities in the study area ([15]).

Assigning a WLA to a MS4 provides reasonable assurances that the WLA will be met, provided all discharges covered by the WLA enter the MS4’s conveyance. In a case where much of the MS4’s discharge originates from private property, the MS4 will have to implement activities to control or treat these discharges. For example, a permitted MS4 may develop ordinances to cover discharges from private properties.

Industrial stormwater

MPCA’s multisector industrial general permit was issued in 2010. The permit includes language that shortens the time permittees have to install BMPs when their stormwater discharges cause or contribute to a water quality violation (e.g. impairment).

There are some examples of TMDLs that have individual WLAs for industrial stormwater. The Ballona Creek TMDL states that each storm water permittee enrolled under the general construction or industrial storm water permits will receive an individual waste load allocation on a per acre basis, based on the acreage of their facility ([16])(This is a dead link). “The general industrial storm water permits shall achieve final wet-weather waste load allocations no later than 10 years from the effective date of the TMDL, which shall be expressed as NPDES water quality-based effluent limitations. Effluent limitations may be expressed as permit conditions, such as the installation, maintenance, and monitoring of Regional Board approved BMPs if adequate justification and documentation demonstrate that BMPs are expected to result in attainment of waste load allocations.” “BMP effectiveness monitoring will be implemented to determine progress in achieving interim wet-weather waste load allocations.” The storm water waste load allocations are apportioned between the MS4 permittees, Caltrans, the general construction and the general industrial storm water permits based on an area weighting approach.

Permits for storm water discharges associated with industrial activity are to require compliance with all applicable provisions of Sections 301 and 402 of the CWA, i.e., all technology-based and water quality-based requirements. EPA also recognizes that the available data and information usually are not detailed enough to determine waste load allocations for NPDES-regulated storm water discharges on an outfall-specific basis. In this situation, EPA recommends expressing the wasteload allocation in the TMDL as either a single number for all NPDES-regulated storm water discharges, or when information allows, as different WLAs for different identifiable categories, e.g., industrial stormwater as distinguished from storm water discharges from construction sites or municipalities. These categories should be defined as narrowly as available information allows (e.g., for industrial sources, separate WLAs for different types of industrial storm water sources or dischargers).

Much of the discussion for construction stormwater applies to industrial stormwater. This includes the difficult nature of calculating loads from industrial facilities, the relatively small contribution from industrial stormwater if permit conditions are met, and the variability in types of industrial facilities.

Consequently, the recommended options for construction stormwater apply to industrial stormwater. These are summarized below.

• Estimate a WLA for industrial stormwater. This differs from calculating a WLA because monitoring data is not used to set the WLA. Instead, the WLA is set using other general criteria, such as percent of land area in the watershed that is currently under industrial activity covered by a permit. This is a reasonable approach. The TMDL should contain additional language to indicate industrial stormwater that is in compliance with the permit meets water quality standards and therefore has no load reduction requirement. The following standard language should be included: The WLA for stormwater discharges from sites where there is industrial activity reflects the number of sites in the watershed for which NPDES industrial stormwater permit coverage is required, and the BMPs and other stormwater control measures that should be implemented at the sites to limit the discharge of pollutants of concern. The BMPs and other stormwater control measures that should be implemented at the industrial sites are defined in the State's NPDES/SDS Industrial Stormwater Multi-Sector General Permit (MNR050000) or facility specific Individual Wastewater Permit (MN00XXXXX) or NPDES/SDS General Permit for Construction Sand & Gravel, Rock Quarrying and Hot Mix Asphalt Production facilities (MNG490000). If a facility owner/operator obtains stormwater coverage under the appropriate NPDES/SDS Permit and properly selects, installs and maintains all BMPs required under the permit, the stormwater discharges would be expected to be consistent with the WLA in this TMDL. It should be noted that all local stormwater management requirements must also be met.

The load for industrial stormwater can be lumped with municipal stormwater. This approach is acceptable although it creates difficulty for MPCA when reviewing SWPPPs, since the WLA is not clear. This problem can be addressed by including the following language in the TMDL: The stormwater wla includes loads from industrial stormwater. Loads from industrial stormwater are considered to be less than 1 percent of the total wla and are difficult to quantify. The WLA for stormwater discharges from sites where there is industrial activity reflects the number of sites in the watershed for which NPDES industrial stormwater permit coverage is required, and the BMPs and other stormwater control measures that should be implemented at the sites to limit the discharge of pollutants of concern. The BMPs and other stormwater control measures that should be implemented at the industrial sites are defined in the State's NPDES/SDS Industrial Stormwater Multi-Sector General Permit (MNR050000) or facility specific Individual Wastewater Permit (MN00XXXXX) or NPDES/SDS General Permit for Construction Sand & Gravel, Rock Quarrying and Hot Mix Asphalt Production facilities (MNG490000). If a facility owner/operator obtains stormwater coverage under the appropriate NPDES/SDS Permit and properly selects, installs and maintains all BMPs required under the permit, the stormwater discharges would be expected to be consistent with the WLA in this TMDL. It should be noted that all local stormwater management requirements must also be met.

Individual permits

Minneapolis and St. Paul are Phase 1 communities and require individual permit coverage. The discussion in Section 4biii covers recommended TMDL language for these cities.

Minnesota issues a number of individual stormwater permits, such as those issued to commercial facilities. In general, pollutant loading from these facilities will be small compared to other permitted sources. Additionally, discharge from these facilities often enters a publicly-owned (MS4) conveyance, which may be addressed through the MS4 permit assuming reasonable assurances can be provided. However, in small watersheds and in situations where private permittees discharge directly to an impaired water, a separate WLA is warranted.

Choosing categorical or individual WLAs and the form of the WLA

The Wayland memo states “It may be reasonable to express allocations for NPDES-regulated storm water discharges from multiple point sources as a single categorical wasteload allocation when data and information are insufficient to assign each source or outfall individual WLAs. See 40 C.F.R. § 130.2(i). In cases where wasteload allocations are developed for categories of discharges, these categories should be defined as narrowly as available information allows.”

In certain cases, TMDLs will assign categorical WLAs . A categorical WLA may be desirable under four circumstances. First, a categorical WLA is appropriate if pollutant loading from all permitted stormwater sources is likely to be similar in nature. This would be the case for construction stormwater and for industrial stormwater within the same SIC category. For example, the San Gabriel Metals TMDL assigns a pollutant load per day to all construction activity. A categorical allocation for construction or industrial should take the form of a pollutant load per unit area.

Second, a categorical WLA is appropriate when each permittee can perform the same stormwater management activities to accomplish the requirements of the TMDL. For example, this situation applies to MS4s when Pollution Prevention, Good Housekeeping, and Education BMPs alone are likely to achieve the WLA. These activities will typically be the same for each permittee and a categorical approach is therefore appropriate. This situation also occurs when the TMDL prescribes a set of BMPs for more than one stormwater entity and those BMPs alone will achieve the WLA.

Third, categorical WLAs are also appropriate when data are inadequate for assigning individual WLAs. This will often be the case for very large watersheds where the modeling cannot achieve sufficient detail to allocate individual WLAs. Examples include the lower Minnesota River Dissolved Oxygen TMDL, the Lake Pepin TMDL (in progress), and the Lower Minnesota Turbidity TMDL (in progress).

Finally, categorical WLAs may be appropriate when a single MS4 or other entity will track BMP implementation and associated load reductions. An example would be a watershed district. However, MPCA has developed guidance that suggests the tracking entity should have regulatory authority and a proven history of implementation.

The WLA can be considered categorical in nature if it includes the load for two or more of the following sectors:

• Construction stormwater;
• Industrial stormwater;
• Individually-permitted MS4 stormwater; and
• MS4 stormwater covered under a General Permit.

Categorical WLAs can be problematic for two reasons. First, consider the example shown in the figure below, which is an allocation from the Lower Mississippi River Fecal Coliform TMDL. The MS4 allocations are shown in pink. Four MS4s in this sub-basin were given these categorical WLAs. This treats each MS4 equally, which may be an incorrect assumption if there are pollution hotspots within some MS4s. There is also an issue of dividing the WLA among the four MS4s. The problem of dividing up the WLA can be overcome by stating the WLA on a per unit area basis or as a required reduction, stated as a percentage.

Categorical WLAs for four MS4s in a sub-watershed of the Lower Mississippi River fecal coliform TMDL

A second concern with categorical allocations is that adjustments in the WLA will be required if the categorical WLA cannot be achieved. For example, the Shingle Creek Chloride TMDL requires a categorical 71% reduction in chloride loading. This applies to all MS4s in the TMDL study area. The BMPs used to achieve this reduction are prevention and good housekeeping BMPs that can be implemented equally by each MS4. If these BMPs do not achieve the desired reduction, it may be necessary to adjust the WLA to target areas where pollutant loading is greatest, which means individual WLAs.

If data support it, individual WLAs for each MS4 are desired. This will most commonly be done for small watersheds with a small number of MS4s and for which pollutant loading is well understood. For example, the Lake Independence Nutrient TMDL provides individual WLAs to the cities of Loretto, Independence, and Medina (figure below).

Recommended WLAs for the cities of Loretto, Independence, and Medina for the Lake Independence TMDL

Individual WLAs can be in the form of a required reduction from current loading, a mass load, or a load per unit area or per unit time basis. In addition to the traditional units of quantity per unit time, MPCA recommends expressing the TMDL as a reduction from a defined baseline. This is consistent with requirements in the MS4 general permit. An example baseline would be a year or a set of BMPs.

In some cases, the WLA can be expressed as a mass that has been translated from a desired reduction. For example, assume the goal is to implement a particular suite of BMPs. We can determine load reductions associated with those BMPs. The total reduction, as a percent, can be multiplied by the estimated current load. This yields a WLA expressed as a mass, even though the WLA will be achieved by implementing a specific set of BMPs. This may be a desirable approach for large watershed TMDLs. These TMDLs can be a concern because required reductions are often very large and modeling methods are not sufficient for calculating current loads for individual MS4s. Consequently, it is preferred to identify a suite of BMPs that would result in load reductions but not drive local stormwater management.

Sections 4b.ii., 4b.iii, 4b.iv, and 4b.v. summarize recommended policy on form of the WLA.