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

Introduction

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 documentis 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.