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Fax Transmission

By Gladys Woods,2014-05-29 22:05
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Fax Transmission

Memo

    Project Name: NPDES Phase II MS4 Permit Client: Mn/DOT Metro District

To: Dennis Larson File No: 1399-03-102

From: Jeanette Powell, Pat Collins & Randy Date: July 8, 2004

    Neprash

Re: Outfalls, Inventory, & Mapping

This memo is to provide clarification on the subjects of:

    o definition of the term “outfall”

    o how that definition affects mapping, inspections, and the infrastructure inventory

    o discussion of other features that should be included in mapping, inspections, and the

    infrastructure inventory

OUTFALL DEFINITION

The MS4 Permit includes a definition of the term “outfall”:

    “Outfall” means the point where a municipal separate storm sewer system discharges from a pipe,

    ditch, or other discrete conveyance to receiving waters, or other municipal separate storm sewer

    systems. It does not include diffuse runoff or conveyances, which connect segments of the same

    stream or other water systems.

    Section V.G.3.a.4 of the Permit requires that MS4s develop a storm sewer system map showing the locations of:

    Outfalls, including discharges from your system to other MS4s, or waters and wetlands that are not

    part of your system (where you do not have operational control); structures that discharge storm

    water directly into groundwater; overland discharge points and all other points of discharge from

    your system that are outlets, not diffuse flow areas.

Receiving Waters

    This definition raises the question of the definition of the term “receiving waters”. An explicit definition of this term is not provided in Appendix B of the Permit. The preamble to the Permit provides clarification:

    This permit establishes conditions for discharging storm water and specific other discharges to

    waters of the state.

This intent is reiterated in the stated goals for the Permit:

    The primary goal of this permit is to restore and maintain the chemical, physical, and biological

    integrity of waters of the state through management and treatment of urban storm water runoff.

Bonestroo, Rosene, Anderlik and Associates, Inc.

    2335 West Highway 36 ; St. Paul, MN 55113 ; Phone: 651-636-4600 ; Fax: 651-636-1311

Memo

    Clearly, the intent of the Permit is that “waters of the state” should be considered the “receiving

    waters”.

The definition of the term “waters of the state” is remarkably broad (MN Statute 115.01.22):

    "Waters of the state" means all streams, lakes, ponds, marshes, watercourses, waterways, wells,

    springs, reservoirs, aquifers, irrigation systems, drainage systems and all other bodies or

    accumulations of water, surface or underground, natural or artificial, public or private, which are

    contained within, flow through, or border upon the state or any portion thereof.

    The breadth of this definition raises problems that have not been formally addressed by the MPCA. For example, it is very common for cities or Mn/DOT to build stormwater ponds for the purpose of receiving and treating stormwater runoff before it is discharged to another water body. These ponds are specifically designed to receive low-quality runoff and are widely recognized as one of most effective and common treatment methods to improve water quality. Under the definition of “waters of the state”, every stormwater pond is a water of the state and thus should be protected

    from receiving low-quality runoff. Obviously, this creates an absurd situation where runoff should be pretreated before it enters the treatment ponds.

    We have discussed this situation with the MPCA staff in the course of our work with the League of Minnesota Cities Guide Plan project. We have proposed an alternative definition of “receiving waters” for the outfall definition:

    water bodies that are under the jurisdiction of the Dept. of Natural Resources or the

    Wetland Conservation Act, and not exempt under the WCA.

This proposal has received informal acceptance from the MPCA staff.

    Under this definition, outfalls would include discharge points to natural lakes, ponds, streams, rivers, and wetlands. Discharge points to constructed treatment ponds and constructed ditches within the ROW would not be considered outfalls. This appears to be a reasonable distinction and we recommend that it be adopted by Mn/DOT Metro District for the purpose of identifying outfalls for mapping and inspections under the MS4 Permit.

Other “Discharges From Your System”

    The other major types of outfalls are the points where stormwater runoff leaves the Mn/DOT system. The definition of outfalls makes it clear that every location where runoff leaves the Mn/DOT ROW via a pipe or ditch should be considered an outfall for the purpose of identifying outfalls for mapping and inspections under the MS4 Permit.

    There appear to be several exceptions to the concept of discharges being outfalls. These include:

Diffuse runoff (or diffuse flow area).

    Bonestroo, Rosene, Anderlik and Associates, Inc.

    2335 West Highway 36 ; St. Paul, MN 55113 ; Phone: 651-636-4600 ; Fax: 651-636-1311

Memo

    Based on language from the Preamble of the Federal Final Rule 40 CFR Parts 9, 122, 123,

    and 124, dated December 8, 1999, and our discussions with MPCA staff, this exception is

    intended to apply to sheet flow leaving a jurisdiction without channelization. Under this

    exception, sheet flow down a hillside leaving the Mn/DOT ROW without forming any

    channels would not be considered an outfall. If sheet flow from a hillside entered a ditch

    before leaving the Mn/DOT ROW, the flow in that ditch would be considered an outfall.

Groundwater

    Although all surface waters can be hydraulically connected to groundwater, the intent of

    requiring MS4s to identify outfalls to aquifers/underground waters is to locate points where

    the storm sewer system is directly discharging into an underground water body. In some

    instances, injection wells are used to divert stormwater runoff directly into groundwater in

    order to recharge the aquifer. This is an example of an outfall to groundwater. To the best

    of our knowledge, Metro does not implement these types of practices and, therefore,

    outfalls to aquifers/underground waters should not be of concern when mapping the Metro

    storm sewer system. Infiltration basins would not be considered direct discharges to

    groundwater.

Segments of the same stream or other water systems

    Simply having a stream or other water system enter Metro’s jurisdiction does not mean that

    where it leaves Metro’s jurisdiction becomes an outfall of your system.

    If a stream or river entered Metro’s system and was conveyed under a roadway through a

    pipe or under a bridge, where it leaves the ROW would not be considered an outfall. If

    runoff from the road was discharged to the stream at one or more points, each of those

    points would be considered an outfall.

    Additionally, if stormwater from an adjacent MS4 was piped under a roadway, the point

    where that pipe left the ROW would not be considered an outfall. If runoff from the road

    was discharged to that pipe at one or more points, each of those points would be

    considered an outfall.

THE OUTFALL DEFINITION AND MAPPING, INSPECTIONS, AND THE INFRASTRUCTURE

    INVENTORY

    Outfalls are specifically addressed in three requirements of the MS4 Permit.

    Section V.G.3.a.4 states that outfalls must be included in the storm sewer system map. These outfalls would include:

     discharge points to natural lakes, ponds, streams, rivers, and wetlands

     every location where runoff leaves the Mn/DOT ROW via a pipe or ditch.

Bonestroo, Rosene, Anderlik and Associates, Inc.

    2335 West Highway 36 ; St. Paul, MN 55113 ; Phone: 651-636-4600 ; Fax: 651-636-1311

Memo

    Section V.G.6.b.3 states that, at minimum, 20% of the outfalls must be inspected each year. Section V.G.6.b.5 states that the results of these inspections must be summarized in each Annual Report.

    Outfalls should also be included in the database of the Metro stormwater system structures.

OTHER FEATURES THAT SHOULD BE INCLUDED IN MAPPING, INSPECTIONS, AND THE

    INFRASTRUCTURE INVENTORY

    The MS4 Permit has specific requirements for stormwater system components that must be mapped and inspected. These include the following items if they are part of your system:

     ponds (including stormwater treatment ponds, sediment basins, and infiltration basins)

     streams

     lakes

     wetlands

     structural pollution control devices

     pipes and conveyances (as a goal), 24 inch diameter pipes as a minimum

     outfalls

    The term “structural pollution control devices” is not clearly defined in the Permit. Devices listed in the Permit and guidance documents include:

     grit chambers

     separators

     trap manholes

     sumps

     floatable skimmers and traps

     settling and filtering devices

     swirl separators

     oil and grease separators

     filters

     infiltration trenches

     stormwater inlet traps

    This term appears to include any type of constructed device or structure that is intended to control pollution in stormwater runoff.

    The MPCA has provided a guidance document titled “Minnesota’s Municipal Separate Storm Sewer System (MS4) Storm-water Permit Inspection and Record Keeping”. It provides some

    specific and useful guidance. It is available on the World-Wide Web at:

     http://www.pca.state.mn.us/publications/wq-strm4-05.pdf.

    This guidance document provides some detailed instructions on the subject of basin and ponds inspections. Though many pond inlets and outlets are not considered outfalls under the definition of this Permit, this guidance document calls for all inlets and outlets to be inspected as part of the pond inspections.

    Bonestroo, Rosene, Anderlik and Associates, Inc.

    2335 West Highway 36 ; St. Paul, MN 55113 ; Phone: 651-636-4600 ; Fax: 651-636-1311

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