Stormwater Ponds

Ponds have been widely used for years to reduce the risk of flooding and handle stormwater runoff. Portions of Golden Valley’s stormwater system direct runoff and any tagalong pollutants (bits of metal, spilled oil and pesticides, fertilizers, pet waste, grass and leaves, etc) into stormwater ponds instead of Bassett Creek, Sweeney Lake, Twin Lake, Wirth Lake, or other natural waterways. The ponds allow time for pollutants to attach to sediment (sand and dirt) and settle at the bottom of the pond.

ducks on pondMost newer residential and business areas are built around ponds. Many of these ponds were not formed naturally, and they are not there solely for aesthetic purposes. To help control phosphorous and other stormwater runoff pollutants, Golden Valley's Surface Water Management Plan requires stormwater detention ponds and sound water management practices on all construction sites throughout the Bassett Creek Watershed. The objective is to reduce the pollutants flowing into Sweeney Lake, Twin Lake, Wirth Lake, Bassett Creek, and other water bodies in the watershed.

Ponds are vital to the health of our community. Although residents are strongly encouraged to minimize pollution, stormwater ponds help deal with it once it has entered the environment.

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+Anatomy of a Stormwater Pond

anatomy of a pond diagram

Anatomy of a Stormwater Pond
(click on photo for larger image)

Several elements make stormwater ponds unique:

  • The main body is often 3 to 8 feet deep and varies in size depending on how much runoff is directed into it. The greater the surface area of a pond, the more pollutants it can remove.
  • Two main pond components are inlet (where the runoff enters) and outlet (where the runoff exits) pipes. They must be far enough apart so pollutants have time to settle before leaving the pond. This is why stormwater ponds are usually oblong. A pond three times as long as it is wide allows the polluted sediment more time to settle before the outlet pipe releases the water.
  • Ponds are constructed with a shallow safety shelf around the edges. These shelves are often filled with aquatic plants to filter more pollutants and make the pond less attractive to swimmers and young children.
  • Where space permits, forebays are constructed between the inlet pipe and the main pond. A forebay, usually a four- to six-foot-deep basin, holds about 15% of the pond’s volume. As water flows from the forebay into the main pond, much of the polluted sediment is filtered out, providing for easier pond maintenance.

+Pond Landscaping

Landscaping around stormwater ponds is not just for good looks. Gentle side slopes encourage runoff to enter the pond and discourage anyone from accidentally entering. Natural, un-mowed landscaping with plants in and around the pond:

  • stabilizes the pond by preventing erosion
  • provides an environment for microorganisms that remove nutrients/pollution from the water
  • improves the pond’s appearance by hiding debris and water level changes
  • provides a habitat for insects, such as dragonflies, that eat mosquitoes
  • discourages geese from visiting and contributing to pollution
  • makes ponds less attractive for wading or swimming

Read more on landscaping around bodies of water »

+Stormwater Pond Contaminant Litigation (FAQs)

Together with several other Minnesota cities, the City of Golden Valley has filed a federal lawsuit against seven refiners of coal tar for allegedly contaminating numerous stormwater ponds with chemicals called polycyclic aromatic hydrocarbons (PAH).

The city is seeking damages to fund proper disposal of PAH contaminants.

PAH is found in coal tar sealant, which is a product that was commonly applied to driveways and parking lots before the State of Minnesota banned the sale and use of sealants containing PAH in 2015.

Regardless of the lawsuit’s outcome, the City plans to begin testing ponds for PAH in 2019. Cleanup will begin following testing.

Why Do Cities Have Stormwater Ponds?

Stormwater ponds were built to perform flood control and water quality functions.

Why Do Cities Have To Test For PAHs?

Cities must test for PAHs before they dredge their ponds, and there are thousands of ponds. Also, starting in around 2023, cities will be required to perform work to restore the water quality function of their ponds. Again, the cost of complying with these regulations will be higher because of the presence of PAHs in the sediment from coal tar sealant runoff.

What Is Coal Tar?

Coking facilities produce raw coal tar, which they sell to steel and aluminum refiners. Refiners use about 95 percent of the raw coal tar in their production processes. Coal tar is the waste byproduct of these coking facilities.

What Is Coal Tar Sealant?

It is a thin black coating applied to paved surfaces (typically driveways, parking lots, playgrounds, etc). It is not commonly used to pave roads. Sealant manufacturers claim their product protects the paved surfaces below from oil damage and weathering.

How Does Coal Tar Sealant Migrate From Driveways And Parking Lots And Into Stormwater Ponds?

Within several years of applying coal tar sealant, the combination of friction from vehicles driving on the surface and environmental/weather exposure wears the sealant off into fine particles that wash from the driveway, down roads, and into stormwater ponds.

Did The Refiners And Sealant Manufacturers Know Their Product Would Erode?

Yes, the product manufacturer recommends reapplying the sealant every few years because they know the products break down over time.

Is Coal Tar Waste Toxic?

Yes, coal tar contains high concentrations of PAHs (polycyclic aromatic hyrdocarbons). PAHs make up more than 50 percent of the raw coal tar. The EPA has designated 16 PAHs as priority pollutants and seven of those as probable human carcinogens. Nearly all seven of these PAHs are in raw coal tar. While in sediment, coal tar is not a public health concern. Once it is dredged, coal tar could pose a health risk if the dredged soils are not properly disposed of in an authorized landfill designed to ensure the waste cannot migrate into the environment.

Are There Alternatives To Coal Tar Sealant, And Were There Alternatives Before The Minnesota Ban?

Yes, asphalt is, and has always been, an alternative to coal tar sealant. And while asphalt also contains PAHs, coal tar sealants have 1,000 times more PAHs than asphalt sealants.

Will Residents Who Used Coal Tar Sealants On Their Driveways Be Responsible For Damages In This Lawsuit?

No, the City is not suing users of the coal tar sealant. Residents are not responsible for the damages the City alleges because they were not warned of the dangers of using the sealant. To the contrary, residents were led to believe it was a safe product to use because it was sold as an option for surfacing and re-surfacing driveways.

Why Litigation?

It is very costly to remediate stormwater ponds contaminated with coal tar and no end in sight. Even though there is now a ban, coal tar sealants were used for years and remain on many properties. Litigation is appropriate because polluters should pay for remediation of the damage they cause. Without a lawsuit, remediation costs would be shouldered by innocent taxpayers.

Why Are Minnesota Cities Filing This Case First?

Minnesota cities are the first to systematically address the problem and to incur disposal costs. Part of this is due to the statewide ban, which also evidences the state’s concern for human and ecosystem health. While in the ponds, sediment coal tar waste does not create a public health concern. But it could if the sediment, once dredged, was not properly disposed of in contained landfills. That is why the cost of disposal is so high. Moreover, statewide thousands of ponds still need to be dredged; it is important to address the issue now before tens of millions of dollars in costs are incurred.

What Damages Are The Cities Seeking?

The cities are seeking compensatory damages for past and future costs of testing waste/sediment and removing and disposing of that waste from the stormwater ponds.