New fees will pay for study of Farmington's storm ponds

Talk about it

Farmington homeowners will pay an extra $6 in storm water utility fees next year so the city can begin testing on ponds around the community. The tests are to see if Farmington’s ponds are filling up with sediment, and if so, if that sediment is contaminated.

Farmington has more than 120 stormwater ponds around the community. Many have been in place for years. Others were built as part of more recent housing developments. All the same, storm water ponds are in place for a reason.

According to Farmington city engineer Kevin Schorzman, the stormwater ponds are first a collector for the water that accumulates during a heavy rainfall event. When heavy rain falls, it moves quickly, often bringing with it everything from garbage to lawn fertilizer and chemicals in the soil. The water around Farmington ultimately drains into the Vermillion River or one of its tributaries. Filters in the ponding system stop much of the debris and chemically-treated soils from reaching the river.

The other main purpose of the ponds, Schorzman said, is to control the volume of water that moves downstream. When rain falls, the same amount falls whether on a hill or in a valley. However, the water on hills naturally flows downward, which means it would pool at the bottom, along with the rain that had already fallen in that area. The pond system allows a lower point for the water to gather, rather than in a yard or an intersection, Schorzman said.

In a perfect world, the ponds would be dredged periodically because, as time goes by, the amount of sediment that settles on the bottom of the ponds reduces the amount of water those ponds can hold.

“The point of, ‘Why do I care?’ is because of that concept, each one is designed to hold so much back and let it go down slowly,” Schorzman said. “If you can imagine a cupful of water, and you fill half of that cup with sand, a 12-ounce can of pop isn’t going to fit. It’s going to flow over the side and get into everything. That same exact thing is going to happen with our ponds.”

That probably doesn’t mean homes will flood, but it does mean some yards could have standing water for longer periods of time following a significant rainfall, if the ponds are not maintained. And that will mean dredging the bottoms of some of Farmington’s ponds in the next several years.

Fee increase

The fee increase approved this week, which equals 50 cents per month, will be used to pay for the initial surveying and testing of the sediment in Farmington’s ponds. City staff will conduct the surveys and the testing, but the current fee structure does not include funding for the work.

Schorzman expects to begin the surveying this winter, as it is easier to stand on ice and drill down to get a more accurate reading. If there are ponds that have a significant amount of sediment present, samples will be taken to determine whether the sediment contains any contaminants.

Specifically, the sediment will be tested for polyaromatic hydrocarbons, or PAHs, which have been found in the cold tar used to resurface driveways. Two or three years ago, Schorzman said, the Minnesota Pollution Control Agency determined the PAH’s were pollutants.

The MPCA now requires that contaminated sediment be removed and transported to a landfill, which is rather expensive, Schorzman said. However, if a city or municipality passes an ordinance that bans the type of driveway sealant that contains PAHs, that community becomes eligible for state funding to help cover the sediment removal costs.

All that is a year or better down the road, though. At this time, Schorzman is working on identifying which ponds should be examined first this winter.

“We’re working with a combination of age, where it is located and how close are developments that have been sitting,” he said, adding that grass in yards helps to filter some of the runoff after rain events, but homes without sod — mostly, new construction — don’t have the same filters. That means ponds in areas where new housing is present could have more sediment, and more contamination, than some of the existing ponds.

Schorzman said staff is putting together a database that will track the “as built” condition of the pond, with its original volume rates, to the “as is” volume. That way, engineering staff will be able to determine which ponds need to be dredged first.

“Honestly, I think it’s going to be a process that is ongoing for a long time. We’re not at a crisis point, but we’re at the stage where, look, we know we have ponds that are 20 to 30 years old. We need to have a program to maintain them on a consistent basis, and part of that is having the funding available to maintain a program like that. It’s another one of those things we need to get on our radar,” Schorzman said.

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