Study aims to track sources of downtown runoff

Mississippi Watershed Management Organization researchers track runoff from a site near U.S. Bank Stadium. Submitted photo
Mississippi Watershed Management Organization researchers track runoff from a site near U.S. Bank Stadium. Submitted photo

A new Mississippi Watershed Management Organization study is aiming to better understand the types and amounts of pollutants that come from different surfaces in downtown Minneapolis.

The agency is collecting and analyzing water samples from four types of impervious surfaces: streets, sidewalks, parking lots and rooftops. Its goal is to utilize the data to better target pollution-mitigation efforts.

“It helps us make decisions where to put projects,” said Stephanie Johnson, MWMO projects and outreach director. “By targeting our efforts on managing the areas that contribute the most pollution, we ensure a wise investment of the public tax dollars that finance our work.”

Most stormwater runoff from downtown goes untreated directly into the Mississippi River, Johnson said. That’s contributed to the river becoming polluted with road salt and other nutrients, sediments and metals.

The agency is using a rain simulator to create its runoff samples for the study.
The agency is using a rain simulator to create its runoff samples for the study.

The MWMO typically uses models that rely on national datasets to figure out where to put projects, Johnson said. Those models assume the same amount of pollution comes from all types of impervious surfaces, which isn’t necessarily the case.

The MWMO hopes to better understand those differences with its new study. It’s collecting samples from streets, sidewalks, parking lots and rooftops and testing those surfaces in different seasons.

The agency is using a rain simulator borrowed from the Minnesota Department of Agriculture to help collect the data. The apparatus imitates natural rainfall, allowing the agency to collect a workable sample in a more predictable fashion.

Johnson said the agency expects runoff from rooftops will be cleaner than runoff from roads, parking lots and sidewalks. She said she generally anticipated they would see more polluted samples in the spring because of the accumulation of sediment, salt and other pollutants, but that it’s hard to know for sure.

The data could help the MWMO identify potential opportunities for green infrastructure, Johnson said, which can range from permeable pavers to rain gardens and other vegetation.

Plants and trees soak up the nutrients in stormwater, while microbes near their roots utilize other pollutants, Johnson said. Soils can collect some of the sediments in stormwater, she said.

The study comes as the MWMO works to develop models for all sub-watersheds within its boundaries, an effort it says will help it better understand stormwater runoff patterns.

The MWMO, a local governmental unit, works with Minneapolis and other government agencies on stormwater treatment efforts within its boundaries, which stretch from the Tangletown neighborhood to Fridley. Past projects have included the “green campus” at Edison High School, a water reuse system at the Minneapolis Sculpture Garden and the St. Anthony Regional Stormwater Treatment and Research System.

Johnson said the agency’s main concern is salt pollution, noting that it can’t be removed from water bodies once it’s in them. The agency works with cities, the state Department of Transportation and private appliers to lessen the impact of road salt.

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