Minneapolis rain gardens were built to reduce water pollution. Research shows they’re making it worse.

The unusual construction project in northeast Minneapolis in 2020 was billed by city officials as an environmental win.

Newly built rain gardens in the Hoyer Heights neighborhood would help prevent flooding and filter out pollutants — including phosphorus, one of the main culprits behind the increasingly common toxic algae blooms on city lakes, according to the city.

But a 2021 study conducted by the city and the University of Minnesota, and recently shared with the Minnesota Star Tribune, suggests the new rain gardens are likely making the city’s phosphorus problem worse.

Minneapolis has installed hundreds of rain gardens, tree trenches and other green infrastructure in recent years, aiming to reduce flood risks and clean up stormwater before it reaches the city’s lakes and rivers. On the city’s south side, nearly 200 gardens were built along Grand Avenue in 2022 and more than 100 along Bryant Avenue in 2023.

The city has to add compost to get those gardens started. The test results now raise questions on whether the rain gardens could be solving one issue while exacerbating another.

“We didn’t expect the compost, what’s in the rain garden media mix itself, to actually increase how much phosphorus comes out,” said Andy Erickson, the stormwater research manager at the University of Minnesota’s St. Anthony Falls Laboratory, which is helping the city research potential solutions. “That is the exact opposite of what we’re trying to do, we’re trying to reduce pollutants.”

Despite that problem, Erickson and city officials defend the rain gardens. Monitoring of the Hoyer Heights gardens by Erickson’s team showed they have filtered out other pollutants, including toxic metals such as lead.

“The rain gardens are still highly beneficial, and they’re doing their job,” said Minneapolis spokesperson Haley Foster. “We’re not seeing catastrophic pollution. We’re seeing a chance to make the system work even better.”

Environmentalists have hailed rain gardens for their abilities to store water, thereby easing pressure on aging stormwater systems that can get overwhelmed during heavy downpours. They also capture oil, heavy metals and other toxic material that gets washed off streets and yards during storms.

City officials also wanted to filter out nutrient pollution, specifically phosphorus and nitrogen, which are commonly found in fertilizers and compost. While those nutrients are important for plant growth, they also feed the toxic algae blooms that forced city officials to shut down three beaches in 2020 for 12 days. The blue-green algae blooms have been known to kill pet dogs and sicken children, who are more vulnerable to the toxins.

Since 2020, Minneapolis officials have issued dozens of swimming advisories or warnings at city beaches, spanning more than 400 days, city data shows.

For many of the city’s lakes, Erickson said, the goal is to keep phosphorus levels below 0.09 milligrams of phosphorus per liter of water — sometimes as low as 0.04 milligrams per liter. The 2021 test results showed 2.7 milligrams of phosphorus per liter coming out of some of the rain gardens, far above what’s considered an acceptable level.

Currently, the city is exploring several options to address that problem, Erickson said. Some compost is needed for the gardens to thrive, he said, but research shows they can likely use less. The researchers have also tested using ground-up iron and other materials that can capture and store the phosphorus before it escapes, he said.

Another easy fix, Erickson said, is to cap the rain gardens’ underdrains, which are pipes beneath the gardens that carry the water to the city’s storm sewers. Those drains have become standard features of newly constructed rain gardens, but the unintended consequence is that they prevent the water from soaking more deeply into the ground, where phosphorus tends to get stuck or natural processes break it down.

Erickson’s research team is now recommending to the city that it seal up all the underdrains of its rain gardens, including all the newly built rain gardens in Hoyer Heights, along Bryant Avenue and elsewhere.

Managing stormwater and nutrient pollution has always been a trade-off for local governments, said Chip Small, a biology professor at the University of St. Thomas. Plants help reduce flooding by soaking up and storing water, creating healthier soil compositions and preventing erosion. They also benefit bees and other pollinators. But cultivating those plants also means introducing nutrients like phosphorus, which then can be leached into the water and fuel algae blooms in lakes.

“Sometimes these things are pitched like a panacea. Like, this is going to solve all the problems,” Small said. “But really, it solves some problems and it creates some other problems.”

One way to minimize nutrient runoff, Small added, is to use city-made compost in the rain gardens rather than compost made from manure, which tends to have higher concentrations of phosphorus. City compost, on the other hand, tends to be made of food and yard waste, he said.

Ultimately, Erickson said, the rain gardens are still worth it for their flood protections and filtration of other pollutants. And some studies have shown that more greenery in neighborhoods can have positive effects on mental health, he said.