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City in Iowa to use man-made islands to clean creek

August 12, 2017

DUBUQUE, Iowa (AP) — Think of them as nature’s kidneys, Autumn Boos said of the matrix of recycled drinking bottles that resemble giant pot scrubbers sitting next to Dubuque’s 16th Street detention basin.

Over time, the porous, raft-like structures made with nontoxic post-consumer plastics will be teeming with native plants and aquatic life, she told the Telegraph Herald .

“The wonderful symmetry is we take around 67,000 plastic bottles that otherwise would go into a landfill and instead use them to clean the water and create a floating ecosystem,” said Boos, director of sales and marketing at Midwest Floating Island.

The St. Paul, Minnesota-based company started installing a system of “floating islands” in the Bee Branch Creek this month to target excess nutrients in the water and increase biodiversity.

Part of the City of Dubuque’s Bee Branch Creek restoration effort, the project will consist of 14 islands — varying in size — with a total combined area of 2,674 square feet to provide habitat and wetland restoration. The two largest islands will be 44 feet long by 17 feet wide.

“The city is excited to use this green infrastructure tool to improve water quality without chemicals,” said city civil engineer Deron Muehring. “This is a more natural way to deal with the nutrient buildup,” while providing habitat for butterflies, insects, fish, waterfowl, turtles and frogs.

Inspired by natural floating peat bogs, the islands will help treat stormwater before it flows into the Mississippi River.

The man-made wetlands will be covered with more than 5,000 native plants that grow roots below the water surface. The plastic matrix and suspended root systems of sedges, wild rye, blue flag iris, New England aster, marsh marigold, swamp milkweed and cardinal flowers create an ideal growing surface for biofilm and microbes to break down pollutants such as phosphorus and nitrogen that cause serious odor and algae issues. And they will help manage and remove heavy metals and suspended solids that create murkiness in the water, Boos said.

“We’re creating a floating ecosystem. ... It’s more than just a short-term fix to clean the water,” she said.

The islands will be anchored to the creek and detention basin bottom and made to adjust to changing water levels. That makes them ideal for stormwater ponds and other bodies of water that rise and fall after rain, ensuring the suspended roots are always in contact with the water and the island’s floating base avoids issues with flooding and dry conditions, Boos said.

Numerous case studies from across the U.S. show that floating wetlands removed substantial concentrations of total nitrogen and total phosphorus from reclaimed water and provided a diverse habitat for invertebrates, wildlife and vegetation. Independent laboratory tests suggest 250 square feet of island translates to an acre’s worth of wetland surface area, Muehring said. That is due to the amount of nutrients and suspended solids circulating in the water that is taken up by the islands’ exposed root systems.

“With our floating island system ... it’s like adding 10 acres of wetland to clean the water and trap sediment,” Muehring said.

The city will use $199,865 in proceeds from a state revolving loan fund and a $46,000 state grant to pay for the $245,865 project, which includes a 10 percent contingency and $30,500 in engineering costs.


Information from: Telegraph Herald, http://www.thonline.com

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