May 25, 2016
By Don Behm of the Journal Sentinel
They look like rafts that might have been launched by children intending to float away the summer.
But the large rafts towed onto an Oak Creek storm-water pond this week are the aquatic equivalent of a Roomba, the vacuum-cleaning robot that works on its own.
Three artificial islands, loaded with native wetland plants, are floating in one pond at Emerald Preserve off S. 6th St., where they will work year-round to cleanse storm water of nutrients and other pollutants, officials said.
Such floating islands are designed to duplicate the effectiveness of natural bogs and other wetlands in removing contaminants from water. While similar wetland islands have been installed and studied in Florida and Minnesota in recent years, they are new to southeastern Wisconsin. Thousands have been launched in Europe, Asia, Canada and New Zealand.
Rain falling on the 85-acre Drexel Town Square drains to several ponds in the preserve. The project south of Drexel Ave. and west of Howell Ave. encompasses commercial and residential development as well as the new city hall and library. A Meijer supermarket, city Civic Center and clock tower, and Water Street Brewery restaurant are already in place. A hotel and apartment buildings are under construction.
Storm water rinses dirt and grit, phosphorus and nitrogen off this developing landscape and carries the load to the ponds.
The trio of islands are tethered to the bottom of the first pond in the system, where they will encounter the bulk of pollutants, said Ron Londre, an ecologist with TRC Environmental Corp. Londre designed the series of storm-water ponds that empty to an 8-acre wetland.
The project was unveiled to the public Wednesday during a meeting at the Oak Creek Civic Center. A $65,000 grant from the Fund for Lake Michigan paid for purchasing and installing the islands.
Each island is a two-layer mat. The bottom layer is shredded plastic loosely woven into a squeezable fabric that looks like a pot scrubber. That is topped by a dense layer of shredded coconut husks.
This week, TRC crews placed swamp milkweed, sedges, bulrush, blue flag iris and other wetland plants into holes in the top layer.
In about two weeks, roots of those wetland plants and others, such as red cardinal flower and boneset, will push through the bottom layer and dangle in the water, said Autumn Boos, marketing director for Midwest Floating Island. The St. Paul-based company supplied the mats.
“The more nutrients in the water, the more the plants grow,” Boos said.
Signs on the Dale Richards Trail through Emerald Preserve provide information on the islands, ponds and wetland.
While roots of island plants will draw up water and nutrients, that is only a small part — maybe 5% — of the cleaning action that takes place here, according to Boos.
The rest of the work is done by microbes. Bacteria and others will colonize the outside of the roots and the bottom of the woven plastic mat. They will digest and break down nutrients, including phosphorus and nitrogen, that spur nuisance algae growth.
Tiny bits of solid organic material suspended in the water will be trapped on the sticky film of microbes and be digested in the same way. Soil, sand and other suspended solids will settle to the bottom.
Studies found that the islands, known commercially as Biohaven floating treatment wetlands, can remove up to 67% of nitrogen, up to 68% of phosphorus and 75% of suspended solids in storm water, Boos said.
Stainless steel cables connect each island to several anchors — plastic buckets filled with cement — to keep them in place. There is enough slack in the cable to enable the islands to float up or down with changing water depths.
The size of the pond, its water quality and temperature all figure into the amount of artificial wetland needed to help cleanse the water, Boos said.
There is a total of 1,370 square feet of islands floating at Emerald Preserve. Their sizes vary: 20 feet by 22 feet; 20 feet by 24 feet; and 15 feet by 30 feet.
In addition to the ponds, rain gardens and porous pavement on streets and parking lots capture some of the storm water within the Drexel Town Square development.
Although a short fence on the perimeter of each island at the preserve will block geese from damaging the property, there is a big enough gap on the fence bottom to allow small turtles to climb aboard and get some sun, Oak Creek environmental engineer Sue Winnen said.
Seeds of native wildflowers and grasses — including switch grass, cup plant, mountain mint and New England aster — were sown on the surface to provide additional habitat for birds and dragonflies, as well as butterflies, bees and other pollinators.