Posted on August 16, 2023
A litany of solutions exist that can rid the region’s waters of contaminants and persistent algae blooms, yet one contender remains relatively unknown.
It’s simple, maybe a little slimy, inexpensive and mimics what’s found in nature: floating wetlands.
Floating wetlands are a natural solution for a growing urban environment, said Rob Zisette, a Herrera Environmental Consultants principal aquatic scientist. They are used in freshwater to treat stormwater ponds, wastewater lagoons and lakes, as well as serve as wave breaks to prevent erosion.
The design is basic: each wetland unit consists of a platform, buoyant devices and aquatic plant species. They are anchored in place and left to stay afloat for more than 20 years without any maintenance — except for occasionally shooing away geese from relieving themselves on the plants.
Depending on whether they are homemade or built by a manufacturer, these platforms can vary in cost, size and sophistication. However, their function remains the same.
Aquatic plant roots spread through the mat and create dense columns that hang underneath. Here, microbes form a layer of gelatinous biofilm where much of the plants’ nutrient absorption occurs, Zisette said. Without the biofilm, nutrients would otherwise feed cyanobacteria.
At least 2 percent of a lake needs to be covered by floating wetlands to improve water quality, according to Floating Islands International.
An unintended benefit of the tool is how it supports critters both above and below the water’s surface. Invertebrates dwell on the biofilm until they meet their demise and become fish food. Platforms can be designed to serve as a breeding ground for waterfowl.
In 2014, the Port of Vancouver researched ways to filter water after quality tests showed high levels of copper in its Terminal 4 stormwater retention pond, which posed a risk to fish in the Columbia River. A 2013 New Zealand study, which reported that floating wetlands sequestered up to 55 percent of metals in test ponds, convinced port officials to build their own.
Matt Graves, port environmental manager, and a colleague constructed 120 cells with foam insulation and cedar board. They filled its top with common rush, a native wetland plant, then anchored each platform in the pond, covering about 20 percent of its surface.
“We were trying to take a more natural approach to cleaning water as opposed to injecting chemicals,” he said.
Preliminary tests showed a slight reduction in metals, indicating that most of the pond’s surface area must be covered to have a significant impact. But any reduction is viewed as a win. The port expects to redesign and rebuild its floating wetlands next year, Graves said.
To Graves’ and Zisette’s knowledge, the Port of Vancouver is the only organization in Southwest Washington to have an established floating wetland project. Farther up north, there has been more success.
A community push to preserve one of Seattle’s most popular lakes, Green Lake, resulted in the installation of floating wetlands in 2022 to clean its water and provide habitat to waterfowl, such as the pied-billed grebe. The University of Washington is researching how wetlands can be used near stormwater entry points in lakes and shipping canals.
Despite floating wetlands not being new technology, the Pacific Northwest seems to be reluctant using them, Zisette said. In Washington, regulatory agencies haven’t approved floating wetlands as a water treatment system, which removes incentives for developers to use them.
“It’s very hard to work at that state government level, but we’re getting there,” he said. “I’m very optimistic.”