Floating Wetlands Create Prospects for Nutrient Mitigation and Wildlife Habitat in Open Waters

According to the Environmental Protection Agency, pollution affects approximately 40 percent of our nation’s waterways, and as such is one of the country’s top environmental challenges. Treating stormwater runoff and urban discharges before they carry heavy metals, nutrients and other contaminants into a river system or estuary has been a major focus of improving water quality over the last three decades. Many scientists now agree that solutions must also consider direct intervention strategies that target ecological degradation in open water. KCI worked with the Maryland Port Administration (MPA) and BlueWing Environmental Solutions & Technologies LLC to launch one such system, a floating treatment wetland (FTW), in the Baltimore harbor.

Photo Courtesy of Jeffrey Sauers of Commercial Photographics
Water quality stewardship and the health of the Chesapeake Bay are at the heart of the MPA’s core mission. Their challenge is managing their properties where pavement is the primary land cover due to the nature of operations.. Jeffrey Sauers of Commercial Photographics

“We have limited opportunities for conventional stormwater management at our terminal sites, so we are aggressively and actively looking at emerging technologies,” said MPA environmental manager Bill Richardson. Of the 1,600 acres that constitute the agency’s 10 facilities, more than 75 percent are impervious, including expanses of pavement needed for vehicle and cargo shipments. MPA originally brought KCI on to assess their existing facilities, evaluate conventional and innovative treatment systems, and develop a facility-wide water quality management plan. “We’re looking for treatments that capitalize on our unique access to waterfront shoreline with plans to upscale them once they are recognized by regulatory organizations,” he said.

FTWs are man-made islands eco-orchestrated to bio-mimic a natural filtering process. The wetland launched by KCI and BlueWing is comprised of native vegetation planted among four layered mats of a non-toxic post-consumer plastic web matrix. Although the plant life takes up nutrients from the water to support its growth, the bulk of pollutant removal actually takes place through naturally occurring bacteria and microbial life that attach to the roots and plastic to create a biofilm.

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As part of KCI’s water quality master plan for the MPA, floating treatment wetlands earned the highest rank by an order of magnitude among area-wide approaches based on the low cost per pound and high removal rate.

“We have a concentrated wetland that is basically real estate for microbes that cleanse the water by ingesting nutrients,” said Ted Gattino, managing partner at BlueWing. “Our FTWs have 198 square feet of available surface area per cubic foot of island for colonization, making our wetlands 800 percent more efficient than plant-based practices.”

Academic and other institutional studies, conducted in closed testing environments, indicate high levels of nutrient reduction even without any vegetation, on the order of one pound of total nitrogen and one tenth of a pound of total phosphorus annually per square foot of wetland. To put that into perspective, a 100-square-foot wetland could remove approximately 10 pounds of phosphorus and 100 pounds of nitrogen each year—one pound of phosphorus and seven pounds of nitrogen can combine with carbon to produce up to 500 pounds of algae that can stretch across acres of water surface.

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In addition to water quality improvements, these man-made islands also create thriving surface and aquatic habitats. Depending on the vegetation type, roots can grow to depths of three-to-four feet and become home to a variety of marine life. At a test site beside the National Aquarium in Baltimore, researchers identified 12 fish species feeding regularly underneath their floating wetland, as well as crustaceans and other organisms.

“For the Port, the wetland serves multiple purposes beyond its primary function as a water quality filter by also supporting habitat, shoreline buffering, aesthetic and awareness objectives,” said KCI project manager William Frost, PE, D.WRE. “The site lies at the mouth of Colgate Creek, which flows between two of the organization’s largest and most active properties—Dundalk and Seagirt marine terminals—and is visible but not accessible from public roadways and bridges.” Its prominent location supports the Port’s public education goals while demonstrating their commitment to water quality and green infrastructure.

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Engineers calculated wind loads and safety factors to determine anchor size and mooring requirements for the wetland.

KCI helped MPA identify the site location and develop conceptual design and permitting documents, as well as assist with installation. The firm’s first open water floating wetland, the largest permitted to date by the Maryland Department of the Environment, spans more than 2,000 square feet covered with more than 4,600 native plants and sits about 70 feet offshore.

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KCI staff used a small boat and canoe to set anchors, securing the wetland within the four-foot to five-foot-deep shoreline area.

Since the launch in July, vegetation has shown noticeable growth, and within weeks, two families of ducks began frequenting the island. KCI will continue to assist the Port in the coming years by monitoring the wetland for invasive species as well as movement following strong storms, while the agency evaluates the treatment system from a management and maintenance standpoint. Within 12 months, the FTW should be self-sustaining with a lengthy expected lifespan.