Monday, March 4, 2024

NC State looking at Wilmington’s Greenfield Lake for state-funded ‘floating wetland islands’ project

If the university team moves forward with the location it will install floating islands on the northern arm of Greenfield Lake, pictured here, which receives drainage water from the surrounding area. (Port City Daily photo/Mark Darrough)

RALEIGH —  A team of engineers from North Carolina State University is leaning toward choosing Greenfield Lake in Wilmington as a research site to install and monitor a series of ‘floating wetland islands’ as part of a $100,000 state-funded stormwater innovation grant.

Governor Roy Cooper announced on Wednesday a total of $14.6 million will be dispersed from the North Carolina Land and Water Fund (LWF) to local governments, universities, and conservation groups to protect 6,710 acres across the state. More than 60 projects were awarded the grant money, including an N.C. State initiative to “optimize floating wetlands” at either Greenfield Lake or a retention pond on the university’s campus.

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Because the school is set to receive $100,000 of a requested $187,000, N.C. State Professor Dr. Bill Hunt, who is leading the project, said he must choose one of the two sites for the project. The on-campus retention pond would be cheaper to send members of this team to measure water quality levels, but he said there is a “better than 50-50” chance he will choose the popular Wilmington lake because of its greater exposure to people and because of its potential to influence a broader effort to improve the lake’s water quality.

“This is a type of project that, if there’s enough effort put in, you should be able to see improvements in the lake eventually,” Hunt said. “So being a part of the beginnings of this effort, to me, is very exciting.”

According to Hunt, the water quality issues at Greenfield Lake stem from excessive nutrients like nitrogen and phosphorus entering the lake via stormwater runoff.

“While those nutrients do exist naturally, in Greenfield Lake they are present in very high numbers. And they get into the lake by running off from the watershed,” Hunt said.

Will Summer, deputy director of the LWF, said a floating wetland island is “essentially a mat constructed in such a way that you can put plants in it through the top and the roots can hang into the water.”

Man-made wetlands created to absorb excessive nutrients are usually installed in shallow waters, he said, because the plants can be rooted to the lake bottom. Floating islands, on the other hand, are intended to filter out excessive nutrients in deeper waters — nutrients that are harmful to the water quality but nutritious for the plants.

Testing of this technology is specifically mentioned in the Stormwater Design Manual published by the N.C. Department of Environmental Quality. A project summary issued by the LWF said the research is needed because while detention ponds are effective at reducing stormwater runoff during peak flows, their removal of pollutants is inconsistent.

“Finding cost-effective retrofits is vital to addressing these shortcomings,” according to the summary, but additional data collection is needed to determine how effective floating wetland islands are at absorbing pollutants.

According to the LWF, the proposed project would include purchasing and installing the wetland islands, installing two groundwater wells at Greenfield Lake, pre- and post-installation water quality monitoring, measuring plant biomass and density to track nutrient absorption, and the evaluation of collected data.

“If the concept proves effective, the use of floating wetland islands would be much more practical, especially for larger sites,” according to the LWF summary. “As common as wet ponds are, this could be applied very broadly across North Carolina.”

Lines of defense

An illustration provided by the N.C. Land and Water Fund shows environmental and aesthetic benefits of a floating wetland island. (Illustration courtesy Iowa State University)

The northern arm of Greenfield Lake receives stormwater drainage from a stream that passes underneath Lake Shore Drive. It collects water from a 173-acre watershed that is largely developed, consisting of numerous home roofs, driveways, and paved roads — all impervious surfaces that contribute to polluted stormwater runoff.

Dr. Hunt said that if the proposal goes through, his team will place lines of connected floating islands stretching across the northern arm: one near the stream’s entrance into the lake, and another approximately 100 feet downstream. He described it within the context of old Revolutionary War tactics.

“Think of floating islands connected as a line of troops that the water has to pass through. If the water goes through the first line of defense, then in a hundred feet it must pass through a second line of defense,” Hunt said.

His team will measure water quality samples taken immediately after the first line to see if there is any improvement, as well as after the second line to see if there is continued improvement as the water moves to the broader areas of the lake. Although the project is not intended to fix the pollution problems at the lake, he said, “we think it is a part of a larger strategy to address them.”

“And if they can work, we might see them more in Greenfield Lake and we might see them more in other ponds in Wilmington,” he said.

Th excessive amount of nitrogen entering the lake is specifically concerning, he said, as nitrogen is found in rainfall that hits rooftops or roadways and there is nothing to filter it out before it enters the lake.

Although he is an engineer unqualified to measure people’s response to the visual aesthetics of the floating island — especially those walking the Greenfield Lake Park Trail that loops around the lake — he said “anecdotally, that will certainly be something we look at.”

Hunt is also helping Cape Fear River Watch on another water quality improvement project on Greenfield Lake, which further motivates him to choose the lake for his own project. On top of that, the city of Wilmington has “been a very good cooperator” with N.C. State in the past, and they are pushing for this project as well. (The city agreed to a small financial match, according to the LWF. If the project had been approved for the initially requested $187,000, the city would have contributed $3,457, with N.C. State’s contribution estimated at $19,090.)

He said the city has agreed to perform maintenance of the floating islands.

Choosing Greenfield Lake would be beneficial not only because the city, along with partners like UNCW and Cape Fear River Watch, have expressed growing interest in improving the lake’s water quality, but because it would encourage use of the floating islands across a region where stormwater runoff has become an increasing concern as development continues to sprawl, according to Hunt.

But plenty still needs to be ironed out in the coming weeks. According to Summer, an LWF program manager will negotiate with Hunt on the project’s scope — what he can do with $100,000 instead of $187,000 — over the next several weeks before anything is finalized.

“It will take a lot more work, trust me, but you gotta start somewhere,” Hunt said. “I feel like this project, plus the one the Cape Fear River Watch is shepherding, are really important starts to an eventual improvement in Greenfield Lake.”

Noah Blanton, who studied marine biology at UNCW, walks along a bridge stretching across Greenfield Lake’s north arm Thursday evening. He said he supports the idea of floating wetland islands if they are scientifically proven to improve the lake’s water quality and will not upset the ecological balance of the lake’s ecosystem. (Port City Daily photo/Mark Darrough)

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