New Recommendation for Stormwater Management Through Subsurface Gravel Wetlands

By Anna Marchessault, Program Assistant
July 29, 2022

As buildings and roads expand across Vermont and storms become more intense due to climate change, more and better stormwater management is needed to avoid floods and protect water quality.

Subsurface gravel wetlands (SGW) are a preferred stormwater storage and treatment practice for roads and buildings in Vermont. SGWs use a saturated bed of gravel and sometimes wetland vegetation to filter incoming water and remove pollutants such as phosphorus. An outlet structure controls water levels, providing retention of stormwater volume and filtering out of pollutants.

Since the 2017 Vermont Stormwater Management Manual recommended SGWs, they have become more popular. To ensure SGW performance is both documented and optimized, Lake Champlain Sea Grant funded Watershed Consulting and the University of Vermont to research how well SGWs retain stormwater and remove pollutants.

In the Field

Lead researcher Andres Torizzo, Principal at Watershed Consulting, and his team monitored 32 storm events over two years at two SGW sites in Vermont—one in South Burlington and the other in Essex. The research team measured flow rates at the inlet and outlet of each system and collected water, soil, and vegetation samples.

The two SGWs successfully reduced flow during storm events, especially during medium-flow storms. However, they did not capture pollutants, specifically phosphorus. On the contrary, data showed that the imported wetland soil media leached soluble reactive phosphorus (SRP) into the discharge. SRP is the most bioavailable form of phosphorus for plant cells and leads directly to algal growth.

“We saw that subsurface gravel wetlands work, but they need improvements,” said Torizzo. “Given the two-year time span of this study and the relatively new installation of the two sites, we recommend further monitoring to better understand change over time.”

At the Lab

Concurrently, Eric Roy, Associate Professor of Environmental Sciences specializing in Ecological Design at the University of Vermont (UVM) focused on the influence of gravel and wetland soil media characteristics on performance in the lab portion of the study. With graduate student Marcos Kubow and additional students from his Advanced Ecological Design course, Roy investigated effects of material selection on stormwater concentrations of total suspended solids (TSS), total phosphorus (TP), soluble reactive phosphorus (SRP), and chloride using a set of experiments.

“Several UVM students received hands-on training in green stormwater infrastructure research in our Ecological Design Makerspace in the Aiken Center as a part of this project,” said Roy. “Our work illustrates how lab studies that enable isolation and testing of certain system components can complement field studies to identify opportunities for better design.”

Roy and his students found that while most engineered soils tested were likely to leach phosphorus, native onsite soils would better limit the introduction of excess nutrients. However, all materials tested failed to achieve the very low infiltration rate that is desirable for the wetland soil material in SGWs, which could lead to some stormwater not receiving full treatment due to “short-circuiting.”

They recommended that the manual be updated to require testing of all soil materials for phosphorus leaching potential, use of native soils when appropriate instead of the current recommendation to import engineered soil media, and design modifications that ensure stormwater moves through the entire subsurface gravel layer.

Next Steps

The Vermont Department of Environmental Conservation will update the Vermont Stormwater Management Manual for contractors and landowners who build new SGWs. The increased cost-effectiveness and water quality protection will help Vermont meet its Lake Champlain total maximum daily load (TMDL) goals for phosphorus.

Torrizzo explained, “With an expanding built environment and more voluminous rainfall and melting events, our goal is to make stormwater infrastructure, such as these subsurface gravel wetlands, as effective, efficient, and sustainable as possible.”

Learn More

Join Andres Torizzo on September 28, 2022 at noon eastern time for a webinar about his research, part of the Lake Champlain Sea Grant research seminar series. Register for this online event: How Effective are Subsurface Gravel Wetlands at Treating Stormwater Runoff in Vermont.

Learn more about this Lake Champlain Sea-funded research project Stormwater Subsurface Gravel Wetlands in Vermont.

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