Subsurface gravel wetlands (SGW) are water treatment practices that utilize a saturated bed of gravel and (sometimes) wetland vegetation to filter incoming water and remove pollutants through a combination of physical filtration, adsorption, biological uptake, and microbial transformation. Water level is controlled by an outlet structure to retain a permanent subsurface pool, providing retention of stormwater volume in addition to pollutant removal. SGW are becoming increasingly popular tools for stormwater treatment in Vermont.
A varied group of partners including academic researchers, municipal stormwater managers, and consulting designers and engineers, have identified questions around stormwater gravel wetland performance as significant in the goal of attenuating the impacts of urban stormwater pollution on Lake Champlain and its subwatersheds. We hypothesized that design variations such as media sourcing, and type of planting media would have significant influence on gravel wetland performance, and guidance on these points should therefore be included in the VSMM. Recommendations for additional information to be included in the VSMM are explained below. Additionally, we suspected that field conditions where chloride loading is highest will result in poor vegetation performance and therefore reduced pollutant removal, particularly as the SGWs age. A thorough assessment of the influence of design variables on treatment practice performance requires control that is only possible in a lab setting. Therefore, we pursued a complementary combination of field monitoring and lab studies in this project.
The research proposed in this study aimed to investigate the following questions:
Part 1 – SGW Hydraulics and Phosphorus Capture
Field Investigation: Do the SGW that are being permitted under the VSMM perform as expected for flow attenuation and phosphorus capture?
Lab-scale Investigation: How do design variables influence SGW flow attenuation and phosphorus capture performance? Design characteristics to be considered include: (a) gravel media sourcing and (b) impermeable wetland “muck” material sourcing.
Part 2 – Effects of Road Salt on SGW Systems
Field Investigation: How is chloride moving through (and being stored) within SGW? Are plants taking up and storing Cl- in the above ground biomass?
Lab-scale Investigation: Is plant productivity and survival impacted by chloride? Do plants assimilate substantial chloride into their biomass?
These questions will be addressed through thorough explanations of methodologies, results, and conclusions over the course of this report.
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Watershed Consulting Associates, LLC and University of Vermont, "Stormwater Subsurface Gravel Wetlands in Vermont: Permitting, Performance, and Chloride Concerns" (2022). Lake Champlain Sea Grant Institute. 6.
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