Subsurface horizontal-flow gravel wetlands in Vermont – permitting, performance, and chloride concerns

Poster 200 – Click on poster below to view presentation from author.

Click on poster to view presentation from author.

Nisha Nadkarni

Watershed Consulting Associates

Co-Authors: Andres Torizzo, Marcos Kubow, Eric Ro,y Donna Rizzo

Subsurface horizontal-flow gravel wetlands (SHGW) are water treatment practices that use a saturated bed of gravel and wetland vegetation to filter incoming water and remove pollutants. With the promise of impressive phosphorus (P) removal, the implementation of SHGWs has proliferated among Vermont municipalities to meet impending P control requirements under state stormwater permit applications. The recent surge in SHGW state permit applications is also influenced by the 2017 Vermont Stormwater Management Manual; however, the design specifications (e.g., treatment cell configurations, types of gravel media, and wetland muck composition) for these permits are vague. The impact of these design nuances on practice performance is not well researched. The lack of performance data poses concerns about P and chloride impacts on surface waters in the Lake Champlain Basin. Two systems located in Burlington and Essex Junction, VT are being monitored over a two-year period to evaluate SHGW hydraulic performance, P capture, and chloride dynamics. Automated water sampling units collected composite samples over 20 individual storm events for subsequent water quality analysis. Area-velocity sensors and volumetric weirs measured total flow entering and leaving each system. Year 1 results suggest that flow attenuation is highly variable in each system and is typically influenced by the total rainfall, the presence of baseflow 24 hours before a storm, and the design characteristics of the selected gravel media and wetland muck. The engineered wetland muck used in both systems contributes to a greater export of total P (TP) from both systems than TP influx. Road salt application near the Burlington-based SHGW introduces acute levels of chloride that likely contribute to poor vegetation performance and therefore reduced pollutant removal. The sites will be monitored for a second year to provide further guidance on SHGW design specifications and inform State stormwater regulators.

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All posts are publicly visible after review by site administrator. Students’ responses to posted questions is factored into scoring for the poster competition. Finalists announced May 25 and awards presented May 26, 2021.

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