Floating Treatment Wetlands – evaluation for pollutant removal improvement in cold climate stormwater ponds

Lake Champlain Sea Grant/University of Vermont, City of South Burlington (2018). Floating Treatment Wetlands – evaluation for pollutant removal improvement in cold climate stormwater ponds (Technical Report No. 88). Grand Isle, VT: Lake Champlain Basin Program.

Floating Treatment Wetland (FTW) units were evaluated for their suitability in north-eastern United States conditions to improve the pollutant removal effect of a wet extended detention stormwater basin. A stormwater pond treating runoff from a residential townhouse development was monitored for chemical (TN, TKN, nitrate/nitrite, TP, TDP, TSS) and physical (Dissolved oxygen (DO), pH, and temperature) parameters for one year (2015) prior to FTW installation and two years (2016-2017) with FTW rafts covering 25% of the pond surface. Flow-weighted composite samples at the inlet and outlet structures of the pond resulted in representative measurements of water quality coming into and leaving the ponds. FTW rafts were designed using three layers of Polyflow biological filter material and a two-part marine foam for floatation. Four plant species were selected based on their referenced use in the FTW literature in other areas: Pondeteria cordata (pickerelweed), Schoenoplectus tabernaemontani (Softstem Bulrush), Carex comosa (Longhaired Sedge), Juncus effusus (Common Rush). Plants were evaluated for survivability through a growth season as well as over one winter. Additionally, species’ biomass was measured as an indicator of robustness of growth. The raft material itself was evaluated for damage after a winter to indicate potential challenges in cold, freezing conditions. The plant that performed the best based on survival and biomass production is the Longhaired sedge (Carex comosa). Water quality performance of the pond was compared between 2015 (pre-FTW) and 2017 (post-FTW and with established root zones). Storm size and antecedent dry days did not differ between the years. Water temperature did not differ between years but DO was lower in the post-FTW than in the time prior to FTW installation (p=0.027). Total nitrogen (TN) influent and effluent values were not affected. Total suspended solids (TSS) influent concentrations were consistent between years but the post-FTW period was characterized by greater TSS concentration in the effluent (p=0.015). Total phosphorus (TP) and total dissolved phosphorus (TDP) had variable influent concentrations between pre- and post-FTW period. As a result, those data were analyzed as a percent difference in concentration between in and out. No difference was detected in percent difference of TP or TDP between years.