Field Scale Investigation of Three Floating Treatment Wetland Designs to Improve Water Quality

Floating treatment wetlands (FTWs) are an innovative stormwater control measure (SCM) used to improve water quality in stormwater retention ponds and receiving surface waters. FTWs contain vast hydroponic root networks which enhance pollutant removal through a slew of treatment mechanisms. This study investigates how strategic FTW placement impacts water quality treatment at three field sites across North Carolina, including two stormwater ponds and one man-made lake (namely, Armory Pond, Merrimont Pond, and Greenfield Lake). Prior to FTW installation, pre-retrofit water quality monitoring was performed at Armory Pond over a six-month period in 2021. FTWs were then installed using designs customized to each waterbody to maximize runoff contact with the FTW root systems at minimal surface coverage rates (i.e., < 5%). Ongoing post-retrofit water quality monitoring is being conducted at all sites to examine how FTW placement influences nutrient and sediment removal during storm events. For Armory Pond, six pre-retrofit and ten post-retrofit paired storm events have been analyzed as of August 2022. Post-retrofit treatment efficiencies were 42%, 36%, and 48% for total nitrogen (TN), total phosphorus (TP), and total suspended solids (TSS), respectively. Further, average post-retrofit effluent concentrations at Armory Pond showed reductions of 30% TN, 20% TP, and 17% TSS compared to pre-retrofit concentrations. A total of ten and twelve post-retrofit storm events have also been collected at Merrimont Pond and Greenfield Lake, respectively, between March - September 2022. Once finalized, results from all sites will be used to inform design recommendations on FTW placement and minimum surface coverage required for economically viable surface water quality improvement.