Abstract:
Nitrogen is widely recognized as a chronic urban stormwater pollutant. In the United
States, wet retention ponds have become widely used to treat urban runoff for quantity
and quality. While wet ponds typically function well for the removal of sediments,
nitrogen removal performance can be inconsistent due to poor design and/or lack of
maintenance. Renovating ponds to improve their nitrogen capture performance, however,
is typically expensive. A relatively untested technology called floating wetlands (FWs)
has been proposed as a sustainable means of improving the nitrogen capture performance
of stormwater wet ponds. The FWs are comprised of an artificial floating island that
supports the hydroponic growth of plants on a pond, lake, or canal. As the plants grow on
the floating island, their roots remove nitrogen directly from the water column and may
trap waterborne sediments. Few studies have been performed on the effectiveness realworld
stormwater systems, however. In this study, the nitrogen and sediment capture
performance of a 50 m2 floating wetland deployed for 137 days on Mason Pond was
investigated. A total of 2684 g of biomass was produced, 3100 g of sediment captured,
and 191 g of nitrogen removed from the pond. Although biomass production was
relatively low (53 g/m2), nitrogen uptake rate by the plants (0.009 g/m2/day) was
comparable to contemporary FW studies. A system model was then developed from the
collected data to simulate nitrogen removal performance of the FW on Mason Pond. The
model was then used to test the nitrogen removal efficiency of the FW over longer
deployment periods and with greater surface area coverage. While the literature suggests
that FWs must cover at least 10-15% of the pond to significantly aid nitrogen removal,
the model suggests only modest nitrogen removal efficiency (~6%) by an FW covering
25% of the surface of Mason Pond. These results may inform municipalities or
developers that are considering the use of FWs on stormwater ponds.
