Since the passage of the Clean Water Act, wet detention ponds have been installed across the United States primarily for peak flow mitigation and sediment reduction. However, there are surprisingly minimal data on water quality benefits of these systems. Additionally, tremendous retrofitting pressure is likely to accompany state of North Carolina-implemented watershed rules associated with the Jordan Lake and Fall Lake reservoirs. Municipalities in these watersheds will be required to meet nutrient load reduction targets, necessitating urban stormwater practice retrofits for existing development. Therefore, it is imperative for many cities in central North Carolina to identify cost effective retrofits. One potential retrofit, floating wetland islands, is particularly attractive because they (1) do not require earth moving, (2) eliminate the need for additional land to be dedicated to treatment, and (3) will not detract from the required storage volume required for wet ponds (because they float). Two wet ponds in Durham, NC, were retrofitted with floating wetland islands. Prior to retrofit, both ponds had little to no vegetation growing inside them. After retrofit, one pond's surface area was comprised of 8% floating wetland islands and the second pond had 19% coverage. The two ponds were monitored in a non-retrofitted state from November 2008 to March 2010. Floating wetland islands were then installed in March-April 2010. After vegetation had established on the islands, monitoring recommenced in July 2010. Area-velocity meters and Manning's equation were used to take flow-proportional, composite water quality samples. Sixteen water quality samples were taken prior to the floating island retrofit, and twelve have been collected thus far post-retrofit. Initial results show that mean effluent concentrations of total nitrogen (TN) were reduced at one pond from 1.05 mg/L to 0.61 mg/L from pre- to post-retrofit. The other pond had relatively low effluent concentrations (0.41 mg/L and 0.43 mg/L) both pre- and post- retrofit. Mean TP effluent concentrations were reduced at both wet ponds from pre- to post-retrofit (0.17 mg/L to 0.12 mg/L and 0.11 mg/L to 0.05 mg/L. Additionally, TSS effluent concentrations at both ponds were modestly lower post-retrofit, presumably due to additional hydraulic resistance provided by the floating islands and their hanging roots, which have been measured to be approximately 0.6 m (2 ft). Initial results show some modest nutrient and sediment removal benefits from the addition of floating islands to stormwater detention ponds. Additional benefits which will be determined in the future include wetland vegetation root mass and potential improvements in benthic macroinvertebrate habitat.
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