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Ecological Engineering 54 (2013) 254– 265

Contents lists available at SciVerse ScienceDirect

Ecological Engineering

j o ur nal homep age : www.elsev ier .com/ locate /eco leng

valuation of floating treatment wetlands as retrofits to existing stormwateretention ponds

yan J. Winstona,∗, William F. Hunta, Shawn G. Kennedya, Laura S. Merrimana,acob Chandlerb, David Brownb

Department of Biological and Agricultural Engineering, North Carolina State University, 3100 Faucette Drive, Raleigh, NC 27695, United StatesCity of Durham, NC Public Works Department, Stormwater Services, 101 City Hall Plaza, Durham, NC 27701, United States

r t i c l e i n f o

rticle history:eceived 20 September 2012eceived in revised form8 December 2012ccepted 16 January 2013vailable online 6 March 2013

eywords:etention basinutrientsedimentrbanloating wetland islandtormwater runoffondsetland treatment

itrogenhosphorusemperaturealmonidsroutTW

a b s t r a c t

Thousands of existing wet retention ponds have been built across the United States, primarily for themitigation of peak flow and removal of sediment. These systems struggle to mitigate soluble nutrientloads from urban watersheds. A simple retrofit for improvement of pond performance for nitrogen andphosphorus removal could become popular. Floating treatment wetlands (FTWs), one such retrofit, are ahydroponic system that provides a growing medium for hydrophytic vegetation, which obtain nutrientsfrom the stormwater pond. Installation of FTWs does not require earth moving, eliminates the need foradditional land to be dedicated to treatment, and does not detract from the required storage volumefor wet ponds (because they float). To test whether FTWs reduce nutrients and sediment, two ponds inDurham, NC, were monitored pre- and post-FTW installation. At least 16 events were collected from eachpond during both monitoring periods. The distinguishing characteristic between the two ponds post-retrofit was the fraction of pond surface covered by FTWs; the DOT pond and Museum ponds had 9% and18%, respectively, of their surface area covered by FTWs. A very small fraction of N and P was taken upby wetland plants, with less than 2% and 0.2%, respectively, of plant biomass as N and P. Temperaturemeasurements at three depths below FTWs and at the same depths in open water showed no significantdifference in mean daily temperatures, suggesting little shading benefit from FTWs. The two ponds pro-duced effluent temperatures that exceeded trout health thresholds. Both the pre- and post-FTW retrofitponds performed well from a pollutant removal perspective. One pond had extremely low total nitrogen(TN) effluent concentrations (0.41 mg/L and 0.43 mg/L) during both pre- and post-FTW retrofit periods,respectively. Floating treatment wetlands tended to improve pollutant capture within both ponds, but notalways significantly. Mean effluent concentrations of TN were reduced at the DOT pond from 1.05 mg/L to0.61 mg/L from pre- to post-retrofit. Mean total phosphorus (TP) effluent concentrations were reduced atboth wet ponds from pre- to post-retrofit [0.17 mg/L to 0.12 mg/L (DOT pond) and 0.11 mg/L to 0.05 mg/L(Museum pond)]. The post-retrofit effluent concentrations were similar to those observed for bioreten-

tion cells and constructed stormwater wetlands in North Carolina. The DOT pond showed no significantdifferences between pre- and post-retrofit effluent concentrations for all nine analytes. The Museum pondhad a statistically significant improvement post-retrofit (when compared to the pre-retrofit period) forboth TP and total suspended solids (TSS). Wetland plant root length was measured to be approximately0.75 m, which had the benefit of stilling water flow, thereby increasing sedimentation. Results suggestedthat greater percent coverage of FTWs produced improved pollutant removal.

∗ Corresponding author. Tel.: +1 9195158595; fax: +1 9195156772.E-mail addresses: rjwinsto@ncsu.edu (R.J. Winston), wfhunt@ncsu.edu

W.F. Hunt), sgkenned@ncsu.edu (S.G. Kennedy), lsmerrim@ncsu.eduL.S. Merriman), Jacob.Chandler@durhamnc.gov (J. Chandler),avid.Brown@durhamnc.gov (D. Brown).

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925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ecoleng.2013.01.023

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Urban stormwater threatens waterways by more efficientlyransporting higher loads of anthropogenic pollutants, such asutrients, heavy metals, sediment, pathogens, and hydrocarbons

Hartigan et al., 1983; Schoonover and Lockaby, 2006; Line and

hite, 2007). The additonal volume of stormwater that is conveyedrom impervious surfaces causes stream bank erosion, degrada-ion of aquatic habitat, and loss of real estate (Paul and Meyer,

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otThe 13.07 ha drainage area consisted entirely of roadway and asso-ciated vegetated shoulders, and was 87.7% impervious (Table 1).The surface area of the pond at the permanent pool elevationwas 0.36 ha. The surface area of the forebay was 11.7% of the

R.J. Winston et al. / Ecologica

001; Davies et al., 2010). Federal, state, and local legislation inhe U.S. mandates the use of stormwater control measures (SCMs)o combat these negative consequences of urban growth.

Examples of SCMs include bioretention, permeable pavement,ater harvesting, and infiltration devices, which are often inte-

rated into Low Impact Development (LID) strategies. Since theassage of the Clean Water Act (1972), wet retention basinsave been used to mitigate increased post-construction peak flowates. As such, there are more than 1000 existing wet ponds inorth Carolina alone (Bradley Bennett, head NCDENR stormwa-

er unit, personal communication, June 2012). Considering theiridespread use nationally, a surprising lack of literature exists on

he performance of wet ponds for removal of pollutants (Mallint al., 2002; Jones and Hunt, 2010; Hancock et al., 2010; Gallaghert al., 2011; Wium-Andersen et al., 2011; DeLorenzo et al., 2012),specially for nutrient removal. Mallin et al. (2002) quantifiedutrient removal at three ponds in the North Carolina Coastal Plain.f the three ponds, one showed promising removal of TN, one

howed negligible removal of TN, and TN increased by 50% at thehird pond. TP concentrations were reduced at two of the ponds,nd increased at a third. Nutrient removal from past studies of wetetention ponds has been uncertain and was less reliable than filter-ased SCMs, such as bioretention (Hunt et al., 2008; Davis et al.,009). Field evaluations of wet ponds haveshown between 41 and3 percent removal of TSS (Wu et al., 1996; Greb and Bannerman,997; Mallin et al., 2002). Sediment trapping was a function of

nfluent particle size distribution (Greb and Bannerman, 1997).One pollutant that is often overlooked in SCM studies is water

emperature, which affects benthic macroinvertebrates and fishopulation and reproduction (Bisson and Davis, 1976; Hokansont al., 1977). Specifically, cold-water fishes, such as trout (a mem-er of the salmonid family), prefer maximum stream temperatureselow 21 ◦C and have a lethal 1 h exposure temperature of 25 ◦CCoutant, 1977; Rossi and Hari, 2007). Previous studies on wetonds have shown that they are a source of thermal load ratherhan a sink (Jones and Hunt, 2010). Therefore, methods of reducingffluent temperatures from wet ponds are needed.

With the passage of the Jordan Lake Rules (North Carolinadministrative Code, 2008) and similar rules in other watersheds

e.g. Chesapeake Bay in the USA and Moreton Bay in Australia),utrient loading reduction goals have been set with strict TN andP load limits for new and existing development. Since existingevelopments often have limited space for retrofitting stormwaterractices, methods to improve existing (in-ground) SCM perfor-ance for nutrient removal are crucial. One potential retrofit for

educing nutrients in wet retention ponds is the use of floatingreatment wetlands (FTWs), also referred to as floating wetlandslands (FWIs). FTWs function as hydroponic systems, where plantsnd microbes inhabit a floating mat and take up nutrients as theyrow. Because they shade the water column, FTWs may also have

thermal benefit. A laboratory study of FTWs treating synthetictormwater showed positive removal of Cu, Zn, dissolved reactive

and fine suspended particulates (Tanner and Headley, 2011). In side-by-side test of two ponds (one with no FTWs and one with0% surface area coverage by FTWs), the pond with FTWs exhib-

ted improved performance, with 41%, 40%, 39%, lower effluentvent mean concentrations for TSS, particulate zinc, and partic-late copper, respectively (Borne et al., 2013). FTWs were used toreat aquaculture effluent in Italy, with a corresponding reductionn TP of 65% (De Stefani et al., 2011). A study using FTWs to treataw domestic wastewater showed removal efficiencies of 22–42%

or total ammoniacal nitrogen (TAN), TN, and TP (Van de Moortelt al., 2010). Van de Moortel et al. (2012) reported that decay-ng biomass in mesocosm scale FTWs may provide enough C toenitrify 70–109 g NO3-N m−2 year−1.

Fwm

neering 54 (2013) 254– 265 255

However, field studies have not been completed on FTWs asetrofits to stormwater wet ponds. The primary objective of thisesearch was to assess the impact of adding FTWs to stormwa-er ponds on pollutant removal, temperature, and effluent wateruality.

. Description of sites

Two existing wet retention ponds in Durham, NC, were identi-ed for monitoring during the summer of 2008. Both ponds wereesigned to treat the water quality volume and mitigate peak flowates from the 1-year, 2-year, and 10-year return period stormvents, as required by NC Department of Environment and Nat-ral Resources (NCDENR, 2007) and the City of Durham. As such,hey were designed to have a forebay, to treat the 25.4 mm rainfallvent without overflow, and to draw down to the permanent poollevation in approximately 2 days.

The first wet pond (Fig. 1a) was installed by the NC Departmentf Transportation (hereafter referred to as the “DOT pond”) duringhe expansion of an interchange at US 15-501 and Interstate-85.

ig. 1. (a) (left). Satellite imagery of the DOT pond with twelve floating treatmentetlands. (b) (right). Satellite imagery of the Museum pond with four floating treat-ent wetlands. Photo credit: Google Earth.

256 R.J. Winston et al. / Ecological Engineering 54 (2013) 254– 265

Table 1Characteristics of two wet retention ponds examined in Durham, NC, USA. Location, watershed characteristics, and design features are presented.

Attribute DOT pond Museum pond

Latitude and longitude 36◦1′28.6′′N, 78◦56′39.2′′W 36◦1′37.1′′N, 78◦54′0.5′′WSurface area (ha) 0.36 0.05Drainage area (ha) 13.07 2.37Loading ratio (unitless) 36.4 47.4Watershed imperviousness (%) 87.7% 54.3%Watershed land use Interstate highway Parking lot, maintenance building, picnic areaForebay area (m2) 421 90Average forebay depth (m) 0.83 0.53Wet pond length (m) 91 36Wet pond average width (m) 32 15Length to width ratio 2.84 2.4Mean depth at permanent pool elevation (m) 1.22 0.93Storage volume at permanent pool elevation (m3) 3869 386

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Brink of overflow storage volume (m3) 7993

Brink of emergency spillway storage volume (m3) 9625

otal surface area of the pond. The pond and forebay had nontentionally planted vegetation; invasive creeping water primroseLudwigia hexapetala) did inhabit the pond prior to commencementf monitoring. Throughout the monitoring periods, the pond wasrequented by approximately 20 Canada geese, Branta canadensis.

The second wet pond (Fig. 1b) was located at the North Carolinauseum of Life and Science (hereafter referred to as the “Museum

ond”) and drained a parking lot, a maintenance building, and aicnic area (Table 1). The drainage area was 2.37 ha and 54.3%

mpervious. The wet pond was 0.05 ha in surface area, and the sur-ace area of the forebay was 18% of the total surface area of theond. The forebay was vegetated with a dense mat of cattails (Typha

atifolia). The DOT and Museum ponds, respectively, had somewhatimilar length to width ratios (2.84 and 2.4), forebay depths (0.83 mnd 0.53 m) and depths at permanent pool (1.22 m and 0.93 m).

. Materials and methods

.1. Data collection

At the DOT pond, stormwater entered through a 152 cm rein-orced concrete pipe (RCP) which was partially submerged atermanent pool elevation. An ISCO 750 area velocity meter (Lin-oln, Neb., U.S.) was fixed to the bottom of the pipe to collect flowata. These probes measure water velocity based upon the Dopplerffect and simultaneously measure stage using a pressure trans-ucer. Similar area velocity flow measuring locations were installedt the submerged outlet to the DOT pond (41 cm RCP), and the inleto the Museum pond (61 cm RCP), which was partially submergedt normal pool. At the Museum pond, stage measurements wereade in the freely flowing 61 cm RCP outlet pipe using an ISCO 730

ubbler module. Stage measurements were converted to flow ratesing Manning’s Equation with known values for pipe slope, pipeoughness, and cross-sectional area (Manning, 1891).

Flow measurements were taken on a 2-min interval, which trig-ered automated samplers to collect flow-weighted, compositeater quality samples. At the DOT pond, ISCO Avalanche® refriger-

ted samplers were used, while at the Museum pond ISCO 6712amplers were employed. Sample intake strainers were locatedn an area of well-mixed flow. A minimum of five aliquots wasequired to adequately represent the entire hydrograph (U.S. EPA,002). Samples were stored in a 10 L glass jar inside the sampler.

Storm events were characterized by a minimum antecedent dry

eriod of 6 h and had rainfall depths between 3 and 149 mm. Rain-all data were collected at each site using both a manual rain gaugend a recording, tipping bucket rain gauge (Davis Rain Collector II,ayward, CA, U.S.).

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.2. Laboratory analysis

Water quality samples were delivered to the laboratory within4 h of the end of a rain event. The composite samples wereispensed into 2 L pre-acidified plastic bottles for nutrient and TSSnalysis. Upon collection, all samples were immediately placedn ice and chilled to <4 ◦C. Samples were delivered to the City ofurham’s EPA-certified laboratory and were analyzed using US EPA

1983) and American Public Health Association (APHA) methodsAPHA et al., 1995) (Table 2). Laboratory analysis was performedor total Kjeldahl nitrogen (TKN), nitrate and nitrite (NO2,3-N), TAN,rthophosphate (ortho-P), TP, and TSS. Organic nitrogen (ON) wasalculated as the difference between TKN and TAN. Particle boundhosphorus (PBP) was calculated by subtracting the ortho-P con-entration from the TP concentration. TN was calculated as the sumf TKN and NO2,3-N.

.3. Floating treatment wetlands

The pre-retrofit wet retention basins were monitored for 14onths spanning December 2008 through February 2010. During

his time, sixteen paired samples were obtained from the inlet andutlet of the DOT pond. Sixteen paired samples were also collectedt the inlet and outlet of the Museum pond.

In late March 2010, FTWs were installed as retrofits at bothhe Museum and DOT ponds. At the DOT pond, twelve floatingreatment wetlands were installed, for a surface coverage of 9%;our islands were installed at the Museum pond (surface cov-rage of 18%). Each island had a surface area of approximately3 m2 and was 25 cm thick. The mats were constructed of extrudedlastic woven together, and they float because of injected closed-ell foam and PVC pipes that were internal to the island. TheTWs were supplied by Floating Island International (Shepherd,T, U.S.).The islands had pre-drilled holes (5.75 cm diameter) on 20 cm

enters that were 13 cm deep. These were half-filled with peatoss, and planted with a mixture of Carex stricta (tussock sedge),

uncus effusus (soft rush), Spartina pectinata (prairie cordgrass),corus gramineus (Japanese sweet flag), Pontederia cordata (pick-relweed), Peltandra virginica (arrow arum), Andropogon gerardiibig bluestem), and Hibiscus moscheutos (marsh hibiscus). A totalf 3550 plugs (2.5 cm diameter) were planted during March 2010,r an average of 225 plants per island. Following planting, the

slands were moved into the ponds (Fig. 2a). FTWs were affixed tohe bottom of the ponds using four cinder block anchors attachedo the islands with metal chains. Fencing was installed at theOT pond to prevent the resident Canada goose population from

R.J. Winston et al. / Ecological Engineering 54 (2013) 254– 265 257

Table 2Analytical methods, preservation methods, and laboratory reporting limit for analysis of stormwater pollutants.

Constituent Laboratory testing methods Preservation Laboratory reporting limit (mg/L)

TAN Std method 4500-NH3-D (APHA et al., 1995) H2SO4 (<2 pH), <4 ◦C 0.05TKN Std method 4500-Norg (APHA et al., 1995) H2SO4 (<2 pH), <4 ◦C 0.3NO2,3-N EPA method 300.0 revision 2.1 (U.S. EPA, 1983) H2SO4 (<2 pH), <4 ◦C 0.1TN Calculated as NO2−3-N + TKN N/A N/AON Calculated as TKN – TAN N/A N/AOrtho-P Std method 4500-P-E (APHA et al., 1995) H2SO4 (<2 pH), <4 ◦C 0.03

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TP Std Method 4500-P-E (APHA et al., 1995)

TSS Std method 2540 D (APHA et al., 1995)

ating the plants (Fig. 2b). Plantings were allowed to maturerom April through June 2010 before monitoring recommenced inuly 2010. Plant growth was very vigorous during the first yearFig. 2a–d).

The post-retrofit monitoring period ended in September 2011. total of sixteen and eighteen paired water quality samples were

aken at the DOT pond and the Museum pond, respectively.

.4. Floating treatment wetland plant sampling and analysis

FTW wetland plant sampling was conducted at both ponds.uncus effusus, Carex stricta, Andropogon gerardii, and Hibiscus

oscheutos were the species with the best survivability at both

onds. Pontedaria cordata was also present at the DOT pond, butot found at the Museum. These species were chosen for sampling;hree samples of each species were somewhat randomly sampledrom all the FTWs at each pond (only plants along the perimeter

maO(

ig. 2. (a) (top left). Launching a floating treatment wetland (March 2010). Islands werenstalling goose prevention fencing (March 2010). (c) (bottom left). Growth of plants afte011). (a)–(d) are all photographs of the DOT pond.

N/A N/AH2SO4 (<2 pH), <4 ◦C 0.03<4 ◦C 2.5

f the FTWs were harvestable). Harvesting occurred on October6, 2011, when the plants were nineteen months old, and prior toenescence. The samples were collected via boat and machete. Thehoot base and root biomass which had grown into the mat werexcluded from the analysis, similar to the methods in Tanner andeadley (2011). The samples were dried in a fan-circulated oven at0 ◦C for at least 48 h. Biomass was measured on an analytical scalend reported in dry weight; biomass ratios were calculated as theuotient of above mat biomass to below mat biomass.

The dried tissue was ground and representative subsamplesf both the above mat (shoots) and below mat (roots) biomassere sent to the North Carolina State University Environmen-

al and Agricultural Testing Service Laboratory to be analyzed for

acronutrients. Nitrogen was measured by Dumas combustion,

nd phosphorus and potassium by Inductively Coupled Plasmaptical Emission Spectrometry (ICP-OES) using the Dry Ash Method

Munter et al., 1984).

anchored to bottom of pond using metal cable and cinder blocks. (b) (top right).r four months (July 2010). (d) (bottom right). Plant growth after 13 months (April

258 R.J. Winston et al. / Ecological Engineering 54 (2013) 254– 265

Table 3Rainfall summary statistics during the pre- and post-retrofit monitoring periods for the DOT and Museum ponds. Statistics are presented for both 1) all storm events and 2)sampled storm events during the respective monitoring periods.

Statistic DOT pond Museum pond

Pre-retrofit Post-retrofit Pre-retrofit Post-retrofit

All stormevents

Events sampledfor waterquality

All stormevents

Events sampledfor waterquality

All stormevents

Events sampledfor waterquality

All stormevents

Events sampledfor waterquality

Number 79 16 64 16 74 16 62 1821

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Mean rainfall (mm) 17 29 19

Median rainfall (mm) 10 19 11Rainfall depth range (mm) 3–149 7–149 3–132

.5. Temperature monitoring

In November 2010, temperature sensors were affixed below theenter of one FTW and at a distance of 2 m from the edge of theame FTW at both the DOT and Museum sites. HOBO TMC50-HDemperature sensors were installed at depths of 2.5 cm, 17.5 cm,nd 77.5 cm below the water surface. Temperature measurementsere collected as a function of pond depth every 5 min using HOBO-12 loggers. The two ponds were located 4 km apart, resulting

n similar observed daily air temperatures at each site. Pair-wiseemperature comparisons were made between the open water andhaded (underneath the FTW) sensors at each depth using a paired-test. Mean, maximum, and minimum daily temperatures wereompared to discern differences in open and shaded water tem-eratures as well as temperature stratification by depth.

.6. Statistical analysis

The water quality data were statistically analyzed to compareaired influent and effluent concentrations. Statistical tests wereompleted separately on pre-retrofit and post-retrofit data forach pond. The difference between each set of paired data wasested for normality using four goodness-of-fit tests (Shapiro–Wilk,ramer-von Mises, Anderson–Darling, and Kolmorogov–Smirnov).

f data were normal or log-normal, a paired t-test was per-ormed. Otherwise, a Wilcoxon signed rank test was utilized.imilar methods were used for paired temperature data at a givenepth.

To determine the effects of the FTWs on nutrient and sedimentoncentrations, statistical comparisons between the pre- and post-etrofit data sets were made. These tests were completed using

ilcoxon rank-sum tests to compare influent concentrations pre-nd post-retrofit and effluent concentrations pre- and post-retrofit.

criterion of 95% confidence ( ̨ = 0.05) was used. Statistical anal-ses were performed using the SAS software version 9.1.3 (SASnstitute, 2006). A value of one-half the detection limit was sub-tituted for concentration data that were below the detection limitGilbert, 1987). Each sampling event was considered a replicate fortatistical purposes.

. Results and discussion

.1. Rainfall

During the pre-retrofit monitoring period, 79 and 74 stormvents occurred at the DOT and Museum ponds, respectively. Ofhese, sixteen events were sampled for water quality analysis.

lightly fewer rain events took place during the post retrofitonitoring period, with 64 and 62 events, respectively, at the DOT

nd Museum ponds. Sixteen and eighteen of these were sampledor water quality analysis. Mean and median rainfall depths for

cwcB

17 31 21 2710 24 12 18

2 3–143 6–143 3–96 3–96

ampled storm events were greater than those for all storm eventsuring the monitoring period (Table 3). The range of rainfall depthsas very similar between sampled and non-sampled storm events.

he median monitored water quality storm event was betweenhe 70th and 80th percentile storm rainfall depth calculated from0 years of rainfall data at Raleigh-Durham International Airport,

ocated approximately 24 km from the DOT and Museum pondsBean, 2005).

.2. Pre-FTW retrofit results

.2.1. Effluent concentrationsThe existing DOT wet retention basin performed well for TN,

P, and TSS reduction during the fourteen month pre-retrofit mon-toring period. The inlet concentrations (Table 4) at the DOT pond

ere representative of previous studies on highway runoff in Northarolina (Wu et al., 1998; Winston et al., 2012). Mean concentra-ion reductions for TN, TP, and TSS at the DOT pond were 36%,6%, and 92%. Statistically significant reductions were observedetween influent and effluent concentration at the DOT pond forO2–3-N, PBP, and TSS. The variability in effluent concentration (asetermined by the standard deviation) was less than 50% that ofhe influent for all pollutants except TAN.

McNett et al. (2010) correlated benthic macroinvertebrateealth to in-stream pollutant concentrations for the three ecore-ions in North Carolina (Mountain, Piedmont, and Coastal Plain).he benthic health was rated on a scale from excellent to poor.xcellent and good water quality support intolerant macroinverte-rate species, including mayflies and caddisflies. Median effluentoncentrations for TN, TP, and TSS for the DOT pond were 0.65 mg/L,.13 mg/L, and 26 mg/L. This corresponded to excellent water qual-

ty for TN and fair water quality for TP. The target of 25 mg/Lstablished for TSS by Barrett et al. (2004) was just exceeded.

The Museum pond had lower mean influent concentrations forN and TSS than the DOT pond (Table 4), perhaps due to differencesn watershed composition (parking lot vis-à-vis interstate high-

ay). The mean TN and TP concentrations were very near those ofight asphalt parking lots in North Carolina [mean TN (1.57 mg/L)nd TP (0.19 mg/L)] reported by Passeport and Hunt (2009). Meanoncentration reductions for TN, TP, and TSS at the Museum pondere 59%, 57%, and 89%, all of which were statistically significant.dditionally, TKN, TAN, ON, and PBP concentrations were all signif-

cantly reduced in the Museum wet pond. The treatment providedy the wet pond reduced the variability in effluent concentrationhen compared to that of the influent.

Median TN, TP, and TSS effluent concentrations for the Museumond were 0.40 mg/L, 0.11 mg/L, and 14 mg/L, respectively. This

orresponded to excellent water quality levels for TN and goodater quality for TP (McNett et al., 2010). Additionally, effluent

oncentrations met the 25 mg/L TSS threshold (Barrett et al., 2004).efore retrofitting with floating treatment wetlands, the ponds were

R.J. Winston et al. / Ecological Engineering 54 (2013) 254– 265 259

Table 4Pre-retrofit monitoring period water quality results for the DOT and Museum ponds. Mean, standard deviation, and median concentrations are presented for each of thenine pollutants that were analyzed.

Sampling Locat ion Sta tistic TKN

(mg/ L) NO2-3-N (mg/ L)

TN (mg/ L)

TAN (mg/ L)

ON (mg/ L)

OP (mg/ L)

PBP (mg/ L)

TP (mg/ L)

TSS (mg/ L)

DOT I nlet Median 0.80 0.15 1.05 0.03 0.98 0.06 0.06 0.15 215 Mean ± Standard Deviation

1.43 ±2.2 1

0.20 ±0. 17

1.64 ±2. 21

0.12 ±0. 23

1.50 ±2.0 4

0.14 ±0.2 1

0.13 ±0. 15

0.26 ±0. 33

354 ±365

DOT Out let

Median 0.60 0.05 0.65 0.07 0.63 0.09 0.04 0.13 26 Mean ± Standard Deviation

0.97 ±0. 98

0.08 ±0. 04

1.05 ±0. 97

0.11 ±0. 18

0.93 ±0. 85

0.12 ±0. 10

0.05 ±0. 02

0.17 ±0. 11

30 ±20

Museum Inlet

Median 0.70 0.05 0.80 0.04 0.75 0.09 0.05 0.18 77 Mean ± Standard Deviation

0.88 ±0. 78

0.12 ±0. 16

1.01 ±0. 81

0.10 ±0. 13

0.89 ±0. 79

0.13 ±0. 11

0.13 ±0. 14

0.26 ±0. 20

216 ±249

Museum Out let

Median 0.35 0.05 0.40 0.03 0.34 0.07 0.03 0.11 14 Mean ± Standard Deviation

0.35 ±0. 17

0.06 ±0. 04

0.41 ±0. 19

0.05 ±0. 05

0.34 ±0. 17

0.07 ±0. 05

0.04 ±0. 02

0.11 ±0. 05 24 ±30

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ote: shaded cells of similar color show a statistically significant difference betweenor normal or log-normal data or a Wilcoxon signed rank test for non-normal data.

erforming better than expected, meeting or exceeding most efflu-nt concentration targets.

.3. Post-FTW retrofit results

Mean pollutant concentrations for the fourteen month post-TW retrofit monitoring period are presented in Table 5. During theost-retrofit period, the DOT pond significantly reduced concentra-ions of TKN, NO2–3-N, TN, ON, OP, TP and TSS. Mean concentrationeductions were 48%, 39%, and 78%, respectively for TN, TP, and TSS.edian effluent concentrations of TN, TP, and TSS were 0.60 mg/L,

.13 mg/L, and 18 mg/L, respectively. This corresponded to excel-ent water quality for TN and fair water quality for TP. Additionally,he pond met the target 25 mg/L TSS concentration. The vari-bility of effluent concentrations tended to be less than influent

Toif

able 5ost-retrofit monitoring period water quality results for the DOT and Museum ponds. Mine pollutants that were analyzed.

Sampling Locat ion Sta tistic TKN

(mg/ L) NO2-3-N (mg/ L)

TN (mg/ L)

TAN (mg/ L

DOT I nlet

Median 0.80 0.35 1.25 0.03

Mean ± Sta ndard Deviation

0.84 ±0. 30

0.34 ±0. 21

1.17 ±0. 34

0.11 ±0. 14

DOT Out let

Median 0.55 0.05 0.60 0.03

Mean ± Sta ndard Deviation

0.55 ±0. 34

0.06 ±0. 02

0.61 ±0. 34

0.05 ±0. 04

Museum Inlet

Median 1.15 0.20 1.25 0.24

Mean ± Sta ndard Deviation

3.32 ±4. 72

0.17 ±0. 11

3.49 ±4. 70

1.60 ±3. 15

Museum Out let

Median 0.40 0.05 0.45 0.03

Mean ± Sta ndard Deviation

0.37 ±0. 25

0.06 ±0. 04

0.43 ±0. 26

0.04 ±0. 02

ote: shaded cells of similar color show a statistically significant difference between influor normal or log-normal data or a Wilcoxon signed rank test for non-normal data. Statist

ent and effluent concentrations. Statistical tests utilized were either a paired t-testical significance was set at ̨ = 0.05.

oncentrations for TP and TSS during the post-retrofit monitoringeriod; this was not the case for nitrogen species.

At the Museum pond, concentrations of all nine analytes wereignificantly reduced between the inlet and outlet of the wet ponduring the post-retrofit monitoring period. Three large influentoncentrations (two of which were statistical outliers) for TKNrange 7.2–15.3 mg/L) and TAN (range 3.4–12.3 mg/L) at the inletf the Museum pond were dramatically reduced by the pond,ontributing to statistically significant reduction of these species.hese higher influent concentrations also resulted in large differ-nces between mean and median nitrogen species values (Table 5).he influent TP concentration was also much higher than any

ther monitoring period/pond combination, suggesting that fertil-zation occurred in the watershed. Mean concentration reductionsor TN, TP, and TSS were 88%, 88% and 95%, respectively; these

ean, standard deviation, and median concentrations are presented for each of the

) ON

(mg/ L) OP

(mg/ L) PBP

(mg/ L) TP

(mg/ L) TSS

(mg/ L)

0.78 0.11 0.05 0.18 100

0.72

±0. 29 0.12

±0. 08 0.07

±0. 06 0.19

±0. 10 101 ±70

0.53 0.06 0.04 0.13 18

0.50

±0. 34 0.07

±0. 06 0.05

±0. 04 0.12

±0. 07 22

±19

0.79 0.09 0.09 0.18 67

1.72

±2. 25 0.24

±0. 34 0.17

±0. 19 0.41

±0. 52 252

±551

0.38 0.02 0.02 0.04 6

0.33

±0. 24 0.02

±0. 01 0.03

±0. 05 0.05

±0. 05 13

±17

ent and effluent concentrations. Statistical tests utilized were either a paired t-testical significance was set at ˛ = 0.05.

260 R.J. Winston et al. / Ecological Engineering 54 (2013) 254– 265

Table 6Mean effluent concentrations of wet ponds in comparison to bioretention and stormwater wetlands that were previously studied in North Carolina.

SCM Name SCM Type Reference Mean effluent concentration (mg/L)

TKN(mg/L)

NO2–3-N(mg/L)

TN(mg/L)

TAN(mg/L)

OP(mg/L)

TP(mg/L)

TSS(mg/L)

DOT (pre-retrofit) Wet pond Herein 0.97 0.08 1.05 0.11 0.12 0.17 30DOT (post-retrofit) Wet pond Herein 0.55 0.06 0.61 0.05 0.07 0.12 22Museum (pre-retrofit) Wet pond Herein 0.35 0.06 0.41 0.05 0.07 0.11 24Museum (post-retrofit) Wet pond Herein 0.37 0.06 0.43 0.04 0.02 0.05 13Lakeside Wet pond Wu et al. (1996) 0.59 NM NM NM NM 0.08 7Waterford Wet pond Wu et al. (1996) 0.73 NM NM NM NM 0.11 44Runaway Bay Wet pond Wu et al. (1996) 0.63 NM NM NM NM 0.08 22Ann McCrary Wet pond Mallin et al. (2002)a NM NM 0.65 0.06 0.03 0.05 4Silver Stream Wet pond Mallin et al. (2002)a NM NM 0.51 0.04 0.02 0.06 6Echo Farms Golf Course Wet pond Mallin et al. (2002)a NM NM 0.62 0.08 0.04 0.07 4Hal Marshall Bioretention Hunt et al. (2008) 0.70 0.43 1.14 0.10 NM 0.13 20Graham High School North Bioretention Passeport et al. (2009) 0.57 0.28 0.76 0.10 0.01 0.05 NMGraham High School South Bioretention Passeport et al. (2009) 0.45 0.38 0.76 0.06 0.02 0.06 NMRocky Mount Sand Bioretention Brown and Hunt (2011) 0.82 0.49 1.31 0.07 0.10 0.23 17Rocky Mount Sandy Clay Loam Bioretention Brown and Hunt (2011) 0.31 0.12 0.43 0.06 0.01 0.09 17Mango Creek Large Cell Bioretention Luell et al. (2011) 0.32 0.08 0.40 0.04 NM 0.11 20River Bend Stormwater Wetland Lenhart and Hunt (2011) 0.94 0.17 1.11 0.08 0.09 0.23 41CMS Stormwater Wetland Line et al. (2008)b 0.87 0.13 1.00 0.14 0.13 0.99 18UNC Stormwater Wetland Line et al. (2008)b 0.79 0.15 0.94 0.08 0.01 0.12 31

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a Monthly sampling conducted. All other studies used event-based sampling.b Median effluent concentrations tabulated.

utrient reductions were clearly skewed by elevated influentoncentrations. Median effluent concentrations for TN, TP, andSS were 0.45 mg/L, 0.04 mg/L, and 6 mg/L, respectively. These val-es corresponded to excellent water quality for both TN and TP.he pond easily met the 25 mg/L TSS target effluent concentra-ion. Effluent concentrations from this pond during the post-retrofit

onitoring period (Table 6) were similar to those observed fromioretention cells (Hunt et al., 2008; Davis et al., 2009).

.4. Comparisons between pre-FTW and post-FTW results

.4.1. Effluent concentrationsInfluent concentrations for the post-retrofit period at the DOT

ond were similar to or less than influent concentrations for there-retrofit period. Since lower influent concentrations are moreifficult to reduce (Strecker et al., 2001), statistical tests were runo compare influent concentrations pre- and post-retrofit at thisond; only NO2–3-N concentrations were significantly higher inhe post-retrofit period. This meant that direct comparison of theffluent concentrations at this pond was reasonable. Mean effluentoncentrations at the DOT pond were 0.44 mg/L TN, 0.05 mg/L TP,nd 8 mg/L TSS lower when comparing the post-retrofit period tohe pre-retrofit period (Fig. 3). FTWs appear to have introducedollutant removal mechanisms, such as plant uptake, microbialespiration, pollutant sorption to roots, and hydraulic resistance,eading to lower pollutant concentrations (and therefore loads).

hile mean concentrations of key pollutants were reduced, nonef the reductions were statistically significant. Perhaps greater than% surface coverage by FTWs would be needed for statistical sig-ificance?

An invasive aquatic weed (Creeping Water Primrose, Ludwigiaexapetala) inhabited the DOT pond both pre- and post-retrofit,hich may have affected the results. During the summer (at maxi-um primrose coverage), roughly 25% of the surface area of the

ond was covered by primrose pre-retrofit. The floating islandscted as footholds for the primrose, and 12 months after instal-

ation (during summer) about 60% of the pond surface was covered

ith primrose (Fig. 4). This is a potential confounding factor for theesults from this pond, since from a pollutant/nutrient perspectiverimrose presence may have been beneficial.

fomp

Mean influent concentrations of TN and TP at the Museum pondere 3.4 and 1.6 times higher, respectively, during the post-retrofiteriod when compared to the pre-retrofit period (Fig. 3), perhapsue to fertilizer use. TN and TAN influent concentration betweenre- and post-retrofit monitoring periods were significantly dif-erent. Because influent concentrations of nitrogen species wereot similar, comparisons between the two data sets were moreifficult. With that caveat, mean effluent concentrations from theost-retrofit Museum pond were 0.02 mg/L higher for TN (essen-ially unchanged), 0.06 mg/L lower for TP, and 11 mg/L lower forSS when compared to the pre-retrofit period (Fig. 3). Thesemprovements in TP and TSS effluent concentrations (which weretatistically significant) suggest greater rates of sedimentation per-aps due to increased hydraulic resistance imparted by the islandshemselves and their associated root mats. Additionally, a sig-ificant improvement in OP was observed at the Museum pond,uggesting that plant uptake occurred. The significant reductionsn OP, TP, and TSS at the Museum pond (when comparing pre- toost-retrofit periods) imply that the 18% coverage provided betterreatment of stormwater by the FTWs than 9% coverage.

Mean effluent concentrations from the ponds pre- and post-etrofit were compared against effluent concentrations from otherCMs studied in North Carolina (Table 6). Concentrations of nitro-en species (TKN, TAN, NO2,3-N) were similar to those previouslyeported for ponds (Mallin et al., 2002), bioretention (Hunt et al.,008; Passeport et al., 2009; Luell et al., 2011), and stormwateretlands (Line et al., 2008). For both ponds (except the pre-retrofiteriod at the DOT pond) effluent concentrations of TN were similaro bioretention with internal water storage (Passeport et al., 2009;rown and Hunt, 2011). Mean concentrations of TP from wetonds in North Carolina ranged from 0.05 to 0.17 mg/L, whichere comparable to both bioretention and stormwater wetlands.ean TSS concentrations from the DOT and Museum ponds were

imilar in magnitude to those reported by Wu et al. (1996), butonsiderably higher than Mallin et al. (2002) for wet ponds. Biore-ention and stormwater wetlands appeared to perform similarly

or TSS to the ponds studied herein. The drawback for the usef wet ponds is that they lack an infiltration component, whichay be provided by SCMs such as bioretention and permeable

avement, limiting pollutant load reduction potential.

R.J. Winston et al. / Ecological Engineering 54 (2013) 254– 265 261

F olids (S nds (FT . Pollu

4

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ig. 3. Boxplots of total nitrogen (TN), total phosphorus (TP), and total suspended science (Museum) ponds both pre- and post-retrofit with floating treatment wetlawo monitoring periods (pre- and post-FTW retrofit) are compared in the boxplots

.5. Water temperature results

Water temperature was compared in each pond both belowhe center of an FTW and 2 m from the edge of an FTW, whichas assumed to be representative of open-water temperature. Fiveinute data were aggregated into daily statistics, and then a grandean, median, maximum, minimum or standard deviation was

pplied (data presented in Table 7). Mean and median water tem-erature and variation in diel water temperature were stratified

y depth; each decreased with increasing depth. Median wateremperature at a 2.5 cm depth was approximately 1.2 ◦C warmerhan at a 79 cm depth at both the DOT and Museum ponds, regard-ess of whether the open water or shaded (i.e. below FTW) sensors

ntpp

Fig. 4. Presence of Primrose around banks and growing from floating t

TSS) for the Department of Transportation (DOT) and Durham Museum of Life andTWs). Paired inlet and outlet samples were taken during storm flow at each pond.tant concentrations are presented on a log scale.

ere observed. Little difference existed between summary statis-ics for open and under FTW sensors at a given depth; median and

ean temperatures varied by less than 0.4 ◦C. At a given depth andithin the same pond, no significant differences existed betweenean temperatures or median temperatures, except the 2.5 cm

epth median temperature at the Museum pond. This suggestedhat while shading by FTWs occurred, heat transfer within pondtrata happened quickly and balanced sub-FTW temperature withpen-water temperature. Since temperature measurements were

ot made prior to FTW retrofit, it cannot be inferred as to whetherhe installation of the FTWs decreased mean temperatures of theonds; it appears that the potential for this is quite low unlessercent coverage of FTWs is high.

reatment wetlands at the DOT pond (photo on August 8, 2011).

262 R.J. Winston et al. / Ecological Engineering 54 (2013) 254– 265

Table 7Measured grand mean, median, minimum and standard deviation of pond temperature at various depths both underneath an FTW and in open water. Differences betweensummary statistics of below-FTW and open water temperatures were very small or zero.

Location Temperaturemeasurementdepth

Measurementlocation

Meantemperature(◦C)

Mediantemperature(◦C)

Maximumtemperature(◦C)

Minimumtemperature(◦C)

Temperaturestandarddeviation

DOT

2.5 cm Open 16.5 15.4 44.8 0.7 9.2Under FTW No Data No Data No Data No Data No Data

17.5 cm Open 16.7 15.3 37.0 1.5 9Under FTW 16.7 15.2 39 1.2 9.1

79 cm Open 16.4 14.6 35.6 2.5 8.4Under FTW 16.3 14.7 40.3 3.5 8.3

Museum

2.5 cm Open 15.1 12.1 34.4 0.1 10.1Under FTW 15.1 11.7 35.7 0.6 10.3

17.5 cm Open 14.9 11.3 33.1 1.2 9.6Under FTW 14.9 11.3 34.4 0.9 9.8

79 cm Open 14.3 10.6 31.8 3.8 9.21

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Under FTW 14.3

The value of pond shading by overhanging trees can be observedn Table 7: at solar noon (when the sun is at the highest point inhe sky), the DOT pond had 0% shading by trees, while the Museumond had approximately 30% shading. This resulted in mean andedian daily temperatures of 1.5–4.0 ◦C lower temperatures at theuseum pond for each respective depth. Perhaps this should be

he foremost way of reducing the negative thermal effects of wetonds on urban waterway temperatures (Jones and Hunt, 2010).

n a study of urban stormwater runoff temperature, a shaded par-ing lot showed reduced runoff temperature when compared to annshaded lot, mirroring the results from the ponds in this studyJones et al., 2012).

Exceedance probability for mean daily temperature at theuseum site is plotted in Fig. 5. It can be readily observed that

ittle difference in probable mean temperatures exist between

aww

ig. 5. Exceedance probability for mean daily temperatures at the Museum site from Novnd 79 cm below the water surface. Trout avoidance threshold (21 ◦C) and trout incipient

0.5 32.3 3.7 9.3

pen and under FTW measurements at a given depth. In fact, thetratification in temperature is not substantial except in the “knee”reas (i.e. those less than 20% and greater than 80% exceedancerobability). This graph also further confirms that wet ponds areetrimental to trout (Jones and Hunt, 2010), which prefer streamemperatures below 21 ◦C, and have lethal incipient temperaturesf 25 ◦C (Coutant, 1977). The 21 ◦C threshold was exceeded 30–35%f the time (depending on measurement depth) in the Museumond, which had the lower of the average temperatures of thewo ponds studied (Fig. 5). The incipient lethal temperature wasxceeded by more than one-fifth of the mean daily tempera-ures measured at all depths in the Museum pond. Thermal load

ssociated with stormwater temperatures greater than 21 ◦Cas substantial, and therefore use of wet ponds in trout streamatersheds should be discouraged.

11, 2010 through July 29, 2011. Temperature sensors were located 2.5 cm, 17.5 cm, lethal temperatures (25 ◦C) are shown as vertical lines.

R.J. Winston et al. / Ecological Engineering 54 (2013) 254– 265 263

Table 8Side-by-side comparison of the mean ± standard deviation for biomass (g) and biomass ratio of the harvested plants at the DOT and Museum ponds. Biomass ratio is definedas the ratio of above to below mat plant biomass.

DOT pond Museum pond

Plant species Below matbiomass (g)

Above matbiomass (g)

Biomass ratio Plant species Below matbiomass (g)

Above matbiomass (g)

Biomass ratio

Mean ± standarddeviation

Mean ± standarddeviation

Mean ± standarddeviation

Mean ± standarddeviation

Juncus spp. 45.0 ± 32.5 106.3 ± 59.1 2.4 Juncus spp. 41.7 ± 13.4 66.2 ± 25.0 1.6Carex Stricta 220.5 ± 50.5 191.0 ± 85.5 0.9 Carex Stricta 194.7 ± 54.7 71.7 ± 5.4 0.4Spartina pectinata 65.5 ± 63.5 159.3 ± 138.9 2.4 Spartina pectinata 30.7 ± 17.7 84.0 ± 42.6 2.7

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Hibiscus moscheutos 74.0 ± 36.4 462.8 ± 171.6 6.3

Pontederia cordata 57.7 ± 52.5 71.8 ± 69.5 1.2

.6. Wetland plant study results

The mean plant biomass (g) by species after nineteen months ofrowth are provided in Table 8. It was observed that the above matiomass was greater than the below mat biomass for all speciesxcept Carex, similar to results for four species of plants (Carexirgata, Cyperus ustulatus, Juncus edgariae, and Schoenoplectus taber-aemontani) studied in FTWs during a mesocosm study (Tanner andeadley, 2011). Carex specimens were found to have large, wooly

oot systems (approximately 0.75 meters in length) compared tohe other species. Similar root lengths (0.9–1.35 m) were observedn northeast Italy for plants grown on FTWs (De Stefani et al., 2011)he maximum biomass ratio was 6.3 (Hibiscus) at the DOT pondnd 2.7 (Spartina) at the Museum pond. For the four species thatanner and Headley (2011) studied, biomass ratios varied from.7 to 4.6, on the higher end of those observed in this study. Per-aps this modest departure is due to differences among wetlandlant species, field versus laboratory conditions, or age at time oflant harvest [nineteen months herein and 230 days in Tanner andeadley (2011)].

At the DOT pond, Hibiscus had the largest total mean plantiomass, followed by Carex. Carex had the largest mean plantiomass at the Museum pond. The Museum pond, which had muchreater canopy cover from surrounding trees, had lower meanbove and below mat biomass for all species sampled when com-ared to the DOT pond. No trees were present along the perimeterf the DOT pond. Sunlight is clearly an important factor in biomassccumulation on FTWs. Pontederia and Juncus, respectively, pro-uced the lowest total biomass at the DOT and Museum ponds.

For each species, the plant nutrient concentration patterns wereimilar above and below mat at both ponds (Fig. 6). Similar toast research, nitrogen and potassium concentrations were muchreater in magnitude than phosphorus concentrations for each

tMwl

ig. 6. Mean plant tissue macro-nutrient concentrations in the above and below mat biondicate above mat biomass, while below mat biomass is shown below the abscissa.

Hibiscus moscheutos 43.8 ± 28.6 74.3 ± 54.5 1.7

pecies (Tanner and Headley, 2011). At both ponds, N and P con-entrations above the mat were less than or equal to 1% of plantiomass, while they were between 1 and 2% for the below matiomass. With the exception of Carex at the Museum pond, theelow mat nutrient concentrations were greater than the aboveat concentrations. On the other hand, the four species studied in

anner and Headley (2011) had a higher percentage of N and P inhe above mat biomass. Mean percent potassium concentrations inhe plant tissue were typically 0.5–1.0% both above and below mat.anner and Headley (2011) found above mat K between 1.5–2.5%nd below mat K between 0.7 and 1.6%. The above mat potassiumoncentration (nearly 5%) for Pickerelweed at the DOT pond was aubstantial outlier.

Lenhart et al. (2012) harvested above-ground biomass for fiveetland species (Pontederia cordata, Sagittaria latifolia, Scirpus

yperinus, Saururus cernuus, and Schoenopletus tabernaemontani)rom two constructed stormwater wetlands in North Carolina. Oneetland was approximately six years old while the other was one

ear old. The percent N in above-ground biomass was 0.78% to.63% (depending on species, Lenhart et al., 2012), compared tohat herein for above mat biomass (0.35% to 1.29%). Below-groundiomass percent N varied from 0.39% to 1.94% (Lenhart et al., 2012),ompared to below mat biomass percent N of 0.92% to 2.19%. Thisuggests that wetland plants on FTWs tend to sequester a similarmount of nitrogen in their roots and shoots when compared tomergent plants in constructed stormwater wetlands.

Juncus had the highest plant biomass nitrogen concentrations athe DOT pond with 1.4%, while Hibiscus had the highest %N (0.9%)t the Museum pond. Pontederia contained the most potassium at

he DOT pond (3.2%), while Hibiscus contained the most K at the

useum pond (1.1%). Phosphorus concentration in plant biomassas very minor and did not appreciably vary by plant species or

ocation.

mass for the DOT (at left) and Museum (at right) ponds. Values above the abscissa

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64 R.J. Winston et al. / Ecologica

. Outcomes and conclusions

Two wet retention ponds in Durham, NC, were evaluated forollutant concentrations for fourteen months prior to and afteretrofitting with floating treatment wetlands. The following con-lusions can be drawn from this study:

(1) The pre-retrofit wet retention ponds performed well, reduc-ng TN (36% and 59%), TP (36% and 57%), and TSS (92% and 89%)

ean concentrations. Concentration reductions and effluent wateruality compared favorably to previous field research on wet reten-ion ponds (Wu et al., 1996; Greb and Bannerman, 1997; Mallint al., 2002). Effluent concentrations for each pond met excellentTN) and fair (TP) aquatic standards devised for NC (McNett et al.,010). During the pre-retrofit period, the DOT pond significantlyeduced concentrations of NO2–3-N, PBP, and TSS. The Museumond significantly reduced TKN, TN, TAN, ON, PBP, TP, and TSS con-entrations. Presumably, sedimentation was the primary removalechanism for TSS and sediment-bound pollutants.(2) When comparing the inlet and outlet pollutant concentra-

ions during the post-retrofit period, the DOT pond significantlyeduced concentrations of seven of nine analytes studied (all exceptAN and PBP), while the Museum pond with FTWs significantlyeduced concentrations of all nine pollutants studied. Mean efflu-nt concentrations were 0.44 mg/L, 0.05 mg/L, and 8 mg/L loweror TN, TP, and TSS, respectively, at the DOT pond during the post-etrofit period. Similar results were observed for TP (0.06 mg/L) andSS (11 mg/L) at the Museum pond. Efficiency ratios improved dur-ng the post-retrofit vis-à-vis pre-retrofit period for TN and TP atoth ponds and TSS at the Museum pond. When compared to there-retrofit period, post-FTW retrofit results would suggest that

larger number of pollutant removal mechanisms were providedy the floating treatment wetlands, including increased hydraulicesistance, plant uptake of nutrients, and microbial biofilm devel-pment on wetland plant roots and the island matrix. Theseiofilms could support nutrient removal processes such as nitri-cation, denitrification, and phosphorus adsorption (Tanner andeadley, 2011).

(3) Statistical comparisons between pre- and post-retrofit efflu-nt concentrations showed no significant improvement for all nineollutants that were sampled at the DOT pond. At the Museumond, OP, TP, and TSS effluent concentrations were significantly

ower in the post-retrofit period. This suggested (1) that the ben-fit to pond performance with the addition of FTWs was modestnd (2) that the Museum pond, which had 18% surface area cov-rage, performed better than the DOT pond, which had 9% surfacerea coverage. Ergo, FTW surface area coverage appeared to affectreatment performance.

(4) Temperature monitoring in open water and directly beneathhe FTWs revealed little difference between open water tempera-ure and under FTW water temperature. The shading provided byTWs was insignificant in reducing pond water temperature at vari-us depths. However, shading provided by overhanging trees at theuseum pond resulted in lower water temperatures than the fully

xposed DOT site.(5) Similarly, the disparities in wetland plant biomass between

he two ponds may be attributed to the amount of sunlighteceived. The shaded Museum pond produced less biomass thanhe DOT pond, which received full sunlight throughout the day.

6. The concentrations of N, P, and K were greater in the belowat biomass in comparison to the above mat biomass. FTW plants

ad similar %N concentrations in their roots and shoots to those in

mergent wetland plants in stormwater wetlands in North CarolinaLenhart et al., 2012). Tanner and Headley (2011) found biomassatios of 3.7 to 4.6 for four wetland plants, while biomass ratios inhis study ranged from 0.4 (Carex) to 6.3 (Hibiscus).

J

neering 54 (2013) 254– 265

As this is one of the first field studies of FTWs in the litera-ure, viewing this study’s findings cumulatively suggests that FTWsave the potential to make modest improvements to water quality

or select pollutants. While still speculative, probable reasons forhis are root systems and their impact on flow and gross filtration.t does appear that more substantial water quality benefits willequire a larger proportion of FTW coverage than tested herein.

cknowledgements

The authors acknowledge the U.S. EPA (319h) program admin-stered by the North Carolina Department of Environment andatural Resources and the City of Durham for funding this study.he staff of the City of Durham, N.C., stormwater services was indis-ensable during installation of the floating treatment wetlands..C. Cooperative Extension faculty and N.C. State University gradu-te students assisted with wetland island installation. The authorsppreciate Floating Islands Southeast for their aid with plant selec-ion and installation instructions. The authors are grateful for theater quality analysis performed by the City of Durham wastewa-

er treatment division.

eferences

merican Public Health Association (APHA), American Water Works Association(AWWA), Water Environment Federation (WEF), 1995. Standard Methods forthe Examination of Water and Wastewater, 19th ed. Washington, DC.

arrett, M.E., Lantin, A., Austrheim-Smith, S., 2004. Stormwater pollutant removalin roadside vegetated buffer strips. Transport. Res. Rec. 1890, 129–140.

ean, E.Z., 2005. A Field Study to Evaluate Permeable Pavement Surface InfiltrationRates, Runoff Quantity, Runoff Quantity and Exfiltrate Quality. M.S. thesis, NorthCarolina State University, Raleigh, NC.

isson, P.A., Davis, G.E., 1976. Production of juvenile chinook salmon, Oncorhynchustshawytscha, in a heated stream. NOAA Fish. Bull. 74, 763–774.

orne, K.E., Fassman, E.A., Tanner, C.C., 2013. Floating treatment wetland retrofit toimprove stormwater pond performance for suspended solids, copper and zinc.Ecol. Eng., in press.

rown, R.A., Hunt, W.F., 2011. Underdrain configuration to enhance bioretentionexfiltration to reduce pollutant loads. J. Environ. Eng. 137 (11), 1082–1091.

outant, C.C., 1977. Compilation of temperature preference data. J. Fish. Res. BoardCan. 34, 740–745.

avies, P.J., Wright, I.A., Findlay, S.J., Jonasson, O.J., Burgin, S., 2010. Impact ofurban development on aquatic macroinvertebrates in south eastern Australia:degradation of in-stream habitats and comparison with non-urban streams.Aquat. Ecol. 44, 685–700.

avis, A.P., Hunt, W.F., Traver, R.G., Clar, M., 2009. Bioretention technology: overviewof current practice and future needs. J. Environ. Eng. 135 (3), 109–117.

eLorenzo, M.E., Thompson, B., Cooper, E., Moore, J., Fulton, M.H., 2012. A long-term monitoring study of chlorophyll, microbial contaminants, and pesticidesin a coastal residential stormwater pond and its adjacent tidal creek. Environ.Monit. Assess. 184, 343–359.

e Stefani, G., Tocchetto, D., Salvato, M., Borin, M., 2011. Performance of floatingtreatment wetland for in-stream water amelioration in NE Italy. Hydrobiologia674, 157–167.

allagher, M.T., Snodgrass, J.W., Ownby, D.R., Brand, A.B., Casey, R.E., Lev, S., 2011.Watershed-scale analysis of pollutant distributions in stormwater managementponds. Urban Ecol. 14, 469–484.

reb, S.R., Bannerman, R.T., 1997. Influence of particle size on wet pond effective-ness. Water Environ. Res. 69 (6), 1134–1138.

ilbert, R.O., 1987. Estimating the mean and variance from censored data sets.In: Statistical Methods for Environmental Pollution Monitoring. Van NostrandReinhold, New York, NY, USA, pp. 177–178.

ancock, G.S., Holley, J.W., Chambers, R.M., 2010. A field-based evaluation of wetretention ponds: how effective are they at water quantity control. J. Am. WaterRes. Assoc. 46 (6), 1145–1158.

artigan, J.P., Quasebarth, T.F., Southerland, E., 1983. Calibration of NPS model load-ing factors. J. Environ. Eng. 109, 1259–1272.

okanson, K.E.F., Kleiner, C.F., Thorslund, T.W., 1977. Effects of constant tempera-tures and diel temperature fluctuations on specific growth and mortality ratesand yield of juvenile rainbow trout, Salmogairdneri. J. Fish. Res. Board Can. 34,639–648.

unt, W.F., Smith, J.T., Jadlocki, S.J., Hathaway, J.M., Eubanks, P.R., 2008. Pollutant

removal by a bioretention cell in urban Charlotte, NC. J. Environ. Eng. 134 (5),403–408.

ones, M.P., Hunt, W.F., 2010. Effect of storm-water wetlands and wet pondson runoff temperature in trout sensitive waters. J. Irrig. Drain. Eng. 136 (9),656–661.

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W

R.J. Winston et al. / Ecologica

ones, M.P., Hunt, W.F., Winston, R.J., 2012. Effect of urban catchment compositionon runoff temperature. J. Environ. Eng. 138 (12), 1231–1236.

enhart, H.A., Hunt, W.F., 2011. Evaluating four storm-water performance metricswith a North Carolina coastal plain storm-water wetland. J. Environ. Eng. 137(2), 155–162.

enhart, H.A., Hunt, W.F., Burchell, M.R., 2012. Harvestable nitrogen accumulationfor five stormwater wetland plant species: a trigger for stormwater controlmeasure (SCM) maintenance. J. Environ. Eng. 138 (9), 972–978.

ine, D.E., White, N.M., 2007. Effects of development on runoff and pollutant export.Water Environ. Res. 79 (2), 185–190.

ine, D.E., Jennings, G.D., Shaffer, M.B., Calabria, J., Hunt, W.F., 2008. Evaluating theeffectiveness of two stormwater wetlands in North Carolina. Trans. Am. Soc.Agric. Biol. Eng. 51 (2), 521–528.

uell, S.K., Hunt, W.F., Winston, R.J., 2011. Evaluation of undersized bioretentionstormwater control measures for treatment of highway bridge deck runoff.Water Sci. Technol. 64 (4), 974–979.

allin, M.A., Ensign, S.H., Wheeler, T.L., Mayes, D.B., 2002. Pollutant removal efficacyof three wet detention ponds. J. Environ. Qual. 31, 654–660.

anning, R., 1891. On the flow of water in open channels and pipes. Trans. Inst. CivilEng. Ireland 20, 161–207.

cNett, J.K., Hunt, W.F., Osborne, J.A., 2010. Establishing stormwater BMP evaluationmetrics based upon ambient water quality associated with benthic macro-invertebrate populations. J. Environ. Eng. 136 (5), 535–541.

unter, R.C., Halverson, T.L., Anderson, R.D., 1984. Quality assurance for plant tissueanalysis by ICP-AES. Commun. Soil Sci. Plant Anal. 15 (15), 1285–1322.

orth Carolina Administrative Code, 2008. Jordan Nutrient Strategy. 15 NCAC 02B,Session Law 2009-216, and Session Law 2009-484, Raleigh, NC.

CDENR, 2007. Stormwater best management practices manual, http://portal.ncdenr.org/web/wq/ws/su/bmp-manual (accessed 12.11.11).

asseport, E., Hunt, W.F., 2009. Asphalt parking lot nutrient characterization foreight sites in North Carolina, USA. J. Hydrol. Eng. 14 (4), 352–361.

asseport, E., Hunt, W.F., Line, D.E., Smith, R.A., Brown, R.A., 2009. Field study of theability of two grassed bioretention cells to reduce storm-water runoff pollution.J. Irrig. Drain. Eng. 135 (4), 505–510.

W

W

neering 54 (2013) 254– 265 265

aul, M.J., Meyer, J.L., 2001. Streams in the urban landscape. Annu. Rev. Ecol. Syst.32 (1), 333–365.

ossi, L., Hari, R.E., 2007. Screening procedure to assess the impact of urbanstormwater temperature to populations of brown trout in receiving water.Integr. Environ. Assess. Manage. 3 (3), 383–392.

choonover, J.E., Lockaby, B.G., 2006. Land cover impacts on stream nutrients andfecal coliform in the lower Piedmont of West Georgia. J. Hydrol. 331 (3/4),371–382.

trecker, E.W., Quigley, M.M., Urbonas, B.R., Jones, E.J., Clary, J.K., 2001. Determiningurban storm water BMP effectiveness. J. Water Resour. Plann. Manage. 127 (3),144–149.

anner, C.C., Headley, T.R., 2011. Components of floating emergent macrophytetreatment wetlands influencing removal of stormwater pollutants. Ecol. Eng.37 (3), 474–486.

.S. Environmental Protection Agency (U.S. EPA), 1983. Methods of chemical anal-ysis of water and waste. EPA-600/4-79-020, Cincinnati, Ohio.

.S. EPA, 2002. Urban storm water BMP performance monitoring: a guidance manualfor meeting the national storm water BMP database requirements. EPA-821-B-02-001, Washington, DC.

an de Moortel, A.M.K., Meers, E., De Pauw, N., Tack, F.M.G., 2010. Effects of veg-etation, season and temperature on the removal of pollutants in experimentalfloating treatment wetlands. Water Air Soil Pollut. 212, 281–297.

an de Moortel, A.M.K., Du Laing, G., De Pauw, N., Tack, F.M.G., 2012. The role of thelitter compartment in a constructed floating wetland. Ecol. Eng. 39, 71–80.

inston, R.J., Hunt, W.F., Kennedy, S.G., Wright, J.D., Lauffer, M.S., 2012. Field evalu-ation of stormwater control measures for highway runoff treatment. J. Environ.Eng. 138 (1), 101–111.

ium-Andersen, T., Nielsen, A.H., Hvitved-Jakobsen, T., Vollertsen, J., 2011. Heavymetals, PAHs and toxicity in stormwater wet detention ponds. Water Sci. Tech-

nol. 64 (2), 503–511.

u, J.S., Holman, R.E., Dorney, J.R., 1996. Systematic evaluation of pollutant removalby urban wet detention ponds. J. Environ. Eng. 122 (11), 983–988.

u, J.S., Allan, C.J., Saunders, W.L., Evett, J.B., 1998. Characterization and pollutantloading estimation for highway runoff. J. Environ. Eng. 124 (7), 584–592.