Comparison of rhodamine WT and bromide in the determination of hydraulic characteristics of constructed wetlands

Comparison of rhodamine WT and bromide in thedetermination of hydraulic characteristics of constructed

wetlands

Angela Yu-Chen Lin a,*, Jean-Francois Debroux b, Jeffrey A. Cunningham a,Martin Reinhard a

a Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305-4020, USAb Kennedy/Jenks Consultants, 622 Folsom Street, San Francisco, CA 94107, USA

Received 26 July 2002; received in revised form 17 January 2003; accepted 30 January 2003

Abstract

Hydraulic tracer tests were performed in the Prado Wetlands, Riverside County, California, USA. The goals of the

tests were (1) to evaluate the suitability of rhodamine WT (RWT) as a tracer for wetlands studies, and (2) to determine

the residence time distribution of the wetlands. The performance of RWT was evaluated by comparing the

breakthrough curve (BTC) of RWT to that of bromide in a pilot-scale test. The BTCs of RWT and bromide indicated

equal results. After the pilot test, a full-scale test was conducted by releasing a RWT pulse at the wetlands inlet and

monitoring for RWT arrival near the wetlands outlet. The BTC indicated 10 and 90% (of the total mass recovered)

breakthrough times of 25 and 112 h, respectively, but these must be considered approximations because only 29% of the

injected RWT mass was recovered. Laboratory experiments suggest irreversible sorption to be the principal loss

mechanism of RWT during transport through the wetlands. RWT is a suitable tracer in wetlands that are relatively

small (less than 1 week residence time) and deep (at least 0.6 m) with limited sediment contact, but RWT yields only

approximate results for the extended wetlands system.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Rhodamine WT; Constructed wetlands; Bromide; Tracer test; Hydraulic characteristics; Residence time distribution;

Treatment wetlands

1. Introduction

Natural and engineered wetlands are emerging

as important treatment systems for wastewater

and agricultural run-off, especially for the removal

of nitrate (Bachand and Horne, 2000; Reilly et al.,

2000), trace metals (Debusk et al., 1996; Kadlec

and Knight, 1996; Webb et al., 1998), pesticides

(Schulz and Peall, 2001), and industrial solvents

(O’Loughlin and Burris, 1999). The factors that

govern contaminant removal in wetlands are

poorly understood, but uptake by plants, sorption* Corresponding author.

E-mail address: [email protected] (A.Y.-C. Lin).

Ecological Engineering 20 (2003) 75�/88

www.elsevier.com/locate/ecoleng

0925-8574/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0925-8574(03)00005-3

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by sediments, microbial degradation and precipi-tation have been implicated. Greater reliance on

wetlands treatment and optimization of existing

wetlands operations require a better understand-

ing of the hydraulic and geochemical factors that

govern contaminant behavior. A precondition for

studying wetlands hydraulics is the availability of

robust tracer methods for assessing the residence

time distribution and mixing conditions.The goals of this study were (1) to evaluate the

use of Rhodamine WT (RWT) as a tracer for

wetlands studies, and (2) to hydraulically charac-

terize the Prado Wetlands (Riverside County,

California) using RWT. The Orange County

Water District (OCWD) diverts a significant (up

to 50%) fraction of the Santa Ana River (SAR) to

the 130-hectare Prado Wetlands. Flow through thewetlands improves the quality of the SAR in that

200 tonnes of nitrate is removed per year (OCWD,

2001). SAR flow is the primary source for

augmenting the Orange County groundwater ba-

sin (OCWD, 1995), and maintaining the basin’s

water quality is one of the primary concerns of the

OCWD.

The suitability of RWT was evaluated in a pilottest by comparing its performance to that of

bromide. Although bromide is a conservative

tracer (Netter and Behrens, 1992; Tanner et al.,

1998), its application for wetlands may be proble-

matic because (1) it is a potential precursor for

disinfection by-products in drinking water sup-

plies, and (2) large additions are necessary if

background concentrations are relatively high(e.g. ca. 0.2 mg/l in the SAR). In addition, on-

site analysis of bromide is difficult. By contrast,

RWT is accepted by regulators and is easily

detectable with a portable fluorescence detector

at concentrations as low as 0.01 mg/l (Wilson,

1968; Smart and Laidlaw, 1977; Kilpatrick and

Wilson, 1982). The interference of background

fluorescence is generally insignificant. RWT haspreviously been used in a mountain stream (Ben-

cala et al., 1983) and for groundwater tracing

(Smart and Smith, 1976; Ptak and Schmid, 1996;

Pang et al., 1998).

RWT is a large aromatic molecule (Fig. 1) with

a maximum fluorescence emission wavelength of

580 nm (Smart and Laidlaw, 1977). The drawback

of RWT as a tracer is that it behaves non-conservatively in certain situations, either by

sorbing to sediments (Smart and Laidlaw, 1977;

Bencala et al., 1983; Trudgill, 1987; Shiau et al.,

1993; Everts and Kanwar, 1994), or by degrading

photochemically (Smart and Laidlaw, 1977; Tai

and Rathbun, 1988) or biologically (Smart and

Laidlaw, 1977). Sorption to vegetation was found

to be relatively unimportant (Turner et al., 1991).Tracer studies with RWT that last longer than a

week may need to consider photochemical decay

(Smart and Laidlaw, 1977). Biological decay is

believed to be negligible in surface water where

microbial populations are relatively low (Smart

and Laidlaw, 1977). Sorptive loss can be signifi-

cant but is difficult to evaluate in field situations.

Except for a study in a tidal marsh (Tarrell, 1997),RWT has not been evaluated as a tracer for

wetlands studies where sediment contact and

sorption by plant material may play a significant

role.

As part of this study, Prado Wetlands sediment

was evaluated to assess the capacity of the sedi-

ments to retard RWT transport and/or to act as an

irreversible sink for RWT. Experimental objectivesof the study were to determine the sorption

isotherm and to evaluate the reversibility of

sorption. In natural environments, RWT sorption

has been shown to depend on initial dye concen-

trations, sediment type, and organic matter con-

tent (Smart and Laidlaw, 1977; Trudgill, 1987;

Everts and Kanwar, 1994). Reported Koc values

(defined as the distribution coefficient, Kd, nor-malized by the organic carbon fraction, foc) are

often on the order of 1000�/4000 ml/g (Sabatini,

1989; Sabatini and Austin, 1991; Trudgill, 1987),

though some studies report apparent values over

10 000 ml/g (Trudgill et al., 1983; Shiau et al.,

1993). Prediction of RWT sorption based on

empirical relationships using the octanol�/water

partitioning coefficient, Kow, and the sedimentorganic fraction, foc, often underpredict the actual

observed RWT sorption (Sabatini and Austin,

1991). It has been speculated that, under certain

conditions, the functional groups of the RWT

molecule (see Fig. 1) may dominate over organic

partitioning as a sorption mechanism (Kasnavia et

al., 1999).

A.Y.-C. Lin et al. / Ecological Engineering 20 (2003) 75�/8876

2. Experimental methods

2.1. Description of study area

The Prado Wetlands (Fig. 2) cover 130 ha of

open surface water and were originally built for

duck hunting. In 1992, the OCWD began studying

the efficiency of the Prado Wetlands for nitrate

removal (OCWD, 1995), and in 1998, the wetlandswere redesigned to improve their performance for

that function.

The Prado Wetlands consist of four subsections

that each consists of inter-connected cells (total of

46 cells). The four subsections are termed East

Cells, North Cells, South Cells, and West Cells,

which cover 22, 26, 28 and 24% of the total area,

respectively. The flow through the wetlands is

indicated schematically in Fig. 2. Each subsection

is comprised of six to 18 cells (OCWD, 1995), and

groups of two to three cells are connected in

parallel or series. At the inlet of the East Cells, theflow is split into two separate streams, with 40%

eventually entering the North Conveyance Chan-

nel and 60% eventually entering the South Con-

veyance Channel. From the Conveyance

Channels, the flow is subdivided into a total of

ten streams, each carrying approximately 10% of

the total flow. Each 10%-portion is fed into a cell

that subsequently feeds into one or two additional

downstream cells. The flow recombines in the

Fig. 2. Map of 130-ha Prado Wetlands, with four subsections that each consist of inter-connected cells. As much as 50% of the SAR

flow is diverted for the wetlands treatment, and is entering the wetlands from the upper right and exiting at bottom left.

Fig. 1. Chemical structure of RWT. Carboxylic groups are

deprotonated at neutral pH. Carboxylic groups are shown in

the meta position.

A.Y.-C. Lin et al. / Ecological Engineering 20 (2003) 75�/88 77

Cattail Channel and then passes through the WestCells with a relative short residence time into

Chino Creek. The average water depth in the cells

is approximately 0.6�/0.76 m, but deeper troughs

and shallow bars are present in some places. The

design flow rate is 2.83 m3/s, and the theoretical

mean residence time through the entire wetlands at

that flow rate is approximately 100 h (based on an

average water depth of 0.76 m).Vegetation in the wetlands is primarily bulrush

(Scripus spp.) and cattails (Typha spp.). Other

plants that may be present include smartweed

(Polygonum spp.), bushy pond weed (Najas mar-

ina ), horned pond weed (Zannichellia palustris ),

and duckweed (Lemna spp.). Plants are harvested

every 3 years or more frequently if necessary.

During normal operation, SAR flow enteringthe wetlands is predominantly tertiary treated

effluent from upstream municipal wastewater

treatment plants. As much as 50% of the SAR

flow is diverted for the wetlands treatment.

Approximately 330 tonnes of nitrate are removed

during wetland treatment annually (Bachand and

Horne, 2000), and during the warm summer

months the nitrate concentration is reduced fromnear 10 mg/l N to less than 1 mg/l (OCWD, 2001).

Cells N6 and N7, two serially connected cells

that are part of the North Cell group, were selected

for the pilot study. Cells N6 and N7 are relatively

large, each covering approximately 3.6 ha. The

water depth in the cells is approximately 0.6�/0.76

m, so the volume of water in each cell is

approximately 22 000�/28 000 m3. Typically, theflow rate through these two cells is maintained at

250�/280 l/s, resulting in a theoretical residence

time of 44�/60 h for the two cells combined. Water

enters N6 through a weir and is dispersed by

transverse mixing inducers. From N6, the flow

enters N7 by a single pipe and exits N7 via a single

weir. Cell width is approximately 140 m, or about

25% of the distance of the combined length of bothcells (520 m).

2.2. Tracer study design

2.2.1. Pilot test

The pilot test was initiated on 22 September

2000. In a 210-l drum, 20.8 kg of bromide (27 kg of

sodium bromide, Fisher) and 1.04 kg of RWT (3.8

l of 21.39/2.5% liquid, Turner Design) were added

to water from the North Conveyance Channel.

The solution was mixed for 30 min with a stainless

steel mechanical mixer. The quantities of bromide

and RWT added were designed to allow detection

above baseline at 95% total mass recovery. The

assumptions and mathematical justification for

selecting the tracer quantities are detailed in the

Appendix A.The 210-l tracer solution was pumped into the

inflow of N6 at a flow rate of approximately 0.32 l/

s. Adding the solution was completed within 15

min and contributed less than 0.1% of the total

flow into Cell N6. The historical data showed that

the average background bromide concentration

and standard deviation (S.D.) at the wetlands

influent is 2009/15 mg/l (OCWD, 1998�/2000).

Two samples taken 4 days prior to the pilot test

indicated background concentrations of 198 and

160 mg/l.

Bromide analysis was performed using ion

chromatography at Stanford University after all

samples were collected. For bromide analyses, 10

ml aliquots were transferred to centrifuge tubes

and centrifuged at 3000 rpm for 30 min. Samples

of supernatant (2 ml) were then transferred to an

analysis vial and capped. Ion chromatography was

performed using a Dionex Ion Pac AS9-HC (4�/

250 mm) column with flow rate of 1.5 ml/min and

bicarbonate eluent (9.0 mM).

RWT was analyzed by a continuous field

fluorometer and in grab samples taken at regular

intervals. The continuous field fluorometer

(Turner Designs 10-AU-005-CE Field Fluorom-

eter) was connected to a peristaltic pump that

pumped water continuously from the center of the

weir at the exit of N7 through the flow-through

fluorometer cell. Concentrations were measured

and logged at 10-min intervals. RWT concentra-

tions were internally temperature-adjusted by the

field fluorometer. Grab samples were stored in the

absence of light at 4 8C. Grab samples were taken

at the exit of N6 and N7 and were analyzed for

both bromide and RWT after the completion of

the pilot tracer test. A pH correction was not

necessary because RWT fluorescence is relatively


independent of pH for pH�/5.0 (Smart and Laid-law, 1977).

2.2.2. Full-scale test

The full-scale test was initiated on 16 October

2000. For the full-scale test, the fluorometer was

installed at the exit of the Cattail Channel where

the flow recombines after passing through the

East, North, and South Cell groups (Fig. 2). TheWest Cells were not included because the flow

splits at the entrance of these cells and discharges

directly into Chino Creek at eight different outlets

(two outlets for each cell). Combined, the East,

North, and South Cells provide almost 90% of the

total wetlands residence time, and measuring the

residence time at the end of the Cattail Channel is,

therefore, deemed to be representative of the entirewetlands.

The amount of RWT tracer needed for the full-

scale test was determined using the same criteria

used in the pilot test, as described in the Appendix

A. The baseline for 95% mass recovery was

calculated by using the mean background RWT

concentrations plus three times its S.D. from the

results of the pilot test. The baseline value was0.124 mg/l in both tests. In the full-scale test, the

total amount of RWT added was 5.72 kg (5.5

times more than in the pilot scale test), and the

total flow rate was ten times higher than in cells

N6 and N7. Every time a set of grab samples was

taken, flow rates through the cells were determined

by reading the water level in the Conveyance

Channel. Grab samples were analyzed using thefield fluorometer, which had been calibrated and

configured for grab sample analysis.

2.3. Laboratory sorption studies

Water for the sorption experiments was col-

lected at the exit of cell N7 and was stored at 4 8Cuntil used. Two sediment samples with signifi-

cantly different organic carbon (OC) content(expressed as the organic carbon fraction, foc)

were used; one was sampled from the top 5 cm

of sediment at the exit of N7, and the other from

the top 5 cm of sediment at the end of the Cattail

Channel. Prior to use, samples were air-dried in

pans for 9 days. The sediments were then segre-

gated according to size by sieving. Although thewetland ponds were covered with detritus and

other organic matter, little detritus remained in the

soil samples as a result of sieving. The effect of

plant detritus on RWT sorption was studied with

detritus separated from sediment samples. The foc

of the N7 and Cattail sediment samples are 45 and

24%, respectively. The fraction of organic carbon

is determined by the difference between totalcarbon and carbonate (inorganic) carbon. Total

carbon is determined by high temperature com-

bustion and carbonate carbon is determined by

acid evolution/coulometry (analyses by Huffmann

Laboratories, Golden, CO).

The sorption study was executed in three phases.

First, kinetic experiments were conducted to

determine the amount of time required to reachequilibrium and to study the percent uptake of

RWT by the sediments. Then, the isotherm curves

were developed to establish the linearity of RWT

sorption at concentrations ranging from 1 to 20

mg/l in the different sediment/water systems.

Lastly, the reversibility of sorption was evaluated

by measuring desorption.

2.3.1. Kinetic uptake experiments

Kinetic uptake experiments were conducted

with site water in 1 l-glass bottles. Conditions

and sediment ratios are indicated in Table 1. Initial

RWT concentrations were 20 and 7 mg/l, and

sediment-to-water ratios were 1/10 and 1/5 (mass/

mass). Concentrations were selected to simulate

conditions of the pilot and full-scale tests. In the

batch with plant detritus, the sediment-to-waterratio was 1/100. The abiotic control contained

0.11% (v/v) formaldehyde (Lee et al., 1992). Two

controls without sediment were included, contain-

ing spiked site water (20 and 7 mg/l). An aliquot of

stock solution (1 mg/l RWT) was added to each

bottle to obtain the desired initial RWT concen-

tration. Bottles were wrapped with aluminum foil

to prevent photodegradation and shaken continu-ously on a shaker table (at 1.8 rpm) at room

temperature. At seven predetermined intervals, 8-

ml aliquots were removed, allowed to settle for 15

min, and centrifuged for 30 min. Then, after 40

min equilibration at room temperature, RWT

concentrations were determined using a Turner


Designs TD700 fluorometer. The fluorometer was

calibrated with a 10 mg/l RWT solution. Centri-

fuged wetlands water served as the blank sample.

2.3.2. Isotherm experiments

Three sets of batch-equilibrium isotherms were

run for 10 days in 15 ml centrifuge tubes. Each set

consisted of four centrifuge tubes with different

initial RWT concentrations (1, 2, 7, and 20 mg/l).

Sets 1 and 2 contained sediment from Cell N7, and

Set 3 contained sediment from the Cattail channel.

Set 1 was run with a sediment-to-water ratio of 1/

10, and Sets 2 and 3 were run with sediment-to-

water ratios of 1/5. Set 3 was run in duplicate to

evaluate uncertainty and repeatability of the

experiment. The duplicate experiments yielded

results within 6% of each other.

2.3.3. Desorption experiments

After 10 days of equilibration time, the kinetic

uptake bottles were used to study RWT desorption

according to the following procedure. The super-

natant was removed to the extent possible, and the

1 l glass bottles were refilled with fresh wetlands

water. The amount of solid sediment that was lost

during decanting and refilling was insignificant

(B/0.5%). Bottles were then equilibrated and

analyzed as described above. A total of five

batches were examined with different initial sorbed

dye concentrations, sediment-to-water ratios, and

soil contents.

2.4. Photodegradation study

A preliminary photodegradation experiment

was performed as a control to approximate the

photochemical loss of RWT. Three vials of pilot

tracer test solution were used to prepare 2 l of 26.5

mg/l RWT solution. The sample was exposed to

ambient light for total experimental time of 166 h(including night time) in a 2 l bottle during a sunny

week in Palo Alto, California. Grab samples of 40

ml each were taken at 12-h intervals and were

wrapped with foil and stored at 4 8C prior to

measurement.

3. Results and discussion

3.1. Tracer test results

3.1.1. Pilot test

Fig. 3 depicts the flow rate during the pilot test,

and Table 2 summarizes the general conditions

and residence time distributions of the two tracer

tests. During the pilot test (151 h), the flow rate

varied between 345 and 79 l/s. For the first 60 h,

the flow rate was relatively constant (in the range

of 310 l/s), and then it steadily decreased to 85 l/s

at 100 h before it increased again to 140 l/s. Thetime-averaged flow rate during the pilot test was

approximately 210 l/s, resulting in a theoretical

residence time of 75 h through Cells N6 and N7

(based on an average depth of 0.76 m).

Fig. 4 shows the RWT and bromide BTCs for

cell N7. The curves are normalized so that areas

Table 1

Experimental conditions for nine batch systems for kinetic uptake experiments

Batch # Initial RWT concentration (mg/l) Sediment location Sediment-to-water ratio (mass/mass)

1 20 N7 1/5

2 20 N7 1/10

3 7 N7 1/5

4 7 N7 1/10

5 7 Cattail Channel 1/5

6 Abiotic 20 N7 1/5

7 Plant detritus 20 N6/Cattail Channel 1/100

8 Control I 20 �/ Site water only

9 Control II 7 �/ Site water only


underneath the curves represent fractional mass

recovery. The response to the tracer pulses in-

dicates 10% breakthrough at 34 h and 90%

breakthrough at 79 h. RWT and bromide yielded

essentially identical results. The mean residence

times, calculated using first moment analysis from

the RWT and bromide BTCs, were 53 and 55 h,

respectively. The difference is less than 4% and

within the uncertainty of experiment. These results

suggest that, for tests at the scale of two wetland

cells (3.6 ha, 0.76 m, and 6 days of residence time),

RWT is an adequate tracer. The factors that are

known to contribute to non-ideal RWT behavior

(sorption by sediment and plant materials, expo-

sure to sunlight, and biological activity) were

relatively insignificant and did not adversely affect

RWT performance.

Fig. 5 shows the cumulative mass recoveries at

the exits of cells N6 and N7. The data indicate

RWT recovery of 90�/100% at the exit of N6 and

Fig. 3. Pilot test: flow rate and RWT concentrations at exit of N6 and N7.

Table 2

Summary of tracer test results

Tracer Pilot test Full-scale test RWT

RWT Bromide

Flow rate range (l/s) 79�/345 2890�/3480

Time weighted average flow rate (l/s) 210 3170

Total run time (h) 151 237

Theoretical retention time (h) 75a 69b

Mean residence time (h) 53 55 60

Percentage mass recovery at N6 90�/100 100 29c

Percentage mass recovery at N7 59 85

Time for 10% breakthrough (h) 34 34 25

Time for 90% breakthrough (h) 79 78 112

a Calculated from using water depth of 0.76 m and time averaged flow rate of 210 l/s.b Calculated from using water depth of 0.76 m and time averaged flow rate of 3170 l/s.c Mass recovery at exit of Cattail Channel.


59% at the exit of N7. Bromide recoveries were

higher, 100% at the exit of cell N6 and 85% at the

exit of cell N7. The 15% bromide loss after N7 is

unexplained but is possibly within the experimen-

tal uncertainty of the test. Bromide was still eluting

at concentrations above background at 150 h

when the experiment was terminated, i.e. a bro-

mide ‘tail’ extended beyond the 6-day experiment.

Also, variations in the background bromide con-

centration (which are unknown) during the 6-day

experiment make it impossible to precisely account

for the bromide mass. RWT data from grab

samples are not shown but are consistent with

the flow-through fluorometer data. Despite the

loss of 41% of the RWT mass, the consistency

between the residence time distributions indicated

Fig. 4. Pilot test result: comparison of the RWT and bromide BTCs for cell N7. The curves are normalized so that areas underneath

represent fractional mass recovery. RWT and bromide yielded essentially identical results.

Fig. 5. Cumulative percent mass recovery for RWT and bromide tracers for cells N6 and N7. Symbols are for grab samples; solid line

is for continuous flow-through data.


by RWT and bromide leads us to conclude thatRWT is a suitable tracer for short-term tracer

tests.

3.1.2. Full-scale test

Fig. 6 shows the BTC of RWT for the full-scale

test, and data are summarized in Table 2. The test

lasted for 237 h and was discontinued when the

measured RWT concentrations reached the base-line level. During the test, the average flow rate

was 3170 l/s, and the flow rate varied within a

relatively narrow range (2890�/3480 l/s). Based on

an average flow rate of 3170 l/s and an average

water depth of 0.76 m in the cells, with injection

and detection points as shown in Fig. 2, the

theoretical residence time is approximately 69 h.

The RWT mass recovery was 29%, and the 10 and90% breakthrough of the recovered RWT oc-

curred at 25 and 112 h, respectively. The mean

residence time calculated based on first-moment

analysis of the BTC was 60 h, 9 h less than the

theoretical hydraulic residence time. The BTC

shows two peaks, presumably indicating two

different flow paths. The two peaks are suspected

to result from flow through the North and SouthConveyance Channels, each with a different resi-

dence time distribution. The peak from the South

Cells appears first.

The RWT mass lost was greater in the full-scale

test (71%) than in the pilot test (41%). Time-

dependent loss (for instance, by first order decay)

leads to an underestimation of breakthrough

times. Therefore, the 10 and 90% breakthrough

times determined from RWT represent lower-

bound estimates. The actual 10 and 90% break-

through times might be longer than the estimated

values of 25 and 112 h.

The increased mass loss (71% in the full-scale

test as opposed to 41% in the pilot test) is

tentatively attributed to the increased sediment-

water contact during flow in the channels. La-

boratory data indicate that exposure to sunlight

and biological activity are relatively insignificant.

For instance, during the 7-day laboratory photo-

degradation study, RWT values dropped from 6 to

5.6 mg/l, less than 10%, suggesting photodegrada-

tion is relatively unimportant. The fact that the

water depth was only about 0.2 m in the Cattail

Channel and in the North and South conveyance

channels (compared with 0.76 m in the wetland

cells) may have contributed to increased sediment

contact during the full-scale test, and, therefore, to

increased RWT loss. Furthermore, since the RWT

mass loss in the full-scale wetlands test was higher

than that in the pilot-scale test, in spite of the

nearly equal residence time, it is concluded that

significant mass loss occurs during the relatively

Fig. 6. Full-scale test result. The RWT mass recovery was 29% with mean residence time of 60 h. The two peaks are suspected to result

from the different residence time distributions of the flow through the North and South Conveyance Channels.


short residence time in the East group of cells andin the Conveyance Channels. As noted above,

water levels in the channels are very shallow, and

sediment contact is more intensive.

Despite the fact that 71% of the RWT mass was

not accounted for, the RWT tracer test provided

useful data. Because RWT sorption is relatively

rapid and, for the most part, irreversible (only 10%

of sorption was reversible in laboratory experi-ments), retarded transport is presumed to be

relatively insignificant. A retarded response would

falsely indicate longer-than-actual residence times.

However, because reversible sorption is insignif-

icant, the apparent 10 and 90% breakthrough

times of RWT represent lower-bound estimates

of the true breakthrough times.

The early response and the long tail probablyindicate short-circuiting and the existence of dead

zones. Dead zones may be located in the corners of

cells. Vegetation has been reported to enhance

dead-zone effects (Andradottir and Nepf, 2000).

3.2. Sorption experiments

Fig. 7 shows the approach to equilibrium for the

kinetic uptake experiments at seven different

conditions. The data indicate that equilibrium

was reached in about 50 h in all cases. Initial

concentrations and sediment-to-water ratios were

such that 57�/80% of the RWT was sorbed during

this time. The experiment shows that sediment

from Cattail Channel sorbed 11% less than that of

cell N7 (cf. lines 3 and 5 on Fig. 7). The experiment

with plant detritus indicates the high sorption

capacity of plant material for RWT. The small

amount of plant detritus (1/100 detritus-to-water

ratio) sorbed more than 80% of RWT in 10 days.

Varying the initial concentrations did not affect

the fractional uptake, indicating that sorption is

essentially linear within the concentration range

studied. In the biotic batches, removals were only

slightly higher (ca. 10%) than in the abiotic batch,

indicating that biological transformation is slow,

and presumably not a significant factor in the field

tests.

Fig. 8 shows isotherm data obtained after 10

days of equilibration. In the aqueous concentra-

tion range of 1�/10 mg/l, RWT adsorption was

essentially linear, consistent with data reported by

Shiau et al. (1993), who have shown linear

isotherms up to 100 mg/l. Deviation from linearity

was reported at concentrations greater that 100 mg/

l (Shiau et al., 1993). The two isotherms with N7

sediment are nearly identical, indicating that

Fig. 7. Kinetic uptake experiments: approaches to equilibrium for seven different batch conditions, which test the effects of sediment

characteristics, amount of sediment contact, initial RWT concentrations and bacteria have on the RWT sorption. The data indicate

that equilibrium was reached in less than 50 h in all cases.


sediment-to-water ratio does not affect the iso-

therm. The distribution coefficient, Kd, derived

from the slope shown in Fig. 4 is 15 l/g for

sediment from cell N7, and 6 l/g for sediment

from the Cattail Channel. The higher Kd value of

sediment from N7 is tentatively attributed to its

higher organic carbon content (45% for N7 sedi-

ment vs. 24% for Cattail channel sediment).

The observed values of Kd imply Koc values

(calculated as Kd/foc) of 33 000 and 25 000 ml/g,

respectively, for sediments from cell N7 and the

Cattail Channel. These Koc values are the same

order of magnitude as those reported by some

previous researchers (Trudgill et al., 1983; Shiau et

al., 1993). However, these values are much higher

than would be predicted from empirical relation-

ships based on the octanol-water partition coeffi-

cient, Kow, which is approximately 0.05 for RWT

(Everts and Kanwar, 1994). The surprisingly high

values of Koc could be an indication that organic

partitioning is only a minor component of overall

sorption, and that other mechanisms might be

dominating RWT sorption. This is consistent with

previous hypotheses that, because of RWT’s func-

tional groups (Fig. 1), ionic interactions with

surface sites may dominate over organic partition-

ing as a sorption mechanism (Shiau et al., 1993;

Kasnavia et al., 1999).

Fig. 9 shows the results of the desorption

experiment. The desorption experiment was termi-nated after 12 days, by which time the aqueous

RWT concentration had apparently reached a

constant value. The data indicate that only about

10% of the sorption was reversible for all five

different batch systems. Therefore, sorption of

RWT onto sediment was mainly an irreversible

process within the time scales of the tracer tests.

4. Conclusions

1) In the pilot test, which covered two adjacent

0.76 m deep wetland cells, mass balances for

RWT and bromide were 59 and 85%, respec-

tively, and the breakthrough responses for

RWT and bromide were similar. The residencetime distributions predicted by RWT and

bromide, defined as the 10 and 90% break-

through points, were essentially equal for both

tracers. The apparent mean residence time

defined by a first moment analysis is 55 h for

bromide and 53 h for RWT. The difference of

Fig. 8. Adsorption isotherm for three different batch conditions. RWT adsorption was linear in the aqueous concentration range of 1�/

10 mg/l. The amount of RWT sorbed is directly proportional to the amount of sediment contacted and is also greatly influenced by

different sediment characteristics.


2 h (B/4%) is within the margin of the

experimental error. Based on these results,

we conclude that RWT is a suitable tracer for

small wetland systems where residence times

are less than approximately 6 days, the water

is relatively deep, and, therefore, sediment

contact is limited.

2) In the full-scale test, the RWT mass balance

was 29%, and the lower limits for 10 and 90%

breakthrough (of the total mass recovered)

were 25 and 112 h, respectively. Actual values

could be significantly higher. Detailed infor-

mation about RWT loss processes is needed to

provide a better interpretation of the data.

The mean residence time defined by a first

moment analysis was 60 h, which is lower than

the theoretical retention time of 69 h under the

conditions of the test. The significant mass

loss in the full-scale test (71%) is tentatively

attributed to irreversible RWT sorption on

sediments.3) Laboratory data indicated that sorption is

largely irreversible. Only 10% of the RWT

sorption was reversible. Biological and photo-

chemical transformations appear to be minor

factors contributing to RWT losses. Prelimin-

ary laboratory experiments aimed at simulat-

ing field conditions suggest losses due to

biodegradation and photochemical transfor-

mation to be less than 10% within 10 and 7

days, respectively.

4) Apparent Koc values indicate the possibility of

the RWT molecule’s functional groups dom-inating over organic partitioning as a sorption

mechanism. Important parameters influencing

the RWT sorption include the sediment-to-

water ratio and the characteristics of the

sediment. The amount of RWT sorbed in-

creases linearly with the amount of sediment

contacted.

5) RWT can be used reliably as a tracer forwetlands systems that are relatively small (less

than 1 week residence time) and deep (at least

0.6 m) with limited sediment contact. For

extended wetlands systems, RWT break-

through data provide lower limits of actual

breakthrough times.

Acknowledgements

This research was funded by Orange CountyWater District (OCWD) located in Fountain

Valley, California. We are grateful for their

support and assistance in field wetland research.

Specially, we thank the staff of the OCWD,

especially Gerry Bischof, Katherine O’Connor,

Patrick Tennant, and Greg Woodside. We also

Fig. 9. Desorption experiment: aqueous RWT concentrations for five different batch conditions. Adsorption of RWT onto sediment

was mainly an irreversible process (with only about 10% of the reversible sorption) in the time scale of 12 days.


thank Tara Schraga and Carry Lopez from USGSlocated in Menlo Park, California, for assisting

and letting us use their fluorometer for sorption

experiments. Any opinions, findings, conclusions,

or recommendations expressed in this publication

are those of the authors and do not necessarily

reflect the views of OCWD or USGS.

Appendix A: Basis for tracer amounts

Prior to initiating the tracer tests, we estimated

the tracer amounts that would be required, ac-

cording to the following criteria.

1) The tracer tests should last until at least 95%

of the injected tracer mass has eluted.2) At the time of 95% bromide elution, the

bromide concentration should be at least two

S.D.s above the background concentration.

OCWD reported background concentrations

of 200 mg/l with a S.D. of 15 mg/l.

3) At the time of 95% RWT elution, the RWT

concentration should be within the dynamic

range of the fluorometer and an order ofmagnitude greater than the background con-

centration.

Tracer amounts to fulfill these criteria were

determined by the following analysis. We assume

that the tracer enters and leaves a wetlands cell at a

single point on each end. The cells are significantly

longer than they are wide, and transverse mixing

inducers are located at the beginning of the flow

path in each cell. Therefore, transverse and verticalmixing are assumed to be rapid and complete.

Under these conditions, the transport of tracer

through a wetlands cell can be described with a

one-dimensional advection-dispersion equation:

@C(x; t)

@t�D

@2C(x; t)

@x2�U

@C(x; t)

@x

where C (x , t) is the tracer concentration as a

function of longitudinal position (x ) and time (t);

D is a longitudinal dispersion coefficient; and U is

the longitudinal water velocity through the wet-

lands cell.

The following initial and boundary conditionswere employed as approximations to the tracer test

conditions.

1) Wetlands cell initially devoid of tracer

C(x; t�0)�0 for x�0

2) Tracer added as a rapid pulse at the cell inlet

C(x�0; t)�M

Ad(t)

where M is the mass of tracer added, A is the

cross-sectional area of the wetlands cell, and

d (t ) is a Dirac delta function.3) At some point far downstream, the tracer

concentration is equal to background

C(x 0 �; t)�0

Kreft and Zuber (1978) have presented the analy-

tical solution to the advective�/dispersive equation

with these initial and boundary conditions. Their

solution can be presented in terms of the dimen-

sionless variables x /L and t //t; where L is the

length of the wetlands cell, and t is the averageresidence time in the cell.

C

�x

L;t

t

��

M

V

x

L

t

tffiffiffiffiffiffiffiffiffiffiffiffiffiffi4p(t=t)

Pe

s66666664

77777775exp

�

[1 � (t=t)]2

4(t=t)

Pe

where V is the volume of the wetlands cell, and Pe

is the well-known Peclet number, defined as UL /

D .

For free-surface wetlands, reported values of Pe

range from 5 to 20 (Kadlec and Knight, 1996). Forour study, we estimated Pe�/10 for the Prado

Wetlands. With this value of Pe , and with the

analytical solution above, it is a simple task to

calculate the time at which 95% of the injected

tracer mass will elute out of the wetlands cell. It is

then another simple task to determine the tracer

mass M that fulfills the criteria listed above.


References

Andradottir, H.O., Nepf, H.M., 2000. Thermal mediation by

littoral wetlands and impact on lake intrusion depth. Water

Resour. Res. 36 (3), 725�/735.

Bachand, P.A.M., Horne, A.J., 2000. Denitrification in con-

structed free-water surface wetlands: I. Very high nitrate

removal rates in a macrocosm study. Ecol. Eng. 14, 9�/15.

Bencala, K.E., Rathbun, R.E., Jackman, A.P., 1983. Rhoda-

mine WT dye losses in a mountain stream environment.

Water Resour. Bull. 19 (6), 943�/950.

Debusk, T.A., Laughlin, R.B., Jr, Schwartz, L.N., 1996.

Retention and compartmentalization of lead and cadmium

in wetland microcosms. Water Res. 30 (11), 2702�/2716.

Everts, C.J., Kanwar, R.S., 1994. Evaluation of rhodamine WT

as an adsorbed tracer in an agricultural soil. J. Hydrol. 153,

53�/70.

Kadlec, R.H., Knight, R.L., 1996. Treatment Wetlands. CRC

Press, Boca Raton, FL.

Kasnavia, T., Vu, D., Sabatini, D.A., 1999. Fluorecent dye and

media properties affecting sorption and tracer selection.

Ground Water 37 (3), 376�/381.

Kilpatrick, F.A., Wilson, J.F., Jr, 1982. Techniques of Water-

Resources Investigations of the United States Geological

Survey. USGS, Denver, CO.

Kreft, A., Zuber, A., 1978. On the physical meaning of the

dispersion equation and its solutions for different initial and

boundary conditions. Chem. Eng. Sci. 33, 1471�/1480.

Lee, C., Hedges, J.I., Wakeham, S.G., Zhu, N., 1992. Effec-

tiveness of various treatments in retarding microbial activity

in sediment trap material and their effects on the collection

of swimmers. Limnol. Oceanogr. 37 (1), 117�/130.

Netter, R., Behrens, H., 1992. Application of different water

tracers for investigation of constructed wetlands hydraulics.

In: Hvtzl, H., Werner, A. (Eds.), Tracer Hydrology,

Proceedings of the Sixth International Symposium on Water

Tracing, Karlsruhe Germany, A.A. Balkema Publishers,

Rotterdam, pp. 125�/129.

OCWD, 1995. Engineers Report on the Prado Constructed

Wetlands Modification Project. Orange County Water

District, Fountain Valley, CA.

OCWD, 1998�/2000. Water quality data.

OCWD, 2001. Prado Wetland. Orange County Water District,

Fountain Valley, CA. http://www.ocwd.com/html/pra-

do.htm.

O’Loughlin, E.J., Burris, D.R., 1999. Reductive dehalogenation

of trichloroethene mediated by wetland DOC-transition

metal complexes. In: Means, J.L., Hinchee, R.E. (Eds.),

Wetlands and Remediation: an International Conference,

Salt Lake City, UT.

Pang, L., Close, M., Noonan, M., 1998. Rhodamine WT and

Bacillus subtilis transport through an alluvial gravel aquifer.

Ground Water 36 (1), 112�/122.

Ptak, T., Schmid, G., 1996. Dual-tracer transport experiments

in a physically and chemically heterogeneous porous

aquifer: effective transport parameters and spatial varia-

bility. J. Hydrol. 183, 117�/138.

Reilly, J.F., Horne, A.J., Miller, C.D., 2000. Nitrate removal

from a drinking water supply with large free-surface

constructed wetlands prior to groundwater recharge. Ecol.

Eng. 14, 33�/47.

Sabatini, D.A., 1989. Sorption and transport of atrazine,

alachlor, and fluorescent dyes in alluvial aquifer sands.

Ph.D. Dissertation, Iowa State University, Ames, IA.

Sabatini, D.A., Austin, T.A., 1991. Characteristics of rhoda-

mine WT and fluorescein as adsorbing ground-water

tracers. Ground Water 29 (3), 341�/349.

Schulz, R., Peall, S.K.C., 2001. Effectiveness of a constructed

wetland for retention of nonpoint-source pesticide pollution

in the Lourens River Catchment, South Africa. Environ.

Sci. Technol. 35, 422�/426.

Shiau, B.J., Sabatini, D.A., Harwell, J.H., 1993. Influence of

rhodamine WT properties on sorption and transport in

subsurface media. Ground Water 31 (6), 913�/920.

Smart, P.L., Laidlaw, I.M.S., 1977. An evaluation of some

fluorescent dyes for water tracing. Water Resour. Res. 13

(1), 15�/33.

Smart, P.L., Smith, D.I., 1976. Water tracing in tropical

regions, the use of fluorometric techniques in Jamaica. J.

Hydrol. 30, 179�/195.

Tai, D.Y., Rathbun, R.E., 1988. Photolysis of rhodamine-WT

dye. Chemosphere 17 (3), 559�/573.

Tanner, C.C., Sukias, J.P.S., Upsdell, M.P., 1998. Organic

matter accumulation during maturation of gravel-bed con-

structed wetlands treating farm dairy wastewaters. Water

Res. 32 (10), 3045�/3046.

Tarrell, A.E., 1997. Field investigation of diffusion within a

submerged plant canopy. Master’s thesis, Department of

Oceanographic Engineering, Massachusetts Institute of

Technology, MA, p. 110.

Trudgill, S.T., Pickles, A.M., Smettem, K.R.J., Crabtree, R.W.,

1983. Soil water residence time and solute uptake, 1. Dye

tracing and rainfall events. J. Hydrol. 60, 257�/279.

Trudgill, S.T., 1987. Soil water dye tracing, with special

reference to the use of Rhodamine WT, Lissamine FF and

Amino G Acid. Hydrol. Proc. 1, 149�/170.

Turner, E.G., Netherland, M.D., Getsinger, K.D., 1991.

Submersed plants and algae as factors in the loss of

rhodamine WT dye. J. Aquat. Plant Manage. 29, 113�/115.

Webb, J.S., McGinness, S., Lappin-Scott, H.M., 1998. Metal

removal by sulphate-reducing bacteria from natural and

constructed wetlands. J. Appl. Microbiol. 84 (2), 240�/248.

Wilson, J.F., Jr, 1968. Techniques of Water-Resources Inves-

tigations of the United States Geological Survey. USGS,

Washington, DC.


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Comparison of rhodamine WT and bromide in the determination of hydraulic characteristics of constructed wetlands