Table S1. Annual average traffic flow data (AATFD) on N7 motorway (NRA, 2016).

Year AATFD % HGV2005 28455 12.92006 29034 11.92007 32125 11.12008 32863 10.42009 33326 9.22010 32919 9.22011 34149 8.82012 34220 8.32013 34418 7.92014 35876 8.1Ave 32739 9.8

NRA, 2012. Traffic Count Data, National Roads Authority, Dublin, Ireland. Available at:

http://www.nra.ie/NetworkManagement/TrafficCounts/

Table S.2. Field sampling results from 2011 and 2014.

A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4

Plan area

(m2)

19.3 19.3 19.3 19.3 19.3 19.3 19.3 19.3 19.3 19.3 19.3 19.3

Plants (per

m2) [2011]

92 144 154 63 105 81 124 119 72 116 118 75

Plants (per

m2) [2014]

102 133 125 89 119 96 103 84 75 102 107 71

Sediment

(mm) [2011]

85 50 65 65 75 50 40 60 75 55 40 40

Sediment

(mm) [2014]

75 60 65 70 75 55 50 50 70 60 60 60

1

10

100

1000

10000

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2021

-23

>24

freq

uenc

y

antecedent dry days

Fig. S.1. Statistical distribution of rainfall events (2005 to 2014) at the site with antecedent

dry weather of <0.5 mm.

Table S.3. Calculated mass in, out and removed for wetland for discrete storm events.

Total mass in (g)TSS Cd Cu Pb Zn

18th Aug 6560 0.246 2.345 3.445 8.63123rd Aug 1235 0.028 0.440 0.633 1.5549th Sept 7545 0.371 2.449 4.382 12.943

26th Sept 12342 0.382 3.415 4.137 15.62928th Sept 4002 0.076 1.255 2.292 6.59510th Oct 16903 0.375 4.283 4.820 12.769

Total mass out (g)TSS Cd Cu Pb Zn

18th Aug 73.3 0.005 0.040 0.141 0.08423rd Aug 14.1 0.000 0.013 0.020 0.0379th Sept 155.7 0.008 0.234 0.272 0.441

26th Sept 1176.7 0.075 0.774 1.179 1.24728th Sept 256.3 0.006 0.197 0.300 0.33410th Oct 1263.4 0.033 0.736 1.396 1.036

Total mass removed (g)TSS Cd Cu Pb Zn

18th Aug 6486.9 0.241 2.305 3.303 8.54723rd Aug 1220.8 0.027 0.426 0.613 1.5179th Sept 7389.4 0.363 2.215 4.110 12.502

26th Sept 11164.9 0.307 2.642 2.958 14.38128th Sept 3745.7 0.070 1.057 1.991 6.26110th Oct 15639.4 0.343 3.547 3.423 11.733

Table S.4. Concentrations of heavy metals in sediment (2011) in the different cells (mg/kg).

Cd Cr Cu Ni Pb ZnA1 1.0±0.2 32.9±1.5 160±30 27.9±1.9 87±9 660±90A2 0.9±0.3 21±4 80±30 23±6 52±20 370±80A3 0.9±0.3 12±2 44±17 18±4 31±11 220±90A4 0.7±0.3 7.4±0.4 22±7 15±2 14.7±1.7 95±12B1 1.0±0.3 18±2 86±19 24±5 44±6 320±60B2 0.8±0.3 15±2 44±23 16.3±1.6 27±9 200±90B3 0.8±0.4 15±3 48±21 17±6 29±12 200±80B4 0.8±0.3 20±3 21±8 20±3 17±8 110±30C1 0.8±0.5 16±4 75±33 20±5 43±14 320±60C2 0.9±0.2 22±5 53±38 23±6 51±20 400±90C3 1.0.±0.3 14±3 38±14 19.9±0.7 28±7 180±60C4 1.1±0.3 12.0±2.0 32±5 20±6 24±3 180±40

clay liner 0.86±0.17 5.2±1.9 10.2±1.8 11±2 9±2 62±19topsoil1 0.25±0.10 7.1±1.3 12.5±2.4 10.5±1.6 12±3 46±14

1. fresh topsoil used to fill wetland during construction

Table S.5. Cross correlation matrix between mass in sediment and mass in the plants.

Cd Cr Cu Ni Pb Znsig 0.044 0.411 0.211 0.044 0.068 0.046

R 0.589 0.262 0.389 0.590 0.544 0.585

1

23

4

0

1

2

C

B

ASample cellref. (length)

Cd in sediment (g)

Sample cellref. (width)

Cd in sediment 1

23

4

0.00

0.01

0.02

0.03

C

B

ASample cellref. (length)

Cd in vegetation (g)

Sample cellref. (width)

Cd in vegetation

1

23

4

020406080100

C

B

A

Sample cellref. (length)

Cr in sediment (g)

Sample cellref. (width)

Cr in sediment 1

23

4

0.0

0.2

0.4

0.6

0.8

C

B

ASample cellref. (length)

Cr in vegetation (g)

Sample cellref. (width)

Cr in vegetation

1

23

4

0

100

200

300

400

C

B

A

Sample cellref. (length)

Cu in sediment (g)

Sample cellref. (width)

Cu in sediment 1

23

4

0

2

4

6

C

B

ASample cellref. (length)

Cu in vegetation (g)

Sample cellref. (width)

Cu in vegetation

1

23

4

0

20

40

60

C

B

A

Sample cellref. (length)

Ni in sediment (g)

Sample cellref. (width)

Ni in sediment 1

23

4

0.0

0.2

0.4

0.6

C

B

ASample cellref. (length)

Ni in vegetation (g)

Sample cellref. (width)

Ni in vegetation

1

23

4

0

100

200

300

C

B

A

Sample cellref. (length)

Pb in sediment (g)

Sample cellref. (width)

Pb in sediment 1

23

4

0.0

0.5

1.0

1.5

C

B

ASample cellref. (length)

Pb invegetation (g)

Sample cellref. (width)

Pb in vegetation

1

23

4

0

600

1200

1800

2400

C

B

A

Sample cellref. (length)

Zn in sediment (g)

Sample cellref. (width)

Zn in sediment 1

23

4

0

5

10

C

B

ASample cellref. (length)

Zn in vegetation (g)

Sample cellref. (width)

Zn in vegetation

Fig. S.2. Net metal accumulation in sediment and vegetation in wetland 2011.

Table S.6. Mass of metals in CW and total accumulated during 9 year operation (g).

Cd Cr Cu Ni Pb ZnA1 2.02 74.7 345.1 60.1 222.7 1945.0A2 1.13 56.9 226.8 37.5 176.9 1160.4A3 1.90 53.5 201.4 48.1 187.8 1382.9A4 1.18 12.9 32.1 22.1 25.7 129.0B1 1.76 77.1 352.1 48.4 247.5 1957.6B2 0.80 25.2 94.9 24.6 79.7 475.0B3 1.03 14.5 40.5 21.5 37.0 192.8B4 1.08 30.9 133.3 26.0 96.5 726.0C1 1.73 72.6 337.9 44.4 242.1 1727.0C2 1.00 21.8 52.0 24.2 51.2 238.4C3 1.00 12.1 19.7 18.5 28.0 121.3C4 1.90 11.5 21.7 17.1 25.0 105.7

Total in wetland (g) 15.54 464.2 1857.3 392.4 1420.3 10161.1

Total accumulated (g) 12.09 366.3 1684.8 247.6 1253.7 9530.8

0 20 40 60 80 100

A1

B1

C1

A2

B2

C2

A3

B3

C3

A4

B4

C4

Weight (%)

Cell

sam

ple

Quartz

Calcite

Dolomite

Mica

Chlorite

Albite

Microcline

Fig. S.3. Mineral composition of sediment samples from XRD analysis.

Sequential extraction of metals from the sediment

The sequential extraction of the metals was then performed in accordance with the procedure

in Zimmerman and Weindorf (2010) adapted from Tessier et al. (1979) as follows:

1. Exchangeable

20mL of 1.0 M MgCl2 solution at pH 7 was added to 2.5 g of the sediment sample

which was then stirred for 1 hour at 20°C. The ionic composition of the water was

changed allowing metals which were sorbed to the surface of the soil to be removed.

2. Carbonate-bound

The residue from the previous step was leached at room temperature with 20 mL of a

1.0 M NaAc solution adjusted to a pH of 5 with HAc by constantly stirring on a

magnetic stirrer for 4 hours. Here, the use of an acid to change the pH helped to

remove metals bound in this phase.

3. Iron-Manganese Oxide bound

Metals bound to Fe and Mn oxides were extracted from the residue of the previous

step by shaking with 50 mL of a 0.04 M NH2OH.HCl in 25% HAc solution. The

extraction was performed at 96°C +/- 3°C for 5.5 hours. At this stage a solution was

used to dissolve the insoluble sulphide salts as the metals bound to Fe-Mn oxides are

sensitive to reducing conditions.

4. Organic Matter bound

Metals bound to the organic matter were extracted by adding 7.5 mL of a 0.02 M

HNO3 solution and 12.5 mL of a 30% H2O2 solution adjusted to pH 2 with HNO3 onto

residue from the previous step. This solution was heated to 85°C for 2 hours with

occasional stirring. A second measure of 7.5 mL of the 30% H2O2 solution adjusted to

pH 2 with HNO3 was then added while keeping intermittent agitation at 85°C for 3

hours. The solution was then allowed to cool to room temperature and 12.5 mL of a

3.2 M NH4Ac in 20% HNO3 was added and agitated for 30 minutes. The organic

material has to be oxidised to allow the release of metals bound in this fraction.

5. Residual

The residue from the previous stage was transferred into a digestion vessel and metals

dissolved in aqua regia using 7 mL of 10 M HCl and 2.3 mL of 15.8 M of HNO3. The

temperature of this mixture was slowly increased until reflux conditions attained and

then sustained at this state for two hours. The strong acids used in this step are

necessary in order to break down the silicate structures where the metals are located.