Duwamish Diagonal: A Comparison of Two Remediation Options

Jenée ColtonJeff Stern Debra Williston

WLRD Science Seminar May, 2009

PCBs and bis(2-ethylhexyl)phthalateElliott Bay/Duwamish Restoration ProgramInterim Remedial Action under the 1991 NRDA Settlement

Purpose

Remediate sediments from CSO/SD, storm drain and old treatment plant outfall

Duwamish/Diagonal Remediation Site

Duwamish/ Diagonal Remediation Site

Areas A&B – partial removal and cap in 2004Perimeter - sampled pre- and post-capENR area – 6” to 9“ placed in 2005 for residuals mgmt.Annual monitoring post-dredgeSite allows comparison of ENR and MNR remedial options

Project Timeline

0

500

1000

1500

2000

2500

3000

Tota

l PCB

s (u

g/kg

dw

)

3C 4C4C

Rep 5C 6C 7C 1C 2C 8C

8C R

ep 9C 10C

11C

12C

October 2003 (Pre-Dredge) March 2004 (Baseline Post-Dredge/Cap) Jan/Feb 2005

Near Cap ANear Cap B

50305030

3275327541804180

0

500

1000

1500

2000

2500

3000

Tota

l PCB

s (u

g/kg

dw

)

3C 4C4C

Rep 5C 6C 7C 1C 2C 8C

8C R

ep 9C 10C

11C

12C

October 2003 (Pre-Dredge) March 2004 (Baseline Post-Dredge/Cap) Jan/Feb 2005

Near Cap ANear Cap B

50305030

32753275418041803275

Comparison of PCBs at Pre- and Post-capping Perimeter Stations

5030

Short-term Residuals Management

Enhanced Natural Remediation (ENR)Thin layer placement strategy in 2-12 m of water

15 cm minimum target (6 “)22 cm average (9”)

Decreased PCB concentrations to < area background

05

10152025303540

4SP

3SP

14SP

26SP6S

P27

SP7S

P24

SP5S

P28

SP15

SP

Stake

Dep

th (c

m)

2004 2004 before before ENRENR

2006 2006 after after ENRENR

Comparison of Area Responses

0

500

1000

1500

2000

2500

Pre-dredge2003

Post-dredge2004

Pre-ENR2005

1yr post-ENR 2006

2yr post-ENR 2007

3yr post-ENR 2008

Tota

l PC

Bs

Mea

n an

d SE

(ug/

kg d

w)

PerimeterENR

n=12

n=8 n=8

n=7 n=7

n=13

n=5

n=5

n=7

n=12

ENR LaidDredging

n=7

n=8

Were Short-term Remedial Goals Met?

ENR successfully covered residualsprovided immediate elimination of increased exposure to biotareduced sediment concentrations to below area background (cleanup goal) immediately

MNR reduced sediment concentrations to pre-remedial levels within 1 year

Response of MNR and ENR

Pre-ENRPost-Cap

Area Responses Relative to Remediated Areas

Pre -ENR

Comparison with Year 10 Model Estimates

STM 10-yr Bed Composition

Pre-ENRPost-Cap

Will Long-term Recovery Goals be Met?

ENR remains below area background (cleanup goal)Suggests that underlying sediments have not mixed inConsistent with measured bioturbation depths

PCB concentrations in MNR at recent higher sedimentation rate will reach background ~10 yearsRecovery in MNR at site’s long term net rate will take longer but approaches goal in 20 yearsThe thin layer (ENR) rapidly reduced the contribution from this area to site risks

Conclusions

ENR helps reduce short term riskRecovery is occurring ENR has accelerated attainment of cleanup goals by 10-20 yearsThe results are in agreement with the numerical sediment transport modelENR can accelerate exposure reductions and reductions of site risks

AcknowledgementsBruce Nairn Fiona McNair John Toll Kirk Ziegler Earl Hayter