Comprehensive Modeling of Dredge Resuspension and Residual Effects of Remedial Alternatives: Portland Harbor Feasibility Study

Transcription

1 Comprehensive Modeling of Dredge Resuspension and Residual Effects of Remedial Alternatives: Portland Harbor Feasibility Study Presented by Michael Werth, Kevin Russell, Kirk Ziegler, and Carl Stivers October 25, 2012

2 Outline Overview of Site Feasibility Study (FS) Summary Summary of FS Model Methods for Simulation of FS Alternatives Dredge residuals and releases Model Application Summary 2

3 Overview of Site Eleven-mile urban/ industrial portion of Lower Willamette River downstream of Portland Modified extensively over many years Channelizing Navigational dredging Wetland draining/filling Numerous sediment contaminants from various historical sources 3

4 Feasibility Study Summary Portland Harbor Draft FS transmitted to EPA (March 2012); substantive EPA comments not yet received Potential risk from four bounding chemical groups (PCBs, dioxins/furans, pesticides, and PAHs) Generally overlap risks from other chemicals Contaminants also exist upstream Eleven alternatives evaluated No action Five removal (dredging) focused Five integrated (mix of capping, in situ treatment, dredging) 4

5 Feasibility Study Summary (cont.) Alternative Total Dredge Volume Removed 1 (cubic yards) Dredge Areas (acres) In Situ Treatment Areas (acres) Engineered Cap Area (acres) B-i 246, B-r 662, C-i 387, C-r 952, D-i 476, D-r 1,118, E-i 1,149, E-r 2,186, F-i 2,640, F-r 5,189, Volume represents midpoint of range presented in the draft FS. 5

6 Feasibility Study Model Comprehensive fate and transport and bioaccumulation model developed and calibrated to support FS Modeling objectives Estimate system response to construction of remedial alternatives Evaluate impact of dredging residuals on remedy effectiveness Estimate potential for downstream recontamination from dredge resuspension 6

7 Feasibility Study Model (cont.) 7

8 Methods for Simulation of Feasibility Study Alternatives General model setup Long-term 45-year simulation Hydrograph representing full range of flows, including event with return period of >100 years External source loading conditions unchanged from calibration Simulation of active remedial alternatives Construction sequencing/duration Impacts of dredge residuals and releases 8

9 Methods for Simulation of Feasibility Study Alternatives: Residuals Potential for generating residuals is well documented Use of traditional multiple cleanup passes often fails to meet project-specific action levels Dredge residuals evaluation in FS Performed using mass balance approach (USACE 2008) Evaluated two residual management strategies Residual cover only Single cleanup pass and cover Residual management strategy assumed for all alternatives One cleanup pass and cover Cleanup pass dredge cut = 1 foot Residual cover = 6 inches 9

10 Methods for Simulation of Feasibility Study Alternatives: Residuals (cont.) Representation of assumed residual management strategy in fate and transport model 6 3 RESIDUAL COVER (Concentration = last dredge pass * 0.05) RESIDUAL LAYER (Concentration = last dredge pass) 48 Reset sediment bed concentration 39 UNDREDGED SEDIMENT Note: Not to scale 10

11 Methods for Simulation of Feasibility Study Alternatives: Resuspension Multiple studies indicate dissolved-phase release most important to fish bioaccumulation Project Dredging Volume BMPs Fox River 2004 Duwamish/Diagonal 2005 Grasse River 2005 Lower Passaic River 2009 Hudson River Dredge Release Case Studies 82,000 cy (Hydraulic) 70,000 cy (Mechanical) 25,000 cy (Hydraulic) 4,000 cy (Mechanical) 280,000 cy (Mechanical) Operation BMPs/ silt curtains Contaminant Mass Released Average 2.2% Operation BMPs Range 1 to 6% Operation BMPs/ silt curtains Operation BMPs/ rinse tank Operation BMPs/ silt curtains Average 3% Range 1 to 6% Average 3 to 4% 11

12 Methods for Simulation of Feasibility Study Alternatives: Resuspension (cont.) Representation of contaminant releases in fate and transport model Simulated as flux of dissolved contaminant to water column Magnitude of release flux dependent on Removal depth Dredge production schedule Assumed fraction of contaminant mass released Assumed 3% release of contaminant mass based on case studies 12

13 Model Application for Feasibility Study Evaluations Flow Multnomah Channel 13

14 Model Application for Feasibility Study Evaluations (cont.) Time series of model-predicted surface sediment PCBs showing example of residual impacts Extreme Flow Event (Year 17) 14

15 Model Application for Feasibility Study Evaluations (cont.) Time series of model-predicted fish tissue PCBs showing example of resuspension impacts 15

16 Model Application for Feasibility Study Evaluations (cont.) Time series of model-predicted fish tissue PCBs showing example of resuspension impacts 16

17 Observed Increases in Duwamish River Fish Tissue PCBs During Dredging Total PCBs(ug/kg wet wt; +/- 2 std. err.) 4,000 3,000 2,000 1,000 Duwamish Waterway Duwamish/Diagonal, East Waterway, Lockheed & Todd Shipyard Dredging (~600,000 cy) Year Data Source: Lower Duwamish Waterway Group (2010) and WDFW (2010) 17

18 Model Application for Feasibility Study Evaluations Short-term changes in surface sediment concentration showing potential downstream sediment recontamination from dredge releases Flow 18

19 Model Application for Feasibility Study Evaluations (cont.) Potential downstream transport during construction 19

20 Summary All alternatives include a large proportion (>45%) of removal All FS alternatives (except No Action) provide long-term overall protection of human health and the environment Removal-focused alternatives have higher amounts of short-term impacts Short-term increases in surface sediment concentrations due to dredge residuals Larger mass of contaminants transported downstream due to release Releases during dredging resulted in short-term increases in fish tissue concentrations; the magnitude of these increases is highly variable throughout the Study Area Alternatives with moderate amounts of dredging focused on most contaminated areas, provided best balance of minimal short-term impacts and long-term effectiveness 20

21 Questions? 21