Remediation of contaminated sediments in open water is a complex and inten-

sive operation. The presence of adjacent industrial activities on land, varied

dockwall and pier design, utility entries into slips, and aging infrastructure,

each complicate the design of an effective remediation strategy. Slip 2 is one

of eight slips in the St. Louis River Area of Concern (AOC) impaired by legacy

contamination from industrial and municipal wastes and harmful land use prac-

tices. Soil samples taken from behind the dockwall of the adjacent Pier B site

contained asbestos and exceeded MPCA recreational soil reference values for

arsenic, mercury, semi-volatile organic compounds (SVOCs), and diesel range

organics (DRO). Dive inspections revealed extensive failures in the existing

dock walls, leaving voids where contaminated site soils leeched into Slip 2 and

out into Lake Superior. The conventional method of slip remediation by dredg-

ing to remove contaminated sediment would have further deepened the slip,

causing complete failure of the adjacent dockwalls and accelerated leeching of

contaminated soil into Slip 2.

AMI consulting Engineers developed a phased approach for the remediation of

Slip 2, coupling the remediation of the slip with that of the adjacent pier. Exca-

vation and removal of contaminated soil from along the dock wall, and repairs

to the dock wall system were critical to the long term success of the Slip 2 sed-

iment remediation. The remediation strategy and lessons learns from Slip 2

should serve as a model for future slip remediation efforts in the St. Louis Riv-

er Area of Concern.

Variable structural designs and failed dock wall conditions

Adjacent contaminated site soil migrates through failed dock walls;

potential recontamination if not properly contained prior to remediation

Sediments below dock walls have typically not been characterized

Adjacent land is often privately owned and dock wall repairs are cost prohibitive

Unknown future land use complicates overall project design

Multi-agency permitting, approvals, and consultations

Historic preservation of dock walls may limit repair options and complicate project timing

Coordination of dredged materials and USACE hired dredging contractor

Pre-project testing of dredge materials may be insufficient

Contaminated sediment containment and/or disposal during remediation

Conflicting property interests due to multiple property owners

The complexities of slip remediation

Assess the condition and design of the terrestrial and water portions of the dock walls

Characterize sediment contours and contamination below dock walls

Perform comprehensive feasibility study, identify alternatives, and prepare cost estimates

Obtain agency permits and perform consultations (RAPs, EAW, Section 106, etc.)

Secure funding, capping material, and establish remediation credits

Execute a project partnership agreement between land owners and agencies

Design the remediation

91 MAIN STREET SUPERIOR, WI 54880 715.718.2193 AMIengineers.com

Authors: LaRae Lehto, Bryan Murdock, Ted Smith, Doug Olson, Seth Johnson, Chad Scott

Abstract

Pre-Remediation Planning

The complexities of sediment remediation of Duluth Superior harbor slips

CONCRETE SLAB CONCRETE RELIEVING

PLATFORM FILL OF UNKNOWN ORIGIN

WOOD RETAINING WALL ROUND WOOD PILE

MIGRATING FILL

IMPACTED WITH

CONTAMINANTS

DAMAGED RELIEVING

PLATFORM

BROKEN UNSUPORTED CONCRETE SLAB

HEAVILY DETERIORATED

FORM LUMBER UPPER SECTIONS OF

RETAINING WALL

CLEAN SOIL FILL EXISTING CAP

CUT DOWN

NEW CONCRETE BOARDWALK

SLIP SHALLOWED TO SUPPORT PILES STRUCTURES AND CAP CONTAMINANTS

STONE FILL

GEOTEXTILE FABRIC

CONCRETE SLAB CONCRETE RELIEVING PLATFORM

FILL OF UNKNOWN ORIGIN

WOOD RETAINING WALL ROUND WOOD PILE

MIGRATING FILL

IMPACTED WITH

CONTAMINANTS

DAMAGED RELIEVING

PLATFORM

BROKEN UNSUPPORTED CONCRETE SLAB

HEAVILY DETERIORATED

FORM LUMBER UPPER SECTIONS OF RETAINING

WALL ROTTED AND MISSING

CLEAN SOIL FILL TO SUPPORT PILES

CLEAN SOIL FILL

NEW CONCRETE BOARDWALK

NEW SHEET PILE

CONTAINMENT WALL

NEW STEEL CAP

STONE FILL

GEOTEXTILE FABRIC

TIMBER FENDER

ROUND WOOD PILE

CONCRETE CAP

TIMBER CRIB

CONCRETE SLAB

FILL OF UNKNOWN

ORIGIN

TIMBER FENDER MISSING

MIGRATING FILL WITH

CONTAMINANTS

DAMAGED CAP

UPPER REGIONS OF CRIB

HEAVILY DETERIORATED

DAMAGED

FOUNDATION

UNSUPPORTED BROKEN

SLAB

SLIP SHALLOWED TO SUPPORT PILES AND CAP CONTAMIANANTS

DETERIORATED SECTIONS REMOVED

STONE RIP RAP

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WOOD PILE / SUPPORTIVE CAP CRIB / CAP REMOVAL & SUPPORTIVE CAP WOOD PILE / SHEETPILE WALL

Arsenic impacts adjacent to Slip 2