Focused Feasibility Study Addendum Monguagon Creek Upper Trenton Channel Detroit River Area of Concern Riverview Michigan


Focused Feasibility Study Addendum

Monguagon Creek - Upper Trenton Channel
Detroit River Area of Concern
Riverview, Michigan

Prepared for
U.S. Environmental Protection Agency
Great Lakes National Program Office

Chicago, IL

Prepared by

inte°

id

consulting inc.

45 Exchange Street

Suite 200
Portland, ME 04101

January 20, 2022

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Focused Feasibility Study Addendum
Monguagon Creek - Upper Trenton Channel

January 20,2022

CONTENTS

LIST OF FIGURES	iii

LIST OF TABLES	iv

ACRONYMS AND ABBREVIATIONS	v

1	INTRODUCTION	1-1

1.1	BACKGROUND	1-2

1.2	UPDATED UNDERSTANDING OF UTILITY LINE	1-3

2	UPDATE TO REMEDIATION ALTERNATIVE 5	2-1

2.1	LONG-TERM EFFECTIVENESS AND PERMANENCE	2-2

2.2	IMPLEMENTABILITY	2-2

2.3	COST	2-2

2.4	COMPARATIVE ANALYSIS	2-2

3	SUMMARY AND RECOMMENDATIONS	3-1

4	REFERENCES	4-1

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LIST OF FIGURES

Figure 1-1.

Vicinity Map

Figure 1-2.

Site Plan

Figure 1-3.

MCUTC and UTC Sites

Figure 1-4.

Annotated Water Line Plan

Figure 1-5.

Annotated Water Line Map

Figure 1-6.

Sidescan Sonar Survey

Figure 2-1.

Conceptual Layout of Remediation Alternative 5

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LIST OF TABLES

Table 2-1. Summary of Remediation Alternatives
Table 2-2. Comparative Analysis of Alternatives

Table 2-3. Advantages and Disadvantages of Each Remediation Alternative

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ACRONYMS AND ABBREVIATIONS

2,4-DP

2,4-di- tert- amylphenol

AOC

area of concern

AOI

area of interest

BATO

Bridgestone Americas Tire Operations, LLC

COC

chemical of concern

FFS

Focused Feasibility Study

MCUTC

Monguagon Creek - Upper Trenton Channel

PAH

poly cyclic aromatic hydrocarbon

PCB

poly chlorinated biphenyl

Ramboll

Ramboll US Corporation

RAO

remedial action objective

RI

Remedial Investigation

TLC

thin layer cover

TU

toxic unit

USEPA

U.S. Environmental Protection Agency

UTC

Upper Trenton Channel

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1 INTRODUCTION

In 2018, Ramboll US Corporation (Ramboll), on behalf of Bridgestone Americas Tire Operations,
LLC (BATO), submitted a Focused Feasibility Study (FFS) for the Monguagon Creek - Upper
Trenton Channel (MCUTC) Site to the U.S. Environmental Protection Agency (USEPA) Great
Lakes National Program Office. The parties' understanding of Site conditions, particularly as
they related to a buried utility line, has evolved since the FFS was completed. The overall
purpose of the 2018 FFS was to identify and evaluate remediation alternatives that address
chemicals of concern (COCs) in Site sediments. Towards that end, the FFS addressed the
following underlying objectives: 1) define remedial action objectives (RAOs); 2) identify
potential remediation technologies and screen based on effectiveness, implementability, and
cost; and 3) develop, evaluate, and compare candidate remediation alternatives.

The overall purpose of this addendum is to determine whether the recommended remediation
alternative differs as a result of updated information about the buried utility line. The vast
majority of the FFS is not affected by the updated information; this addendum addresses only
those portions of the FFS that are affected by the updated information on the buried utility line.
RAOs, cleanup goals, and definition and screening of remedial technologies remain unchanged
as a result of the updated information. The preferred remedy for AOI-D (dredging) also is not
affected by the updated information. It is only the evaluation of Remediation Alternative 5
(Dredge in AOI-C and -D) that warrants updating to reflect the new understanding of the water
line. In addition to the updated evaluation of Remediation Alternative 5, this addendum
provides concise excerpts from the FFS that provide context and aid overall understanding, as
well as replacement of the term "enhanced monitored natural recovery" with "thin layer cover"
because the latter is a more intuitive descriptor of the technology.

Like the FFS, this addendum was prepared in accordance with the Great Lakes Legacy Act
Project Agreement executed on August 12, 2015, between USEPA and BATO for Remedial
Investigation (RI) and FFS of the MCUTC Site. The FFS and this addendum were prepared
voluntarily by BATO, the Non-Federal Partner, to facilitate and define potential voluntary
remedial work in coordination with USEPA. Completion of the RI was led by USEPA, which
contracted CH2M. The RI (CH2M 2017) was completed in August 2017.

The MCUTC Site is located within the Detroit River area of concern (AOC). The Site location is
shown on the Site vicinity map included as Figure 1-1. A Site plan is included as Figure 1-2 and
the MCUTC Site's spatial relationship to the Upper Trenton Channel (UTC) Site is illustrated in
Figure 1-3. COCs that are the focus of the FFS and this addendum are polycyclic aromatic
hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), mercury, and 2,4-d i-ferf-amyl phenol
(2,4-DP).

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1.1 BACKGROUND

This subsection provides context and background regarding the MCUTC Site; it is a condensed
version of Section 2 of the FFS, provided here for ease of access and to highlight aspects of the
Site most relevant to the updated information about the buried utility line. Readers are referred
to the FFS for a more detailed description of the Site, Detroit River, AOC, investigation and
remediation history, sampling data, conceptual site model, COCs, and basis for defining areas
of interest (AOIs).

The MCUTC Site is located within the Detroit River AOC and includes the lower 1,700 feet of
Monguagon Creek and approximately 50 acres of Trenton Channel immediately downstream of
the Grosse He Toll Bridge and across to Grosse He in Riverview, Michigan (Figure 1-1).
Monguagon Creek flows into the Trenton Channel, which is an 8-mile stretch of the lower
Detroit River. The Detroit River flows south into Lake Erie. The Site represents a small portion
of the UTC Site and an even smaller portion of the Detroit River AOC.

The MCUTC Site is divided into three main areas: Monguagon Creek, UTC-West, and UTC-East
(Figure 1-2). UTC-East was included within the MCUTC Site because it may present
opportunities to improve habitat (Detroit River Public Advisory Council 2014). The presence of
the navigation channel between UTC-East and UTC-West likely limits or prevents contributions
of contamination to UTC-East from any of the industrial sources adjacent to UTC-West. Rather,
UTC-East reflects conditions that are generally consistent with urban background, as opposed
to direct releases from point sources from within the MCUTC Site.

Monguagon Creek is approximately 0.7 mile long and 30 to 40 feet wide, discharging to UTC-
West in Riverview, just south of Bridge Road and the Grosse He Toll Bridge. Largely
channelized and partially culverted, Monguagon Creek is located within a highly industrialized
area of Riverview. Monguagon Creek is one of five tributaries that flow into the Detroit River
AOC. Flow within Monguagon Creek is largely derived from the Huntington Drain, which
conveys stormwater from the City of Riverview.

Monguagon Creek receives untreated stormwater runoff from the streets and industrial
properties that surround it via the storm sewer system. Huntington Drain receives stormwater
from an approximately 1 square mile area of Riverview; it flows into the northern reach of
Monguagon Creek. Other storm sewer drains that discharge to Monguagon Creek are located
north of the West Jefferson Avenue Bridge and approximately 300 feet downstream of the
bridge on the east bank. A total of five stormwater outfalls, originally installed in 1919 and the
1960s, discharge to Huntington Drain or Monguagon Creek.

During the period from 1951 to 1982, manufacturing wastes and products were released to
portions of the Huntington Drain (upstream of the Site) and adjacent to Monguagon Creek. In
addition, this area was used as a surface impoundment for sludge, chromium, and corrosive

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waste. A reach of the creek was subsequently capped as a landfill, and another section became a
parking lot. During the 1970s, 2,4-DP, oil and grease, lead, and zinc were discharged to
Monguagon Creek from an outfall at the former ATOFINA Chemicals, Inc., West Plant facility.
This outfall was redirected to the Trenton Channel in January 1997 (CRA 1997).

The shoreline of Monguagon Creek and UTC-West is a mixture of natural shoreline, concrete
and rock fill, and sheet pile. Some embankments are quite steep, particularly at the mouth of
Monguagon Creek. Monguagon Creek's shoreline vegetation is dominated by invasive shrubs
(e.g., honeysuckle [Lonicera maackii] and buckthorn [Rhamnus spp.]). A few mature cottonwood
trees (Populus spp.) grow along the banks. There is minimal herbaceous cover. Some
undercutting of both banks and exposed tree roots are evidence of bank erosion throughout the
creek. Concrete debris is prevalent in the creek. There is minimal woody debris within
Monguagon Creek, though shrubs overhang the creek by about 5 to 10 ft.

Much of the bank of UTC-West is lined with a steel retaining wall, concrete debris, and/or
riprap. The vegetation along the bank of UTC-West is represented by invasive shrubs, a few
mature trees, and some herbaceous understory. The banks appear relatively stable, though the
use of a retaining wall and riprap for bank stabilization limit the quality and quantity of
foraging and cover habitat for wildlife.

AOI-C is located at the mouth of Monguagon Creek and appears to be strongly influenced by
backwater/eddy effects of UTC. Elevated concentrations of most COCs occur within the PAH
toxic unit (TU)=1 footprint of AOI-C and, for that reason, the PAH TU=1 isopleth preliminarily
defines the boundary of AOI-C.

Work is currently under way to support a pre-design investigation, including sampling,
analysis, and modeling. The outcomes of that work will be reported separately and may result
in changes to the AOI boundaries, which in turn may result in changes to remediation costs.

1.2 UPDATED UNDERSTANDING OF UTILITY LINE

When the 2018 FFS was prepared, the main source of information about a buried utility line at
the mouth of Monguagon Creek was derived from CRA (1997), which described the 1994
remedial action of Monguagon Creek. The remedial action was conducted under a voluntary
agreement between the Michigan Department of Environmental Quality, Elf Atochem North
America Inc., Bridgestone/Firestone Inc., and Jones Chemicals Inc. Approximately 34,500 tons of
contaminated, nonhazardous sediments was excavated to the underlying clay. The sediment
removal also included a portion of the Huntington Drain. CRA (1997) reported that a water
main utility crossing near the confluence of Monguagon Creek and UTC prevented sediment
removal from occurring in the mouth of the creek. That description suggested the utility line
crossed the mouth of Monguagon Creek.

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The FFS noted that subsurface utility clearance and mapping would be a critical component of
engineering design for Remediation Alternative 4; the same is true for Remediation
Alternative 5. To support such mapping and clearance, USEPA identified historical maps and
plans that noted the location of an 8-in. welded steel pipe that crosses UTC at the Grosse He Toll
Bridge and flows into a pumping station on Grosse He (Figures 1-4 and 1-5). Given its
connection to a pumping station, the utility line is inferred to be a water line. Both the map and
the plan depict this water line as tracking directly under the full length of bridge, following the
southern side of the bridge.

Representatives of Grosse He Township indicate that two inactive water lines exist near AOI-C.
The original water main that supplied water to the township of Grosse He was a buried 8-inch
diameter line that ran parallel to the toll bridge, 4 feet south of the centerline of the bridge. That
line was replaced by a 12-inch diameter line. There are no records related to the
decommissioning of the 8-inch diameter line, though it was presumably cut and capped at both
ends. The 12-inch diameter replacement line ran parallel to and approximately 50 feet south of
the bridge. In 1943, the 12-inch diameter line was replaced by a 16-inch diameter line that
remains active and runs directly under the north side of the bridge. Though there also are no
decommissioning records for the 12-inch diameter line, Grosse He representatives were
confident that it had been cut and capped at both ends.

During the completion of hydrographic bathymetry, magnetometry, and side scan sonar
surveys of the project area in the fall of 2020, a linear feature was observed south of, and
running parallel to, the bridge in the sediment of the river bottom (Figure 1-6). This feature
appears to be the inactive 12-inch diameter water line, which is partially buried. The
magnetometry survey provided limited information about the water line due to high
interference from the nearby bridge.

The inactive water lines do not influence the preferred remediation alternative for AOI-D. The
FFS determined that dredging is the preferred remedy for AOI-D. The northern boundary of
AOI-C, as defined in the FFS, tracks closely to the southern side of the Grosse He Toll Bridge.
Work that is being conducted in 2022 related to the remedial design will refine the dredging
footprint, debris removal, and other activities necessitated by the presence of the inactive water
lines.

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2 UPDATE TO REMEDIATION ALTERNATIVE 5

The FFS developed five remediation alternatives:

• Remediation Alternative 1: No Action

• Remediation Alternative 2: Thin Layer Cover in AOI-C and Cap in AOI-D ("TLC &
Cap")

• Remediation Alternative 3: Thin Layer Cover in AOI-C and Dredge in AOI-D ("TLC &
Dredge")

• Remediation Alternative 4: Dredge in AOI-C and Cap in AOI-D ("Dredge & Cap")

• Remediation Alternative 5: Dredge in AOI-C and -D ("Dredge").

Institutional controls were incorporated into all remediation alternatives except No Action.
Similarly, habitat restoration in AOI-C and -D was incorporated into all remediation
alternatives except No Action.

Table 2-1 summarizes the five remediation alternatives. Of the five remediation alternatives,
only Remediation Alternative 5: Dredge in AOI-C and -D is affected by the new information
about the water line near AOI-C. Therefore, the evaluation of this alternative is updated in this
addendum to accurately reflect conditions in and near AOI-C.

Remediation Alternative 5 involves four main components: a) dredging followed by backfilling
with a 6-in. sand cover in AOI-C; b) dredging followed by backfilling with a 6-in. sand cover in
AOI-D, with optional monitored natural recovery adjacent to AOI-D; c) habitat restoration in
AOI-C and -D1; and d) institutional controls. The updated information on the two inactive
water lines affects only the first of the four components, and only with respect to the dredge
area and dredge volume. The inactive water lines track along the length of bridge, parallel to
and south of the bridge's southern side.

Of the six National Contingency Plan criteria considered in the FFS, the updated information
did not affect three criteria, namely:

• Overall protection of human health and the environment

• Reduction in mass, toxicity, mobility, or volume through treatment

• Short-term effectiveness.

1 Optional restoration work in UTC-East, described in the Focused Feasibility Study (Ramboll 2018), has since been
initiated by Friends of Detroit River, independent of the MCUTC project. That optional restoration work therefore is
removed from Remediation Alternative 5.

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This section, therefore, updates the evaluation of Remediation Alternative 5 relative to the
remaining three criteria: a) long-term effectiveness and permanence; b) implementability; and
c) cost. Figure 2-1 illustrates the conceptual layout of Remediation Alternative 5. This section
closes with an update to the overall comparative analysis of the five remediation alternatives.

2.1 LONG-TERM EFFECTIVENESS AND PERMANENCE

Remediation Alternative 5 involves dredging in AOI-C and -D. Management of residuals and
sequencing relative to the UTC remediation may influence recontamination and therefore
effectiveness of the remedy. Within AOI-D, it is assumed that complete dredging would be
possible and the remedy therefore would be permanent.

2.2 IMPLEMENTABILITY

Remediation Alternative 5 involves dredging in AOI-C and -D. Dredging in AOI-C is
implementable, provided the two inactive water lines that cross the dredge footprint are
removed prior to dredging. Dredging in AOI-D would be technically and administratively
implementable. For both AOI-C and -D, implementability would depend on vertical and lateral
delineation sampling during engineering design. Treatability studies also may be necessary
during engineering design, to optimize dewatering and any amendments that may be
warranted prior to disposal of dredged materials.

2.3 COST

The cost of implementing Remediation Alternative 5 is the highest of the active options. Its
estimated cost is $7.1M. This estimated cost is contingent on the dredging approach specified in
the FFS, particularly with respect to dredging footprint, dredging depth (i.e., to native clay plus
6-in. overdredge), and application of a 6-in. sand backfill layer after dredging to control
residuals. There is uncertainty in costs associated with the dredge volume (to be refined during
engineering design).

2.4 COMPARATIVE ANALYSIS

Table 2-2 compares the five remediation alternatives relative to six National Contingency Plan
criteria. Based on the detailed evaluation of the remediation alternatives presented in the FFS,
combined with updates to two criteria relative to Remediation Alternative 5, the key
advantages and disadvantages of each are summarized in Table 2-3.

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3 SUMMARY AND RECOMMENDATIONS

This FFS Addendum updates the evaluation of Remediation Alternative 5 to reflect current
understanding of the two inactive water lines. When the FFS was prepared in 2018, the main
source of information about a utility line at the mouth of Monguagon Creek was derived from
CRA (1997), which reported that utility crossings near the confluence of Monguagon Creek and
UTC prevented sediment removal from some portions of the creek.

Representatives of Grosse He Township subsequently shared information about two inactive
water lines in the vicinity of AOI-C. The original water main was an 8-inch diameter line that
ran parallel to the toll bridge, 4 feet south of the centerline of the bridge. That line was replaced
by a 12-inch diameter line that ran parallel to and approximately 50 feet south of the bridge. The
12-inch diameter line was replaced by a 16-inch diameter line that remains active and runs
directly under the north side of the bridge.

The FFS identified the following five remediation alternatives:

• Remediation Alternative 1: No Action

• Remediation Alternative 2: Thin Layer Cover in AOI-C and Cap in AOI-D (TLC & Cap)

• Remediation Alternative 3: Thin Layer Cover in AOI-C and Dredge in AOI-D (TLC &
Dredge)

• Remediation Alternative 4: Dredge in AOI-C and Cap in AOI-D (Dredge & Cap)

• Remediation Alternative 5: Dredge in AOI-C and -D (Dredge).

This addendum focuses on Remediation Alternative 5, which is the only remediation alternative
affected by the improved understanding of the buried water line. The 2018 FFS had identified
Remediation Alternative 3 (TLC & Dredge) as the preferred alternative. With the improved
understanding of the inactive water lines, Remediation Alternative 5 (Dredge) is equally
appropriate. The relatively small increase in cost ($300,000) of Remediation Alternative 5
(compared to Remediation Alternative 3) is balanced by improved long-term effectiveness;
consequently, this addendum recommends changings the preferred alternative to Remediation
Alternative 5. If, however, the cost of dredging differs markedly from that assumed in the FFS—
for example if the unit cost of disposal increases substantially—the added cost of dredging AOI-
C may not be justified and the preferred alternative would revert to Remediation Alternative 3.

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4 REFERENCES

CH2M. 2017. Final Remedial Investigation Report, Monguagon Creek, Upper Trenton Channel,
Detroit River Area of Concern, Riverview, Michigan. July.

CRA. 1997. Completion of Construction Report Monguagon Creek Remedial Action. Conestoga-
Rovers & Associates. July.

Detroit River Public Advisory Council. 2014. Targets for Removal of the Loss of Fish & Wildlife
Habitat and Degradation of Fish & Wildlife Populations Beneficial Use Impairments of the
Detroit River Area of Concern. Fish & Wildlife Technical Committee. Submitted to Michigan
Department of Environmental Quality, Office of the Great Lakes, Lansing, MI. Originally
adopted April 17, 2009; revised May 12, 2014.

Ramboll. 2018. Focused Feasibility Study. Monguagon Creek, Upper Trent Channel, Detroit
River Area of Concern. Riverview, Michigan. Prepared for U.S. Environmental Protection
Agency, Great Lakes National Program Office, Chicago, IL. Ramboll US Corporation.

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MCUTC and UTC Sites

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/ rS



INACTIVE 12- INCH WATER LINE
ON SEDIMENT SURFACE

SIDESCAN
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Source: Seaworks Group, LLC. 2021. Monguagon Creek,
Riverview, Ml. Hydrographic & Topographic Investigation

Figure 1-6.

Sidescan Sonar Survey

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Near-Shore Fish Habitat

Service Layer Credits: Woridjmagery: Maxar

Benthic Habitat
Bank Restoration
Near-Shore Fish Habitat

• Inactive water line
IZJ AOIs

Remediation Alternative

~~ Dredge with Backfill
I Optional MNR

CONCEPTUAL LAYOUT
REMEDIATION ALTERNATIVE 5

A

Habitat Enhancement

200
	I Feet

FIGURE 2-1

RAMBOLL US CONSULTING, INC.

A RAMBOLL COMPANY

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January 2022

Table 2-1. Summary of Remediation Alternatives, Monguagon Creek Upper Trenton Channel Site

Remediation
Alternative

Remedy Description

Total Remedy
Area (acres)

TLC Area
(acres)

Cap Area
(acres)

Dredge Area
(acres)

Estimated
Dredge Volume
(cubic yards)3

6-in. Overdredge
Volume
(cubic yards)3

Total Estimated
Dredge Volume
(cubic yards)3

1

No Action

0

0

0

0

0

0

0

2

TLC in AOI-C and Cap in AOI-D

2.8

0.95

1.8

0

0

0

0

3

TLC in AOI-C and Dredge in AOI-D

2.8

0.95

0

1.8

35,900

1,460

37,360

4

Dredge in AOI-C and Cap in AOI-D

2.6

0

1.8

0.8

1,590

615

2,205

5

Dredge in AOI-C and AOI-D

2.6

0

0

2.6

37,490

2,075

39,565

Notes:

AOI = area of interest
TLC = thin layer cover

a Dredge volumes are estimated based on spatial interpolation of soft sediment thickness. Because soft sediment thickness is not constrained in all areas of the Site, these
numbers are uncertain and will be evaluated during the remedy design.

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Table 2-2. Comparative Analysis of Alternatives, Monguagon Creek Upper Trenton Channel Site

REMEDIATION ALTERNATIVES

Remediation Alternative 1
No Action

Remediation Alternative 2
TLC & Cap

Remediation Alternative 3
TLC & Dredge

Remediation Alternative 4
Dredge & Cap

Remediation Alternative 5
Dredge

Threshold Criteria

Overall Protection of Human Health and
the Environment

No action would not detectably change conditions from baseline. Baseline

nditions include PAH I Us greater than 1 to 4, which suggest that the
bioavailable fraction of PAHs has potential to impact benthic invertebrates.
Risks to human health, to the extent that they may be posed in part by current
conditions at the MCUTC Site, are currently managed through a fish
uiiiMimplinn .ulvjsoiy. N<> At. lion. ihcicluu:. K pioict.iivc of hum.in Iu.mIUi. bill
not the environment.

Remediation of AOI-C and -D through a combined remedy using TLC, cap, and/or dredge, or through dredging alone, would replace the existing biologically active zone with clean material -.u u a 0
of the environment. Risks to human health, to the extent that they may be posed in part by current conditions at the MCUTC Site, are currently managed through a fish consumption advisory.

Attain RAOs:

1. Support restoration of beneficial uses
within Detroit River AOC by reducing the
mass, volume, and concentration of
COCs in MCUTC sediment

No l (•()( in
MCUTC sediment.

Ri;iiu;(li,iiioii ol A( )l (. .ind 1) Ihioiigh .i i.ombiiu:tl ii;iiu;dy using 11 (.. uip. . in M( .IJ1 (. scdimcnl.

2. Reduce short- and long-term risks to
human health and the environment

No action would not reduce risks, though its implementation also would not
cause any short-term risks to human health or the environment.

Remediation of AOI-C and -D through a combined remedy using TLC, cap, and/or dredge, or through dredging alone, would reduce risks, while limiting physical, chemical, or biological harm to the ecosystem associated with implementation of Remediation Alternatives 2 through 5.

3. Improve habitat of the Site through
targeted restoration

No action would not improve habitat of the Site through restoration.

Under Remediation Alternatives 2 through 5, habitat restoration would be implemented in AOI-C and -D following remediation, as well as within UTC-East.

4. Manage contaminated sediments that are
susceptible to scour and downstream
transport

No action would not manage any sediments, including those that are
susceptible to scour and downstream transport

Remediation of AOI-C and -D through a combined remedy using TLC, cap, and/or dredge, or through dredging alone, would manage contaminated sediments that are currently susceptible to scour and downstream transport.

Balancing Criteria

Long-Term Effectiveness and Permanance

No action would not be effective in the long-term and does nol piovide a
permanent remedy.

1 lu; long-term effectiveness and permanunou ol 1 Lc in AOl c and ol cap in
AOI-D depend on proper engineering design to ensure that both covers can
withstand high energy events.

1 hu ulluolivuness of Remediation Allui native 3 would be a function of
sequencing of the UTC remedy. Assuming recontamination risk is mitigated,
the long-term effectiveness of TLC in AOI-C would depend on proper
engineering design to ensure that the thin cover can withstand existing
conditions.

Also assuming recontamination risk is mitigated, the dredging proposed for
AOI-D would be effective in the long-term and would be essentially permanent.

1 hu effectiveness of Remediation Alternative 4 would bu a function of
sequencing of the UTC remedy.

In addition, the long-term effectiveness of the AOI-D cap would depend on
proper engineering design to ensure that the cap can withstand high energy
events. The steep bathymetry within AOI-D and high energy in the channel
pose some risks to the long-term effectiveness of a cap over AOI-D,
particularly in case of extreme events, such as flooding, earthquake, or ice
scour.

Because contaminated sediments would be left in place underneath the AOI-D
cap, this remediation alternative would not be permanent.

The effectiveness of Remediation Allui native b would be a function of
sequencing of the UTC remedy. Dredging would be permanent.

Reduction in Toxicity, Mobility, or Volume
through Treatment

No action would not reduce the toxicity, mobility, or volume of COCs.

Amendments may be integrated into the cover materials used with TLC and
capping, in order to reduce bioavailability of PAHs and PCBs. Cover materials
will be selected during engineering design.

Dredged materials may be treated prior to disposal in order to yield physical
and chemical characteristics that will reduce mobility.

Amendments may be integrated into the cover materials used with TLC and
capping, in order to reduce bioavailability of PAHs and PCBs. Cover materials
will be selected during engineering design.

Dredged materials may be treated prior to disposal in order to yield physical
and chemical characteristics that will reduce mobility.

Short-Term Effectiveness

Because no action does not require any Site work, its implementation would
not pose short-term risks. It also would not mitigate any short-term risks that
are potentially posed under current conditions.

Site work associated with TLC, capping, and/or dredging would involve Site access and equipment staging, thin cover placement, cap placement, sediment removal, treatment, transport and disposal, and long-term monitoring. Site access and equipment staging could harm wetland vegetation. Vegetative restoration
typically requires at least one growing season. Placement of thin cover and/or cap and dredging can have short-term effects on the benthic community. Benthic recolonization with suitable substrate typically occurs within weeks to months following the completion of construction. Most recolonization occurs through
natural import of biological species. To the extent practicable, materials used for TLC, cap or backfill would be selected based on compatibility with habitat preferences for local invertebrates to support rapid recolonization.

Short-term risks to field technicians include the potential for exposure to chemicals in sediment and the hazards of offshore construction. These risks would be mitigated through compliance with OSHA regulations and Site-specific health and safety plans to reduce on-site construction risks and the risk of chemical
exposure. Therefore, short-term risks of remedy implementation would be mitigated. Short-term community risks also exist due to the need to transport cap/cover materials to the Site via local roads—these risks could be reduced by developing a Site-specific transportation plan during design.

Integral Consulting Inc.

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Focused Feasibility Study Addendum
Monguagon Creek Upper Trenton Channel

January 2022

Table 2-2. Comparative Analysis of Alternatives, Monguagon Creek Upper Trenton Channel Site

REMEDIATION ALTERNATIVES

Remediation Alternative 1
No Action

Remediation Alternative 2
TLC & Cap

Remediation Alternative 3
TLC & Dredqe

Remediation Alternative 4
Dredqe & Cap

Remediation Alternative 5
Dredqe

Implementability

No implementation would be associated with the No Action alternative. There
are therefore no constraints on the No Action alternative's technical or
administrative feasibility.

TLC would be implementable in AOI-C, requiring the design and installation of
a thin cover layer, and preparation and approval of a long-term monitoring plan
and implementation of that plan. Field characterization would be needed during
engineering design to confirm lateral extent and to assess geotechnical
characteristics of the existing soft sediment layer to ensure stability of the thin
cover.

Capping would be implementable in AOI-D, though there are some factors
unique to AOI-D that would create challenges. Implementation would require
the design and installation of the cap, and preparation and approval of a long-
term monitoring plan and implementation of that plan. Some portions of AOI-D
may not be conducive to capping due to steep bathymetry and high energy.
Field characterization would be needed during engineering design to confirm
lateral extent and to assess the geotechnical characteristics of the existing soft
sediment layer and bathymetry to ensure cap stability. Hydrological modeling
also may be warranted during engineering design to ensure that reduced water
depths would not significantly affect flood storage capacity or flow.

TLC in AOI-C would be implementable, requiring the design and installation of
a thin cover layer, and preparation and approval of a long-term monitoring plan
and implementation of that plan. Field characterization would be needed during
engineering design to confirm lateral extent and to assess geotechnical
characteristics of the existing soft sediment layer to ensure stability of the thin
cover.

Dredging would be implementable in AOI-D, requiring vertical and lateral
delineation sampling during engineering design, remedy design and
implementation, and preparation and approval of a long-term monitoring plan
and implementation of that plan. Treatability studies also may be necessary
during engineering design, in order to optimize dewatering and any
amendments that may be warranted prior to disposal of dredged materials.

Dredging would require vertical and lateral delineation sampling during
engineering design, remedy design and implementation, and preparation and
approval of a long-term monitoring plan and implementation of that plan.
Treatability studies also may be necessary during engineering design, in order
to optimize dewatering and any amendments that may be warranted prior to
disposal of dredged materials.

Constraints on the implementability of capping in AOI-D may be related to
hydrodynamics, bathymetry, and maintenance of water depths. Testing to be
undertaken during engineering design may find that some portions of AOI-D
are too steep or subject to sheer stresses too great to allow capping
throughout the entire AOI. Hydrological modeling also may be warranted
during engineering design to ensure that reduced water depths would not
significantly affect flood storage capacity or flow. Permitting may place some
limits on administrative implementability, particularly if capping cannot be
permitted without first conducting some dredging to maintain water depth.

Dredging in AOI-C and -D would be technically and administratively
implementable. Dredging would require vertical and lateral delineation
sampling during engineering design, remedy design and implementation, and
preparation and approval of a long-term monitoring plan and implementation of
that plan. Treatability studies also may be necessary during engineering
design, in order to optimize dewatering and any amendments that may be
warranted prior to disposal of dredged materials.

Cost

No costs are associated with the No Action alternative.

Least expensive of the active remediation alternatives.

Cost uncertainty more likely to lead to higher than lower actual costs; current
assumption is that no dredging is necessary in AOI-D prior to capping. If
hydrological modeling determines otherwise, costs will increase.

Intermediate in expense, relative to the other active remediation.

Cost uncertainty is associated with dredge depth, which is currently assumed to extend to native clay. Vertical delineation sampling conducted during
engineering design may reduce dredge depth and costs.

Most expensive of the active remediation alternatives.

Cost uncertainty is associated with dredge depth, which is currently assumed
to extend to native clay. Vertical delineation sampling conducted during
engineering design may reduce dredge depth and costs.

Notes:

AOI = area of concern
COC = chemical of concern
CUG = cleanup goal

MCUTC = Monguagon Creek Upper Trenton Channel

MNR = monitored natural recovery

OSHA = Occupational Safety and Health Administration

PAH = polycyclic aromatic hydrocarbon

PCB = polychlorinated biphenyl

RAO = remedial action objective

Rl = remedial investigation

TLC = thin layer cover

TU = toxic unit

UTC = Upper Trenton Channel

Integral Consulting Inc.

Page 2 of 2

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Focused Feasibility Study Addendum
Monguagon Creek Upper Trenton Channel

January 2022

Table 2-3. Advantages and Disadvantages of Each Remediation Alternative

Remediation
Alternative

Advantages

Disadvantages

1. No Action

No cost.

Not adequately protective of the environment.

No implementation and therefore no short-
term risk associated with implementation.

Does not achieve RAOs.

2. TLC & Cap

Least expensive of the active remediation
alternatives.

Some portions of AOI-D may not be conducive to
capping, due to steep bathymetry and high energy.

TLC is readily implementable in AOI-C, as it
will not disturb underground utilities.

If hydrodynamic modeling indicates that TLC in AOI-C
and/or capping in AOI-D will reduce flood storage
capacity, then it may be necessary to dredge before
installing the thin cover and/or cap, which could

substantially increase costs.

If hydrodynamic modeling indicates that TLC in AOI-C
will reduce flood storage capacity, then it may be
necessary to dredge before installing the thin cover,
which could pose implementability challenges and
increase costs.

Vertical and lateral delineation of AOI-D necessary
during engineering design.

More expensive than TLC & Cap and Dredge & Cap,
though costs may be reduced based on delineation of
AOI-D.

Some portions of AOI-D may not be conducive to
capping, due to steep bathymetry and high energy.

If hydrodynamic modeling indicates that capping in
AOI-D would reduce flood storage capacity, it may be
necessary to dredge before capping in AOI-D, which
could substantially increase costs.

5. Dredge Dredging, including overdredging and Most expensive of the active remediation alternatives,

backfilling, enhances this remedy's long-term
effectiveness and reduces risk of scour and
downstream transport of COCs.

The steep bathymetry of AOI-D likely poses
fewer implementation challenges for dredging
than for capping.

Notes:

AOI = area of interest
COC = chemical of concern
TLC = thin layer cover

3. TLC & Dredge TLC is readily implementable in AOI C, as it
will not disturb underground utilities.

Dredging in AOI-D, including overdredging
and backfilling, enhances this remedy's long-
term effectiveness and reduces risk of scour
and downstream transport of COCs.

The steep bathymetry of AOI-D likely poses
fewer implementation challenges for dredging
than it does for capping.

4. Dredge & Cap Dredging in AOI-C enhances this remedy's
long-term effectiveness and reduces risk of
scour and downstream transport of COCs.

Integral Consulting Inc.

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