NC0003573_Appendix F - Ex-Situ Capture 60 Percent Design_20210921

Geosyntec °
consultants
Geosyntec Consultants of NC, P.C.
NC License No.: C-3500 and C-295
GE#S
GEOSemcel, 11C, Ge0te[1.1,1,0l in0 H enah Engineers
Appendix F
Seep Ex -Situ Capture Remedy Design Report
TR0795
Aug-2021

GEtS
Geotechnical, Environmental and Materials Engineers
Ex -Situ Capture Remedy 60% Design
Submittal
Chemours Fayetteville Works
Prepared for
The Chemours Company FC, LLC
22828 NC Highway 87
Fayetteville, NC 28306
Prepared by
GEOServices, LLC
5559 North Lee Highway
Cleveland, TN 37312
August 2021

GE :^S
G6elefln ioaI, Enuironmenlal and Materials Engineers
Table of Contents
1. Introduction and Objectives 1
2. Seeps Capture Remedy Design 1
2.1 Seep Capture Basis of Design 2
2.2 Seep Ex -Situ Capture Design 2
2.2.1 Implementation 3
2.2.2 Mitigation of Sediment Loading 5
2.2.3 Operation and Maintenance 5
3. Summary and Closing 6
List of Tables
Table 1 Hydraulic Loading of Representative Groundwater and Seep Sources
List of Figures
Figure 1 Concrete Channel Capture Detail (Plan View)
Figure 2 Concrete Channel Capture Detail (Section)
Figure 3 Collection Structure Detail
Attachment 1 60% Plan Set
Attachments
Ex -Situ Capture Remedy 60% Design
i August 2021

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&6efenn ioaI, Enuironmenlal and Materials Engineers
List of Abbreviations
BOD Basis of Design
COA Consent Order Paragraph 12 Addendum
ft feet
gpm gallons per minute
GWTP groundwater treatment plant
HFPO-DA hexafluoropropylene oxide dimer acid
mg/L milligram per liter
MSL Mean Sea Level
NCDEQ North Carolina Department of Environmental Quality
PFAS per- and polyfluoroalkyl substances
PFMOAA perfluoro- 1 -methoxyacetic acid
PMPA perfluoro-2-methoxypropanoic acid
TSS total suspended solids
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GE :^S
&6efenn ioaI, Enuironmenlal and Materials Engineers
1. Introduction and Objectives
GEOServices, LLC (GEOS) has prepared for The Chemours Company, FC, LLC (Chemours) this
60% Engineering Design Report for the long-term seep remediation system planned to be installed
at the Chemours Fayetteville Works facility in Bladen County, North Carolina. This report has
been prepared pursuant to the Consent Order Paragraph 12 Addendum (COA) which requires a
60% design report for the Barrier Wall and Groundwater Extraction and Treatment System be
submitted by August 15, 2021 to North Carolina Department of Environmental Quality (NCDEQ)
for approval. This report describes the basis of design (BOD) for the long- term seep capture
system portion of the overall system.
The COA provides that Chemours shall proceed with the design and the installation of a barrier
wall and groundwater extraction and treatment system to reduce per- and polyfluoroalkyl
substances (PFAS) loading from groundwater flow from under the Facility to the Cape Fear River
and Willis Creek. The primary objective of the long-term seep remedy is to reduce the total annual
mass loading of PFAS (as measured by the indicator parameters hexafluoropropylene oxide dimer
acid [HFPO-DA], perfluoro-2-methoxypropanoic acid [PMPA], and perfluoro- 1 -methoxyacetic
acid [PFMOAA]) to the Cape Fear River from Seeps A through D.
The remedy objectives outlined in the COA results in a remedy design with three components:
• The groundwater interception remedy
• The Seeps A and B ex -situ capture remedy
• The groundwater treatment plant
The following sections of this document and the supporting figures outline the design basis,
capture basis, implementation, and methods utilized for the ex situ seeps capture.
Per paragraph 2(c)(i) of the COA, the seeps ex -situ capture remedy design consists of two capture
locations where the seep flow headwaters exist above ground above the barrier wall alignment at
Seep A and Seep B. The design uses impoundment and pumping systems along with equalization
basins sized to accommodate dry weather flows and stormwater flows from rainfall events up to
0.5 inches over 24 hours. The captured water is then conveyed to the groundwater treatment plant
(GWTP) which is being designed to treat collected water and remove 99% of indicator parameters.
2. Seeps Capture Remedy Design
Pursuant to paragraph 2(c)(i) of the COA, the seep remedy at Seeps A and B will consist of the
ex -situ capture of dry weather flow and stormflow up to one-half inches in a 24-hour period. The
seep remedy was developed based on flume data and catchment modeling prepared by Geosyntec
Consultants of NC, P.C. (Geosyntec; see Appendix E of the 60% Design Report). The flows
Ex -Situ Capture Remedy 60% Design 1 August 2021

GE :^S
&6efenn ioaI, Enuironmenlal and Materials Engineers
utilized for design were based on the flows submitted in Table 8 of Appendix E. The table
outlining these flows is shown below.
Flow
Flow Rate to
GWTP (gpm)
Flow Volume During
0.5" storm (gallons)
Seep A Rainfall
76
110,000
Seep B Rainfall
32
46,000
Subtotal
108
156, 000
Seep A Dry Weather Flow
31
NA
Seep B Dry Weather Flow
130
NA
Subtotal
161
NA
Total
269
156,000
The remaining sections of this document discuss the basis for the design, the collection, storage,
conveyance, solids control and maintenance requirements for the Ex -Situ Capture 60% design.
2.1 Seep Capture Basis of Design
The preliminary design of the seep capture system is shown on the attached plans (Sheets SC-0.1
through SC-2.2). attached plans. The site slopes east toward the proposed barrier wall. The upper
elevation of the drainage area for each seep is approximately 148 feet (ft) Mean Sea Level (MSL).
The preliminary alignment of the barrier wall (Appendix C) crosses both Seep A & B at elevation
64ft (MSL). The seeps will be collected in the existing channels/low areas at elevation 80ft (MSL)
above the barrier wall elevation of 72 ft (MSL). The collection system will gravity drain to a
proposed basin system shown on the attached plans. The required volume for treatment from each
seep will be collected and stored upstream of the barrier wall.
The collection system for each seep will function as an equalization basin allowing the continuous
flow to the treatment plant. The basin system will capture and store both the dry weather flow and
the stormwater flows from rainfall events up to 0.5 inches over 24 hours.
2.2 Seep Ex -Situ Capture Design
The dry weather flow for each seep will be collected via a gravity system flowing from the channel
to the equalization basin system (Figure 1). The additional seep flow from rainfall events up to 0.5
inches over 24 hours will be collected via a second gravity system that drains to an upper basin
that will function as a detention / equalization basin. The upper basin will provide steady flow into
the lower equalization basin removing spikes in flow resulting from stormwater flows from rainfall
events up to 0.5 inches over 24 hours.
Ex -Situ Capture Remedy 60% Design 2 August 2021

GE :^S
G6elefln ioaI. Enuironmenlal and Materials Engineers
2.2.1 Implementation
2.2.1.1 Collection/Storage
A concrete channel will be constructed to provide the means for collecting the dry weather flow.
The concrete channel will include a small diversion weir to divert the dry weather flow into the
first catch basin of the collection structure (Figures 1 - 2).
30'
Elev. 82.30
Elev_ 80.307
Concrete Channel
Elev. 80.30 —
a a 4 1" Rainfall Peak
Dry Weather How • Capture Structure
Diversion Weir (Catch Basin)
+.
\
"• O Cry Weather Floe
i• Capture Structure
(Catch Basin) --
lev. 82.30—'1"
Pipe to Lower
Equalization Basin
r
0 0
0 =
00
00
00 0
00
00
= =
00
00
00
}" Rainfall Peak
Runoff Diversion
Weir
Elev. 80.00
Elev. 80.00
E
Pipe to Upper
Basin
Elev. 82.00
Elev. 82.00
Figure 1— Concrete Channel Capture Detail (Plan View flow left to right)
Concrete Channel
Each structure shall
include FABCO
Screen Box (or
approved equal)
Dry Weather Flow
Capture Structure
(Catch Basin)
Dry Weather Flow
Diversion Weir
i" Rainfall Peak
Runoff Diversion
Weir
? Rainfall Peak
Capture Structure
(Catch Basin)
Figure 2 — Concrete Channel Capture Detail (Section)
Flow from the collection structure to the equalization basin will be controlled via an orifice (Figure
3). The orifice will be sized to allow the dry weather flow from each seep into the gravity pipe
system. The gravity pipe will be oversized to allow adjustments to the collection system including
Ex -Situ Capture Remedy 60% Design 3
August 2021

GE :^S
G6elefln ioaI, Enuironmenlal and Materials Engineers
enlarging the orifice if necessary. A trash rack type system will be installed at the upstream end of
the concrete channel to reduce effects of unwanted material in the collection structures. A detail
of the collection structure is provided in Figure 3 below.
Chamber 2
Solid Top -
Pipe to Basin
" Rainfall Peak
Capture Structure
(Catch Basin)
Flow Control
Orifice
Chamber 1
Grated Top
Figure 3 — Collection Structure Detail
All flow above the dry weather flow rate will overtop the dry weather flow diversion weir and
continue to flow in the concrete channel (Figure 2). A second diversion weir will be included in
the channel to direct all flow rates more than the dry weather flow and up to the peak flow from
rainfall events up to 0.5 inches over 24 hours into the second collection structure. This structure
will also include an orifice to control the flow into the gravity system. The gravity system from
the second collection structure drains into the upper basin. The upper basin will include an outlet
structure that detains the rainfall runoff volume and outlets the water into the lower equalization
basin over a 24-hour period. Because the flow rate between the upper and lower basin is controlled,
the upper basin also adds the benefit of acting as a settling basin to reduce the sediment/ total
suspended solids (TSS) load that is transferred to the lower equalization basin. The basin system
includes a float -controlled gate that will minimize capture of volumes above the t/2" rainfall in 24
hours runoff volume. Once the volume in the upper basin reaches the required collection volume,
the float will close the gravity pipe from the second collection structure. The pipe will open again
once the water level has lowered via the detention outlet structure.
Both basins are sized to accommodate the conservative BOD collection volumes. Both basins will
also include emergency spillways to maintain integrity in the event that there is a malfunction of
the system. All basins will include a liner system to prevent infiltration of captured water back into
the soil.
Ex -Situ Capture Remedy 60% Design 4 August 2021

GE :^S
&6efenn ioaI, Enuironmenlal and Materials Engineers
An overview of the anticipated seep capture system outlining the location of the collection channel
and equalization basins can be seen on Sheets SC-1.0 and SC-2.0 in the Attachments.
2.2.1.2 Conveyance
The equalization basin will gravity flow into the wet well for the pump station. The pump station
will provide a steady/known flow to the treatment plant via a force main system. This force main
is separate from the extraction well force main discussed in Appendix D. Each seep (A and B) will
include a pump station that is sized to function based on the size of the equalization basin of each
seep. The general location of the pump station and conveyance system are shown on Sheet SC-0.1
in the Attachments.
2.2.2 Mitigation of Sediment Loading
The ex -situ capture system will deliver captured flow to the treatment plant discussed in Appendix
G. The hydraulic loading at the treatment plant will include seep dry weather flow, seeps
stormflow, groundwater from the Surficial Aquifer, and Groundwater from the Black Creek
Aquifer transferred via force main from respective collection points. The current technical
specification for the treatment plant assumes 250 milligram per liter (mg/L) peak solids loading at
a max design flow of 1500 gallons per minute (gpm) including all combined sources. The turnkey
treatment facility is not currently planned to have a means of addressing sudden surges in influent
solids loading. As such, the solids handling must be addressed in the ex -situ capture.
The collected stormwater flow is expected to be the most sediment laden component of the
proposed ex situ capture network. Sheets SC-1.2 and 2.2 in the attachments shows how a series of
basins will be utilized for TSS control. The upper basin for each seep is designed to operate as a
dry basin during dry weather storage conditions but will be utilized during stormflow events. The
upper basin for each seep capture system will allow settlement of TSS prior to the water entering
the lower equalization basin.
2.2.3 Operation and Maintenance
As with all systems, general maintenance will be required. A summary of expected weekly
inspections and maintenance is listed below:
• Inspect trash rack system upstream of concrete channel Remove all material from trash
rack screen (debris, leaves, limbs, sticks, etc.).
• Inspect dry weather flow collection structure including inlet grates and orifice. Remove
any debris blocking the inlet grate and/or orifice.
• Inspect 1/2" rainfall collection structure including inlet grates and orifice. Remove any
debris blocking the inlet grate and/or orifice.
• Inspect outlets into each basin. Verify that the gravity systems are discharging freely into
each basin.
Ex -Situ Capture Remedy 60% Design 5 August 2021

GE :^S
&6efenn ioaI, Enuironmenlal and Materials Engineers
• Inspect outlet structure and gravity system from upper basin to lower equalization basin.
• Measure sediment in each basin and record depths. Remove sediment, as necessary.
• Inspect pump station and test each pump.
In addition to the general maintenance listed above, the system shall be monitored after rainfall
events on a routine schedule.
3. Summary and Closing
The 60% design of ex -situ seep capture is part of the overall hydraulic barrier system planned to
accomplish requirements pursuant to paragraph 2(c) (i) of the COA. The hydraulic modeling
results and subsequent hydraulic loading information prepared by Geosyntec has been utilized to
prepare the design documents which are attached. The hydraulic loading information can be found
in the BOD located in Appendix E.
The design plan indicates the collection system for each seep will function as an equalization basin
allowing steady flow to the treatment plant. The basins at each seep will capture and store both the
dry weather flow and stormwater flows from rainfall events up to 0.5 inches over 24 hours. The
upper basin for each seep will operate as a dry basin during dry weather flow conditions but will
be utilized during stormflow events. The control of flow between upper basin and lower basin for
each seep will allow settlement of TSS prior to the water entering the lower equalization basin for
additional solids control.
Specific next steps include developing TSS control measures for the basins and sizing conveyance
infrastructure between basin systems and the water treatment plan. The 90% design submittal will
include plans, calculations, and specifications for the ex -situ seep capture system.
Ex -Situ Capture Remedy 60% Design 6 August 2021

Attachments
60% Plan Set
Ex -Situ Capture Remedy 60% Design August 2021

The Chemours Company
Fayetteville, North Carolina
Ex -Situ Seep Capture Design Plans
August 12, 2021
DRAWING INDEX
GENERAL
G-1.0 COVER SHEET
EX -SITU SEEP CAPTURE
SC- 0.1 LOCATION PLAN
SC-1.0
SC-1.1
SC-1.2
SC-2.0
SC-2.1
SC-2.2
SEEP A
SEEP A
SEEP A
SEEP B
SEEP B
SEEP B
CAPTURE
CAPTURE
CAPTURE
CAPTURE
CAPTURE
CAPTURE
SYSTEM GRADING PLAN
SYSTEM DETAILS
SYSTEM DETAILS
SYSTEM GRADING PLAN
SYSTEM DETAILS
SYSTEM DETAILS
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SCALE:
AS SHOWN
DATE
August 12, 2021
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DRAWING:
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PROJECT NUMBER:
45-20803

Fayetteville, North Carolina

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