NC0003573_Engineering Report_20210921

Chemours.
Chemours Fayetteville Works
Stormwater Capture and Treatment System
Engineering Report
February 2021

Chemours Company — Fayetteville Works Addendum to Consent Order Paragraph 12
STORMWATER CAPTURE AND TREATMENT SYSTEM
DESIGN BASIS AND DATA ANALYSIS
Chemours will treat stormwater runoff from the Chemours Monomers/IXM area (Monomers area) with a
stormwater capture and treatment system (Treatment System) designed to treat for per- and polyfluoroalkyl
substances (PFAS) compounds. The goal of the Treatment System is to capture and treat stormwater, up to
the designated design storm, and reliably achieve a removal efficiency of 99% for concentrations of
indicator parameters GenX (HFPO-DA), Perfluoro-2-methoxypropanoic acid (PMPA), and Perfluoro-1-
methoxyacetic acid (PFMOAA). The Treatment System is to commence operation by June 30, 2021.
The Addendum to Consent Order Paragraph 12 (CO Addendum) specifies (in Paragraph 4) that "By June
30, 2021, Chemours shall complete installation of and commence operation of a system that captures and
treats stormwater from the Monomers/IXM area at the Facility." The CO Addendum also specifies that
"the Monomers/IXM stormwater capture, and treatment system consistently captures stormwater from the
Monomers/IXM area in rain events up to one (1) inch within a 24-hour period and removes PFAS
compounds (as measured by concentrations of indicator parameters GenX, PMPA, and PFMOAA) at a
minimum removal efficiency of 99%."
This document provides the conceptual design and engineering assumptions for the Treatment System.
INFLUENT STORMWATER
The Treatment System will treat stormwater runoff from a 13.9-acre drainage areal in the Monomers area.
The Treatment System will receive stormwater runoff from the Monomers area via the Cooling Water
Channel (CWC), and separate stormwater-only ditch, through an offline configuration. In the CWC,
stormwater and Non -Contact Cooling Water (NCCW) will be separated to create a stormwater-only channel
(NCCW will be in a separate pipe), with a sump and diversion structure located in the southwest corner of
the Monomers area. There will also be a separate sump and diversion structure for the stormwater-only
ditch. This is illustrated in Figure 1.
The CO Addendum specifies (in Paragraph 4) that the Treatment System should capture and treat
stormwater from the Monomers area in rain events up to one inch within a 24-hour period. The data
collected to characterize the influent stormwater (see Data Analysis below) were intended to be
representative of the stormwater that will be treated. These samples, however, were not collected as influent
samples to the Treatment System. Further, actual influent concentrations will be highly variable based on
a number of factors including rainfall intensity, duration, and antecedent conditions.
' Some rainwater is collected within process water secondary containment in the Monomers area, which is presently
disposed of offsite and therefore does not currently flow through the site conveyance network to Outfall 002. However,
13.9 acres was conservatively assumed as the drainage area to the Treatment System.
2 February 2021

Chemours Company — Fayetteville Works
Addendum to Consent Order Paragraph 12
Figure 1. Proposed Treatment System Drainage Area
CONCEPTUAL TREATMENT SYSTEM
Figure 2 shows the potential location of the Treatment System and discharge location where treated effluent
is discharged back into the site conveyance network (and ultimately to Outfall 002). These locations are
preliminary and may be re-evaluated during the design process.
Diverted stormwater will be pumped via an offline pump station(s)2 into a storage tank that will function
as flow equalization and initial pretreatment (via sedimentation) for the Treatment System. Following
treatment, treated stormwater will be piped back into the Site Conveyance Network, comingling with
NCCW and flowing to Outfall 002. This concept is illustrated in Figure 3. When the capacity of the sump
and diversion structure is exceeded during storm events with large total rainfall depths and/or intensities,
stormwater flows will flow over the diversion, comingling with NCCW, and flow to Outfall 002. The
concept for the diversion sump is illustrated in Figure 4.
2 An offline pump station pumps stormwater flows that do not exceed the capacity of the Treatment System.
3 February 2021

Chemours Company — Fayetteville Works
Addendum to Consent Order Paragraph 12
Legend
f• 1 1 1 F
■ Potential Location of Stormwater Treatment System
Potential Effluent from Stormwater Treatment System
Site Boundary
Site Conveyance Network
Ditch Types
Wood Lined Trench
Wastewater Treatment Plant Discharge
Cooling Water Channel
Open Channel to Ouffall 002
DuPont Area
`Location of stormwater treatment system is approximate and subject to change.
Figure 2. Potential Location of Proposed Treatment System
4
February 2021

Chemours Company — Fayetteville Works
Addendum to Consent Order Paragraph 12
NCCW
V retreated
storrnwater
(potentially when
rainfall is in
excess of 1"
design storm)
4- Treated
stormwater
Diversion -Storage -Treatment Flow Diagram
filtration
treatment train
Existing
channel (to +
OF 002)
4- F F
junction
manhole
storage/
pretreatment
tanks
Ul
Stormwater runoff greater
than the design storm
r
Pipe for NCCW (within existing
•Nor to scale channel)
pump
station
diversion
weir
Stormwater flows into a
diversion sump/pump
station. High flows (above
design storm'. continue
through the dmnnel (by
flowing over the exit weir
in the diversion sump'.
Existing concrete -lined channel
(will be stormwater only)
Figure 3. Treatment System Design Schematic
Legend
Stormwater
J NCCW (within pipe)
Stormwater bypassing
the treatment system
(greater than design
storm)
THIS VIEW SHOWN SCHEMATICALLY FOR REFERENCE ONLY
Diversion
sump/pump
station (to
storage tank
& treatment
system)
Figure 4. Diversion Weir Concept (Applicable to the Diversion from the CWC3)
Stormwater Runoff Volume
To determine the necessary storage capacity of the storage tank, the volume of stormwater runoff draining
to the Treatment System was determined, based on guidance from the North Carolina Department of
Environmental Quality (NCDEQ) Stormwater Design Manual (Manual).4 The Simple Method for Runoff
Volume, as outlined in Part B of the Manual, was used to determine the runoff volume from the 1-inch rain
event (as specified in the CO Addendum) from the drainage area.
For hydrologic modeling purposes, the diversion sump and weirs for the stormwater only channel were assumed to
be identical to that of the CWC.
4 North Carolina Department of Environmental Quality. North Carolina Department of Environmental Quality
Stormwater Design Manual — Stormwater Calculations. 2017. https://deq.nc.gov/sw-bmp-manual
5 February 2021

Chemours Company — Fayetteville Works Addendum to Consent Order Paragraph 12
This method first determines the runoff coefficient using the impervious fraction of the drainage area, as
shown in Equation 1. The impervious fraction was determined using aerial imagery. The drainage area to
the Treatment System was divided into the following classifications: building/rooftop, impervious (i.e.,
pavement), gravel/river rock, and open/undeveloped. Table 1 shows the characterization of the drainage
area by land cover. Building/rooftop and impervious (i.e., pavement) areas were assumed to have an
imperviousness of 100%, gravel/river rock areas were assumed to be 80% impervious, and
open/undeveloped areas were assumed to be 11% impervious. These assumed values of imperviousness,
based on land cover, were based on guidance from the NCDEQ Stormwater Design Manual (Part B,
Stormwater Calculations, Table 1)5 and best professional judgement. The resulting impervious fraction of
the drainage area to the Treatment System was 0.83, or 83% impervious.
Table 1. Land Cover of Drainage Area to Treatment System
Land Cover
Area (acre)
Building/rooftop
2.1
Pavement
2.8
Gravel/river rock
8.2
Open/undeveloped
0.8
Total Area
13.9
Equation 1: Runoff Coefficient
Rv = 0.0 5 + 0.9 x IA
where,
R„ is the runoff coefficient (unitless); and
IA is the impervious fraction (unitless).
Rv=0.05+0.9x0.83=0.80
Stormwater Equalization/Storage Tank Volume
The design volume was then determined using Equation 2 with a design storm depth of one inch. The
drainage area to the Treatment System was determined to be approximately 13.9 acres, as illustrated in
Figure 1. Rainwater in process sumps will continue to be disposed of off -site. The process sumps drainage
area has secondary containment that captures rainwater for disposal off -site (i.e., it does not drain to the site
conveyance network and then to Outfall 002). Rainwater captured in non -process sumps will be sent to the
Treatment System in accordance with standard operating procedures. To be conservative, the full 13.9-acre
drainage area is assumed for use in subsequent calculations.
5 Open/undeveloped areas were assumed to be similar to "Lawns, heavy soils, flat (<2%)", in Table 1 in Part B of the
Manual, which references a rational runoff coefficient of 0.15 (and imperviousness of 0.11).
6 February 2021

Chemours Company — Fayetteville Works Addendum to Consent Order Paragraph 12
Equation 2: Design Volume
DV = 3630 x RD X Rv x A
where,
DV is the design volume (cubic feet);
RD is the design storm depth (inches);
R„ is the runoff coefficient (unitless); and
A is the drainage area (acres).
DV=3630 tuft in x ac x1inx0.80x13.9ac
DV = 40, 214 cu ft = 300, 800 gal
The calculated design volume was used to determine the volume capacity of the storage tank, resulting in
a storage tank that will be designed to have a minimum volume capacity of approximately 300,800 gallons.
The capacity of the storage tank is sized to store the entire runoff volume from the 1-inch design storm,
even though stormwater will also be treated by the Treatment System throughout the storm event, allowing
for additional capacity in the storage tank.
Storage Tank Drawdown Time and Design Treatment Flowrate
The maximum treatment flowrate of stormwater that the Treatment System will be capable of treating was
determined based on the flowrate to drain the storage tank (assuming the tank is full) in the desired tank
drawdown time. With a desired drawdown time of 36 hours,6 the discharge rate from the storage tank
(assuming it is full) was calculated to be 139 gallons per minute (gpm) using Equation 3.
Equation 3: Design Flowrate
DV 1 hr
Design Flow = x
T 60 min
where,
Design Flow is the maximum design flowrate for the
Treatment System (gallons per minute [gpm]);
DV is the design volume of the storage tank (gallons); and
T is the drawdown time of the storage tank (hours).
6 For a sand filter, the Manual requires at least two inches per hour media filtration and a maximum ponding depth of
six feet, which results in a maximum drawdown time of 36 hours. To be consistent with guidance in the Manual for
other flow -through treatment Stormwater Control Measures (SCMs), a 36-hour drawdown time was assumed.
7 February 2021

Chemours Company — Fayetteville Works Addendum to Consent Order Paragraph 12
300,800 gal 1 hr
Design Flow = x = 139 gpm
36 hr 60 min
The Treatment System is planned to be designed for a (conservative) treatment flowrate of 150 gpm, or
0.22 million gallons per day, on a continuous basis. This flowrate represents stormwater captured in the
storage tank being slowly released to the Treatment System for treatment, which is why it is assumed as a
constant flowrate. It should be noted that the storage tank and treatment system itself function together with
the diversion structure(s) and pump station(s) to provide adequate capture and treatment of stormwater
runoff from the drainage area.
It should also be noted that design guidance (from the Manual) for other flow -through Stormwater Control
Measure (SCM) types was also examined,' for determination of the storage tank design volume, tank
drawdown time, and subsequent treatment system design flowrate.
Chemours will have a temporary storage tank(s) that will hold the 300,800 gallons with a permanent
equalization tank to be ready for operation in July 2021.
Diversion Sump, Weirs, and Pumps
Stormwater runoff from the CWC and from the stormwater-only ditch will be directed to diversion
sumps/pump stations. The sumps were assumed to be 20 feet long, four feet wide, and eight feet high.
Stormwater runoff will enter the diversion sump at the same elevation of the existing CWC (i.e., the weir
wall was five feet high, as measured from the bottom of the diversion sump, resulting in the weir wall being
at the same elevation of the existing channel bottom), and this weir was seven feet long. The weir allowing
stormwater runoff to exit the diversion sump (i.e., bypass the treatment system) was assumed to be six feet
high as measured from the bottom of the diversion sump, or one foot above the bottom of the existing
channel. This weir was 12 feet long.
The pump rate from the CWC was set at 2,600 gpm (5.8 cubic feet per second [cfs]), and the pump rate
from the stormwater-only ditch was set at 2,000 gpm (4.5 cfs). Additional simulations using the hydrologic
model (presented below) found that slightly lower pump rates produced less desirable stormwater runoff
The design volume for a bioretention cell (Part C-2 of the Manual) is equivalent to the volume that is contained
above the planting surface to the invert of the bypass mechanism for the design storm. Therefore, the required surface
area of the bioretention cell is equal to the required treatment volume divided by the ponding depth (required to be 12
inches or less above the planting surface). Unlike the sand filter, the bioretention cell does not utilize a discount factor
for the design volume. The Manual also specifies that an underdrain should be installed beneath a biofilter if the
underlying soil infiltration rate is less than 2 inches per hour. The proposed design for the Treatment System is
consistent with design specifications for the bioretention cell in that a discount factor for the design volume is not
utilized, and a two inches per hour drain rate is utilized.
Other flow -through SCM types in the Manual include wet ponds and stormwater wetlands. Guidance for both wet
ponds (Part C-3 of the Manual) and stormwater wetlands (Part C-4 of the Manual) specify that the design volume is
equivalent to the volume that is retained for a two to five-day period between the temporary pool elevation and the
permanent pool elevation (also referred to as the temporary pool or temporary inundation zone) (these SCMs have a
permanent pool volume as well). Like the bioretention cell, these SCMs do not utilize a discount factor for the design
volume. Therefore, the proposed design for the Treatment System is consistent with design specifications in that a
discount factor for the design volume is not utilized. A draw down time of 36 hours was assumed for the storage tank
associated with the Treatment System, which is slightly less (i.e., more conservative, in that it results in larger storage
volume and greater long-term runoff volume capture) than the two to five days specified for the wet ponds and
stormwater wetlands.
8 February 2021

Chemours Company — Fayetteville Works Addendum to Consent Order Paragraph 12
capture volumes, while even significantly higher pump rates produced minimal increases in stormwater
runoff capture.
Additionally, it is important to note that the diversion sumps, pump stations, and weirs were designed to be
able to accommodate the peak stormwater runoff rate from the design storm (1 inch in 24 hours) for the
drainage area to the Treatment System. The peak runoff rate for the design storm was determined using the
hypothetical storm developed by the Soil Conservation Service (SCS), or National Resources Conservation
Service (NRCS). These hypothetical storms show typical distribution of rainfall intensity throughout a rain
event of a certain total depth and duration (in this case, 1 inch in 24 hours). This curve was used to estimate
the peak runoff flowrate from the drainage area, which is illustrated approximately by the portion of the
curve circled in red in Figure 5.
The design storm was modeled in the hydrologic model (to be discussed) using the rainfall distribution
shown in Figure 5 (for type II). The peak runoff rates from the portion of the drainage area draining to the
CWC was 4.4 cubic feet per second (cfs), and the peak runoff rate from the portion of the drainage area
draining to the stormwater only ditch was 3.7 cfs. As previously noted, the pump rate from the CWC was
set at 2,600 gpm (5.8 cfs), and the pump rate from the stormwater-only ditch was set at 2,000 gpm (4.5 cfs).
Therefore, the pump rates were greater than the peak stormwater flowrates from each drainage area draining
to the Treatment System. It should be noted that the diversion sump, weirs, and pump station(s) work
collectively to capture stormwater flows, and results from the hydrologic modeling (discussed below)
demonstrated that they were sized to be able to capture the peak stormwater runoff rates.
It should be noted that these runoff rates correspond to a rainfall intensity of 0.84 inches per hour, based on
the rainfall distribution curve in Figure 5 and the design storm depth of 1 inch. The type II designation was
determined based on geographic location within the U.S., as shown in Figure 6.
1.0
0.9 -
0.8-
0.7
r, F 0.6 -
n y 0.5-
r
- 0.4-
0.3 -
0.2 -
0.1 -
0.0
0 2 4 6 8 10 12 14 16 18 20 22 24
t (hr)
SCS (NRCS) Rainfall Types (Source Nicklow el al., 2006)
Figure 5. SCS (NRCS) Rainfall Distribution Curve
9 February 2021

Chemours Company — Fayetteville Works
Addendum to Consent Order Paragraph 12
.o Owe
h-fir,t
�I� oiso-tutlan
lRainfall
o`, i/ Ty
�v ei
Type IA
Type II
Type 111
Figure 6. Determination of Appropriate Type of Rainfall Distribution
These pump rates and weir design parameters may be refined in subsequent design of the pump stations.
The pumps will be designed to pump stormwater flowrates to maintain similar capture as if the treatment
system were an on-line system.
HYDROLOGIC MODELING
After completion of the design sizing above, a long-term continuous simulation hydrologic model was
developed with the conceptual Treatment System design inputs to evaluate the overall long-term runoff
volume capture performance of the proposed treatment system. The continuous simulation model assesses
long-term performance using historical precipitation data, which accounts for back-to-back storms and
storms lesser and greater than the design storm. The amount of runoff volume that can be captured by a
diversion weir and pump, storage tank, and flow -through Treatment System from a series of storms varies
considerably based on the high variability in the total rainfall depths and intensities that are associated with
rain events, including the antecedent dry period between rain events. In order to characterize the volume
capture performance with a variety of rain event durations, depths, intensities, and antecedent dry periods,
a long-term simulation of the drainage area hydrology and Treatment System hydraulics was conducted.
The system performance was modeled continuously over 13 years of historical precipitation data to evaluate
whether the system was consistently capturing the 1-inch, 24-hour storm, as required by the CO Addendum.
Model Setup and Inputs
The U.S. Environmental Protection Agency (USEPA) Storm Water Management Model (SWMM) was
used to develop a long-term continuous simulation hydrologic model of the drainage area and proposed
storage tank to evaluate long-term runoff capture. The subsections below outline input data for the model.
Meteorological Data
Historical hourly precipitation data from the Fayetteville Regional Airport Grannis Field, NC US gauge
(93740) were downloaded from the Climate Data Online database from the National Oceanic and
Atmospheric Administration (NOAA). Thirteen years of rainfall data from January 1, 2006 to December
31, 2018 were modeled.' The model used a one -hour time step during dry weather and a one -minute time
8 The average annual rainfall in Fayetteville, NC is approximately 45.3 inches (based on historical rainfall data from
1930 to 2018). The average annual rainfall from 2006 through 2018 was 44.3 inches. Therefore, the modeled time
period was representative of typical rainfall. The modeled period did include several hurricanes. The most significant
10 February 2021

Chemours Company — Fayetteville Works Addendum to Consent Order Paragraph 12
step during wet weather. Monthly averages of daily evaporation loss rates were incorporated based on
evapotranspiration values for the Fayetteville area.9
Drainage Area Characteristics
A portion of the drainage area to the Treatment System drains to a concrete -lined stormwater-only ditch,
which is located approximately on the northern and western boundaries of the Monomers/IXM area and
flows from east to west, then north to south. The remaining portion of the drainage area drains to the CWC,
which is approximately located on the eastern and southern boundaries of the Monomers/IXM area and
flows from north to south, then east to west. The hydrologic model includes details of the CWC (portion
flowing east to west), the drainage areas to each channel, and the proposed storage tank. The two drainage
areas were provided by Chemours, and imperviousness was determined using aerial imagery, as described
above.
Geospatial files downloaded from the U.S. Department of Agriculture Natural Resources Conservation
Service (NRCS) Web Soil Survey indicate that the soil type in the drainage area to the Treatment System
is Hydrologic Soil Group (HSG) A. Based on this information, the soil suction head was assumed to be 2.9
inches and the initial deficit was assumed to be 0.32.1° However, the hydraulic conductivity was assumed
to be 0.05 inches per hour.' 1 This assumption was determined during calibration of the full site hydrologic
model to be more representative of compacted soils, and as a conservative approach. For each drainage
area, the width of the drainage area was calculated using the estimated longest flow path. The slope of each
drainage area was assumed to be 0.4%. Modeling parameters for the two drainage areas to the proposed
Treatment System are shown in Table 2.
Table 2. SWMM Modeling Drainage Area Parameters
Parameter for SWMM
Model
Drainage Area to
CWC
Drainage Area to
Stormwater-only Ditch
Area (ac)
7.7
6.2
Width (ft)
502
490
Slope (%)
0.4
0.4
Imperviousness (%)
86
79
Suction Head (in)
2.9
2.9
Conductivity (in/hr)
0.05
0.05
Initial Deficit (fraction)
0.32
0.32
(recorded) rainfall was due to Hurricane Matthew (which resulted in 16.2 inches of recorded rainfall in Fayetteville
on October 8, 2016).
9 Miller, Grady, et al. "Water Requirements of North Carolina Turfgrasses." NC State Extension Publications, NC
State Extension, 2018.
10 Geosyntec Consultants and Larry Walker Associates. Ventura County Technical Guidance Manual for Stormwater
Quality Control Measures. Prepared for the Ventura Countywide Stormwater Quality Management Program. June
2018.
11 Corresponds to HSG D (Hydrology National Engineering Handbook, Chapter 7, Natural Resources Conservation
Service, U.S. Department of Agriculture, January 2009).
11 February 2021

Chemours Company — Fayetteville Works Addendum to Consent Order Paragraph 12
Channel and Pipe Characteristics
To account for the proposed 30-inch pipe that will be installed within the CWC to convey NCCW, the width
of the modeled CWC segment was reduced such that the resulting cross -sectional area was equal to the
cross -sectional area of the proposed design (accounting for the 30-inch NCCW pipe). The assumed
parameters for each channel and pipe segment are shown in Table 3. The existing stormwater-only ditch
(which is concrete lined) will not be modified.
Table 3. SWMM Modeling Channel and Pipe Parameters
Channel/Pipe Segment
Flow
Direction
Length
(ft)
Width
(ft)
Depth
(ft)
Slope
1. Precast concrete trench (rectangular)
West
99.7
2.04
2.5
0.21%
2. Precast concrete box culvert (rectangular)
West
32
3.36
3
0.25%
3. Precast concrete trench (rectangular)
West
340
4.36
3
0.25%
4. Cast -in -place channel to headwall (rectangular)
West
13.2
4.36
3
0.00%
5. Pipe buried through headwall.
West
142
3
3
0.06%
6. Pipe downstream of ditch
South
32
3
3
0.25%
Equalization Storage Tank, Pump, and Weir Sizing
The proposed storage tank was modeled with a storage volume of 300,800 gallons (as calculated by the
Simple Method for Runoff Volume, as previously described) and an outflow pump to the CWC with
constant flow to represent the treatment flow rate (139 gpm). In the hydrologic model, stormwater runoff
from the CWC and from the stormwater-only ditch was directed to diversion sumps/pump stations. As
previously described, the pump rate from the CWC was set at 2,600 gpm, and the pump rate from the
stormwater-only ditch was set at 2,000 gpm. The diversion sumps and weirs were modeled as previously
described.
The hydrologic model was setup to stop pumping (from the CWC and stormwater-only ditch) when the
storage tank reached capacity, and pumping resumed when there was some available capacity in the storage
tank (i.e., when there was 7,000 gallons of capacity in the storage tank).
These pump rates and weir design parameters may be refined in subsequent design of the pump stations.
The pumps will be designed to pump stormwater flowrates to maintain similar capture as if the treatment
system were an on-line system.
Treatment System Bypass Assumptions
Rain events vary in intensity, depth, duration, and antecedent dry period, so the hydrologic model accounts
for stormwater runoff potential bypassing the Treatment System, by the following:
1. Pumping rate is not adequate compared to the influent flowrate of stormwater runoff;
2. Runoff volume overtops the diversion weir in the diversion sump/pump station12; and
3. Storage capacity of the storage tank reaches capacity.
12 The pump sizing was selected to be similar to the weir diversion flowrate.
12 February 2021

Chemours Company — Fayetteville Works Addendum to Consent Order Paragraph 12
As previously mentioned, sizing of the pumps and design of the diversion weir may be further refined
during design of the diversion sump/pump station.13 As a result, modeling results presented herein may
vary slightly from the final design.
Long-term Runoff Volume Capture
The long-term hydrologic simulation was executed, based on the aforementioned parameters and
assumptions, and the long-term runoff capture from the drainage area to the Treatment System was
calculated to be 76%. This includes full treatment of runoff from storms less than the design storm and
partial treatment of runoff from storms greater than the design storm.
Capture of 1-inch Storms
As previously described, and as noted in the CO Addendum, the Treatment System was collectively sized
to capture stormwater runoff from the 1-inch, 24-hour design storm. A design storm is a hypothetical
discrete rainstorm (in this case, characterized by a specific rainfall depth of one inch and 24 hours of
duration) that is used in the design of a stormwater control measure. Sizing a stormwater control measure
involves calculating the volume and/or flowrate of runoff resulting from the specified design storm that
will drain to the control measure. Therefore, the Treatment System will be sized to capture and treat runoff
equivalent to the NCDEQ design storm. However, the Treatment System will not necessarily capture and
treat all runoff from storms with depths of one inch in 24 hours due to some storms occurring in close time
proximity to each other or with high intensity. Therefore, an analysis has been performed, using the
aforementioned continuous simulation hydrology model, to evaluate the fraction of 1-inch, 24-hour storms
that will be completely versus partially captured.
The capture of individual storm events less than or equal to the design storm (i.e., storms with precipitation
up to 1-inch in 24 hours) was analyzed over the 13-year simulation period. The analysis defined storm
events (over the 13-year simulation) as calendar days with at least 0.1 inches of rainfall. The analysis
demonstrated that the proposed design resulted in 100% capture of runoff volume from 97% of all
calendar -day storms with up to one inch of precipitation. An analysis was also performed on an hourly
basis where storm events with up to one inch of rainfall and durations of 24 hours or greater were
considered. The analysis defined storm events (over the 13-year simulation) as having 48 hours of dry
weather between storm events (i.e., if there was less than 48 hours of dry weather between wet weather
periods, the wet weather periods were defined as the same storm event). Dry weather was defined as having
less than 0.1 inches of rainfall in a 24-hour period. This analysis demonstrated that the proposed design
captured 100% of runoff volume from 100% of storms (defined on an hourly basis, not calendar days)
with up to one inch of precipitation in at least 24 hours.
Even though the Treatment System is sized based on a discrete design storm, the long-term simulation
shows that a very high percentage, between 97% and 100%, of storms equivalent to or smaller than the
design storm will result in capture and treatment of all of the stormwater runoff from the event. This
demonstrates that the proposed design performs well in capturing storms equivalent to the design storm
over the long-term, even when there is the potential to have short dry weather periods between successive
storm events.
13 Design parameters were established for these components (e.g., diversion weir overflow, pumping flowrate) during
hydrologic modeling, to be used as design goals during subsequent design stages.
13 February 2021

Chemours Company — Fayetteville Works Addendum to Consent Order Paragraph 12
Hydrologic Modeling Summary
The proposed design for the storage tank and Treatment System includes a storage tank with 300,800
gallons of storage capacity and a design flowrate (i.e., maximum flowrate) for the Treatment System of at
least 139 gpm. A conservative treatment system design flowrate of 150 gpm will be used for design of the
Treatment System.
This design is anticipated to provide approximately 76% long-term average annual capture of stormwater
runoff from the drainage area, which includes variable storm events, ranging in precipitation depths,
durations, and intensities, from the historical 13-year simulation. This proposed design resulted in the
capture of 100% of the runoff volume from approximately 97 to 100% of historical storm events equivalent
to, or smaller, than the design storm of one inch in 24 hours.
As previously mentioned, sizing of the pumps and design of the diversion weir may be further refined
during design of the diversion sump/pump station in the future. As a result, modeling results presented
herein may vary slightly.
TREATMENT SYSTEM
The Treatment System, shown in Figure 3, will contain the following components:
• Prefiltration system to remove total suspended solids (TSS), turbidity, and other constituents that
could potentially have an adverse impact on the PFAS removal by downstream unit operations.
• Granular activated carbon (GAC) system or other similar treatment to remove PFAS compounds.
The GAC system will have vessels in lead/middle/lag configuration and a minimum empty bed
contact time (EBCT) of 10 minutes per vessel.
• Post -filtration to remove GAC fines if needed.
• Settling tanks and solids handling system for the backwash waste from the prefiltration system,
including necessary chemical dosing systems.
• Ancillary equipment (such as pumps and break tanks) as needed for a fully functional Treatment
System. All pumps will be electrically operated.
• Equipment in pre-engineered containerized or trailer -mounted systems. All equipment material of
construction (MOC) will be compatible with anticipated influent water quality.
• Instrumentation (level, flow and pressure transmitters), controls (PLC system with HMI), and other
appurtenances necessary for a fully functional system. The Treatment System will be operated
based on a level control with continuous observation during treatment by a qualified operator.
14 February 2021

Chemours Company — Fayetteville Works Addendum to Consent Order Paragraph 12
Pre -Filtration
Backwash
150 GPM of
Stormwater Stormwater
Collection Tank
To Landfill
Solids Handling System j
Pre -Filtration step for
Suspended Solids
(i.e., Clarifier, UF,MM,
Sand Filter)
150GPM Filtrated
Stormwater
GAC
Adsorption/Similiar
Technology
Backwash Flow
150 GPM Treated
Stormwater to
Outfall 104
Adsorption (GAC) or
Similiar Technology
4
Figure 7. Conceptual Monomers/IXM Stormwater Treatment System
Following treatment, treated stormwater will be piped back into the site conveyance network, comingling
with NCCW and flowing to Outfall 002. When the capacity of the sump and diversion structure is exceeded
during storm events with large total rainfall depths and/or intensities, stormwater flows will flow over the
diversion, comingling with NCCW, and flow to Outfall 002.
DATA ANALYSIS
The data analysis presents information on influent stormwater data and treatment performance goals for the
stormwater Treatment System.
The influent data set consists of stormwater-only samples collected from within the Monomers area during
six storm events. Samples were collected between June 2019 and September 2020. All samples were grab
samples. The June 2019 stormwater sampling data were reported in the Assessment of HFPO-DA and
PFMOAA in Outfall 002 Discharge and Evaluations of Potential Control Options, submitted as Attachment
3 of the Cape Fear River PFAS Loading Reduction Plan (Geosyntec, 2019). The October 2019 and March
2020 stormwater sampling data were included in the final Paragraph 11 quarterly report, for the third quarter
of 2020 (Geosyntec, 2020a). Maps of the stormwater sampling locations within the Monomers areas are
included in the aforementioned reports.l4
These samples were collected from numerous stormwater locations within the drainage area to the
Treatment System. The approximate drainage area to each stormwater sampling location was determined,
and calculations were performed (accounting for varying imperviousness of the sampling location drainage
areas) to estimate the approximate flow fraction from each drainage area. The minimum, average, and
maximum concentrations of analyzed parameters, over all sampled storm events, were first calculated for
each individual stormwater sampling location. These statistics were then used, with the estimated flow
contribution from the drainage area to each sampling location, to calculate minimum, average, and
maximum flow -weighted concentrations. It is important to note that that these minimum, average, and
maximum flow -weighted concentrations are intended to be representative of influent stormwater to the
14 The September 2020 stormwater sampling was conducted to gather more data for planning and design of the
stormwater Treatment System. A subset of the locations that were sampled in March 2020, including locations 60, 61,
65, 68, 71, and 80, were sampled during three rain events in September 2020.
15 February 2021

Chemours Company - Fayetteville Works Addendum to Consent Order Paragraph 12
Treatment System. However, the reported data were not collected as influent samples to the Treatment
System. Results of the stormwater sampling and influent water quality analysis are included in Table 4.
Standard deviations are also reported in Table 4. For stormwater sampling locations that were sampled
during multiple stormwater sampling events, the standard deviation of sample results (over all sampled
events, for each parameter) was calculated. The average of these standard deviations (for all sampling
locations where a standard deviation was calculated) was then reported for each parameter, in Table 4.
Therefore, the standard deviations are not flow -weighted.
Table 4. Summary of PFAS Indicator Compounds in Stormwater Influent
Compound
Units
Minimum
Average
Maximum
Standard
Deviation
Count of
Results
pH
SU
6.8
7.5
8.0
0.67
43
Turbidity
NTU
60
116
208
104
43
Specific Conductivity
mS/cm
35
48
61
22
14
Total Suspended Solids
mg/L
31
88
148
48
30
Bromide
mg/L
0.014
0.021
0.028
0.012
17
Chloride
mg/L
1.2
1.2
1.3
0.87
10
Fluoride
mg/L
0.10
0.23
0.35
0.12
17
Iron
mg/L
0.52
0.75
0.98
0.24
17
Nitrate
mg/L
0.11
0.19
0.28
0.23
17
Sulfate
mg/L
2.2
2.9
3.6
1.4
17
Total Organic Carbon
mg/L
1.8
2.5
3.1
1.1
17
EVE Acid
ng/L
154
415
725
1,543
38
HFPO Dimer Acid
ng/L
2,289
7,754
14,925
16,974
38
Hydro -EVE Acid
ng/L
166
403
834
2,223
37
Hydrolyzed PSDA
ng/L
422
691
945
1,006
28
Hydro -PS Acid
ng/L
281
519
758
476
28
NVHOS
ng/L
427
567
670
452
36
PEPA
ng/L
527
743
1,170
1,589
38
PES
ng/L
53
77
94
24
39
PFECA B
ng/L
53
78
95
25
39
PFECA-G
ng/L
53
77
94
24
39
PFMOAA
ng/L
2,085
8,355
13,836
14,391
39
PFO2HxA
ng/L
2,900
4,543
6,167
4,348
39
PFO3OA
ng/L
1,618
2,703
3,849
2,765
40
PFO4DA
ng/L
1,159
2,151
3,633
3,035
40
PFOSDA
ng/L
743
2,036
4,104
3,196
38
PMPA
ng/L
704
1,146
1,857
1,629
39
PS Acid
ng/L
1,471
2,662
3,858
3,385
28
R-EVE
ng/L
159
274
384
452
39
R-PSDA
ng/L
1,069
2,903
5,361
8,810
28
R-PSDCA
ng/L
86
95
106
19
28
*Note: Minimum, average, and maximum were calculated as flow -weighted concentrations. Standard deviation was calculated as
the average of the standard deviations from each sampling location (where the variation was among different sampling events).
16
February 2021

Chemours Company — Fayetteville Works Addendum to Consent Order Paragraph 12
As noted in the CO Addendum, the treatment goal for the Treatment System is to achieve 99% concentration
reduction of PFAS (as measured by concentrations of indicator parameters HFPO-DA, PMPA, and
PFMOAA). The Sampling Plan (Geosyntec, 2020b) included the proposed procedures for quantifying the
effectiveness of the Treatment System. The Sampling Plan outlines how the stormwater influent to the
Treatment System (flow and concentration) will be characterized, how the stormwater effluent from the
Treatment System (concentration) will be characterized, and how the Treatment System PFAS removal
efficiency will be assessed (for comparison to the CO Addendum requirement of 99% removal).
References
Geosyntec Consultants (Geosyntec), 2020a. Characterization of PFAS in Process and Non -Process
Wastewater and Stormwater Initial Characterization — Final Quarterly Report. December 18, 2020.
Geosyntec, 2020b. Stormwater Treatment System Sampling Plan. September 30, 2020.
Geosyntec, 2019. Cape Fear River PFAS Loading Reduction Plan. August 26, 2019.
17 February 2021