NC0003573_Appendix E - Seep Flow at Barrier Wall Memo_20210921

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GEOSemcel, 11C, Ge0te[1.1,1,0l in0 H enah Engineers
Appendix E
Seep Flow at Barrier Wall Memo
TR0795
Aug-2021

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NC License No.: C-3500 and C-295
Memorandum
Date: August 13, 2021
To: The Chemours Company, FC, LLC
From: Geosyntec Consultants of NC, P.C.
Subject: Assessment of Seep Flows at Barrier Wall
2501 Blue Ridge Road, Suite 430
Raleigh, NC 27607
PH: 919.870.0576
FAX: [phone]
www.geosyntec.com
Introduction and Objectives
Geosyntec Consultants of NC, PC (Geosyntec) has prepared this memorandum for The Chemours
Company, FC, LLC (Chemours) to describe the assessment of expected Seep flow rates where the
seeps will cross the underground barrier wall to be constructed pursuant to paragraph 2 of the
Addendum to Consent Order Paragraph 12 (CO Addendum) among Chemours, the North Carolina
Department of Environmental Quality (NCDEQ) and Cape Fear River Watch. Chemours operates
the Chemours Fayetteville Works facility in Bladen County, North Carolina (the Site) where there
are four onsite groundwater Seeps A, B, C, and D (Figure 1) that originate on the bluff at Site and
discharge into the Cape Fear River. Seeps C and D occur down gradient of the proposed barrier
wall location and do not cross the barrier wall. Therefore, this assessment considers Seep A and
Seep B flow rates that are present upgradient of the barrier wall. Chemours will capture and treat
total dry weather base flow plus rain events up to 0.5 inches in a 24-hours period, for the two seeps
that daylight upgradient of the Barrier Wall. There are four onsite groundwater Seeps A, B, C, and
D that originate on the bluff at the facility and discharge into the Cape Fear River. Seeps C and D
occur down gradient of the proposed barrier wall location. Therefore, Seep A and Seep B flow
rates and rain events up to 0.5 inches in a 24-hour period will be captured and treated.
The objectives of this report are:
1. To summarize measurement of the flowrates at Seep A and Seep B locations that
daylight upgradient of the barrier wall under dry weather conditions.
2. To summarize modeling predictions of the stormwater runoff volumes at Seep A and
Seep B locations that daylight upgradient of the barrier wall for rain events up to 0.5
inches over a 24-hour period.
3. Estimate volumes and flow rates from seeps for a preliminary design basis for seeps ex
situ capture, retention and conveyance designs.
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These objectives are addressed through analysis of historical onsite flume and rainfall data, in
addition to assessment of stormwater runoff volumes and flowrates. The Stormwater flows and
volumes from the drainage areas to Seep A and Seep B (upgradient of the barrier wall) are assessed
based on North Carolina stormwater design guidance, in addition to the development and
execution of a long-term continuous hydrologic model in combination with static calculations.
The hydrologic model will provide estimates of stormwater runoff, but not seep flow rates under
dry conditions.
The remainder of this memorandum is organized into the following sections as follows:
• Flume Data Assessment — describes data collection methodology and results of the flume
data relevant to the assessment of seep flows upgradient of the Barrier Wall.
• Stormwater Assessment — describes the stormwater runoff volumes that may be
experienced in the Seep A and Seep B catchment areas during rain events, based on both
static calculations and the hydrologic model.
• Seep Basis of Design Flow Rates and Volumes — describes the recommended flow rates
and stormwater volumes to use as the basis of design for the ex -situ capture systems.
Flume Data Assessment
Chemours has previously installed several flumes at Seeps A and B including locations at the end
of each seep, as close as practicable to the Cape Fear River, to estimate Seep flow rates entering
the river. Additionally, several other flumes were installed at various tributaries that feed the main
seep channels. The detailed locations and analysis of this flume data was presented in the Interim
Seep Remediation System Plan (Geosyntec, 2020).
For the purposes of this assessment, only a subset of the flume installations were applicable in
estimating the flow at the proposed barrier wall intersection. Figure 2 depicts the locations of flume
installation at locations Seep A-4 and Seep B-2. These locations, while downgradient of the
planned barrier wall route, are the closest representative flume locations to where the upgradient
portions of Seep A and Seep B intersect the barrier wall, respectively. Therefore, these locations
are used as a substitute for understanding potential flows at the intersection of the seep channels
and the barrier wall.
These locations are interpreted to provide a conservative estimate (i.e., overestimate) of flows at
the barrier wall as these two locations are substantially downgradient the barrier wall and therefore
encompass larger drainage areas. The location of the flume measurement at Seep A-4 was
approximately 500 feet (ft) downgradient of the proposed barrier wall location. A few minor
tributaries join the main channel within that 500 ft. Similarly, Seep B-2 flume location was
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approximately 250 ft down gradient of the barrier wall location. However, there are no tributaries
joining the seep channel within that 250 ft stretch, so measurement at Seep B-2 may be more
representative than Seep A-4.
Data Collection
Flow rate data were monitored at Seep locations A-4 and B-2 (Figure 2) for the period of August
2019 to November 2020 and September 2019 to October 2020, respectively. The flow rates were
measured by converting the depth of water in the flume using a formula based on the instrument
geometry. The Extra -large 60° trapezoidal flume was used at Seep B-2 and a Large 60° trapezoidal
flume was used at Seep A-4. These were installed using a pond liner in front of and below the
flume to mitigate the potential for underflow and using plywood panels and sandbags at the sides
to channel the seep flow and mitigate the potential for flow around the flumes. Level loggers
(Solinst 3001 LT F30/M10) were installed to measure the water elevations in the flumes, these
data were then barometrically corrected and used in flow rate estimates. Periodically, the flumes
were inspected and maintained, particularly when field teams observed occurrences of bypass
around the flumes or other obstructions near the measurement location.
Precipitation data and weather conditions for the monitored periods were assessed using the onsite
meteorological station and supplementing that data with the existing USGS weather monitoring
station at the W.O. Huske Dam (gage 02105500) when there were data gaps.
Flume Uncertainties
Uncertainties in the observed data may be present when using flumes to measure flow rates in
channels for several reasons. Some of these potential reasons include:
Missed flows:
o Bypass of the flume due to leakage under the flume or new flow channels routing water
around the flume.
Inaccurate water depth estimates based on pressure:
o Incorrect pressure measurement due to placement of pressure sensor.
o Sediment buildup at the flume base that moves the elevation of zero water depth.
Flowrate outside of flume accuracy ranges estimates due to flume sizing:
o Flow can periodically be outside of the accuracy range of the flume resulting in too
high or low ranges in pressure/depths.
- Inaccurate flowrate estimates due to compatibility limitations of the hydraulic assumptions
behind the flume equations with field conditions:
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o Flow turbulence upstream of flume installation, inadequate length of straight channel
or variable cross section size of channel prior to flume installation.
o Obstructions (e.g., by branches, leaves, rocks) that alter the assumed cross -sectional
area of flow (or that could affect the pressure measurement).
Maintenance events were completed approximately once every month. After maintenance events
flumes were operating as per desired conditions (limited to no obstructions, no sediment
accumulation, flow directed into flume, and liner to minimize underflow). As time extended past
maintenance events, the potential for factors causing bias likely increases. A standard practice to
reduce flume uncertainty is perform frequent maintenance.
Seep Flow Rate Assessment Methodology
Flume data underwent organization and preparation to represent flow readings on 30-minute
intervals. Interval lengths were kept constant across the analysis for each flume to reduce potential
bias when calculating statistics. The flowrate data were then paired with the corresponding
precipitation data for that date and time. Precipitation data were taken from the onsite
meteorological station and supplemented with precipitation data from the United States Geological
Survey (USGS) monitoring station at the W.O. Huske Dam if there were no onsite precipitation
data available.
Certain data points were excluded from the data set for each flume. Data were excluded when: (a)
the flume was not operational, (b) the flume was inundated by elevated Cape Fear River water
levels, (c) the flume data exhibited a low bias, and (d) the measured flow was above the upper
limit or below the bottom limit of the flume's measurement range.
Categories based on precipitation were established to analyze the data under different weather
conditions. The data were then assessed statistically and graphed based on the categories
established. These categories included:
• Total data — All data except excluded data.
• Dry weather (i.e. No Rain) — Data that has a period of 24 hours prior to measurement with
no precipitation measured.
• Rain < 0.5 inches — Period of 24 hours with 0.5 inches or less of measured precipitation.
• Rain > 0.5 inches — Period of 24 hours with greater than 0.5 inches of measured
precipitation.
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Results
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The data for Seep A-4 are shown Figure 3 and the data for Seep B-2 are shown in Figure 4. These
time series (Figures 3 and 4) include an indication of the upper and lower flow rates that the flume
can accurately measure. Data outside of these limits were excluded during the data preparation
process prior to the statistical assessment. However, understanding where the data exceeds or falls
below these limits may still be helpful with interpretation of the data, so the excluded data was
plotted on Figure 3 and Figure 4 as grey data points to aide in visual interpretation of flume flow
trends.
A table summarizing the numerical assessment of the flume flowrates at flume locations Seep A-
4 and Seep B-2 is presented in Table 1 below.
Table 1: See u A-4 and See u B-2 Flow Rate Summar
Seep A-4
Weather
Conditions
Number of
days
25th
percentile
(gpm)
Median
Flow
(gpm)
95th
percentile
(gpm)
99th
percentile
(gpm)
Peak Flow'
(gpm)
Total
273
14
18
34
66
110
Dry Weather
179
14
18
31
36
49
Rain < 0.5"
64
14
19
36
56
110
Rain > 0.5"
30
16
23
83
110
110
Seep B-2
Weather
Conditions
Number of
days
25th
percentile
(gpm)
Median
Flow
(gpm)
95th
percentile
(gpm)
99th
percentile
(gpm)
Peak Flow2
(gpm)
Total
276
68
83
170
290
680
Dry Weather
184
65
77
130
150
260
Rain < 0.5"
64
74
92
160
190
450
Rain > 0.5"
28
95
130
340
470
680
The peak flow rate measured at Seep A-4 was 110 gallon per minute (gpm) and the median flow
for this period was 18 gpm. The peak flowrate measured at Seep B-2 over 276 observed days was
680 gpm and the median flow from this period was 83 gpm.
To estimate the baseline flow rates at Seep A-4 and Seep B-2, the "Dry Weather" data in Table 1
should be considered. Seep A-4 had a median flowrate was 18 gpm and a 95th percentile flow of
' The Seep A-4 Flume has an upper limit of measurement of 116 gpm and a lower limit of 5 gpm
2 The Seep B-2 Flume has an upper limit of measurement of 695 gpm and a lower limit of 0.4 gpm.
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31 gpm while Seep B-2 had a median flow rate of 77 gpm and a 95th percentile flow of 130 gpm,
during 24-hour periods of no measured rainfall.
The dry weather flowrates at Seep A-2 had a measured peak flow of 49 gpm and storms up to 0.5
inches increased the measured peak flow to 110 gpm (124% increase), while dry weather flowrates
at Seep B-2 had a peak of 260 gpm and storms up to 0.5" increased the measured peak flow rate
to 450 gpm (73% increase). Rainfall will increase total seep runoff volumes as well as peak flow
rates due to overland flow.
The CO Addendum requires the capture and treatment of total dry weather flow plus rain events
up to 0.5 inches in a 24-hours period. Figure 5 and Figure 6 show the time series of dry weather
data in addition to data for rain event up to and equal to 0.5 inches of rain in a rolling 24 hour
period for Seep A-4 and Seep B-2, respectively. These figures show the representation of the flow
rates from the observation period that need to be captured and treated upgradient of the barrier
wall.
Stormwater Assessment
The seeps ex situ capture systems must capture seeps flow during rainfall events up to 0.5" in
depth over 24-hours. As stormwater flows are variable in nature and can occur at relatively high
intensities the seeps ex situ capture systems are expected to utilize equalization storage to meter
out flows to a groundwater treatment plant (GWTP) so as to not overwhelm plant capacity.
Consequently, stormwater runoff volumes were assessed, for both the Seep A and Seep B drainage
areas upgradient of the barrier wall, for rain events up to 0.5 inches over a 24-hour period.
Stormwater is defined as wet weather -driven flows that exclude baseflows (such as groundwater
exfiltration or seeps).
Methodology and Calculations
Sizing a stormwater control measure involves calculating the volume and/or flowrate of runoff
resulting from the specified design storm, or the hypothetical discrete rainstorm. Guidance from
the NCDEQ Stormwater Design Manual (Manual) was followed to perform static calculations for
the CO Addendum specified design storm of 0.50 inches in 24-hours, and these calculations are
outlined in the following subsections. These static calculations were followed by an analysis of
results from a long-term continuous, non -calibrated simulation with a hydrologic model as a check,
as outlined in the "Hydrologic Model" section.
Stormwater Runoff Volume
Stormwater runoff volumes from the Seep A and Seep B drainage areas upgradient of the barrier
wall were first assessed for storm events with 0.5-inches of rainfall. Stormwater runoff volumes
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based on total rainfall depth were calculated using the Simple Method for Runoff Volume based
on guidance from the Manual (Part B, Stormwater Calculations, Simple Method for Runoff
Volume). This method first determines the runoff coefficient, which reflects the runoff potential,
using the impervious fraction of the drainage area (discussed further in the "Drainage Area
Characteristics" subsection), as shown in Equation 1.
Equation 1: Runoff Coefficient
R„=0.05+0.9XIA
where,
R„ is the runoff coefficient (unitless); and
IA is the impervious fraction (unitless).
R„ (Seep A) = 0.05 + 0.9 x 0.24 = 0.27
R„ (Seep B) = 0.05 + 0.9 x 0.13 = 0.17
The stormwater runoff volumes were then calculated using Equation 2. For comparison purposes,
the stormwater runoff volumes were also calculated for storm depths of 0.25, 0.75, and 1.00-inch,
in addition to the 0.50-inches specified in the CO Addendum. Table 2 shows the calculated
stormwater runoff volumes for the Seep A and B drainage areas for the designated design storm
depths, based on guidance from the Manual.
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).
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Table 2. Stormwater Runoff Volumes for Seep A and Seep B based on the Simple Method
from the Manual
Seep
Design Storm
Depth (in)
Stormwater
Runoff Volume
(cubic feet)
Stormwater
Runoff Volume
(gallons)
A
0.25
7,200
54,000
0.5
14,000
110,000
0.75
22,000
160,000
1
29,000
220,000
B
0.25
3,100
23,000
0.5
6,200
46,000
0.75
9,300
69,000
1
12,000
92,000
Hydrologic Model
A long-term continuous simulation hydrologic model was developed with the Seep A and B
drainage area inputs to verify the calculated stormwater runoff volumes (Table 2) and estimate
peak stormwater runoff rates draining to the Seep A and B capture points (Figure 7).
Stormwater runoff rates can vary considerably based on the high variability in the total rainfall
depths, durations, and intensities that are associated with storm events, including the antecedent
dry period between storm events. Therefore, a long-term simulation of the hydrology of the
drainage areas was conducted. The United States Environmental Protection Agency (USEPA)
Storm Water Management Model (SWMM) was used to develop a long-term continuous
simulation hydrologic model of the drainage areas. The subsections below outline input data for
the model and model results.
Meteorological Data
Historical hourly precipitation data from the Fayetteville Regional Airport Grannis Field, NC US
gauge (USW00093740) were downloaded from the Climate Data Online database from the
National Oceanic and Atmospheric Administration (NOAA). Fifteen years of rainfall data from
January 1, 2006 to December 31, 2020 were modeled3. The model used a one -hour time step during
3 The average annual rainfall in Fayetteville, NC is approximately 45.5 inches (based on historical rainfall data from
1930 to 2020). The average annual rainfall from 2006 through 2020 was 45.6 inches. Therefore, the modeled time
period was considered representative of typical rainfall. The modeled period included several hurricanes. The most
significant (recorded) rainfall was due to Hurricane Matthew (which resulted in 16.2 inches of recorded rainfall in
Fayetteville on October 8, 2016).
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dry weather and a one -minute time step during wet weather. Monthly averages of daily evaporation
loss rates were incorporated based on evapotranspiration values for the Fayetteville area4.
Drainage Area Characteristics
The hydrologic model includes details of the drainage areas to the seep capture points for Seeps A
and B. The drainage areas were delineated using a digital elevation model (DEM) and the proposed
seep capture points as shown in Figure 7. The impervious fraction and runoff coefficient for each
drainage area were evaluated using aerial imagery. The drainage areas were divided into the
following classifications: asphalt/concrete; building/rooftop; gravel/river rock; unimproved;
wooded areas; and lawns, sandy soil, flat. Table 3 shows the characterization of the drainage areas
by land cover.
Runoff coefficients (C) and values of imperviousness were assumed based on land cover
classification. The values used for each land cover classification were based on guidance from the
Manual (Part B, Stormwater Calculations, Table 1). The gravel/river rock land cover classification
was not included in the Manual; assumed values for this land cover classification are consistent
with those used at other sites. These values are shown in Table 3. The land cover -based runoff
coefficients and impervious fractions were area -weighted for each drainage area. In the Seep A
drainage area, the runoff coefficient was assumed to be 0.27 and the impervious fraction was 0.24,
or 24% impervious. In the Seep B drainage area, the runoff coefficient was assumed to be 0.17
and the impervious fraction was 0.13, or 13% impervious.
Table 3. Land Cover of Seep Drainage Areas
Land Cover
Runoff
Coefficient (C)
Imperviousness
Drainage Area (acre)
Seep A
Seep B
Asphalt/concrete
0.95
1.0
2.4
0
Building/rooftop
0.9
0.94
0.56
0.10
Gravel/river rock
0.77
0.8
1.9
0.46
Unimproved
0.35
0.33
1.9
0
Wooded area
0.15
0.11
15
19
Lawns, sandy soil, flat
0.15
0.11
8.1
0.36
Total Area
29.4
19.9
Geospatial files downloaded from the U.S. Department of Agriculture Natural Resources
Conservation Service (NRCS) Web Soil Survey indicate that 87% of the Seep A drainage area has
4 Miller, Grady, et al. "Water Requirements of North Carolina Turfgrasses." NC State Extension Publications, NC
State Extension, 2018.
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soils classified as Hydrologic Soil Group (HSG) A and 13% of the Seep A drainage area contains
soils classified as HSG C. In the Seep B drainage area, 29% of the area is classified as HSG A and
71% is classified as HSG C. Soil suction head, initial deficit, and hydraulic conductivity values
were obtained from the SWMM User's Guide. However, the industrial portions of the drainage
areas were assumed to be more compacted than typical HSG A or C soils, so hydraulic
conductivities were adjusted in the on -site industrial areas to present more conservative (i.e.,
lower) estimates of hydraulic conductivity. Soil suction head, initial deficit, and hydraulic
conductivity values assigned for each soil type and drainage area are shown in Table 4.
Table 4. Soil Prouerties
Seep
Area
(acres)
HSG
Suction
head (in)
Conductivity
(in/hr)
Initial
deficit
(fraction)
A
26
A
2.90
0.24
0.32
3.7
C
8.60
0.10
0.24
B
5.7
A
2.90
0.32
0.32
14
C
8.60
0.10
0.24
Area -weighted averages were calculated for each parameter for each drainage area. The width of
each drainage area was estimated by dividing the total area by the length of the estimated longest
flow path. The slope for each drainage area was assessed by computing the slope throughout the
drainage areas, using a digital elevation model, and then calculating the average slope of the
drainage area. Input parameters for the hydrologic models of the Seep A and B drainage areas are
shown in Table 5.
Table 5. SWMM Modeling Drainage Area Parameters
Parameter for SWMM Model
Drainage Area to Seep A
Drainage Area to Seep B
Area (ac)
29.4
19.9
Width (ft)
424
561
Slope (%)
4.7
11
Imperviousness (%)
24
13
Soil Suction Head (in)
3.62
6.97
Hydraulic Conductivity (in/hr)
0.23
0.16
Initial Deficit (fraction)
0.31
0.26
Model Uncertainties
There are uncertainties in the ability of the hydrologic model to accurately predict stormwater
runoff volumes and rates from the drainage areas. All input parameters to the hydrologic model
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(e.g., drainage area boundaries, imperviousness, soil types and associated infiltration rates,
depression storage, parameters that govern interflow) have a degree of uncertainty, which
contribute to the overall uncertainty of model results. However, the most uncertain and sensitive
parameters for determining runoff volumes include Green-Ampt infiltration parameters and
depression storage. For examining stormwater runoff flowrates, flow width is also a sensitive
parameter, in addition to the parameters noted to be sensitive for determining runoff volumes.
Importantly, the hydrologic model was not calibrated with measured flow data. Modeled flow rates
are more accurate when calibrated, but since there were no flume measurements for the portion
modeled, i.e., the catchment upgradient of the barrier wall only, the model was not calibrated.
Model Results
Stormwater Runoff Volumes
Historical storm events (during the modeled period of record from 2006 through 2020) with total
rainfall depths approximately equal to 0.50 inches were examined The average total stormwater
runoff volumes during these storm events, as predicted by the hydrologic model, are shown in
Table 6.
Table 6. Model Predicted Stormwater Runoff Volumes for the 0.50-inch Storm Event
Seep
Storm Event
Total Rainfall
Depth (in)
Average Model -Predicted
Stormwater Runoff
Volume (cubic feet)
Average Model -Predicted
Stormwater Runoff Volume
(gallons)
A
0.45 - 0.55
11,000
80,000
0.40 - 0.60
10,000
77,000
B
0.45 - 0.55
4,000
30,000
0.40 - 0.60
4,000
29,000
Results from the hydrologic model (Table 6) were compared to the estimated stormwater runoff
volumes using the Simple Method for Runoff Volume from the Manual (Table 2) for the 0.50-inch
design storm. The stormwater runoff volumes for the 0.50-inch design storm estimated using
guidance from the Manual are slightly higher (i.e., more conservative) than the model -predicted
runoff volumes. This is consistent with expectations that the approximate calculation methodology
(outlined in the Manual) would result in conservative (i.e. higher) estimates of runoff volumes than
a hydrologic model.
Seep Basis of Design Flow Rates
This section presents the recommended basis of design flow rates and stormwater volumes for the
seeps ex situ capture systems. First, the recommended seeps baseflow are described, then the
stormwater volumes for 0.5" or less rainfall events.
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Dry weather flows were measured using flumes in both seeps down gradient of the planned barrier
wall location. The 95th percentile dry weather flows from both seeps are recommended to be used
as the basis of design for dry weather flow capture. For Seep A this flow rate is 31 gpm and for
Seep B 130 gpm for a total of 161 gpm.
Stormwater runoff volumes for a 0.5-inch design storm were calculated using both the Simple
Method for Runoff Volume and by using a hydrologic model to serve as a check on the Simple
Method. The Simple Method was the more conservative design basis with a higher total
stormwater flow volume; these results are recommended to be used as the basis of design for
stormwater capture volumes. For 0.5" rainstorms during a 24-hour period, Seep A was estimated
to receive up to 110,000 gallons of flow and Seep B receive up to 46,000 gallons of flow for a total
volume of 156,000 gallons. The ex -situ seeps capture system will include equalization storage.
Assuming the ability to store and equally meter out the entire 24-hr, 0.5" rain event flow volume
to the GWTP over a 24-hour period yields estimated flow rates of 76 gpm for Seep A and 32 gpm
for Seep B for a total of 108 gpm. This calculation is shown below in Table 7.
Table 7: Storm Flows Basis - Simple Method
Seep A
Seep A - 0.5" Rain Volume
110,000
gal
0.5" Rain Volume over 24-hrs
76
gpm
Seep B
Seep B - 0.5" Rain Volume
46,000
gal
0.5" Rain Volume over 24-hrs
32
gpm
Combined
Combined Volume - 0.5" Rain
156,000
gal
0.5" Rain Volume over 24-hrs
108
gpm
Taken together the dry weather and the stormflows for rainfall events up to 0.5" over 24-hrs
comprise a total design flowrate of 269 gpm (Table 8), which will be directed to the GWTP.
Table 8: Design Flowrates and Volumes
Flow Source
Flowrate to GWTP m
(gpm)
Stormwater Volume During
0.5" Storm (gallons)
Seep A Rainfall
76
110,000
Seep B Rainfall
32
46,000
Subtotal
108
156, 000
Seeps A Baseflow
31
-
Seeps B Baseflow
130
-
Subtotal
161
-
Total
269
156,000
Assessment of Seep Flow at Barrier Wall
engineers I scientists I innovators

Assessment of Seep Flows at Barrier Wall
August 2021
Page 13
References
Geosyntec°
consultants
Ceasynttec Cor ]ianis of NC, P.C.
NC: License Na.: C-3500 and C:-295
Geosyntec 2020, Interim Seep Remediation System Plan, Prepared for The Chemours Company
FC, LLC.
NCDEQ, 2020. NCDEQ Stormwater Design Manual. Part B: Calculations Guidance. Available
at: https://deq.nc.gov/about/divisions/energy-mineral-and-land-
resources/stormwater/stormwater-program/stormwater-design.
Larry W. Mays, 2010. Water Resources Engineering. Second Edition.
Assessment of Seep Flow at Barrier Wall
engineers I scientists I innovators

Assessment of Seep Flows at Barrier Wall
August 2021
Figures
Geosyntec°
consultants
Ceasynttec Cor ]ianis of NC, P.C.
NC: License Na.: C-3500 and C:-295
Assessment of Seep Flow at Barrier Wall
engineers I scientists I innovators

Legend
❑ Site Features
Site Boundary
Nearby Tributary
— — Observed Seep (Natural Drainage)
Site Conveyance Network
Areas at Site
Chemours Monomers IXM
❑Chemours Polymer
Processing Aid Area
DuPont Polyvinyl Fluoride
Leased Area
Former DuPont PMDF Area
Kuraray SentryGlas®
Leased Area
❑
Kuraray Trosifol®
Leased Area
❑ Wastewater Treatment Plant
❑
Power - Filtered and
Demineralized Water
Production
Kuraray Laboratory
Notes:
1. The outline of Cape Fear River is approximate and is based on open data from ArcGIS Online and North Carolina Department of
Environmental Quality Online GIS (MajorHydro shapefile).
2. Basemap sources: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS
User Community
1,000 500 0
1,000 Feet
Site Location Map
Chemours Fayetteville Works, North Carolina
Geosyntec
consultants
Geosyntec Consultants of NC, P.C.
NC License No.: C-3500 and C-295
Figure
Raleigh
August 2021
1

Legend
•
Proposed Flow
Measurement Location3
A Flow Measurement
Location
Planned Groundwater
Remedy Route
Site Boundary
Observed Seep
Nearby Tributary
Notes:
1. Topographic surface was generated using LiDAR scans performed on December 1,
2019 and December 19, 2019 by Spectral Data Consultants, Inc.
2. Seep locations identified visually as reported in Geosyntec, 2019. Seeps and
Creeks Investigation Report. Chemours Fayetteville Works. 26 August 2019.
3. Proposed flow measurement locations are placed at points where the observed
seeps cross the planned groundwater remedy route.
4. The outline of Cape Fear River is approximate and is based on open data from
ArcGIS Online and North Carolina Department of Environmental Quality Online
GIS (MajorHydro shapefile).
5. Basemap source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics,
CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.
1,000
500
0
1,000 Feet
Surrogate Flow Measurement Locations
Chemours Fayetteville Works, North Carolina
Geosyntec'>
consultants
Geosyntec Consultants of NC, P.C.
NC License No.: C-3500 and C-295
Figure
Raleigh
August 2021
2

140
120
100
0_80
C7
60
40
20
0
—Flume Upper Limit —Flume Lower Limit
Seep A-4
Flow Data Flow Data (Excluded) Daily Cumulative Rainfall
•
••
.•
•
1
•
•
•
•
•
• •
a
•
•
•
•
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.
•.
•
s
•
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•
.
• •
•
• •
•
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•
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•
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8 i
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•
•
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.•
•
•
t .•
•
•.••
•
•
•
2 ;
;
• -
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•
•i
•
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s
•
•8
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a
:
•
•
4
4
•
••
•.
•
i
•
•
•
•
•
•
44
r
i
�.n
-�
.
.F
1
.IP
In
09.
610 o��o�o w��o�o ���o�o ���o�o ���o�o o��o�o o�o or()
��ti ,�ti 4 v o\ v \ v 4,\ v , v tio\�'
Notes:
1. Time spans without data are associated with periods when the flume was not operational.
2. Daily cumulative rainfall is the cumulative rainfall in one calendar day.
3. Excluded flow data was determined from river inundation and data outside of the flume limits.
Abbreviations:
gpm - gallon per minute
in - inch
2.5
2
0.5
0
Seep A-4 Time Trends
Chemours Fayetteville Works, North Carolina
Geosyntec'>
consultants
Geosyntec Consultants of NC, P.C.
NC License No.: C-3500 and C-295
Raleigh August 2021
Figure
3

800
700
600
500
2
400
0
1 300
200
100
0
Seep B-2
—Flume Upper Limit —Flume Lower Limit Flow Data
Flow Data (Excluded) Daily Cumulative Rainfall
••
1
••i
•
•
•
•
•
•
_
• •
•
•
i•
••
•
•
•
•
•
_
•
••
•
S
•
t •Ill•
•
.i
•
•
•
'
4
e •
• • •
•
,
•
•
•
•
• .
_
• •
o
0 0 0 0 °\,p c,\,°�,O �°.LO �°.LO �°.LO �\�°.LO �\�°.LO \�°.LO 6\�°.LO �\,°.Lo <\,
°\
Notes:
1. Time spans without data are associated with periods when the flume was not operational.
2. Daily cumulative rainfall is the cummulative rainfall in one calendar day.
3. Excluded flow data was determined from river inundation and data outside of the flume limits.
Abbreviations:
gpm - gallon per minute
in - inch
2.5
2
co
1.5 .ro
a)
ro
1 E
U
ri
0
0.5
0
Seep B-2 Time Trends
Chemours Fayetteville Works, North Carolina
Geosyntec t>
consultants
Geosyntec Consultants of NC, P.C.
NC License No.: C-3500 and C-295
Raleigh August 2021
Figure
4

140
Seep A-4
Flow Data Flow Data (Excluded) —Flume Upper Limit —Flume Lower Limit —Cumulative 24-hour Rainfall
120 -
100 -
•
•
•
• •
•
•
•
•
•
•
•
80 -
° 60 -
•
•
•
•
•
•
•
•
• •
• •
• •
40 .•
•
•
20 -Ill 4111 ,
• I • 1 Ii.iiH
0 - gill 1 oll If ■III=I_I!■WI tINOMIENIMIMIll
• •
•
•
•
•
•
1
••
••
•
•
•
• •
• • •
�• •• 7/0
a •
•
••
••
•
•
•
•
\ti 0)\eti �ti �� ti„ ti 1,\ti 0,\ti \ti \ti (0\1, I\ti cb\ti ti oSti
Notes:
1. Time spans without data are associated with periods when the flume was not operational.
2. Cumulative 24-hour rainfall is the cumulative rainfall over a rolling 24 hour period.
3. Excluded flow data was determined from river inundation and data outside of the flume limits.
Abbreviations:
gpm - gallon per minute
in - inch
t�
•
2.5
2
1.5
1
0.5
Cumulative 24-hour Rainfall
Seep A-4 Time Trends
Cumulative 24-hour rainfall up to 0.5 inches
Chemours Fayetteville Works, North Carolina
Geosyntec
consultants
Geosyntec Consultants of NC, P.C.
NC License No.: C-3500 and C-295
Raleigh
August 2021
Figure
5

800
Seep B-2
• Flow Data • Flow Data (Excluded) Flume Upper Limit —Flume Lower Limit —Prior 24 hour Rainfall
700 - •
600 -
500 -
400 -
0
300 -
200 -
100 -
0
•
i
•
•
•
•
•
•
•
•
•
•
•
•
•
•
• •
••
•
•
•
•
• •-
•
a•
•
•
11
i
Q.c) „„e e 91 1 5) C)
�\ti cp, Cbl �o�ti � titi�� ��ti �ti 1)\1' 41-�ti \ti c5\ge
Notes:
1. Time spans without data are associated with periods when the flume was not operational.
2. Cumulative 24-hour rainfall is the cumulative rainfall over a rolling 24 hour period.
3. Excluded flow data was determined from river inundation and data outside of the flume limits.
Abbreviations:
gpm - gallon per minute
in - inch
2.5
2
4-
1
Seep B-2 Time Trends
Cumulative 24-hour rainfall up to 0.5 inches
Chemours Fayetteville Works, North Carolina
Geosyntec
consultants
Geosyntec Consultants of NC, P.C.
NC License No.: C-3500 and C-295
Raleigh August 2021
Figure
6

Legend
0
0
-
Seep A Upstream Capture
Point
Seep B Upstream Capture
Point
Seep A-4 Flume Location
Seep B-2 Flume Location
Planned Groundwater Remedy
Route
Seep
Estimated Watershed
L_
Ezz
Seep A - Modeled Catchment
Seep B - Modeled Catchment
Seep A - Additional Catchment
Measured by Flume
Seep B -Additional Catchment
Measured by Flume
Notes:
1. Seep catchment areas were estimated using
geospatial methods and tools, and
based on lidar data collected for Chemours.
2. Seep locations identified visually as reported in
Geosyntec, 2019. Seeps and
Creeks Investigation Report. Chemours
Fayetteville Works. 26 August 2019.
3. Basemap source: Esri, DigitalGlobe, GeoEye,
Earthstar Geographics,
CNES/Airbus DS, USDA, USGS, AeroGRID, IGN,
and the GIS User Community.
500 250 0 500 Feet
Seep Drainage Areas
Chemours Fayetteville Works, North Carolina
Geosyntec° GeosYntec CensWramsofNC.PC.
NC License No.: C 3500 end C 205
consultants
Raleigh
August 2021
Figure
7
Projection: NAD 1983 StatePlane North Carolina FIPS 3200 Feet; Units in Foot US