NC0003573_Onsite Seeps Long-Term Loading Calculation Plan_20210921

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Geosyntecl>
consultants
Geosyntec Consultants of NC, P.C.
NC License No.: C-3500 and C-295
ONSITE SEEPS LONG-TERM
LOADING CALCULATION PLAN
Chemours Fayetteville Works
Prepared for
The Chemours Company FC,
22828 NC Highway 87
Fayetteville, NC 28306
Prepared by
Geosyntec Consultants of NC, P.C.
2501 Blue Ridge Road, Suite 430
Raleigh, NC 27607
Project Number TR0795A
October 2020
LLC
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NC License No.: C-350D and C•295
TABLE OF CONTENTS
1 INTRODUCTION AND BACKGROUND 1
2 CONSENT ORDER ADDENDUM REQUIREMENTS 2
3 BASELINE AND COMPLIANCE PERIOD 5
3 1 Baseline Period Timeline 5
3.2 Annual Compliance Period Timeline 5
4 SAMPLING AND MEASUREMENTS 5
4.1 Measurement Locations 6
4.2 Dry Weather Baseflow and Increased Wet Weather Flow 7
4.3 Weather and Flow States 8
4.4 Baseline and Compliance Period Sampling Types 9
4.4.1 Dry Weather Flow Samples 9
4.4.2 Wet Weather Flow Samples 10
4.5 Upgradient Capture and Treatment Seeps Compliance Sampling 10
5 CALCULATIONS 11
5.1 Determination of Weather -Flow State 11
5.2 Annual Mass Discharge Calculation 12
5.3 Compliance Demonstration — Dry Weather Loading Reduction 15
5.3.1 Estimating Compliance After Seep Remedy Removal 15
5.4 Compliance Demonstration — Wet Weather Flow Loading Reduction 16
5.4.1 Estimating Compliance After Seep Remedy Removal 16
5.5 Compliance Demonstration — Upgradient Seeps Capture and Treatment17
5.5.1 Compliance Demonstration — Capture Requirements 17
5.5.2 Compliance Demonstration — Treatment Requirements 17
6 SCHEDULE AND REPORTING 19
7 POTENTIAL ADJUSTMENTS 20
8 REFERENCES 21
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LIST OF TABLES
Table 1: Concurrent Seep Sampling Programs, Sampling Frequencies and Approximate
Durations
Table 2: Anticipated Baseline and Annual Compliance Period Dates
Table 3: Sampling Locations
Table 4: Seep Weather and Flow States
Table 5: Sampling and Measurement Program
Table 6: Upgradient Capture Compliance Sampling and Measurement
Table 7: Annual Mass Discharge
Figure 1: Onsite Seeps
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LIST OF FIGURES
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LIST OF ABBREVIATIONS
CFRW Cape Fear River Watch
CO Addendum Addendum to Consent Order Paragraph 12
HFPO-DA hexafluoropropylene oxide dimer acid
M measure in mass
MT-1 mass per time
NCDEQ North Carolina Department of Environmental Quality
NOAA National Oceanic and Atmospheric Administration
NPDES National Pollutant Discharge Elimination System
PFAS per- and polyfluoroalkyl substances
PFMOAA perfluoro-2-methoxyacetic acid
PMPA perfluoromethoxypropyl carboxylic acid
T measured in time
USGS United States Geological Survey
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1 INTRODUCTION AND BACKGROUND
Geosyntec Consultants of NC, PC (Geosyntec) has prepared this Onsite Seeps Long -
Term Loading Calculation Plan ("the Plan") on behalf of The Chemours Company FC,
LLC (Chemours) pursuant to the requirements of paragraph 2(c)(ii) of the Addendum to
Consent Order Paragraph 12 (CO Addendum). The objective of this Plan is to describe
the calculation methodology for demonstrating compliance with the Long -Term Seep
Remediation Objective described in CO Addendum paragraph 2(c)(i).
At the Chemours Fayetteville Works site (the Site) there are four onsite Seeps A, B, C
and D that have been identified that discharge into the Cape Fear River. Per- and
polyfluoroalkyl substances (PFAS) from the Site reach the Cape Fear River (Geosyntec,
2020a) from these seeps. Between late 2020 and early 2021, Chemours intends to install
interim seep remediation systems, pursuant to CO Addendum paragraph 2(a), to reduce
the loadings from these seeps to the river. These seep remediation systems are planned to
be in situ flow -through cells. The performance of these interim systems will be evaluated
pursuant the Interim Seep Remediation System Plan (Geosyntec, 2020b). Performance of
the interim seep remedy is not assessed as part of this document.
By March 15, 2023 Chemours must complete the installation and commence operation
of a groundwater remedy which will include hydraulically controlling groundwater
migrating towards both the onsite seeps and the Cape Fear River. This remedy is
anticipated to significantly reduce groundwater seepage downgradient of a potential
barrier wall (Figure 1) and may significantly reduce flow to Seeps C and D leading to
periods of time where measurable flow may be absent in these seeps. Seeps A and B are
expected to still be fed by seepage from the Surficial Aquifer and potentially the Perched
Zone water bearing units upgradient of the groundwater remedy. These waters will be
captured and treated pursuant to CO Addendum paragraph 2(c)(i).
The remainder of this document describes the sampling program and calculations to be
performed to evaluate baseline mass loadings from these seeps and then subsequently
outline a calculation methodology to evaluate compliance with the Long -Term Seep
Remediation Objective. The remainder of this document is organized as follows:
• Section 2 — Consent Order Addendum Requirements which describes the Long -
Term Seep Remediation Objective and the requirements of this Plan;
• Section 3 — Baseline and Compliance Period which describes the different
evaluation time periods of this plan;
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• Section 4 — Sampling and Measurements which describes sampling and flow
measurements conducted to support implementation of this plan;
• Section 5 — Calculations which describe how loadings will be calculated and
compliance evaluated;
• Section 6 — Schedule and Reporting which describes how and when loading data
will be reported; and
• Section 7 — Potential Adjustments which describes how this Plan may be adjusted
in the future based on knowledge gained and/or changing conditions at the Site.
2 CONSENT ORDER ADDENDUM REQUIREMENTS
The Long -Term Seep Remediation Objective requires Chemours to show that the long-
term seep remedial actions and the groundwater remedy required pursuant to Paragraph
3 of the CO Addendum achieve the following for Seeps A, B, C and D combined by
March 15, 2025 per CO Addendum paragraph 2(c)(i):
• during dry weathers, reduce total mass loading by at least 99%2,
• during dry weather and following rain events of 0.5 inches or less [in a 24 hour
period], reduce total mass loading by at least 95%2, and
• for a seep upgradient of the groundwater remedy capture total dry weather flow
plus rain events of 0.5 inches or less in a 24-hour period treat PFAS with a removal
efficiency of at least 99%2.
To evaluate compliance with these mass loading reduction requirements, this Plan
discretizes the approach into two time periods, the Baseline Period and Annual
Compliance Periods. For the seeps, "Baseline Period" measurements will commence with
the installation of the interim remediation systems. During the interim seep remediation
system implementation, there will be frequent, high -resolution flow measurement data
and PFAS sampling specified by this plan.
1 Dry and wet weather flows are described in Sections 4.2 and 4.3.
2 Removal efficiency evaluated using indicator compounds hexafluoropropylene oxide dimer acid (HFPO-
DA), perfluoromethoxypropyl carboxylic acid (PMPA) and perfluoro-2-methoxyacetic acid (PFMOAA).
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Site Feature
Interim Seep Remedy
Lo cation
Assumed Upstream
Capture Location
- Observed Seep
• Nearhy Tributary
■ Site Boundary
Potential Groundwater
• ■ ■•
Remedy Roue
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Old Outfall 002
Estimated Catchment to
Seep Remedies
Seep A
Seep B
1=11Seep C
Seep D
Figure 1: Onsite Seeps
3
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D=)-.tfaI 0.:2
Seep C
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Annual compliance periods will be time periods of 12 consecutive months, where samples
are collected to calculate to evaluate reductions against the Baseline loads. The Baseline
and Annual Compliance loadings and reductions will be calculated as detailed in the
calculations of Section 5. These calculations exclude time periods when the seep remedies
or seep measurement locations are inundated by the Cape Fear River. River water levels
vary over the course of the year as they are influenced by releases of water upstream and
rainfalls in the surrounding areas. River levels may rise to such a level that the seep
remediation systems may be flooded, and capture or treatment of seep water is
challenged; or in a long-term scenario, seep channels which were dried up may experience
temporary flows as the river water recedes.
Sampling of the seeps described in this Plan occurs concurrently with two other sampling
programs required by the CO Addendum: the updating of the PFAS loading model
pursuant to paragraph 1(b) and the Interim Seep sampling and effectiveness plan pursuant
to paragraph 2(a)(iii). Samples collected between these other programs may potentially
be used for other programs if the sampling conditions are suitable (e.g. proper duration,
weather type, etc.,). For comparison, the sampling programs and their approximate
durations and sampling frequencies for each seep are shown below in Table 1.
Table 1: Concurrent Seep Sampling Programs, Sampling Frequencies and
Approximate Durations
Year
2021
2022
2023
2024
Quarter
1 2 3 4
1
2 3 4
1
2 3 4
1
2 3 4
CO Addendum
Paragraph
1(b)
Loading
Model
Monthly
Quarterly
2(a)(ii)
Interim
S & E Plan
2X Monthly
Mani
As
Needed 2
2(c)(ii)
This
Plan
Monthly
(Baseline Period)
LModified
None
Monthly
(Compliance Period(s))
1— pursuant to paragraph 2(a)(iv) the Sampling and Effectiveness plan may be modified after six months
of operating all seep interim remedies.
2 — pursuant to paragraph 2(c)(ii) and 2(c)(iii) Chemours must operate the seep interim remedies and
implement the sampling and effectiveness plan until the initial demonstration of the Long -Term Seep
Remediation Objectives has been achieved.
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3 BASELINE AND COMPLIANCE PERIOD
This section describes the Baseline and Annual Compliance Periods. The start and end
dates for both these periods will depend on seep and groundwater remedy construction
schedules and the dynamics of groundwater seepage at the Site. Table 2 below and the
following subsections describe these anticipated dates.
Table 2: Anticipated Baseline and Annual Compliance Period Dates
Onsite
Seep
Baseline
Period —1 year per Seep
First Annual Compliance
Period
Anticipated
Start Date'
Start Date2
End Date2
Seep A
Interim Seep
Remediation Systems
Operational
By
Mar. 15, 2024
By
Mar. 15, 2025
Seep B
SeepC
Seep D
1 — Baseline Period start dates will be based on CO Addendum Interim Seep Remediation System
implementation dates. Dates may vary depending on permitting or construction schedules.
2 — First annual compliance must be attained by March 15, 2025 per CO Addendum; therefore, the start
date can be no later than March 15, 2024. Compliance may potentially be achieved earlier.
3.1 Baseline Period Timeline
The Baseline Period for each seep will begin when the seeps interim remediation systems
are fully operational. The interim remediation systems per the CO Addendum and the
Interim Seeps Remediation System Plan (Geosyntec, 2020a) will include flow
measurement and sampling of both the influent and effluent of each seep. The Baseline
Period for each seep will conclude after twelve consecutive months of Baseline Period
sampling at each seep.
3.2 Annual Compliance Period Timeline
CO Addendum paragraph 2(c)(i) requires compliance with the Long -Term Seep
Remediation Objective to be demonstrated by March 15, 2025. Therefore, the first annual
compliance period at latest will start on March 15, 2024.
4 SAMPLING AND MEASUREMENTS
This section includes information on the sampling and flow measurements that will be
collected during the Baseline and Compliance periods. Section 4.1 describes the
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measurement locations, Section 4.2 describes how dry weather and wet weather flows
will be determined, Section 4.3 describes the different combinations of seep flow, river
and weather conditions, and Section 4.4. describes the sample collection weather types
(dry and wet) and collection frequency. Last, Section 4.4 describes the sampling and
measurements to be performed at the long-term upgradient seep capture and treatment
locations.
4.1 Measurement Locations
Sampling during the Baseline and Compliance Periods will be performed at the treatment
system influent and effluent locations of each seep interim remedy. During the Baseline
Period this sampling enables a clear demonstration of loadings before any seep treatment,
and during the Compliance Period samples from this location provide a clear comparison
to Baseline Period loads. The locations are described below and summarized in Table 3.
Table 3: Sampling Locations
Period
Seep Treatment
Status
Sampling Locations
Influent
Effluent
Baseline
Flow -Through Cell
✓
__
Compliance
Flow -Through Cell
✓
✓
System Removed
__
✓
Baseline Period
Baseline Period samples will be collected at the influent of the Flow -Through Cells. The
influent samples enable evaluating the seep mass loading before any PFAS removal
occurs.
Compliance Period — Flow -Through Cell Interim Remedy
Compliance Period sampling before the removal of the Flow -Through Cells system will
be conducted at the influent and effluent of the Flow -Through Cells. The influent and
effluent samples are collected for different purposes. The effluent sample will be used to
evaluate compliance with the Long -Term Seep Remediation Objective; in other words,
during dry weather flows is loading to the River reduced by 99% and during wet weather
flows is loading to the River reduced by 95% .
The Flow -Through Cell influent sample during the initial stages of the Compliance Period
will be used to evaluate if the Long -Term objectives will continue to be achieved after
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the Flow -Through Cell systems are removed. In other words if the influent samples,
which are taken before any treatment, demonstrate a 99% dry weather flow loading
reduction and demonstrate a wet weather flow loading reduction of 95% then the Flow -
Through Cell systems can be removed as the Long -Term Seep Remediation Objective
will continue to be met after removal.
Compliance Period — Interim Seep Remedies Removed
Compliance period sampling after the removal of the Interim Seep Remediation systems
will be performed at the same location as the effluent sampling location prior to removal
of the remedies. This location at each Seep enables a direct comparison between Site
conditions before and after seep remedy removal.
4.2 Dry Weather Baseflow and Increased Wet Weather Flow
The duration of increased wet weather flows in each of the seeps will be evaluated using
flow data collected during the Baseline Period to differentiate between dry weather only
baseflows and additional flows due to wet weather events. Onsite seep channels
experience both dry weather baseflow which is groundwater fed and increased wet
weather flows due to rainfall events. The volume and duration of increased wet weather
flows after a rainfall event is expected to be a function of rainfall volume and duration,
the size of the drainage basin, the underlying aquifer system and the length of the
drainage/stream network.
Wet weather flow and baseflow components can be separated from each other using one
of several baseflow separation techniques described in hydrology textbooks (e.g. Bras
1990, Gupta 2008, Dingman 2015, McCuen 2017). Baseflow separation is part of unit
hydrology theory, which is a framework used to understand and predict streamflow that
will occur in a specific basin following a precipitation event.
During the baseline time period for each seep, flow data will be collected from both the
Flow -Through Cell weir and the overflow bypass spillway to develop a higher resolution
flow data set alongside the high resolution precipitation data sets from the onsite
meteorological station and United States Geological Survey (USGS) rain gauge station at
the W.O. Huske Dam. These data will be analyzed to determine how long wet weather
flow conditions persist after rainfall events for each seep before dry weather baseflow
conditions are re-established. A similar evaluation will be performed to determine how
long after river inundation the seep flow rates may be increased directly as a result of the
inundation. The results of these analyses will be described in reports to be submitted as
described in Section 6.
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4.3 Weather and Flow States
There are three categories of seep conditions (river conditions, weather conditions, and
presence/absence of seep flow) that lead to seven unique states under which the seeps
may exist. These categories are described below, and the resultant seep weather and flow
states listed in Table 4.
River Conditions: Inundation — Not Inundated
• River Inundated: a time period where elevated Cape Fear River water levels are
affecting the ability to measure flow or collect samples at a given measurement
and sampling location, and waters emplaced in soils adjacent to the seeps by
elevated river stages are draining from soils into the seeps.
• River Not Inundated: a time period where the Cape Fear River does not affect the
ability to measure flow or collect samples at a given measurement and sampling
location, nor does the river affect baseflow in the seep.
Weather Conditions: Dry, Wet -Rain 0.5 inches or less, Wet -Rain greater than 0.5 inches
• Dry weather condition;
• Wet weather less than or equal to 0.5 inches rainfall in 24 hours (wet weather
sampling conducted for this Plan) condition. The duration of wet weather flows
after rain events will be evaluated during the Baseline Period as described above
in Section 4.2; and
• Wet Weather greater than 0.5 inches rainfall in 24 hours condition. The duration
of wet weather flows after rain events will be evaluated during the Baseline Period
as described above in Section 4.2.
Seep Flow Conditions: Flow, No Flow
• Flow: a time period where there is measurable flow in a seep.
• No flow condition: a time period when there is no measurable flow in a seep;
Seep flow conditions will be assessed using flow monitoring devices (e.g. weirs and
transducers in interim seep remedy devices). River elevations will be assessed using river
elevation data from the USGS river monitoring station at the W.O. Huske Dam (gage
02105500). Weather conditions will be assessed using precipitation data from either the
existing USGS weather monitoring station at the W.O. Huske Dam (gage 02105500) or
the onsite meteorological station. Snowfall and subsequent snowmelt may require
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modification to the definitions of wet and dry weather flows. A large snowfall with
delayed snowmelt could lead to large volumes of runoff.
Table 4: Seep Weather and Flow States
River
Condition
Weather
Condition
Flow
Condition
Considered in
Calculation
Sampled
Condition
Inundated
--
--
No
No
Not Inundated
Dry
Flow
Yes
Yes
Not Inundated
Dry
No Flow
Yes
No
Not Inundated
Wet; Rain < 0.5"
Flow
Yes
Yes
Not Inundated
Wet; Rain < 0.5"
No Flow
Yes
No
Not Inundated
Wet; Rain > 0.5"
Flow
No
No
Not Inundated
Wet; Rain > 0.5"
No Flow
No
No
Notes: -- Not Applicable
4.4 Baseline and Compliance Period Sampling Types
Dry and wet weather flow samples will be collected in the Baseline and Compliance
Periods. Table 5 below summarizes the sample types to be collected and the subsections
below describe the specifics of how each sample type will be collected on a monthly
basis.
Table 5: Sampling and Measurement Program
Period
Seep Treatment
Sample Type(s)
Flow
Dry
Wet
Baseline
Flow Through Cell
✓
✓
✓
Compliance
Flow -Through Ce11
✓
V
V
System Removed
✓
V
✓
4.4.1 Dry Weather Flow Samples
Dry weather flow samples will be collected on a monthly basis as composite samples with
a minimum compositing time period of 24 hours when measurable flow is present in a
given seep. Dry weather flow sampling will be initiated when the preceding 72 hours
have experienced no more than 0.1 inches of rain in any 24 hour time period, and the
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weather prediction two days before sampling indicates a 20% or less likelihood
precipitation over the targeted 24 hour sampling period. National Oceanic and
Atmospheric Administration (NOAA) National Weather Service hourly forecasts will be
used to track rainfall predictions at the site3. If no data are collected for a given month
due to weather conditions or equipment malfunctions, additional sampling will be
conducted in subsequent months as needed to generate a data set of at least eight sampling
events over the course of the year.
4.4.2 Wet Weather Flow Samples
Wet weather flow samples will be collected on a monthly basis as composite samples
with a minimum compositing time period of 4 hours. Wet weather flow samples will also
be collected during rainfall events with a minimum nominal rainfall depth of 0.1 inches
and up to 0.5 inches when measurable flow is present in a given seep. Wet weather flow
sampling will be planned when rainfall is predicted two days before with at least a 70%
likelihood of 0.1 inches to 0.6 inches of rainfall depth over a 24 hour period. NOAA
National Weather Service hourly forecasts will be used to track rainfall predictions at the
site3
If no qualifying rain events occur in a month, then a wet weather flow sample will not be
collected. Periodic equipment malfunctions (e.g. rain sensor malfunctions) or inconsistent
rainfall patterns (e.g. only sudden flash storms) may also result in a potential lack of data
for a given month. If no data were collected for a given month due to weather conditions
or equipment malfunctions, additional sampling will be conducted in subsequent months
with greater rainfalls as needed to generate a data set of at least eight sampling events
over the course of the year.
4.5 Upgradient Capture and Treatment Seeps Compliance Sampling
The portion of Seeps A and B flow that originates upgradient of the anticipated barrier
wall will be captured and treated by an ex situ system. The captured flow volumes and
the overflow volumes (i.e., not captured volumes) for each system will be measured.
Precipitation data will be recorded from either the existing USGS weather monitoring
station at the W.O. Huske Dam (gage 02105500) or the onsite meteorological station.
System influent and effluent samples will be collected pursuant to the National Pollutant
Discharge Elimination System (NPDES) permits issued for these systems.
3 Hourly weather forecast graphs for Fayetteville Regional Airport, North Carolina can be found here:
https://forecast.weather. gov/MapClick.php?lat=34.99&lon=-78.8 8 &lg=english&FcstType=graphical
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Table 6: Upgradient Capture Compliance Sampling and Measurement
Period
Seep Treatment
Compliance Monitoring Actions
Compliance
Upgradient
Upgradient Capture
and Treatment
Systems Y
Measure captured and overflow volumes
Record precipitation data
Collect influent and effluent samples per NPDES
5 CALCULATIONS
This section presents the calculations for developing the onsite seeps loading baseline,
calculating post -remedy mass loadings and evaluating compliance against the Long -Term
Seep Remediation Objectives.
The mass loading quantities used in these calculations are the annual mass discharge
[mass per time; MT-1] across all the onsite seeps for dry and wet weather flow conditions
over an evaluation period. Mass discharge is the chosen compliance metric rather than an
estimate of annual mass load (i.e., total mass) since the time duration of dry versus wet
weather flow conditions in any given year are unlikely to be equal and therefore not
directly comparable. Some years will be drier, others wetter, and others experience more
river inundation events. Therefore, normalizing the mass loads for each weather type by
the duration of time for each weather type yields the annual mass discharge, a quantity
directly comparable between different monitoring years.
In the remainder of this section, sub -section 5.1 presents the determination of weather -
flow state, while sub -section 5.2 describes the calculation of the annual mass discharge
calculation. Last, sub -sections 5.3 to 5.5 describe how compliance with the Long -Term
Seep Remediation Objectives will be evaluated for dry weather flow, wet weather flow
and for the upgradient seep capture and treatment system, respectively.
5.1 Determination of Weather -Flow State
The first step in determining baseline loadings and assessing compliance will be
determining the weather and flow conditions for each time increment being considered
for each seep. There are seven unique states that any seep can exist under per the weather
and flow types described in Section 4.3. Of these seven states, four are considered in
calculations. These four states are organized by weather type, Dry and Wet with 0.5
inches or less than rain, and then variations of whether there is or is not flow in the seep(s).
The four states are listed below:
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• Dry Weather with either:
o Flow in seep(s)
o No flow in seep(s)
• Wet Weather, < 0.5 inches rain, with either:
o Flow in seep(s)
o No flow in seep(s)
For calculations involving any given seep, flow and seep concentration data will not be
considered when that seep is either inundated by the Cape Fear River or when rainfalls
exceed 0.5 inches of rain in a 24 hour period and the subsequent wet weather flow
durations associated with these rainfall events as described in Section 4.2.
5.2 Annual Mass Discharge Calculation
The annual mass discharge quantities for wet and dry weather during both the baseline
and compliance periods are calculated following the same approach. The different forms
of the annual mass discharge equation will be used to calculate up to six separate mass
discharge quantities described in Table 7. Equation 1 below first calculates for each
scenario the annual mass load (e.g. kilograms in that year) from all the seeps for a given
weather -time period scenario. The mass load is calculated per seep on a monthly basis by
calculating the average measured mass loading rate in a month for the weather and flow
condition and applying this rate to all the sum of all time increments in the month (i.e.
days in a month) for that weather and flow condition. The sum of all these mass loads is
then divided by the sum of time increments (i.e. number of days) the seeps experienced
the scenario weather condition during the year (wet or dry) to yield the annual mass
discharge value.
Equation 1 also assesses if the seeps have been dried up (i.e. no flow or presence of flow).
Periods of no flow are included in determining the annual mass discharge value. A period
with no flow conditions from a given seep represents a termination of mass loading from
that seep consistent with the Long -Term Seep Remediation Objective.
The monthly samples described in Section 4 will be used to calculate the annual mass
discharge quantities following Equation 1 below:
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Equation 1: Annual Mass Discharge
MScenario
MDscenario =
tScenario
MDscenario = M
�m=1 tm
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S M fs,m,n
N I cs,m,n,iVX
s,m,n 1
Es=1 Em=1 Js,m,n Lan=1 Lam,
i=1 ts,n
where,
MDscenario = is the annual mass discharge for a given scenario (Table 7) from all four
seeps based on samples collected at a particular measurement location for a
selected weather condition, measured in mass per unit time [MT-'];
MScenario = is the annual mass load, for a given 12-month period, for a given scenario
from all four seeps based on samples collected at a particular measurement
location for a selected weather condition, measured in mass [M];
tscenario = is the duration, during the given annual 12-month period, where the seeps
experienced the scenario weather condition (dry or wet, rain 0.5 inches or less),
measured in time [T];
s = represents each of seeps being evaluated;
S = represents the total number of seeps being evaluated (e.g., four (4) — Seeps A,
B, C and D);
m = represents each of the individual months in a period (Compliance or Baseline)
where samples are collected;
M = represents the total number of months in a period (Compliance or Baseline)
where samples are collected (e.g., twelve);
n = represents each individual sample collection interval during a month;
N = represents the total number of individual sample collection intervals during a
month (e.g., one);
fs,m,n
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= represents an indicator variable which takes on the value of:
0 if a given seep "s" has no flow in a given month "m" during a given sample
collection interval "n" (i.e., no sample is collected for a given interval
"n") • or
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1 if a given seep "s" has flow in a given month "m" during a given sample
collection interval "n" (i.e., a sample is collected for a given interval "n");
i = represents each of the three indicator parameters hexafluoropropylene oxide dimer
acid (HFPO-DA), perfluoromethoxypropyl carboxylic acid (PMPA), and
perfluoro-2-methoxyacetic acid (PFMOAA);
I = represents the total number of indicator parameters, i.e., three (3) - HFPO-DA,
PMPA, and PFMOAA;
c is the measured concentration of agiven indicatorparameter "i" at a given
s,m,n,i =
seep "s " in a given month "m " during a given sample collection interval "n";
Vs,m,n = is the volume of flow for a given seep "s" in a given month "m " during a
given sample collection interval "n";
ts,m,n = is the length of time over which the composite sample was collected at a
given seep "s" in a given month "m " during a given sample collection interval
ts,m = is the duration of time of the scenario weather condition (dry or wet) for a given
seep "s" in a given month "m "; and
tm = is the duration of time of the scenario weather condition' for a given month "m ".
Table 7: Annual Mass Discharge Quantity Scenarios
Mass Discharge Quantity
Period
Weather
Sample Location
MDB dry
Baseline
Dry
Influent
MDB wet
Baseline
Wet
Influent
MDef-dry
Compliance
Dry
Effluent
MDcf wet
Compliance
Wet
Effluent
MDB dry
Compliance
Dry
Influent
MDc wet-nf
-
Compliance
Wet
Influent
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4 Weather conditions are assumed to be equal across the seeps.
14
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5.3 Compliance Demonstration — Dry Weather Loading Reduction
CO Addendum Paragraph 2(c)(i) requires a 99% total mass loading reduction during dry
weather; dry weather flow conditions were described in Sections 4.2 and 4.3. The annual
compliance period mass loading reductions percentage during dry weather will be
calculated following Equation 2 below:
Equation 2: Annual Dry Weather Mass Loading Reduction Percentage
MDeff
Rdry = 1 of ry X 100%
MDB-dry
where,
Rdry = is the dry weather mass loading reduction;
MDef dry = is the Compliance Period dry weather annual mass discharge to the Cape
Fear River as measured at the effluent location; and
MDB dry = is the Baseline Period dry weather annual mass discharge as measured at
the influent.
5.3.1 Estimating Compliance After Seep Remedy Removal
Prior to terminating operation of seep interim remedies, the estimated percentage loading
reduction that would occur after interim seep remedy termination and removal will be
calculated following Equation 3 below. The calculation presented in Equation 3 may be
performed on a seep by seep basis depending on the relative performance of the long-
term remedy at the various seeps. For evaluating discharges on a seep by seep basis, the
relevant annual mass discharge quantities will be calculated on a per seep basis as well
by adjusting in Equation 1 the value of "s" and "S" the individual and total number of
seeps being evaluated.
Equation 3: Annual Dry Weather Mass Loading Reduction After Remedy Removal
MD inf
C-dry 0
Rdry,wo remedy = 1 inf X 100/o
MDB-dry
where,
Rdry,wo remedy - is the estimated dry weather mass loading reduction with the interim
seep remedies removed;
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MDB drY = is the Compliance Period dry weather annual mass discharge entering the
Interim Remediation System; and
MDB drY = is the Baseline Period dry weather annual mass discharge as measured at
the influent.
5.4 Compliance Demonstration — Wet Weather Flow Loading Reduction
CO Addendum Paragraph 2(c)(i) requires a 95% total mass loading reduction during wet
weather where rainfall depths are nominally less than 0.5 inches in a 24 hour time period
and during the subsequent wet weather flow duration as described in Sections 4.2 and
4.3. The annual compliance period mass loading reductions during wet weather will be
calculated following Equation 4 below:
Equation 4: Annual Wet Weather Mass Loading Reduction
MDeff
Rwet(1 MD of et x 100%
—
B-wet
where,
Rwet = is the wet weather mass loading reduction;
wet = is the Compliance Period wet weather annual mass discharge to the Cape MDcf
Fear River as measured at the effluent location; and
MDB Wet = is the Baseline Period wet weather annual mass discharge as measured at
the influent.
5.4.1 Estimating Compliance After Seep Remedy Removal
Prior to terminating operation of seep interim remedies, the estimated wet weather
percentage loading reduction that would occur after interim seep remedy termination and
removal will be calculated following Equation 5 below. The calculation presented in
Equation 5 may be performed on a seep by seep basis depending on the relative
performance of the long-term remedy at the various seeps. For evaluating discharges on
a seep by seep basis, the relevant annual mass discharge quantities will be calculated on
a per seep basis as well by adjusting in Equation 1 the value of "s " and "S" the individual
and total number of seeps being evaluated.
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Equation 5: Annual Wet Weather Mass Loading Reduction After Remedy Removal
MD in f
C—wet
Rwet,wo remedy = 1 inf X 100%
MDB—wet
where,
is the estimated wet weather mass loading reduction with the interim
Rwet,wo remedy =
seep remedies removed;
wet = is the Compliance Period wet weather annual mass discharge entering the MDC
Interim Remediation System; and
= is the Baseline Period wet weather annual mass discharge as measured at MDB Wet
the influent.
5.5 Compliance Demonstration — Upgradient Seeps Capture and Treatment
CO Addendum Paragraph 2(c)(i) requires that for any seep that daylights upgradient of
the Barrier Wall, the total dry weather flow will be captured plus rain events of 0.5 inches
or less in a 24-hour period upgradient of the Barrier Wall and during the subsequent wet
weather flow duration as described in Section 4.2 and 4.3. Additionally, the CO
Addendum requires treating captured water with a PFAS removal efficiency of at least
99% for indicator compounds HFPO-DA, PMPA and PFMOAA. The following sub-
section describes the methodology for calculating compliance with these capture and
treatment requirements.
5.5.1 Compliance Demonstration — Capture Requirements
Long -Term Seep Remedy Objective compliance with capturing upgradient seep flow
volumes during dry weather flow and wet weather flow with rainfalls nominally less than
0.5 inches in 24 hours will be evaluated by first calculating the captured and overflow
volumes during the three weather flow conditions described in Sections 4.2 and 4.3.
Compliance will be evaluated by inspecting the results of these calculations. If no
overflow volume is recorded during dry or wet weather flows with rainfall depths of 0.5
inches or less in 24 hours, then compliance will have been demonstrated.
5.5.2 Compliance Demonstration — Treatment Requirements
Long -Term Seep Remedy Objective compliance with demonstrating PFAS removal
efficiency requirements of captured upgradient seep waters will be performed using
influent and effluent samples captured at each seep pursuant to NPDES permits for each
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seep. Compliance will be evaluated on an annual basis. The removal efficiency
calculation uses volume weighted concentrations of influent and effluent samples to
calculate the percentage of mass removal. Volume weighted concentrations were
developed in the event that either the influent and effluent sampling periods have different
compositing durations and timings due to differing NPDES permit requirements,
potential equipment malfunction, or severe weather events. Weighting by volume
provides a representative assessment of mass present in both the influent and effluent
over time; samples corresponding to greater flow volumes will have a proportionately
higher weight. The calculation will be performed following Equation 6 below:
Equation 6: PFAS Removal Efficiency
Rupyr = 1 _ceff X 100%
cin f
M i=I ref
1 Em=1 �i=1 Cm,i X Wm X 100%
= (
S N i=1 Cs
X W
�s=1 �n=1 �i=1 s,n,i s,n
/ M i=I eff Vm
Em=1 Lai=1'm,1 X VM
= 1 La m=1 Vm X 100%
�S N i=I inf Vs,n
s=1 Lan=1 �i=1's,n,i X C`N
Lan=1 Vs,n/
where,
Rupyr = is the combined PFAS removal efficiency across all captured and treated
upgradient seeps.
ceff= is the volume weighted effluent concentration for a given annual compliance
period;
cin f = is the volume weighted influent concentration of the seeps combined for a
given annual compliance period;
m = represents an individual effluent composite sample time interval during a given
annual periods;
5 This calculation assumes that the influents from the upgradient seeps are combined into a common
treatment system with one effluent location. If this condition is different upon implementation, then this
calculation will be adjusted accordingly.
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M = is the total number of effluent composite sample time intervals during a given
annual period;
n = represents an individual influent composite sample time interval during a given
annual period;
N = is the total number of influent composite sample time intervals during a given
annual period;
i = represents the each of the three indicator parameters HFPO-DA, PMPA, and
PFMOAA;
I = represents the total number of indicator parameters, i.e., three (3) — HFPO-DA,
PMPA, and PFMOAA;
ceff = is the measured concentration of agiven indicatorparameter "i" fora given
m,i
effluent composite sample6 "m";
clef. = is the measured concentration of agiven indicatorparameter "i" fora given
s,n,i
seep "s ", for a given influent composite sample6 "n";
Wm = is the effluent concentration volumetric weighting factor calculated for and
applied individually to each effluent composite sample concentration;
= is the volume of water exiting the treatment system during the effluent composite
sample collection period;
u's,n = is the influent concentration volumetric weighting factor for a given seep "s "
and a given influent composite sample "n "; and
Vs,n = is the volume of water being captured for a given seep "s " and a given influent
composite sample "n ";
6 SCHEDULE AND REPORTING
The results from this sampling program will be provided to North Carolina Department
of Environmental Quality (NCDEQ) and Cape Fear River Watch (CFRW) on a quarterly
basis where outputs for the previous quarter are provided within ninety (90) days of the
6 Non -detect influent and effluent sample results will be assigned a value of zero for the calculation and the
values from duplicate samples will be averaged together.
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end of the previous quarter pursuant to CO Addendum paragraph 2(c)(v). Information to
be reported includes the following:
"results of all sampling data, information on extraction, treatment, and
flow; an evaluation of the performance of the Barrier Wall, Groundwater
Extraction System, and seep remediation system(s) installed pursuant to
Subparagraphs 2(a) or 2(b) or compliance with the Long -Term Seep
Remediation Objective following removal of a seep remediation system
pursuant to Subparagraph 2(c)."
The first quarterly report is anticipated to be submitted to NCDEQ by March 31, 2021
and will be for the period from October to December 2020. After the initial compliance
demonstration conditions have occurred and are then subsequently reported, Chemours
will repeat this demonstration for the first five years of the groundwater remedy's
operation. Chemours may request NCDEQ to consider semi-annual compliance reporting
rather than quarterly reporting after the initial compliance demonstration has been made.
7 POTENTIAL ADJUSTMENTS
The calculation methodologies described in this Plan have been developed based on the
present understanding of Site conditions. If conditions or methods change, modifications
may need to be made to this Plan. For example, future NPDES permits or Site remedy
implementation may require modifications to this Plan. Modifications to the calculation
methodologies will be described in submitted reports described in Section 6.
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8 REFERENCES
Bras, R. L. Hydrology: An Introduction to Hydrologic Science, 1990.
Dingman, S. L. Physical Hydrology, 3rd ed, 2015.
Geosyntec, 2020a. Cape Fear River PFAS Mass Loading Assessment — Second Quarter
2020 Report. Chemours Fayetteville Works. September 2020.
Geosyntec, 2020b. Interim Seep Remediation System Plan. Chemours Fayetteville
Works. August 31, 2020.
Gupta, R.S. Hydrology and Hydraulic Systems, 3rd ed., Waveland Press, 2008.
McCuen, R. H. Hydrologic Analysis and Design, 4th ed., 2017.
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