NC0004987_SOC S17-009_20191122

,
tv., DUKE James Wells
w' ENERGY Vice President
® Environmental,Health and Safety
Programs&Environmental Sciences
526 South Church Street
Charlotte,NC 28202
[phone]
November 22, 2019
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Linda Culpepper NOV 2 9 2019 J
Director of Water R ources
North Carolina D artment of Environmental Quality
1617 Mail Service Center DIVISION OF WATER RESOURCES
DIRECTOR'S OFFICE „
Raleigh, NC 27699-1617
Subject: Report Under Special Order by Consent—EMC SOC WQ S 17-009
Duke Energy Carolinas, LLC—Marshall Steam Station
NPDES Permit NC0004987
Seep S-1 Re-Assessment Report Submittal
Dear Ms. Culpepper:
On behalf of Duke Energy Carolinas, LLC (DEC), I am submitting the enclosed report containing
the results of a re-assessment of seep S-01 at the Marshall Steam Station, as required by Section
2(c)(1) of the Marshall Steam Station SOC No. S 17-009.
Following the receipt of third quarter sampling results for S-1, which indicated an exceedance of
the SOC's Interim Action Limit(IAL)for hardness,Duke Energy initiated two projects to evaluate
potential corrective action. The first evaluated the use of water softening resin, which was
available for immediate deployment and was known to be effective at rapid reduction of water
hardness. The second was to retain SynTerra to evaluate more permanent options to reduce
hardness over a longer term, as detailed in the attached report.
Regarding the use of resin, resin-filled socks deployed in October were effective at reducing
downstream hardness; however, reduction below the IAL required the use of a significant amount
of resin (approximately 5 cubic feet) and the resin was depleted within 24 hours of installation.
Based on this, Duke Energy concluded that it would not be sustainable to replace the resin socks
at a sufficient rate to maintain downstream hardness below the IAL under environmental
conditions as they existed in October and early November. Accordingly,the resin was left in place
and not replaced after October. To evaluate the amount of resin required and the duration of the
effect, Duke Energy collected multiple samples upstream and downstream of the S-1 sample point
at varying intervals during October. The results of that investigation will be reported under a
separate cover with the October monthly sampling data.
Regarding the SynTerra evaluation, as detailed in the report, SynTerra identified four potential
options for longer term corrective action: (1) impoundment and capture of the unnamed tributary

Ms. Culpepper
November 22, 2019
2IPage
upstream of S-1, (2) trenching and capture of the unnamed tributary upstream of S-1, (3) use of
extraction wells to capture groundwater flow upgradient of the unnamed tributary, and (4) use of
extraction wells and clean water injection to capture groundwater flow upgradient of the unnamed
tributary. Based on the report, Duke Energy proposes the fourth option, to be implemented on a
schedule determined through the groundwater corrective action process. The first and second
options are not recommended because they would eliminate a significant portion of the unnamed
tributary, causing harm to the resource the SOC and the IAL are intended to protect. The fourth
option, extraction wells and clean water injection, is anticipated to be an effective method of
addressing the migration of groundwater to S-1 based on modelling and other analysis conducted
for the groundwater corrective action plan (CAP) required by the Coal Ash Management Act
(CAMA). Duke Energy anticipates that the Marshall CAP, which is scheduled for submission in
mid-December 2019, will contain a detailed plan for such a well system, focused on the area
between the ash basin and the unnamed tributary. Duke Energy requests that the Water Quality
Permitting Section work in conjunction with the Department's CAMA groundwater team as it
evaluates the CAP proposal.
As shown in the report, the increase in hardness at S-1 is potentially due to seasonal drought
conditions experienced during this period, as Duke Energy has not made any operational changes
at Marshall other than the decanting required under the SOC, that would increase the hardness of
any discharge to S-1. To the extent seasonal drought is the driving factor, the concentration of
hardness at S-1 is due to events beyond Duke Energy's control. Duke Energy has retained a third
party to further evaluate the effects of seasonal rainfall. Following the IAL exceedance in
September, in accordance with the SOC, Duke Energy has taken monthly samples at S-1 to
evaluate the effects of treatment and of natural variations in rainfall on concentrations of hardness
and other constituents. We anticipate that increased rainfall in November and December may
result in a return to baseline hardness concentrations in the short term without further intervention.
Until then, Duke Energy will continue monthly monitoring to assess how conditions are changing
at S-1; however, it is our understanding that these assessment samples will not be used for
determining compliance with the IALs.
As required by the SOC,I certify under penalty of law that this document and all attachments were
prepared under my direction or supervision in accordance with a system designed to assure that
qualified personnel properly gather and evaluate the information submitted. Based on my inquiry
of the person or persons who manage the system, or those persons directly responsible for
gathering the information, the information submitted is, to the best of my knowledge and belief,
true, accurate, and complete. I am aware that there are significant penalties for submitting false
information, including the possibility of fine and imprisonment for knowing violations.
Please direct any questions concerning the Marshall Steam Station NPDES Permit NC0004987
SOC No. S 17-009 to Robert Wylie at [phone] and specifically to this report to Chris
Hallman at(980) 373-7892.

FINDINGS OF RE-ASSESSMENT
D EVALUATION OF REMEDIAL ACTIONS
UNDER SPECIAL ORDER BY CONSENT -
E M C SOC WQ S17-009
AT
MARSHALL STEAM STATION
8320 NC HIGHWAY 150
TERRELL, NORTH CAROLINA 28682
NOVEMBER 2019
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
TABLE OF CONTENTS
SECTION PAGE
1.0 INTRODUCTION 1-1
2.0 RE-ASSESSMENT APPROACH 2-1
3.0 RE-ASSESSMENT ACTIVITIES AND FINDINGS 3-1
3.1 Stream Conditions 3-1
3.2 Surface Water g
Sam lin and Data Evaluation 3-1
Sampling
3.3 Groundwater Sampling and Data Evaluation 3-4
3.4 Ash Basin Conditions and Decanting 3-5
3.5 Local Precipitation Data 3-5
3.6 Ion Exchange Resin Evaluation and Deployment 3-6
4.0 PRELIMINARY CONCLUSIONS 4-1
5.0 PROPOSED REMEDIAL OPTIONS 5-1
5.1 Short-Term 5-1
5.1.1 Normal Streamflow Capture, Pump and Treat-Impoundment 5-2
5.1.2 Normal Streamflow Capture, Pump and Treat-Trench 5-2
5.1.3 Stream Channel Dewatering, Pump and Treat-Extraction Wells 5-2
5.1.4 Stream Channel Dewatering, Pump and Treat- Extraction Wells with
Clean Water Injection 5-2
5.2 Long-Term 5-3
5.2.1 Ash Basin Decanting and Dewatering 5-3
5.2.2 Ash Basin Closure and Groundwater Corrective Action 5-3
6.0 REMEDIAL EFFECTIVENESS MONITORING AND REPORTING 6-1
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent-EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
LIST OF TABLES
Table 1 Historic and Recent Hardness Results Comparison
Table 2 Historical Hardness Levels and Average
Table 3 Groundwater Quality Data
Table 4 Drought Condition Summary
LIST OF FIGURES
Figure 1 S-1 Re-Assessment Area of Investigation
Figure 2 S-i Re-Assessment Background Sample Locations
Page ii

Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent-EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
1.0 INTRODUCTION
Duke Energy Carolinas, LLC (Duke Energy) and the North Carolina Environmental
Management Commission entered into Special Order By Consent SOC No. S17-009, which
was approved on April 18, 2018.
Monitoring of classified waters of the State receiving flow from non-constructed seeps,
found in Attachment B of the SOC, and reporting of analytical results have been
completed as required.
Certain interim action levels (IALs) were established in the agreement, including, at
Seep ID Number S-1: pH 5-10 standard units (S.U.) and hardness 200 mg/L. An
exceedance of any interim action level by 20%in a single sampling event or for two
consecutive monitoring events requires Duke Energy to take specific actions, involving
increased monitoring and seep re-assessment.
Samples collected at S-1 on September 25, 2019 for the third quarter 2019, indicated
hardness at 316 mg/L, exceeding the hardness interim action level by approximately 55
percent.
This document fulfills the reporting requirement described in section 2. c.Additional
Compliance Measures 1)of the SOC:
If the monitoring of any classified water of the State receiving flow from
seeps regulated by this Special Order indicates exceedance of any interim
action level established by the Special Order, Duke Energy shall increase
monitoring at that location from quarterly to monthly until concentrations
of monitored characteristics return to those observed at the initiation of the
Special Order. If any interim action level established by the Special Order
is exceeded by more than 20%in a single sampling event, or exceeded for
two (2) consecutive monitoring events, in addition to paying the
associated stipulated penalty, Duke Energy shall conduct a re-assessment
of the contributing seep(s), including, but not limited to, evaluation of
proposed remedial actions for treatment and/or control of the seep such
that impacts to the receiving waters are quickly mitigated. A report
compiling the findings of the re-assessment, including proposed remedial
actions, shall be provided to the Director of DWR within 60 days of any
applicable exceedance. Following its review, DWR shall notes Duke
Energy of its concurrence or disapproval of Duke Energy's proposed
remedial actions.
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
Activities for the re-assessment of S-1, evaluation of interim remedial actions and
proposed short- and long-term corrective actions in pursuit of exceedance mitigation
are compiled in the remainder of this document.
•
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent-EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
2.0 RE-ASSESSMENT APPROACH
Planning for the re-assessment of the S-1 seep began on September 27, 2019, upon
receipt of the laboratory report containing the third quarter SOC sample data from the
September 25, 2019 sampling event.
The following activities were outlined to evaluate current S-1 seep conditions and
develop preliminary remedial options to quickly mitigate affects to receiving waters, as
required by the SOC.
• Evaluate conditions upstream and downstream of the S-1 location
• Collect surface water samples upstream of the S-1 location and analyze for the
SOC quarterly parameter list
• Collect surface water samples at previously-sampled background locations SW-7
and SW-8 and analyze for the quarterly SOC parameter list
• Collect groundwater samples in the vicinity of S-1 (19 monitoring wells: BG-
2S/BR, GWA-7S/D, MW-14S/D/BR/BRL, GWA-10S/D, AL-1S/D/BR/BRL, GWA-
11S/D/BR, GWA-15S/D) and analyze for Interim Monitoring Plan (IMP)
parameters
• Collect information regarding ash basin decanting
• Review and evaluate historical and current surface water data
• Review and evaluate historical and current groundwater data
• Review and evaluate historical, local precipitation data
• Develop preliminary conclusions
• Develop conceptual remedial actions
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
3.0 RE-ASSESSMENT ACTIVITIES AND FINDINGS
Re-assessment field activities began on October 10, 2019. Duke Energy and SynTerra
staff performed multiple tasks to quickly evaluate current conditions affecting the water
quality at S-1 and remedial options for mitigating the hardness levels at the sample
location. Surface water and groundwater samples were collected, visual reconnaissance
of the stream reach, including adjacent wetlands and upland drainage was conducted,
bench- and field-testing of an ion exchange resin was performed and information on
site conditions was gathered.
3.1 Stream Conditions
Visual assessment of upstream and downstream conditions from S-1, adjacent wetlands
and upland drainage area was conducted on October 15, 2019:
• Stream flow at the location of S-1 was estimated at 1-2 gallons per minute
• Stream flow was not contiguous throughout the reach
o For approximately 200 feet upstream of the S-1 location, the stream was
characterized by alternating flowing and ponded conditions from natural
grade control structures and dams from historic beaver activity
o Beyond approximately 200 feet upstream of S-1, the reach, was
characterized by disappearing stream conditions and pools
• The stream water was visiblyclear, which indicated that the turbiditywas low
throughout the stream reach where water was present
• Iron floc and other evidence of filamentous bacteria was noted, primarily in the
lower two-thirds of the stream reach above the S-1 location
• Apparent wetlands adjacent to the stream were characterized by varied
hydrologic conditions ranging from inundated to dry
• Visual inspection of the immediate drainage area of the stream did not indicate
any discharges of materials considered to impact downstream water quality
P q tY
3.2 Surface Water Sampling and Data Evaluation
Surface water sampling was conducted on October 10, 2019 to provide data as part of
the re-assessment work, which included process control and evaluation samples aimed
at assessing the effectiveness of certain potential remedial options.
Samples were collected at 2 locations upstream of the S-1 seep and 3 locations
considered background (i.e., beyond the influence of groundwater containing coal ash
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
constituents). The sample locations upstream of S-1 were 'S-1 In Process' and'S-1
Springhead'. The background locations were the previously sampled SW-7 and SW-8
(which are located on small streams to the northwest of the ash basin) and a new
location BG-200. Figure 1 illustrates the sampling locations along the tributary to Lake
Norman. Monitoring wells in the vicinity of S-1 that were sampled as part of this re-
assessment are also shown on Figure 1. Figure 2 illustrates the three background
surface water locations that were sampled.
Water quality data collected during the re-assessment sampling, indicate an increase of
historical hardness levels at all locations.
Data provided through monthly and later quarterly samples at S-1 collected under
NPDES permit NC0004987 (Outfall 101) from October 2016 to April 2018 and quarterly
S-1 samples collected under the SOC since June 2018 indicated an average hardness of
203 mg/L.
As shown in Tables 1 and 2, the September 25, 2019 sample result for hardness of 316
mg/L represents a 55% increase compared to the average hardness analytical results of
203 mg/L and a 12% increase from the highest sample result (281 mg/L on 10/6/2016 and
8/2/2017) found in the NPDES and SOC dataset.
It should be noted that during the period of coverage of S-1 (Outfall 101), discharges by
the NPDES permit, a pH limit between 6.0 S.U. and 9.0 S.U. was required. To maintain
compliance, the seep S-1 flow was actively treated using limestone. The use of
limestone in the unnamed tributary above S-1 apparently increased the hardness
concentrations for those sample events. Therefore, the elevated hardness values during
the period of NPDES coverage were not considered when determining the SOC IALs
for S-1. Any increases and averages noted in this report were derived using the
hardness data collected during the NPDES coverage period discussed above.
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent-EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
TABLE 1
HISTORIC AND RECENT HARDNESS RESULTS COMPARISON
Hardness (mg/L)
Re-Assessment NPDES and
Sample ID September Sampling SOC Average 2018 Increase
25, 2019 October 10, (October 2016 Average Over
2019 to May 2019) Earlier
Data
S-1 (Outfall 101) 316 203 55
S-01 In Process 305
S-1 Springhead 92.6
SW-7 18 13 38
SW-8 28.2 18 57
BG-200 38.4
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te:
Any increases and averages noted in this report were derived using the hardness data collected during the NPDES coverage period
discussed above.
TABLE 2
HISTORICAL HARDNESS LEVELS AND AVERAGE
Sample ID Sample Collection Date Hardness (mg/L)
OUTFALL 101 10/06/2016 281
OUTFALL 101 11/07/2016 244
OUTFALL 101 12/09/2016 241
OUTFALL 101 01/09/2017 209
OUTFALL 101 02/02/2017 153
OUTFALL 101 03/06/2017 201
OUTFALL 101 04/11/2017 142
OUTFALL 101 05/04/2017 163
OUTFALL 101 06/09/2017 190
OUTFALL 101 07/12/2017 222
OUTFALL 101 08/02/2017 281
OUTFALL 101 09/05/2017 278
OUTFALL 101 10/02/2017 268
OUTFALL 101 11/15/2017 220
OUTFALL 101 12/05/2017 226
OUTFALL 101 01/04/2018 216
OUTFALL 101 04/02/2018 139
5-01 06/07/2018 159
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
TABLE 2
r HISTORICAL HARDNESS LEVELS AND AVERAGE
I Sample ID Sample Collection Date Hardness (mg/L)
5-01 09/11/2018 195
5-01 10/31/2018 137
S-01 03/27/2019 138
S-01 05/07/2019 166
L S-01 i AVERAGE 203
Prepared by: CBC Checked by: WCG
Five samples had been collected at background location SW-7 during the month of
August 2018. As shown in Table 1, the October 10, 2019 sample resulted in a hardness
level of 18 mg/L, a 38%increase when compared to the average 2018 hardness analytical
results of 13 mg/L.
Five samples had been collected at background location SW-8 during the month of
August 2018. As shown in Table 1, the October 10, 2019 sample resulted in a hardness
level of 28.2 mg/L, a 57% increase when compared to the average 2018 hardness
analytical results of 18 mg/L.
Hardness data variations in surface water appear to be seasonal (Table 2), with the
highest levels of hardness recorded during August through November. This is likely
attributable to decreased precipitation during the last summer and fall months.
According to the U.S. Drought Monitor, the Marshall Steam Station area was considered
to be in moderate to severe drought conditions during September and October 2019.
Additional information on precipitation can be found in Section 3.5 below.
3.3 Groundwater Sampling and Data Evaluation
Groundwater samples were collected on October 14, 2019 from monitoring wells
considered to be proximal to the S-1 location. The following wells were sampled to
assess water quality conditions in the area: BG-2S/BR, GWA-7S/D, MW-14S/D/BR/BRL,
GWA-10S/D, AL-1S/D/BR/BRL, GWA-11S/D/BR, GWA-15S/D. The groundwater
samples were analyzed for IMP parameters,including calcium and magnesium. The
monitoring well locations are shown on Figure 1.
Data review and comparison with the historical dataset indicate stable groundwater
quality conditions in the S-1 vicinity. No increases in constituent concentrations that
might affect hardness at S-1 were noted. The groundwater quality data are summarized
on Table 3.
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
3.4 Ash Basin Conditions and Decanting
To accommodate closure of the ash basin, decanting of free water from the basin via
gravity flow commenced on July 16, 2019 from a pond elevation of 787.94 feet.
Mechanical decanting commenced on September 13, 2019 from a pond elevation of
786.44 feet. On October 15, 2019, the date of the initial re-assessment site visit, the pond
level was 783.10 feet and decanting operations had removed 83.61 million gallons of
water from the ash basin for discharge.
Visual conditions of the ash basin during the re-assessment sampling reflected the
decrease in pond elevation—pond surface area appeared diminished and additional
coal ash was exposed compared to previous site observations.
At nearby monitoring well (AL-1S), the water elevation decreased more than 3 feet
since gravity-flow decanting began.
3.5 Local Precipitation Data
Approximately 2 years of regional and local precipitation data was reviewed as part of
this re-assessment.
According to data available from the U.S. Drought Monitor
(https://droughtmonitor.unl.edu/Maps/MapArchive.aspx), a product produced by the
National Drought Mitigation Center, National Oceanic and Atmospheric
Administration and United States Department of Agriculture, the Marshall Steam
Station area, during the following time periods shown in Table 4, was characterized by
the indicated drought conditions:
TABLE 4
DROUGHT CONDITION SUMMARY
Marshall Steam Station Area
Time Period Drought Condition
October 2016 Abnormally Dry
November 2016 - December 2016 Moderate
January 2017 (First 3 weeks) Abnormally Dry
February - April 2017 Moderate
Mid-November to December 2017 Abnormally Dry
January to February (First Week) 2018 Moderate
• Februa Second Week to March First Week 2018 Abnormal) Dr
September (Weeks 2 and 3) 2019 Abnormally Dry
September (Week 4) to October (Week 1) 2019 Moderate
October (Weeks 2 - 4) 2019 Severe
October (Week 5) 2019 Moderate
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order byConsent—EMC SOC WQS17-009 November 2019
P
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
Data collected by a Duke Energy automated weather station located at the Marshall
Steam Station indicated decreased precipitation totals when compared to average
historical, regional rainfall records and generally aligned with the drought conditions
published by the Drought Monitor.
3.6 Ion Exchange Resin Evaluation and Deployment
Preliminary evaluation of remedial options was performed as part of the subject re-
assessment effort. A cation exchange resin, widely utilized in water-softening and
demineralization treatment, was selected for bench-testing to determine the
effectiveness in addressing the hardness levels in the S-1 waters. Test results indicated
promising treatment. Authorization was received from the North Carolina Department
of Environmental Quality, Division of Water Resources and United States Army Corps
of Engineers to place cation exchange resin socks and related materials in the stream
above the S-1 location.
On October 16, 2019, 1 sock was deployed at each of 2 locations upstream of the S-1
sample point; 1 sock approximately 5 feet upstream of S-1 and 1 sock approximately 20
feet upstream of S-1. Deployment of the socks was perpendicular to streamflow.
After approximately 24 hours, on October 17, 2019, a total hardness field screen was
utilized to evaluate treatment effectiveness at the S-1 location. Upstream of the
treatment socks,hardness tested as 320 mg/L and at S-1 hardness was tested as 280
mg/L. One additional treatment sock was deployed at the 5-foot upstream location and
2 additional treatment socks were deployed at the 20-foot upstream location to increase
treatment effectiveness.
On October 18, 2019, a total hardness field screen test was utilized to evaluate treatment
effectiveness at the S-1 location. Two separate hardness tests indicated 160 and 200
mg/L at S-1. A sample was collected at S-1 and analyzed per quarterly SOC parameters.
An additional sample, 'S-1 Above' (upstream of the treatment socks),was analyzed for
hardness. Analytical results indicated hardness of 297 mg/L at S-1 and 310 mg/L at'S-1
Above'. Reliability of the field hardness screening information was reconsidered after
receipt of the October 18 sample dataset.
After conferring with the cation exchange resin vendor, it was decided to deploy new
socks in an alignment parallel with streamflow, rather than perpendicular to flow as
initially deployed. On October 25, 2019, 5 new socks were deployed just above the S-i
location parallel with streamflow and the previously-deployed socks were repositioned
parallel with the stream channel, as well. Once stream conditions were suitable for
sampling (e.g., flow and turbidity normalized), approximately 1 hour after sock
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
installation, samples were collected at S-1 and analyzed per quarterly SOC parameters
and 'S-1 Above' for hardness. Analytical results indicated hardness at S-1 at 122 mg/L
and'S-1 Above' at 272 mg/L.
In addition to the SOC parameter sampling described above, 4 hourly hardness samples
were collected at S-1 on October 25, 2019, to assess the effectiveness of the ion exchange
socks over time. Sample results are listed below:
• Time 12:10 Result: 121 mg/L
• Time 13:10 Result: 145 mg/L
• Time 14:10 Result: 149 mg/L
• Time 15:10 Result: 155 mg/L
The resin socks remained in-place to allow for continued treatment of seep waters.
Upon determining the effectiveness of the resin to reduce hardness to levels below the
established SOC interim action levels, Duke Energy ordered additional supplies to
maintain the treatment materials, while more sustainable remedial options are
evaluated, designed and implemented.
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
4.0 PRELIMINARY CONCLUSIONS
Recent and historical data evaluated for the re-assessment of seep S-1, indicate the
reported exceedance of hardness from Q3 SOC (9/25/19) sample at S-1 (316 mg/L)
exceeds historical hardness concentrations at this location.
Groundwater data collected on October 14, 2019 indicated generally enerall stable conditions
across all well locations and did not reveal any increases in constituents that might have
affected the hardness levels at S-1 (Table 3)
Ash basin mechanical decanting operations,which began on September 13, 2019,
decreased the ash basin free water elevation by 4.84 feet as of October 15, 2019 by the
removal of over 83 million gallons of water. Decanting of the ash basin (the assumed
source of the elevated hardness concentrations) is part of the long-term corrective action
to reduce groundwater migration from the basin toward S-1.
Correlation between hardness levels at S-1 (Outfall 101) and seasonal effects due to
precipitation variation is evident, as shown in Table 2. Severe drought conditions
reduce natural infiltration to groundwater and are likely a contributory factor in the
recent increase in hardness at S-1.
Based on historical trends,hardness levels at S-1 would be expected to decrease during
the late winter, early spring months when precipitation increases.
Immediate actions were taken by Duke Energy to reduce hardness levels at S-1 and to
mitigate affects to receiving waters. Further remedial action development and
implementation will be required to develop a more permanent remedy.
The exceedance of the IAL for hardness in the small low-flow tributary,has no known
detrimental effect on the environment. There is no human health or aquatic life concern
with increased hardness. Water quality standards (WQS) for hardness have only been
established in waters classified as Water Supply where hardness is considered
undesirable for economic or aesthetic reasons.
Hardness levels recorded in Lake Norman as part of SOC sampling requirements
indicate hardness levels well below WQS. Hardness concentrations recorded at the SOC
Lake Norman sample locations have been 20.7 mg/L or less.
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
5.0 PROPOSED REMEDIAL OPTIONS
Re-assessment actions conducted per the SOC requirements included evaluation of
remedial options for treatment and/or control of the S-1 seep to quickly mitigate
possible affects to receiving waters.
After receiving the data from the September 25, 2019 sampling event, Duke Energy
immediately pursued mitigation options to reduce hardness levels at the S-1 seep
location. Duke Energy staff identified the option to utilize the cation exchange resin
socks and began procurement of the materials to conduct bench-scale and field-scale
testing. As described in Section 3.6, use of treatment socks did prove acceptable to
decrease hardness levels;however only for short durations. Operation and
maintenance requirements are very high due to frequent replacement needs and their
use can increase TDS levels in the treated area. Thus, treatment using resin socks is not
considered a viable solution.
Evaluation of short-term and long-term proposed remedial actions is underway with
the alternatives being considered described below.
All options under consideration require further analysis of local hydrology and
hydrogeology, planning and engineering, which will be completed in the upcoming
phase of the response,before selection of the most effective and implementable
remedial option.
Once approval of the selected remediation system has been received, permitting, final
designs and planning will begin, followed by implementation of the remedial selection.
5.1 Short-Term
Ash basin decanting, a part of ash basin closure, is currently underway and will reduce
the hydraulic head near the topographic divide, located to the west of S-1, that
separates the basin from the stream. The reduction in hydraulic head in the ash basin
will reduce the hydraulic driving force for potential constituents of interest (COI)
migration in groundwater from the basin toward S-1, supporting short-term remedial
efforts.
Currently, development of long-term corrective action plans for the Marshall Steam
Station are underway. The short-term remediation approaches below are expected to
address the hardness levels in excess of the SOC interim action levels until alternate
remediation efforts are implemented.
Page 5-1

Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
Capture of stream channel flow or control of groundwater discharge to the stream
channel in the vicinity of S-1 were considered the two most reliable and effective
methods to remediate the hardness. Under both approaches, the collected water would
be conveyed to the Marshall Steam Station wastewater treatment system.
5.1.1 Normal Streamflow Capture, Pump and Treat -
Impoundment
This approach utilizes an impounding structure to capture normal stream base
flow upstream of the S-1 location and eliminate (or significantly reduce) the
discharge to receiving waters below. Impounded water would be collected via a
mechanical pump and delivered to the Marshall Steam Station wastewater
treatment system.
Dam design would include a mechanism to bypass storm water flow during
elevated precipitation events.
5.1.2 Normal Streamflow Capture, Pump and Treat — Trench
This approach utilizes a subsurface trench to capture normal stream base flow
upstream/upgradient of the S-1 location and eliminate (or reduce) discharge to
receivingwaters below. The trench would be constructed perpendicular to the p p
stream channel and contain porous material (e.g., washed stone). Captured
water would be collected via a mechanical pump and delivered to the Marshall
Steam Station wastewater treatment system.
Trench design would include a mechanism to bypass storm water flow during
elevated precipitation events.
5.1.3 Stream Channel Dewatering, Pump and Treat —
Extraction Wells
This conceptual approach utilizes a line of 6 groundwater extraction wells
installed in the stream valley adjacent to the run of the tributary to capture
groundwater in the vicinity of the S-1 location. These wells would reduce the
groundwater base flow discharge to the stream channel and to receiving waters
below. Extracted groundwater would be collected via a mechanical means and
delivered to the Marshall Steam Station wastewater treatment system.
5.1.4 Stream Channel Dewatering, Pump and Treat —
Extraction Wells with Clean Water Injection
As of the date of this report, the basin closure plan and groundwater corrective
action plan have not been submitted to NCDEQ. Therefore, groundwater
Page 5-2

Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
remediation alternatives have not been finalized. However, a robust
groundwater remedial design is being prepared as part of the Corrective Action
Plan Update to control migration of dissolved phase constituents of interest
beyond the ash basin compliance boundary at concentrations above the
respective 02L standards. The eastern portion of the Marshall Steam Station site,
between the Phase I Dry Ash Landfill and the unnamed tributary, is included in
the remedial design. The groundwater remedy in this portion of the site is
comprised of a network of vertical groundwater extraction wells and clean water
P
infiltration/recharge wells, associated pumps, piping and control systems.
It is recommended that any groundwater remedy installed to address IAL
exceedance in the tributary be compatible with the Remedial Alternative design
in the CAP Update (in progress).
Accelerated implementation of portions of the site-wide groundwater remedial
design along the eastern side of ash basin,between the unnamed tributary and
the Dry Ash Landfill (Phase I) could also help address the discharge of
groundwater to the tributarywith elevated concentrations of hardness. Based on
the groundwater remedial alternative being proposed in the CAP Update, it is
estimated that 10 extraction wells and 10 clean water infiltration/recharge wells
would be installed. Installation of an active groundwater extraction and
infiltration system should result in short-term affects to surface water quality in
the tributary.
Before implementation of a groundwater remedy, additional field investigations
(e.g., aquifer pumping tests and groundwater/surface water interactions from
both banks of the stream) should be performed to inform the design.
5.2 Long-Term
5.2.1 Ash Basin Decanting and Dewatering
Ash basin decanting, a part of ash basin closure, is currently underway and will
reduce the hydraulic head near the topographic divide, located to the west of S-1,
that separates the basin from the stream. The reduction in hydraulic head in the
ash basin will reduce the hydraulic driving force for potential COI migration in
groundwater from the basin toward S-1.
5.2.2 Ash Basin Closure and Groundwater Corrective Action
As presented in Section 5.1.4, above, as of the date of this report, the basin
closure plan and groundwater corrective action plan have not been submitted to
Page 5-3

Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
NCDEQ. Therefore, groundwater remediation alternatives have not been
finalized. However, a robust groundwater remedial design is being prepared as
part of the Corrective Action Plan Update to control migration of dissolved
phase constituents of interest beyond the ash basin compliance boundary at
concentrations above the respective 02L standards. The eastern portion of the
Marshall Steam Station site,between the Phase I Dry Ash Landfill and the
unnamed tributary, is included in the remedial design. The groundwater
remedy in this portion of the site is comprised of a network of vertical
groundwater extraction wells and clean water infiltration/recharge wells rechar
/ g
associated pumps, piping and control systems.
It is recommended that any groundwater remedy installed to address IAL
exceedance in the tributary compatible atible with the Remedial Alternative design
gn
in the CAP Update (in progress).
Accelerated implementation of portions of the site-wide groundwater remedial
design along the eastern side of ash basin, between the unnamed tributary and
the Dry Ash Landfill (Phase I) could also help address the discharge of
groundwater to the tributary with elevated concentrations of hardness. Based on
the groundwater remedial alternative being proposed in the CAP Update, it is
estimated that 10 extraction wells and 10 clean water infiltration/recharge wells
would be installed. Installation of an active groundwater extraction and
infiltration system should result in short-term affects to surface water quality in
the tributary. Ongoing operation and maintenance of the remedial system
would have a long-term effect in reducing the discharge of site-related
constituents to the unnamed tributary as ash basin closure is implemented.
Before implementation of a groundwater remedy, additional field investigations
(e.g., aquifer pumping tests and groundwater/surface water interactions from
both banks of the stream) should be performed to inform the design.
Page 5-4

Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
6.0 REMEDIAL EFFECTIVENESS MONITORING AND REPORTING
Following remedy selection and system startup, monthly system effectiveness updates
will be provided to NCDEQ.
Monitoring for effectiveness would begin after system startup and would potentially
include the volume of water collected and conveyed for treatment, groundwater
elevation monitoring data and stream water quality data.
If system performance is not adequate, optimization of existing components and
utilization of additional methods would be considered. Modifications or additions to
treatment would be implemented and reported in a timely manner.
Page 6-1

Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent—EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
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DRAFT FOR DISCUSSION
:`QUALITY DATA
RE-ASSESSMENT AND EVALUATION OF RDEEDLAL ACTIONS UNDER
SPEC42 ORDER BY CONSENT
MARSHALL STEAM STATION
DUKE ENERGY CARDuNAS,LLc,TERRELL ND
MORO.
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Findings of Re-Assessment and Evaluation of Remedial Actions Under
Special Order by Consent-EMC SOC WQ S17-009 November 2019
Duke Energy Carolinas,LLC,Marshall Steam Station SynTerra
FIGURES

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1' GWA-11S WELL IN SHALLOW ZONE ` %Mw-1aBk�( ., � ,�; \ - ��`--"
S GWA-11D WELL IN DEEP ZONE \ MW-14BRL '' \ l,GWA-1OS \ - s
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0 IS-1l UNCONSTRUCTED SEEP \ f
IS-1 ABOVE] I .._...SEE INSET ' �
-— ASH BASIN COMPLIANCE BOUNDARY +.!,i 0 4� l � /,
4 ° Is-ill _ ,/ I
LANDFILL COMPLIANCE BOUNDARY /
L ---J 1
WETLAND BY McKIM&CREED AL-1s 4 /
STREAM BY McKIM&CREED \
AL-1BRL �-v,
\ , v AL-1BR' \V`�, H� J / 1 i
NOTES: v
'. \
1. THE TOPOGRAPHY IS SHOWN FOR REFERENCE PURPOSES ONLY AND SHOULD NOT �. ��: 1
BE USED FOR DESIGN OR ENGINEERING PURPOSES.TOPOGRAPHY IS BASED ON s\\ r„ \ ,- `` t`
LIDAR BARE EARTH DATA OBTAINED FROM THE NORTH CAROLINA SPATIAL DATA SITE \ \ ,- / i \ X 1
AT https://sdc.nc.gov/sad/DataDownload.aspx �s \A _GWA-15S _ � , g." \
2. THE WATERS OF THE US DELINEATION HAS NOT BEEN APPROVED BY THE US ARMY \� �'-�-- F. - _GWA-15D ,-- / ��
CORPS OF ENGINEERS AT THE TIME OF THE MAP CREATION.THIS MAP IS NOT TO BE \ ) �. 45y, \ r
USED FOR JURISDICTIONAL DETERMINATION PURPOSES.THE WETLANDS AND \\ - _ 7'
STREAMS BOUNDARIES WERE OBTAINED FROM STREAM AND WETLAND \\ LAKE
DELINEATION CONDUCTED BY McKIM&CREED MARCH,2016. \ NORMAN
efaN DUKE
GRAPHIC SCALE
200 0 200 400
U E
IN FEE
T ENERGY - FIGURE 1
CAROLINAS DRAWN BY:J.CHASTAIN DATE:5/03/2019 S-1 RE-ASSESSMENT
REVISED BY:C.NEWELL DATE:11/19/2019
CHECKED BY:E.WEBSTER DATE:11/19/2019 AREA OF INVESTIGATION
0 APPROVED BY:W.GERALD DATE:11/19/2019 MARSHALL STEAM STATION
PROJECT MANAGER:B.WILKER TERRELL, NORTH CAROLINA
synTerra www.synterracorp.com
L

!. ., . -\
/
LEGEND
' ...
r • SEEP LOCATION
■ BACKGROUND SURFACE WATER SAMPLING LOCATION
• 0 MONITORING WELL TO BE SAMPLED
--•ASH BASIN WASTE BOUNDARY
2, J ___._ --•ASH BASIN COMPLIANCE BOUNDARY
1[ -LANDFILL BOUNDARY
---,_ ,�• —STRUCTURAL FILL BOUNDARY
• CAL-15NIBR/BR LANDFILL COMPLIANCE BOUNDARY
/ 4 , ,,, ✓ • e. ;; .rrr-"t S : DUKE ENERGY CAROLINAS MARSHALL PLANT
a .:I,.., .P.X. mrl{ ;w,x; °.. '1 M —.•SITE BOUNDARY
GW1-11SIDIE - STREAMIMCKIM B CREED 20161
\ "`gym. '�w yl ; ,; .� YM#"t'"w'�'Lty:a •* =a:: WETLAND IMCKIM N CREED 20161
•
' DUKE r.-. "ECG.. FIGURE 2
•'' 't"ENERGY
'`*-, m S-1 RE-ASSESSMENT
` BACKGROUND LOCATION MAP
`,� o MARSHALL STEAM STATION
'� TERRELL,NORTH CAROLINA
1 SyfITEYfd CM can