Marshall Background Groundwater Dataset Evaluation - December 2016_20161231

Technical Memorandum
Date: Tuesday, December 13, 2016
Project: Marshall Steam Station Ash Basin System
To: Tyler Hardin
From: Chad Hearn
Subject: Additional Background Monitoring Well Determinations
The purpose of this memorandum is to provide justification for additional monitoring wells that should be
considered as background wells for the Marshall Steam Station (MSS) ash basin. Monitoring wells MW-4,
MW-4D, MS-10, BG-1 S, BG-1 D, BG-2S, BG-2BR, BG-3S, and BG-3D were previously evaluated and will
continue to represent background groundwater quality as documented in Appendix B of the MSS
Corrective Action Plan (CAP) Part 1, dated December 7, 2015.
The potential additional background monitoring wells at MSS include GWA-4S/D, GWA-5S/D, GWA-6S/D,
GWA-8S/D, GWA-12S/D/BR, BG-1 BR, and BG-313R. These potential background wells are being
proposed based on evaluation of the following criteria: groundwater flow direction, groundwater elevation
as compared to the nearest ash basin surface water or porewater elevation, historical analytical results,
and horizontal distance to the ash basin system. Distances and elevations pertinent to the evaluation are
provided in Table 1. Locations for each monitoring well discussed below are shown on Figure 1.
Additional details are provided for each monitoring well cluster on the following pages of this
memorandum.
Historical analytical data for previously evaluated background wells (CAP Part 1) and the potential
background wells listed above are provided in Table 2. The background wells and analytical data are
separated by groundwater flow layer (shallow, deep and bedrock) and a valid sample count is provided
for each constituent. Samples with field measured turbidity greater than 10 nephelometric turbidity units
(NTU) and/or pH greater than 8.5 standard units (SU) are denoted with red -colored text and were not
included in the valid sample counts.
GWA-4S and GWA-4D
Monitoring wells GWA-4S and GWA-4D were installed as part of the Comprehensive Site Assessment
(CSA) activities in May and June 2015 to evaluate groundwater quality and flow direction northwest of the
MSS ash basin. Water level measurements and calculated groundwater elevations from March, June,
and September 2016 have been considered to confirm groundwater elevations compared to the ash
basin and for determining groundwater flow direction in this area of the site.
Monitoring wells GWA-4S/D are located approximately 550 feet west-northwest of the ash basin
waste boundary. The ground surface elevation at the monitoring well cluster is approximately 899
feet.
• The closest ash basin surface water elevation was measured to be 817.46 feet in June 2016, in
the northwest portion of the ash basin.

GWA-4S is screened from approximately 49 to 64 feet below ground surface (ft bgs) in the
shallow flow layer (approximately 850 to 835 feet elevation). The average groundwater elevation
calculated from the March, June and September 2016 gauging events was 843.95 feet
(approximately 27 feet higher than the nearest ash basin surface water elevation measured in
June 2016). The lowest groundwater elevation calculated for these sampling events was 843.42
feet (March 2016).
GWA-41D is screened from approximately 81 to 86 ft bgs in the deep flow layer (approximately
818 to 813 feet elevation). The average groundwater elevation calculated from the March, June
and September 2016 gauging events was 844.54 feet (approximately 27 feet higher than the
nearest ash basin surface water elevation measured in June 2016). The lowest groundwater
elevation calculated for these sampling events was 844.22 feet (March 2016).
Analytical data included in Table 2 indicate that boron was not reported in GWA-4S and GWA-41D
at concentrations that equal or exceed the laboratory method reporting limit. Chloride was
reported at concentrations approximately one to two orders of magnitude less than the 2L
Standard in GWA-4S and GWA-41D samples. Sulfate was either not detected or reported at
concentrations more than two orders of magnitude less than the 2L Standard in GWA-4S
samples, and at approximately one order of magnitude less than the 2L Standard in GWA-41D.
Based on the justification provided above, groundwater passing through monitoring wells GWA-4S/D
originates upgradient and northwest of the ash basin and is not impacted by the ash basin system.
Therefore, groundwater samples collected from GWA-4S/D represent background water quality relative to
the MSS ash basin system and have been included in the valid background sample counts.
GWA-5S and GWA-5D
Monitoring wells GWA-5S and GWA-51D were installed as part of the CSA activities in July 2015 to
evaluate groundwater quality and flow direction north of the MSS ash basin. Water level measurements
and calculated groundwater elevations from March, June, and September 2016 have been considered for
determining groundwater flow direction in this area of the site. Note that GWA-5S was dry in September
2016.
• Monitoring wells GWA-5S/D are located approximately 500 feet north of the ash basin waste
boundary. The ground surface elevation at the monitoring well cluster is approximately 844 feet.
• The closest ash basin surface water elevation was measured to be 803.8 feet in June 2016, in
the northeast portion of the ash basin.
• GWA-5S is screened from approximately 24.5 to 34.5 ft bgs in the shallow flow layer
(approximately 820 to 810 feet elevation). The average groundwater elevation calculated from the
March, June, and September 2016 gauging events was 813.21 feet (approximately 9.5 feet
higher than the nearest ash basin surface water elevation measured in June 2016). The lowest
groundwater elevation calculated for these sampling events was 813.19 feet (March 2016). Note
that GWA-5S was dry during the September 2016 gauging event.
• GWA-51D is screened from approximately 43.5 to 48.5 ft bgs in the deep flow layer (approximately
801 to 796 feet elevation). The average groundwater elevation calculated from the March, June,
and September 2016 gauging events was 813.08 feet (approximately 9 feet higher than the
nearest ash basin surface water elevation measured in June 2016). The lowest groundwater
elevation calculated for these sampling events was 812.69 feet (September 2016).
• Analytical data included in Table 2 indicate that boron was not reported in GWA-5S and GWA-51D
at concentrations that equal or exceed the laboratory method reporting limit. Chloride and sulfate
2

were reported at concentrations approximately two orders of magnitude less than the 2L
Standard.
Based on the justification provided above, groundwater passing through monitoring wells GWA-5S/D
originates upgradient and north of the ash basin and is not impacted by the ash basin system.
Therefore, groundwater samples collected from GWA-5S/D represent background water quality relative to
the MSS ash basin system and have been included in the valid background sample counts.
GWA-6S and GWA-6D
Monitoring wells GWA-6S and GWA-61D were installed as part of the CSA activities in June and July 2015
to evaluate groundwater quality and flow direction northeast of the MSS ash basin. Water level
measurements and calculated groundwater elevations from March, June, and September 2016 have
been considered for determining groundwater flow direction in this area of the site.
• Monitoring wells GWA-6S/D are located approximately 480 feet northeast of the ash basin waste
boundary. The ground surface elevation at the monitoring well cluster is approximately 843 feet.
• The ground surface elevation at the closest portion of the ash basin waste boundary is
approximately 800 feet. Standing water has been observed in this "finger" of the ash basin, but a
surface water elevation has not been measured. The closest portion of the ash basin waste
boundary is at the upgradient end of this "finger" of the ash basin. Therefore, the surface water
elevation in this finger of the ash basin should be considered less than or equal to 800 feet (or a
maximum of 800 feet).
• GWA-6S is screened from approximately 38 to 48 ft bgs in the shallow flow layer (approximately
805 to 795 feet elevation). The average groundwater elevation calculated from March, June, and
September 2016 gauging events was 804.34 feet (approximately 4 feet higher than the nearest
maximum ash basin surface water elevation). The lowest groundwater elevation calculated for
these sampling events was 803.74 feet (September 2016).
• GWA-61D is screened from approximately 102 to 107ft bgs in the deep flow layer (approximately
741 to 736 feet elevation). The average groundwater elevation calculated from the March, June,
and September 2016 gauging events was 804.14 feet (approximately 4 feet higher than the
nearest maximum ash basin surface water elevation). The lowest groundwater elevation
calculated for these sampling events was 803.49 feet (September 2016).
• Groundwater flow direction in the vicinity of GWA-6S/D is from the northeast and flows southwest
toward the ash basin.
• Analytical data included in Table 2 indicate that boron was not reported in any of the GWA-6
wells at concentrations which equal or exceed the laboratory method reporting limit. Chloride and
sulfate were reported at concentrations approximately one to two orders of magnitude less than
the 2L Standard.
Based on the justification provided above, groundwater passing through monitoring wells GWA-6S/D
originates upgradient and northeast of the ash basin and is not impacted by the ash basin system.
Therefore, groundwater samples collected from GWA-6S/D represent background water quality relative to
the MSS ash basin system and have been included in the valid background sample counts.
GWA-8S and GWA-8D
Monitoring wells GWA-8S and GWA-81D were installed as part of the CSA activities in July 2015 to
evaluate groundwater quality and flow direction north of the MSS ash basin. Water level measurements

and calculated groundwater elevations from March, June, and September 2016 have been considered for
determining groundwater flow direction in this area of the site.
• Monitoring wells GWA-8S/D are located approximately 1,075 feet north of the ash basin waste
boundary. The ground surface elevation at the monitoring well cluster is approximately 850 feet.
• The closest ash basin surface water elevation was measured to be approximately 803.95 feet in
June 2016, in the northeast portion of the ash basin.
• GWA-8S is screened from approximately 23 to 38 ft bgs in the shallow flow layer (approximately
827 to 812 feet elevation). The average groundwater elevation calculated from the March, June,
and September 2016 gauging events was 819.37 feet (approximately 15.5 feet higher than the
nearest ash basin surface water elevation measured in June 2016). The lowest groundwater
elevation calculated for these sampling events was 818.97 feet (March 2016).
• GWA-81D is screened from approximately 56 to 61ft bgs in the deep flow layer (approximately
794.5 to 789.5 feet elevation). The average groundwater elevation calculated from the March,
June, and September 2016 gauging events was 818.37 feet (approximately 14.5 feet higher than
the nearest ash basin surface water elevation measured in June 2016). The lowest groundwater
elevation calculated for these sampling events was 818.09 feet (September 2016).
• Analytical data included in Table 2 indicate that boron was not reported in any of the GWA-8
wells at concentrations which equal or exceed the laboratory method reporting limit. Chloride and
sulfate were reported at concentrations approximately two orders of magnitude less than the 2L
Standard.
Based on the justification provided above, groundwater passing through monitoring wells GWA-8S/D
originates upgradient and north of the ash basin and is not impacted by the ash basin system.
Therefore, groundwater samples collected from GWA-8S/D represent background water quality relative to
the MSS ash basin system and have been included in the valid background sample counts.
GWA-12S, GWA-12D, and GWA-12BR
Monitoring wells GWA-12S, GWA-12D, and GWA-12BR were installed offsite in May 2016 to evaluate
groundwater quality and flow direction west of the MSS ash basin. These wells were first sampled in June
2016. Water level measurements and calculated groundwater elevations from June and September 2016
have been considered for determining groundwater flow direction in this area.
• Monitoring wells GWA-12S/D/BR are located approximately 950 feet west of the ash basin waste
boundary. The ground surface elevation at the monitoring well cluster is approximately 876 feet.
• The closest ash basin surface water elevation was measured to be approximately 800.17 feet in
June 2016, in the southwest portion of the ash basin.
• GWA-12S is screened from approximately 10 to 25 ft bgs in the shallow flow layer (approximately
866 to 851 feet elevation). The average groundwater elevation calculated from the June and
September 2016 gauging events was 860.73 feet (approximately 60 feet higher than the nearest
ash basin surface water elevation measured in June 2016). The lowest groundwater elevation
calculated for these sampling events was 859.92 feet (September 2016).
• GWA-12D is screened from approximately 88 to 93 ft bgs in the deep flow layer (approximately
788 to 783 feet elevation). The groundwater elevation calculated from the September 2016
gauging event was 860.41 feet (approximately 60 feet higher than the nearest ash basin surface
water elevation measured in June 2016). Note that a water level measurement was not provided
for June 2016.
4

GWA-12BR is screened from approximately 113 to 118 ft bgs in the bedrock flow layer
(approximately 763 to 758 feet elevation). The average groundwater elevation calculated from the
June and September 2016 gauging events was 858.97 feet (approximately 59 feet higher than
the nearest ash basin surface water elevation measured in June 2016). The lowest groundwater
elevation calculated for these sampling events was 858.51 feet (June 2016).
Analytical data included in Table 2 indicate that boron was not reported in any of the GWA-12
wells at concentrations which equal or exceed the laboratory method reporting limit. Chloride was
reported at concentrations approximately two orders of magnitude less than the 2L Standard.
Sulfate was reported at approximately one to two orders of magnitude less than the 2L Standard.
Based on the justification provided above, groundwater passing through monitoring wells GWA-12S/D/BR
originates upgradient and west of the ash basin and is not impacted by the ash basin system. Therefore,
groundwater samples collected from GWA-12S/D/BR represent background water quality relative to the
MSS ash basin system and have been included in the valid background sample counts.
BG-1 BR
Monitoring well BG-1 BR was installed adjacent to existing background wells BG-1 S/D in April 2016 to
further evaluate groundwater quality and flow characteristics in the bedrock flow layer to the northeast of
the ash basin. As documented in Appendix B of the MSS CAP Part 1, BG-1 S/D were evaluated and
confirmed to be background wells. BG-1 BR was first sampled in April 2016. Water level measurements
and calculated groundwater elevations from June and September 2016 have been considered for
determining groundwater flow direction in this area.
• Monitoring well BG-1 BR is located approximately 5,700 feet northeast of the ash basin waste
boundary. The ground surface elevation at the monitoring well cluster is approximately 815 feet.
• The closest ash basin surface water elevation was measured to be approximately 786.81 feet in
June 2016, in the southeast portion of the ash basin.
• BG-1 BR is screened from approximately 84 to 89 ft bgs in the bedrock flow layer (approximately
731 to 726 feet). The average groundwater elevation calculated from the June and September
2016 gauging events was 800.79 feet (approximately 14 feet higher than the nearest ash basin
surface water elevation measured in June 2016). The lowest groundwater elevation calculated for
these sampling events was 799.68 feet (June 2016).
• Analytical data included in Table 2 indicate that boron was not reported in monitoring well BG-
1 BR at concentrations which equal or exceed the laboratory method reporting limit. Chloride and
sulfate were reported at concentrations approximately two orders of magnitude less than the 2L
Standard.
Based on the justification provided above, groundwater passing through monitoring well BG-1 BR
originates side -gradient and northeast of the ash basin and is not impacted by the ash basin system.
Therefore, groundwater samples collected from BG-1 BR represent background water quality relative to
the MSS ash basin system and have been included in the valid background sample counts.
=IeSQ=]:7
Monitoring well BG-3BR was installed adjacent to existing background wells BG-3S/D in April 2016 to
further evaluate groundwater quality and flow characteristics in the bedrock flow layer to the northeast of
the ash basin. As documented in Appendix B of the MSS CAP Part 1, BG-3S/D were evaluated and
confirmed to be background wells. BG-3BR was first sampled in April 2016. Water level measurements

and calculated groundwater elevations from June and September 2016 have been considered for
determining groundwater flow direction in this area.
• Monitoring well BG-3BR is located approximately 4,100 feet northeast of the ash basin waste
boundary. The ground surface elevation at the monitoring well cluster is approximately 865 feet.
• The closest ash basin surface water elevation was measured to be approximately 786.81 feet in
June 2016, in the southeast portion of the ash basin.
• BG-3BR is screened from approximately 105 to 110 ft bgs in the bedrock flow layer
(approximately 760 to 755 feet). The average groundwater elevation calculated from the June
and September 2016 gauging events was 832.18 feet (approximately 45 feet higher than the
nearest ash basin surface water elevation measured in June 2016). The lowest groundwater
elevation calculated for these sampling events was 831.29 feet (June 2016).
• Analytical data included in Table 2 indicate that boron was not reported in monitoring well BG-
3BR at concentrations which equal or exceed the laboratory method reporting limit. Chloride and
sulfate were reported at concentrations approximately one to two orders of magnitude less than
the 2L Standard.
Based on the justification provided above, groundwater passing through monitoring well BG-3BR
originates side -gradient and northeast of the ash basin and is not impacted by the ash basin system.
Therefore, groundwater samples collected from BG-3BR represent background water quality relative to
the MSS ash basin system and have been included in the valid background sample counts.
6

Table 1. Background Monitoring Well Information
Marshall Steam Station Ash Basin System
F)2
Distance and Direction
Closest Ash
from Source Area
Groundwater
Ground
Elevation
Basin
Well ID
Flow
Surface
of
Screened
Elevation of
Groundwater
Surface
Water
Groundwater Elevation Monitoring
Dry Ash
Ash Basin
Landfill
Direction
Elevation
Interval
(feet)
Elevation to
Events
(Unit II)
(feet)
(feet)
(feet)
Well
feet
CAP Part 1 Established Background Monitoring Wells
MW-4
500 ft NW
1,000 ft
SE
864
823.5
829.56
803.95
2015 - July
NW
813.5
2016 - March, June, September
MW-4D
500 ft NW
1,000 ft
SE
863
804 - 799
829.86
803.95
2015 - July
NW
2016 - March, June, September
MS-10
1,700 ft
SE
848
835 - 825
833.95
800.17
2015 - July
NW
2016 - March, June, September
BG-1 S
5,700 ft NE
--
SE*
815
807 - 797
807.31
786.81
2015 - July
2016 - March, June, September
BG-1 D
5,700 ft NE
--
SE*
815
780 - 775
806.56
786.81
2015 - July
2016 - March, June, September
BG-2S
3,500 ft NE
--
SE*
812
805 - 795
809.58
786.81
2015 - July
2016 - March, June, September
BG-2BR
3,500 ft NE
--
SE*
812
762 - 757
808.28
786.81
2015 - July
2016 - March, June, September
BG-3S
4,100 ft NE
--
SE
861
838.5
835.27
786.81
2015 - July
828.5
2016 - March, June, September
BG-3D
4,100 ft NE
--
SE
862
802.5
837.49
786.81
2015 - July
797.5
2016 - March, June, September
Proposed Additional Background
Monitoring Wells
GWA-4S
550 ft NW
--
SE
899
850 - 835
843.95
817.46
2016 - March, June, September
GWA-4D
550 ft NW
--
SE
899
818 - 813
844.54
817.46
2016 - March, June, September
GWA-5S
500 ft N
--
S
844
820 - 810
813.21
803.81
2016 - March, June, September
GWA-5D
500 ft N
--
S
844
801 - 796
813.08
803.81
2016 - March, June, September
GWA-6S
480 ft NE
--
SW
843
805 - 795
804.34
800 (max)
2016 - March, June, September
GWA-6D
480 ft NE
--
SW
843
741 - 736
804.14
800 (max)
2016 - March, June, September
GWA-8S
1,075 ft N
--
N
850
827 - 812
819.37
803.95
2016 - March, June, September
GWA-8D
1,075 ft N
--
N
851
794.5
818.97
803.95
2016 - March, June, September
789.5
GWA-12S
950 ft W
--
ENE
876
866 - 851
860.73
800.17
2016 - June, September

Table 1. Background Monitoring Well Information
Marshall Steam Station Ash Basin System
F)2
Distance and Direction
Closest Ash
from Source Area
Groundwater
Ground
Elevation
Basin
Flow
Surface
of
Elevation of
Surface
Groundwater Elevation Monitoring
Well ID
Dry Ash
Direction
Elevation
Screened
Groundwater
Water
Events
Ash Basin
Landfill
(feet)
(feet)
Interval
(feet)
Elevation to
(Unit II)
(feet)
Well
feet
GWA-12D
950 ft W
--
ENE
876
788 — 783
860.41
800.17
2016 — September
GWA-12BR
950 ft W
--
ENE
876
763 — 758
859.97
800.17
2016 — June, September
BG-1 BR
5,700 ft NE
--
SE*
815
731 — 726
800.79
786.81
2016 —June, September
BG-3BR
4,100 ft NE
--
SE*
865
760 — 755
832.18
786.81
2016 —June, September
Notes:
1. Elevations presented in North American Vertical Datum (NAVD) 88.
2. The groundwater elevations presented are the averaged based on available groundwater elevation measurements from sampling/gauging events at the site.
3. Ash basin surface water elevations closest to monitoring wells were measured during the June 2016 gauging event, except for GWA-6S/D (see note below).
4. The closest ash basin surface water elevation to GWA-6S/D should be considered less than or equal to 800 feet due to topography and the lateral extent of
the ash basin waste boundary.
* = Groundwater flow direction estimated based on topography and slope -aquifer system.

Table 2. Marshall Ash Basin System Background Groundwater Baseline Analytical Results
POTENTIAL BACKGROUND WELLS TO BE ADDED JUSTIFICATION ATTACHED
(Ind ding GWA-4S,
•r
�n'�t����:m�����E:an��fa���s��a��rmerrxmi���a���s��r:��asrr��r���m���a��s���iirFn��ms�����®����
sum.0
Valid Sample Count 30 30 30 30 30 30 30 is 30 30 30 30 30 30 so 30 0 30 30 30 30se 0 30 21 30 30 30 30 30 30 3030 30 30 30 30 6 6 1
gym.9
®s29
�1�
�tas�e�
i
s29
�iecn�tais
iiirt��xsssi
Valid Sample Count 29 23 5 29 29 16 29 29 29 16 29 29 11 16 29 29 23 29 29 16 29 is 23 29 23 Is a
Notes:
Red [e# indicates samples that will not be used in determining proposed provisional background threshold values (PPBNs) due to pH > 8.5 or turbidity >30 NTL
- Bold highlighted concentration indicates exceedance of the 15A NCAC 02L Standard or IMAC, Appendix 2, April 1, 201:
pm —
n pg/L =micrograms per liter
p5/cm = miaosiemens per centimeter
ft = feet
mg/L = milligrams per liter
Ph values were Calculated by adding 200 mV to the field -measured ORP value. Depending on the field meter used, Eh values calculated using this formula can vary by +/- 5 ml
my = mlllivar,
NTU - nephelometric turbidity unit
pC5/L = pimcudes per liter
SU = standard unit
I = laboratory estimated concentration
I- = estimated concentration, biased low
I+ = estimated concentration, biased high
U = results not detected at concentrations which equal the laboratory's method reporting limi
02L = North Carolina groundwater standards as specified in T15A NCAC 02L
D = dissolved
Deg C = degree Celsius
DHHS = North Carolina Department of Health and Human Service
DO = dissolved oxygen
GW = groundwater
GWQS = groundwater quality standarC
IMAC = Interim Maximum Allow Concentration
N = normal
NS = no standard
PPBC = Proposed Previsional Background Concentration!
T = total
TDS = total dissolved solids
TOC =total organic carbon
'BG-1BR is scheduled for replacement due to potential grout mnWminatiar
Page 2 of 2

a
LEGEND
IT 717147
CAMAASSESSMENT
GROUNDWATER MONITORING WELL; 9s> r
NPDES COMPLIANCE , Do
GROUNDWATER MONITORING WELL
NPDES VOLUNTARY �: f
GROUNDWATER MONITORING WELL
LANDFILL GROUNDWATER
MONITORING WELL
CAMAASSESSMENT
GROUNDWATER OBSERVATION
WELL
LANDFILL GROUNDWATER RAI
OBSERVATION WELL [�Y�IKaJ
ASH BASIN WASTE BOUNDARY
ASH BASIN COMPLIANCE
BOUNDARY
ASH BASIN COMPLIANCE
BOUNDARY COINCIDENT WITH
DUKE ENERGY PROPERTY
L,,>,
BOUNDARY?s
,
z;
LANDFILL/STRUCTURAL FILL
_
BOUNDARY
LANDFILL COMPLIANCE
BOUNDARY
DUKE ENERGY PROPERTY
BOUNDARY
HYDROGRAPHY
W
NOTES:
1. WASTE BOUNDARY AND ASH STORAGE AREA BOUNDARIES ARE APPROXIMATE
2. THE COMPLIANCE BOUNDARY IS ESTABLISHED ACCORDING TO THE DEFINITION FOUND IN
15A NCAC 02L .0107 (a)
3. AERIAL IMAGERY IS A COMBINATION OF A WSP AERIAL SURVEY (2014) AND NC ONEMAP (2010)
4. HYDROGRAPHY WAS DIGITIZED FROM USGS 1:24,000 SCALE TOPOGRAPHICAL MAPS
5. AS -BUILT CAMA ASSESMENT AND NPDES SERIES WELLS HAVE BEEN PROVIDED BY WSP AND
DUKE ENERGY
SCALE (FEET)
500 0 500 1,000
ITION MW-3
F.1LL
DRY AS•H
LANDFILL
AIMS
�= f
FEZ
AL-3S
AL-3D
AL-3BR
OB-2
(M W-6)
rn
WELL LOCATION MAP
DUKE ENERGY CAROLINAS, LLC
MARSHALL STEAM STATION ASH BASIN
CATAWBA COUNTY, NORTH CAROLINA
r
DATE
DECEMBER 2016
FIGURE
1