Riverbend Additional BG Wells Tech Memo_REV1_20170526

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Technical Memorandum
Date:
Friday, May 26, 2017
Project:
Riverbend Steam Station
To:
Sean DeNeale
From:
Jacob Ruffing
Subject: Additional Background Monitoring Well Determinations — Revision 1
The purpose of this revised technical memorandum (TM) is to address the North Carolina
Department of Environmental Quality's (NCDEQ's) requirement identified below. In addition, this
revised TM includes the justification for additional monitoring wells that should be considered as
background wells for the Riverbend Steam Station (RBSS) ash basin system as originally
presented in the TM dated December 12, 2016.
Following submittal of the TM dated December 12, 2016, Duke Energy submitted the Statistical
Methods for Developing Reference Background Concentrations for Groundwater and Soil at
Coal Ash Facilities (Background Methodology) to NCDEQ on January 20, 2017. NCDEQ
subsequently requested additional information regarding the Background Methodology via a
letter dated April 28, 2017. As part of the April 28, 2017 letter, NCDEQ requested the following
information:
Provide up-to-date digital spreadsheets of raw background groundwater data for each
facility by May 26, 2017, and include, within the raw background data spreadsheet as
"strike-throughs", the individual monitoring data results that Duke Energy believes
should be omitted from the background dataset due to (a) high pH, (b) high turbidity,
(c) auto -correlation, (d) outlier designation, (e) non -detect values that are above
2L/IMAC, or (f) other reasons.
The up-to-date raw background groundwater data for background determinations at RBSS are
provided in Table 2 of this revised TM. Note the remainder of the requirements in the April 28,
2017 NCDEQ letter are addressed in a revision to the Background Methodology under separate
cover.
Monitoring wells MW-7D, MW-7SR, MW-7BR, BG-1S, and BG-1D were previously evaluated
and chosen to represent background groundwater quality as documented in Appendix B of the
RBSS Corrective Action Plan (CAP) Part 1, dated November 16, 2015. Well locations are
shown on Figure 1.
Wells BG-2S, BG-2D, BG-2BR, BG-3S, and BG-3D were previously background wells and
should no longer be used to represent background water quality. The results of the evaluation of
these monitoring wells concluded that it was undetermined if the location of these wells

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represent background water quality relative to the ash disposal areas at the site and that
subsequent sampling and groundwater measurements would aid in determining the suitability of
these wells for use in determining background concentrations. An additional evaluation of these
monitoring wells is provided on the following pages of this memorandum.
The proposed additional background monitoring wells include BG-4S, BG-4D, BG-4BR, BG-5D,
BG-5BR, GWA-5S, GWA-14S, and GWA-14D. These 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 source area. Distances and elevations pertinent to
the evaluation are provided on Table 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. Sample data associated with turbidity reported to
be greater than 10 nephelometric turbidity units (NTU), samples without a recorded turbidity,
samples with a pH greater than 8.5, non -detect samples with a method detection limit greater
than the respective 2L Standard or IMAC, or samples with less than a 60 day interval between
sample events are denoted with "strike-throughs" and were not included in the valid sample
counts as requested by NCDEQ.
BG-2S, BG-2D, and BG-2BR
Monitoring well BG-2S, BG-2D, and BG-2BR were installed as background monitoring wells
during the CSA assessment. June 2015, August 2015, April 2016, and June 2016 groundwater
gauging events have been considered for determining groundwater elevations and flow direction
in this area of the site.
• Monitoring wells BG-2S/D/BR are located approximately 1,450 feet east of the eastern
side of the ash basin waste boundary. The ground surface at the monitoring wells
(approximately 702.5 feet) is 17.5 feet lower than the ash basin waste boundary (720
feet).
• The historical surface water elevation in the ash basin was 713 feet.
• BG-2S is screened from approximately 49.5 feet to 65.5 feet below ground surface (bgs)
in the shallow flow layer (approximately 653 to 637 feet elevation). The lowest
groundwater elevation calculated from gauging events was 648 feet during the
September 2015 event (water elevation in well was 64 feet lower than the historical ash
basin surface water elevation).
• BG-2D is screened from approximately 155 feet to 160 feet bgs (approximately 547 to
542 feet elevation) in the deep flow layer. The lowest groundwater elevation calculated
from gauging events was 648 feet during the July 2015 event (water elevation in well
was 64 feet lower than the historical ash basin surface water elevation).
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BG-2BR is screened from approximately 203 feet to 208 feet bgs (approximately 500 to
495 feet elevation) in the bedrock flow layer. The lowest groundwater elevation
calculated from gauging events was 648 feet during the September 2015 event (water
elevation in well was 64 feet lower than the historical ash basin surface water elevation).
Based on groundwater flow and groundwater/surface water elevation data, groundwater
in monitoring wells BG-2S/D/BR may originate upgradient and to the west from the ash
basin.
Monitoring wells BG-2S, BG-21D, and BG-2BR may not represent background water quality
relative to the ash disposal areas at the site and have not been included in the valid background
sample counts.
BG-3S and BG-3D
Monitoring well BG-3S and BG-31D were installed as background monitoring wells during the
CSA assessment. June 2015, August 2015, April 2016, and June 2016 groundwater gauging
events have been considered for determining groundwater elevations and flow direction in this
area of the site.
• Monitoring wells BG-3S/D are located approximately 1,350 feet east of the eastern side
of the ash basin waste boundary. The ground surface at the monitoring wells
(approximately 672 feet) is 26 feet lower than the edge of waste at the ash basin waste
boundary (698 feet).
• The historical surface water elevation in the ash basin was 713 feet.
• BG-3S is screened from approximately 21 feet to 37 feet bgs in the shallow flow layer
(approximately 651 to 635 feet elevation). The lowest groundwater elevation calculated
from gauging events was 644 feet during the September 2015 event (water elevation in
well was 69 feet lower than the historical ash basin surface water elevation).
• BG-31D is screened from approximately 100 feet to 105 feet bgs (approximately 572 to
567 feet elevation) in the deep flow layer. The lowest groundwater elevation calculated
from gauging events was 642 feet during the September 2015 event (water elevation in
well was 71 feet lower than the historical ash basin surface water elevation).
• Based on groundwater flow and groundwater/surface water elevation data, groundwater
in monitoring wells BG-3D/BR may originate upgradient and to the west from the ash
basin.
Monitoring wells BG-3S and BG-31D may not represent background water quality relative to the
ash disposal areas at the site and have not been included in the valid background sample
counts.
BG-4S, BG-4D, and BG-4BR
Monitoring wells BG-4S, BG-41D, and BG-4BR were installed as background monitoring wells
during the installation of the additional assessment wells. The June and September 2016
groundwater gauging events have been considered for determining groundwater elevations and
flow direction in this area of the site.

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• Monitoring wells BG-4S/D/BR are located approximately 920 feet southwest of the ash
storage area waste boundary. The ground surface at the monitoring wells (approximately
705 feet) is 25 feet higher than the edge of the ash storage area waste boundary (680
feet).
• The historical full pond elevation in the primary cell of the ash basin was 713 feet.
• BG-4S is screened from approximately 50 feet to 65 feet bgs in the shallow flow layer
(approximately 655 to 640 feet elevation). The lowest groundwater elevation calculated
from gauging events was 664 feet during the September 2016 event (water elevation in
well was 49 feet lower than the historical ash basin surface water elevation).
• BG-41D is screened from approximately 127 feet to 137 feet bgs (approximately 578 to
568 feet elevation) in the deep flow layer. The lowest groundwater elevation calculated
from gauging events was 664 feet during the September 2016 event (water elevation in
well was 49 feet lower than the historical ash basin surface water elevation).
• BG-4BR is screened from approximately 164 feet to 169 feet bgs (approximately 541 to
535 feet elevation) in the bedrock flow layer. The lowest groundwater elevation
calculated from gauging events was 538 feet during the June 2016 event (water
elevation in well was 175 feet lower than the historical ash basin surface water
elevation).
• Although the water level in the wells is lower than the historical ash basin water
elevation, based on the slope aquifer system groundwater at the wells would originate to
the northwest of the monitoring wells along Horseshoe Bend Beach Road, and not from
the ash disposal areas on the site.
• Boron was not reported in any of these wells at concentrations which equal or exceed
the laboratory method reporting limit. Sulfate reported at one to two orders of magnitude
less than the 2L Standard. Chloride reported at approximately two orders of magnitude
less than the 2L Standard. BG-4BR had a TDS exceedance reported; however, the pH
and turbidity values were not recorded an may have had an effect on the results of this
sample.
• Based on analytical data, groundwater flow, and groundwater/surface water elevation
data, groundwater in monitoring wells BG-4S/D/BR originates northwest of well locations
at Horseshoe Bend Beach Road and is not impacted by ash disposal areas.
Based on the justification provided above and analytical data included in Table 2, monitoring
wells BG-4S, BG-41D, and BG-4BR represent background water quality relative to the ash
disposal areas at the site and have been included in the valid background sample counts.
BG-5D and BG-5BR
Monitoring wells BG-51D and BG-5BR were installed as background monitoring wells during the
installation of the additional assessment wells. The June and September 2016 groundwater
gauging events have been considered for determining groundwater elevations and flow direction
in this area of the site.
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• Monitoring wells BG-5D/BR are located approximately 2,875 feet southwest of the ash
storage and cinder storage area waste boundaries. The ground surface at the monitoring
wells (approximately 716 feet) is 36 feet higher than the edge of the ash storage area
waste boundary (680 feet).
• The historical full pond elevation in the primary cell of the ash basin was 713 feet.
• BG-51D is screened from approximately 97.5 feet to 102.5 feet bgs in the deep flow layer
(approximately 618.5 to 613.5 feet elevation). The lowest groundwater elevation
calculated from gauging events was 684 feet during the September 2016 event (water
elevation in well was 29 feet lower than the historical ash basin surface water elevation).
• BG-5BR is screened from approximately 127.5 feet to 132.5 feett bgs (approximately
588.5 to 583.5 feet elevation) in the bedrock flow layer. The lowest groundwater
elevation calculated from gauging events was 678 feet during the June 2016 event
(water elevation in well was 35 feet lower than the historical ash basin surface water
elevation).
• Although the water level in the wells is lower than the historical ash basin water
elevation, based on the slope aquifer system groundwater at the wells would originate to
the west of the monitoring wells to the east of State Route 1911, and not from the ash
disposal areas on the site.
• Boron was not reported in either of these wells at concentrations which equal or exceed
the laboratory method reporting limit. Sulfate and chloride reported at one order of
magnitude less than the 2L Standard.
• Based on analytical data, groundwater flow, and groundwater/surface water elevation
data, groundwater in monitoring wells BG-51D and BG-5BR originates upgradient and
west of the well locations and is not impacted by ash disposal areas.
Based on the justification provided above and analytical data included in Table 2, monitoring
wells BG-51D and BG-5BR represent background water quality relative to the ash disposal areas
at the site 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 an upgradient monitoring well during
the CSA assessment. June 2015, August 2015, April 2016, and June 2016 groundwater
gauging events have been considered for determining groundwater elevations and flow direction
in this area of the site.
• Monitoring wells GWA-5S/D are located approximately 225 feet southeast of the ash
storage area waste boundary. The ground surface at the monitoring wells (approximately
785 feet) is 25 feet higher than the edge of waste at the ash storage area waste
boundary (760 feet).
• The historical full pond elevation in the primary cell of the ash basin was 713 feet.
• GWA-5S is screened from approximately 60 feet to 75 feet bgs in the shallow flow layer
(approximately 725 to 710 feet elevation). The lowest groundwater elevation calculated

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from gauging events was 710 feet during the November 2015 event (water elevation in
well was 3 feet lower than the historical ash basin surface water elevation).
• GWA-51D is screened from approximately 212.5 feet to 217.5 feet bgs in the deep flow
layer (approximately 572.5 to 567.5 feet elevation). The lowest groundwater elevation
calculated from gauging events was 716 during the September 2015 event (water
elevation in well was 3 feet higher than the historical ash basin surface water elevation).
• Boron was not reported in any of these wells at concentrations which equal or exceed
the laboratory method reporting limit. Sulfate and chloride reported at one to two orders
of magnitude less than the 2L Standard.
• Based on analytical data, groundwater flow, and groundwater/surface water elevation
data, groundwater in monitoring wells GWA-5S/D originates upgradient and south of the
well location and the ash basin and is not impacted by ash disposal areas.
Based on the justification provided above and analytical data included in Table 2, monitoring
wells GWA-5S and GWA-51D represent background water quality relative to the ash disposal
areas at the site and have been included in the valid background sample counts. Monitoring well
GWA-51D appears to be grout contaminated and the historical results from this well will not be
utilized. The well is scheduled for re -development. If the pH in the well is lowered to within the
acceptable range following re -development the future results will be considered for incorporation
of background concentration determinations.
GWA-14S and GWA-14D
Monitoring wells GWA-14S and GWA-14D were installed as upgradient wells during the
installation of the additional assessment wells. The June and September 2016 groundwater
gauging event has been considered for determining groundwater elevations and flow direction in
this area of the site.
• Monitoring wells GWA-14S/D are located approximately 540 feet southeast of the ash
basin waste boundary. The ground surface at the monitoring wells (approximately 776
feet) is 46 feet higher than the edge of the ash basin waste boundary (730 feet).
• The historical full pond elevation in the primary cell of the ash basin was 713 feet.
• GWA-14S is screened from approximately 65 feet to 80 feet bgs in the shallow flow layer
(approximately 711 to 696 feet elevation). The lowest groundwater elevation calculated
from gauging events was 713 feet during the September 2016 event (water elevation in
well equal to the historical ash basin surface water elevation).
• GWA-14D is screened from approximately 202.5 feet to 207.5 feet bgs (approximately
573.5 to 568.5 feet elevation) in the deep flow layer. The lowest groundwater elevation
calculated from gauging events was 712 feet during the September 2016 event (water
elevation in well was 1 feet lower than the historical ash basin surface water elevation).
• Boron was not reported in GWA-14S at a concentration which equals or exceeds the
laboratory method reporting limit. Sulfate and chloride were reported at two orders of
magnitude less than the 2L Standard in monitoring well GWA-14S.
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The pH of monitoring well GWA-14D was reported at 9.6 and 10 SU. The well is grout
contaminated and the data has not been included in the valid background sample
counts.
• Based on groundwater flow and groundwater/surface water elevation data, groundwater
in monitoring wells GWA-14S/D originates upgradient and south of the well location and
the ash basin and is not impacted by ash disposal areas.
Based on the justification provided above and analytical data included in Table 2, monitoring
well GWA-14S represents background water quality relative to the ash disposal areas at the site
and has been included in the valid background sample counts. The pH measured in GWA-14D
indicates that this well is grout contaminated and it has not been included in the valid
background sample counts.

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MW-15
/MW-15D
MW-15BR
MW-12*
S-4/GWA-10
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S-4
GWA-10S
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GWA-10BRU
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S-9
MW-3S
MW-3D
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GWA-1BRU
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MW-11SR •
BMW-11DR
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M W-2S*
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MW-2D
GWA-2S
GWA-2BRU
/ VGWA-2BR
/ MW-10
l �S-1/S-2 MW-1S
A ` IN, J•� J MW-1D AB-5S*
alo� AB-5SL*
AB-6S AB-5D*
Q QEF� I I AB-6BRU
PRO MW-9 S-1
{�.MW-9D
GWA-3S* I J MW-9BR
GWA-3S-A A13-8S
GWA-3D GWA-15S
GWA-313R GWA-15D
AB-8D
• SW-3
GWA-11D
C-113RUO
GWA-11S
C-1S
C-2S
C-2D
CINDER
STORAGE
ASH STORAGE
GWA-12S
GWA-12D
AS-2S*
AS-2D*
AS -IS*
AS -ID*
GWA-23BR
GWA-23S
GWA-23D
GWA-4S
OGWA-4D
GWA-4BR
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RBSWO01 STEAM STATION
BG-4S BG-41)
BG-4BR 1
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NOTES:
1. PARCEL DATA FOR THE SITE WAS OBTAINED FROM DUKE ENERGY REAL ESTATE AND IS APPROXIMATE.
2. ASH STORAGE BOUNDARY AND CINDER STORAGE BOUNDARY ARE APPROXIMATE.
3. AS -BUILT MONITORING WELL LOCATIONS PROVIDED BY DUKE ENERGY.
4. SHALLOW MONITORING WELLS (S) - WELL SCREEN INSTALLED ACROSS THE SURFICIAL WATER TABLE.
5. SHALLOW LOWER MONITORING WELLS (SL) - BOTTOM OF WELL SCREEN SET AT THE ASH-REGOLITH INTERFACE.
6. INTERMEDIATE MONITORING WELLS (1) - WELL SCREEN INSTALLED IN REGOLITH BELOW WATER TABLE AND BELOW ASH IN ASH BASIN.
7. TOPOGRAPHY DATA FOR THE SITE WAS OBTAINED FROM NC DOT GEOGRAPHIC INFORMATION SYSTEM (GIS) WEB SITE.
8. ORTHOPHOTOGRAPHY WAS OBTAINED FROM WSP (DATED APRIL 2014).
9. THE COMPLIANCE BOUNDARY IS ESTABLISHED ACCORDING TO THE DEFINITION FOUND IN 15A NCAC 02L .0107 (a).
10. HYDROGRAPHY WAS OBTAINED FROM USGS 7.5-MINUTE SERIES TOPOGRAPHIC MAPS MOUNTAIN ISLAND LAKE, NC.
AS-3SA*
AS-3D*
ASH BASIN
SECONDARY CELL
l
MW-4D� OGti T,9 O
M W-13 �Fc5l
pA�
MW-5D 0��� IrF
�AB 1D � \
S-10 1 \
S-11 ►� \ `�
• S-10/S-12
M W`S,S
MW- D
AB-2S0- 0 S-12 ( \.
AB-21)
GWA-9S
GWA-9D \
AB-4S*
GWA-913R
-�
AB-4D*
MW-14 -
S-6 \
SW-2
v
oGWA-8S
GWA-8D
AB-3S*
F
AB-3D*
f S-13 I
ASH BASIN
AB-3BR*
OBG-3S
PRIMARY CELL
w
•a d r2 t '. ti
'`
GWA-7S
a�
GWA-713R
BG-2S /
1;�,BG-2D
013-1 OB-2
BG-2BR
AB-7S*
AB-71
/'•,
AB-7D*
M W-7S*
/
GWA-6S .
MW-7BR
7
GWA-6D
MW-71)
MW-7SR
/
.� BG-1S
BG-ID
GWA-14S
/
GWA-5SL
GWA-14D
GWA-5D
GWA-22S HORSES OH E BE D BEACH ROAp i
GWA-20S G WA-22 D
GWA-20D �, MW-85
GWA-20BR MW-8D GWA-22BR-A \ I
MW-81 GWA-21S,
GWA-211) '` \
GWA-13S GWA-21BR
GWA-13D
Y ir
y !. '� • r-. ` _ + ,., r "rGy`• �k tbra" _
r
ey
-r
4
y^ xF�_ _
SCALE (FEET)
200' 0 200' 400'
I" = 400'
ABANDONED WELLS
WELL ID
NORTHING
EASTING
AB-3BR
593978.52
1415271.06
AB-313
593974.72
1415275.76
AB-3S
593982.21
1415267.15
AB-413
594396.84
1414855.06
AB-4S
594393.61
1414857.36
AB-513
593709.72
1414280.27
AB-5S
593710.50
1414273.99
AB-5SL
593711.23
1414269.22
AB-713
593068.71
1415045.58
AB-7S
593047.91
1415046.88
A B-7S I
593054.54
1415045.99
AS-1D
592498.83
1413188.18
AS-1S
592513.58
1413205.28
AS-2D
592600.94
1413757.51
AS-2S
592617.99
1413760.43
AS-3D
592656.95
1414310.71
AS-3S
592637.18
1414312.33
GWA-3S
593361.15
1412342.59
MW-2S
595037.80
1414508.00
MW-7S
592781.00
1415852.00
MW-12
595927.30
1414835.00
* DENOTES THAT WELL HAS BEEN ABANDONED
LEGEND:
DUKE ENERGY PROPERTY BOUNDARY
ASH BASIN COMPLIANCE BOUNDARY
ASH BASIN COMPLIANCE BOUNDARY COINCIDENT
WITH DUKE ENERGY PROPERTY BOUNDARY
ASH BASIN WASTE BOUNDARY
ASH OR CINDER STORAGE AREA BOUNDARY
STREAM
ASH BASIN COMPLIANCE
GROUNDWATER MONITORING WELL
ASH BASIN VOLUNTARY
GROUNDWATER MONITORING WELL
CSA GROUNDWATER MONITORING WELL
POST-CSA ADDITIONAL ASSESSMENT
GROUNDWATER MONITORING WELL
ABANDONED WELL
OBSERVATION WELL (WATER LEVEL ONLY)
SURFACE WATER SAMPLE LOCATION
AREA OF WETNESS SAMPLE LOCATION
NCDENR SAMPLE LOCATION
SAMPLE LOCATION MAP DATE
CAMA AND NPDES PROGRAMS mnvzon
DUKE ENERGY CAROLINAS, LLC
RIVERBEND STEAM STATION ASH BASIN FIGURE
cAsoN roUri-Y. NORTH r_;aRouN= �

Table 1. Monitoring Well Information
Distance and
Closest
Direction from
Ground-
Ground
Elevation of
Lowest
Ash Basin
Well ID
Source Area
water
Surface
Screened
Elevation of
Water
Groundwater Elevation Gauging Events
(Primary/Secondary
Flow
Elevation
Interval
Groundwater
Elevation
Cells, Ash Storage
Direction
(feet)
(feet)
(feet)
to Well
Area
feet
CAP 1 Evaluated Monitoring Wells
MW-7SR
380 feet SE
NE
763
696-716
710.15
713
2015 - July, September
2016 — June, September
MW-7D
380 feet SE
NE
763
660-665
710.12
713
2015 - July, September, November, December
2016 — March, June, September
MW-7BR
380 feet SE
NE
763
561-566
709.76
713
2015 - July, September, December
2016 — March, June, September
BG-1S
1,600 feet ESE
N
701
656-671
669.2
713
2015 - July, September, November, December
2016 — March, June, September
BG-1D
1,600 feet ESE
N
701
502-507
667.01
713
2015 - July, September, November, December
2016 — March, June, September
BG-2S
1,450 feet E
N
702.5
637-653
648.95
713
2015 - July, September, November, December
2016 — March, June, September
BG-2D
1,450 feet E
NE
702.5
542-547
647.96
713
2015 - July, September, November, December
2016 — March, June, September
BG-2BR
1,450 feet E
N
702.5
495-500
647.58
713
2015 - July, September, November, December
2016 — March, June, September
BG-3S
1,350 feet E
N
672
635-651
643.79
713
2015 - July, September, November, December
2016 — March, June, September
BG-3D
1,350 feet E
N
672
567-572
641.83
713
2015 - July, September, November, December
2016 — March, June, September