Cape Fear TM BG GW 2017-05-26

Date: May 26, 2017
TECHNICAL MEMORANDUM
File: 1026.103.27
To: John Toepfer (Duke Energy)
Cc: Kathy Webb (SynTe a), Ryan Czop (Duke Energy)
From: Chris Suttell :19�
Up-to-date Background Groundwater Data - Cape Fear Steam Electric Plant
The North Carolina Department of Environmental Quality (NCDEQ) provided a response letter
(Zimmerman to Draovitch, April 28, 2017) to the technical memorandum titled Statistical
Methods for Developing Reference Background Concentrations for Groundwater and Soil at Coal Ash
Facilities (HDR Engineering, Inc. and Synterra Corporation, January 2017) requesting the following
technical issues be addressed in order to progress site assessments and corrective actions:
0 Collect a minimum of ten valid background groundwater samples, opposed to eight,
prior to the determination of background threshold values (BTVs) for constituents in
each flow layer; and
`i Provide updated spreadsheets of background groundwater data for each facility.
NCDEQ requests that the updated background data spreadsheets contain strike-throughs of
data that may be omitted from the background data set due to one or more of the following
conditions:
01 Sample pH is greater than or equal to 8.5 standard units (S.U.);
01 Sample turbidity is greater than or equal to 10 Nephelometric Turbidity Units (NTUs);
0 Result is a statistical outlier identified for background sample data collected through
March 2017;
h Sample collection occurred less than a minimum 60 days between sampling events; and
y Non-detected results are greater than 2LAMAC.
Table 1 provides a summary of existing background well data for each flow system and
indicates which samples are considered for use as a minimal dataset in statistical calculations to
determine concentrations of naturally occurring (background) inorganic constituents in
groundwater. Sample results designated as statistical outliers or were collected less than 60
days between sampling events are identified in Table 1.
Table 1 does not include analytical results from wells at the NPDES compliance background
location (BGMW-4 and BGTMW-4) which NCDEQ has called into question. Also, the dataset
on Table 1 does not include wells in the MW -20 cluster. Analytical results from wells at the
P: \ Duke Energy Progress. 1026 \ Statistics_CAMA \ May 26 2017 - Updated Background Data Spreadsheets for
NCDEQ\Tech Memo - Response to NCDEQ 04282017 Letter \Final\ Cape Fear TM BG GW 2017-05-17 Revl.docx

Up-to-date Background Groundwater Data May 19, 2017
Cape Fear Steam Electric Plant Page 2 of 2
MW -20 cluster indicate the ash basins have not affected groundwater quality at this location
and detected constituent concentrations may represent background conditions. Water level
maps and groundwater modeling also indicate that it is unlikely for ash -basin derived
constituents to migrate to groundwater at the MW -20 well cluster. SynTerra recommends each
of these wells (BGMW-4, BGTMW-4, MW -20S and MW-20BR) be further considered for
inclusion in the background dataset during future evaluations. Rationale for inclusion of these
wells has been provided in the CSA Supplement 1 and the January 2017 technical
memorandum. Additional background data would supplement the dataset presented herein.
The following discussion quantifies the number of valid results collected through February 2017
within each flow zone.
BACKGROUND WELL NETWORK
The current background monitoring well network consists of wells installed within two flow
zones; surficial and bedrock.
SURFICIAL ZONE
Three monitoring wells considered to represent background for CAMA (MW-15SU, MW-15SL,
and MW -16S) are screened within the surficial flow zone. The surficial background
groundwater dataset satisfies the request for the minimum number of ten samples to conduct
statistical analysis for each constituent except for radium -228 (eight samples). Samples for
radium -228 will be collected along with other constituents from surficial zone background wells
during the sampling events planned for June 2017 and in the third quarter of 2017. Inclusion of
those results within the dataset provide herein will bring the total number of valid radium -228
samples to ten.
BEDROCK ZONE
Four monitoring wells considered to represent background for CAMA (MW -9, MW-9BR, MW-
15BR, and MW-16BR) are screened within the bedrock flow zone. The bedrock background
groundwater dataset satisfies the request for the minimum number of ten samples to conduct
statistical analysis for each constituent.
ATTACHMENTS: Table 1: Background Analytical Results through February 2017
P: \ Duke Energy Progress.1026 \ Statistics _CAMA \ May 26 2017 - Updated Background Data Spreadsheets for
NCDEQ\Tech Memo - Response to NCDEQ 04282017 Letter \Final\ Cape Fear TM BG GW 2017-05-17 Revl.docx