Roxboro - GW Tech Memo_20170526

Date: May 26, 2017
TECHNICAL MEMORANDUM
File: 1026.107
To: John Toepfer (Duke Energy); Kimberlee Witt (Duke Energy)
Cc: Kathy Webb (SynTerra)
From: Craig Eady (SynTerra) e .
Subject: Up-to-date Background Groundwater Data —Roxboro 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 lower; 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.);
y Sample turbidity is greater than or equal to 10 Nephelometric Turbidity Units (NTUs);
`i Result is a statistical outlier identified for background sample data collected through
March 2017;
01 Sample collection occurred less than a minimum 60 days between sampling events; and
01 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 in statistical calculations to determine
concentrations of naturally occurring (background) inorganic constituents in groundwater. The
following discussion quantifies the number of valid results collected through January 2017
within each flow zone. Sample results designated as statistical outliers or were collected less
than 60 days between sampling events are identified in Table 1. Groundwater sampling was
recently completed in April 2017 and when available these results will be able to augment the
dataset provided herein.
P: \ Duke Energy Progress.1026 \ Statistics_CAMA \ May 26 2017 - Updated Background Data Spreadsheets for
NCDEQ \ Tech Memo - Response to NCDEQ 04282017 Letter \ Final \ Roxboro - Tech Memo 05262017.docx

Up-to-date Background Groundwater Data May 26, 2017
Roxboro Steam Electric Plant Page 2 of 2
BACKGROUND WELL NETWORK
Groundwater across the Site is unconfined and found within the ash basins and three
hydrostratigraphic units: the residuum/saprolite; a transition zone; and fractures/joints within
the underlying bedrock. However, saturated conditions are not evident within the
residuum/saprolite units in background locations due to the nature of the former natural
drainage features in which the ash basins were created. The current background monitoring
well network consists of wells installed within the transition zone and bedrock units.
TRANSITION ZONE
Three background wells (BG-01, MW-15D and MW-18D) are screened within the transition
zone. Table 1 presents analytical results for the transition flow zone. The transition zone
groundwater dataset satisfies the requirements for the minimum number of ten samples to
conduct statistical analysis for each constituent.
BEDROCK
Nine wells (BG-01BR, MW-10BR, MW-13BR, MW-14BR, MW-15BR, MW-16BR, MW-17BR, MW-
18BR and MW-19BRL) are screened in the bedrock flow zone. The bedrock groundwater
dataset satisfies the requirements for the minimum number of ten samples to conduct statistical
analysis for each constituent.
ATTACHMENTS: Table 1: Updated Background Groundwater Analytical Results
Through January 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 \ Roxboro - Tech Memo 05262017.docx