GW Tech Memo_20170526

Date: May 26, 2017
TECHNICAL MEMORANDUM
File: 1026.105
To: John Toepfer (Duke Energy); Kimberlee Witt (Duke Energy)
Cc: Kathy Webb (SynTerra)
From: Jerry Wylie(SynTerra)
Subject: Up-to-date Background Groundwater Data —Mayo 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:
y 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 system; and
y 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:
y Sample pH is greater than or equal to 8.5 standard units (S.U.);
0 Sample turbidity is greater than or equal to 10 Nephelometric Turbidity Units (NTUs);
411 Result is a statistical outlier identified for background sample data collected through
March 2017;
y Sample collection occurred less than a minimum 60 days between sampling events; and
47, Non -detected results are greater than 2L/lMAC.
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 February 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 \ Mayo - GW Tech Memo 05262017.docx

Up-to-date Background Groundwater Data May 19, 2017
Mayo Steam Electric Plant Page 2 of 2
BACKGROUND WELL NETWORK
The current background monitoring well network consists of wells installed within three flow
zones: surficial, transition zone, and bedrock.
SURFICIAL
One background well (MW-12S) is screened within the surficial flow zone. MW-12S has been
sampled eleven times since June 2015. Potentially autocorrelated data, outliers, and an evolving
parameter list result in the number of valid events ranging from four to six, depending on the
constituent. Additional samples will be collected to achieve the desired sample size by the end
of first quarter (Q1) 2018. An analysis of the potentially autocorrelated datasets is
recommended and may demonstrate the dataset is currently sufficient to perform statistical
calculations. Subsequent outlier screening of the surficial background dataset may warrant
additional sample collection.
TRANSITION ZONE
Two background wells (BG-02 and MW-12D) are screened within the transition zone.
Currently, the transition zone background groundwater dataset satisfies the requirements for
the minimum number of ten samples to conduct statistical analysis for all constituent except
radionuclides (Radium-226, Radium-228, Uranium-233, Uranium-234, Uranium-236, and
Uranium-238). The data set will be sufficient to conduct statistical analysis on all parameters
following the Q3 2017 sampling event.
BEDROCK
Five wells (BG-01, MW-10BR, MW-11BR, MW-13BR, and NM-14BR) are screened in the
bedrock flow zone. Data results for MW-11BR have been eliminated due to measured pH
values that exceed the range of acceptable values. The bedrock background groundwater
dataset satisfies the requirements for the minimum number of ten samples to conduct statistical
analysis.
ATTACHMENTS: Table 1: Updated Background Groundwater Analytical Results Through
March 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 \ Mayo - GW Tech Memo 05262017.docx