Background Stats TM Response 4-28-17

Water Resources
Environmental Quality
April 28, 2017
Paul Draovitch
Senior Vice President
Environmental, Health & Safety
Duke Energy Energy
526 South Church Street
Mail Code EC3XP
Charlotte, North Carolina 28202
ROY COOPER
Governor
MICHAEL S. REGAN
Secretary
S. JAY ZIMMERMAN
Director
Subject: Request for Additional Information regarding Statistical Methods for Developing
Reference Background Concentrations for Groundwater and Soil at Coal Ash
Facilities (HDR Engineering, Inc. and Synterra Corporation, January 2017)
Dear Mr. Draovitch:
The North Carolina Department of Environmental Quality (DEQ) received the technical
memorandum (TM) titled Statistical Methods for Developing Reference Background
Concentrations for Groundwater and Soil at Coal Ash Facilities (HDR Engineering, Inc. and
Synterra Corporation, January 2017) on January 20, 2017. DEQ grants conditional approval of
the document; however, outstanding technical issues must be resolved to proceed with the site
assessments and corrective action. To address outstanding issues, Duke Energy shall provide the
following information in the form of dataset transmittals and a revised TM.
Datasets
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 (see comment below), (d) outlier designation, (e) non -detect values that are
above 2L/IMAC, or (f) other reasons.
Provide up-to-date digital spreadsheets of raw background soil data for each facility by
May 26, 2017. Any soil data collected since submittal of the Comprehensive Site
Assessments should be included in the table and existing soil data should be reviewed to
identify any quality control issues (i.e., sampling intervals, corresponding boring sample
ID, etc.) along with identification of outliers and revised accordingly.
Once DEQ reviews and approves the raw background datasets, background groundwater and soil
determinations should be completed and provided within 30 days.
State of North Carolina I Environmental Quality I Division of Water Resources
Water Quality Regional Operations Section
1636 Mail Service Center I Raleigh, North Carolina 27699-1636
919-707-9129

Revised Draft of Statistical Methods for Developing Reference Background Concentrations
for Groundwater and Soil at Coal Ash Facilities (HDR, January 2017)
Groundwater Background Comments
Introduction. Page 1. The memorandum states that "For the purpose of establishing
background threshold values at this time, the value which represents the upper threshold
value from the upper tail of the data distribution for a given constituent will be
considered the value representative of a naturally occurring concentration, or the
proposed provisional background threshold valiue." The words "the upper tail of
should be removed. As stated in EPA/600/R-07/041, ProUCL Technical Guide, 2013,
page 17, "The objective is to compute background statistics based upon the majority of
the dataset representing the main dominant background population, and not to
accommodate a few low probability high outliers (e.g. coming from extreme tails of the
data distribution) that may also be present in the background dataset." The memorandum
should reference the ProUCL page 17 language and be updated accordingly.
• Part II, Page 7. Autocorrelation. Duke Energy states that autocorrelation will be
evaluated using a "sample autocorrelation function". The Division position is to use a
minimum 60 -day interval between sample events. Duke may, at any time, submit for
consideration evidence that data from additional sample events spaced closer together in
time are not autocorrelated, but for purposes of the raw background dataset and
background determinations required at this time, the minimum 60 -day interval should be
used. The memorandum should be updated accordingly.
Part IV, Page 14. Step 3. The memorandum states that "If data are normally and
lognormally distributed, but not gamma distributed, preference should be given to upper
tolerance levels (UTLs) produced assuming data are lognormally distributed provided
the logged data have a standard deviation less than or equal to one". The Division does
not concur. Assuming data follow the corresponding UTL distribution type, the
following UTL computation preference should be used: normal UTL, then gamma UTL,
then lognormal UTL, then nonparametric UTL. However, as stated in ProUCL, because
lognormal distributions can result in unrealistically high UTL computations, lognormal
UTL should be avoided when skewness is high (e.g. standard deviation of logged data is
> 1 or 1.5) and sample size is small (e.g., < 20 — 30). In this case, nonparametric UTL
should be preferred over lognormal UTL. The memorandum should be updated
accordingly.
• Part IV, Page 14. Step 5. The memorandum states that "A minimum of eight valid
background groundwater samples should be obtained prior to producing background
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threshold values (BTVs) for each constituent in each flow layer". Eight samples should
be changed to ten samples. The memorandum also states that "In addition, a minimum of
eight additional samples should be obtained prior to evaluating if new background data
should be combined with previous data to produce revised BTVs. " In practice, time
frames needed to collect ten additional background samples may not be available given
CAMA and other deadlines. In these cases, DEQ will determine what data are
appropriate for inclusion in a comprehensive background dataset based on all relevant
considerations. The memorandum should be updated accordingly.
Part IV, Page 14. Step 5. The memorandum states that "If it is deemed necessary to
produce BTVs prior to obtaining eight valid samples, NCDEQ will be consulted and the
maximum observation may need to be used as a BTV". BTVs are expected to be
produced for all constituents and flow systems whether ten valid samples are available.
If less than ten valid samples are available for this determination, no formal UTL
statistics should be run and the provisional background threshold value (PBTV) for a
constituent and flow system should be computed to be either: a) the highest value, or b) if
the highest value is above an order of magnitude greater than the geometric mean of all
values, then the highest value should be considered an outlier and removed from further
use and the PBTV is computed to be the 2nd highest value. The memorandum should be
updated accordingly.
Comments on Duke Energy's January 20, 20/17 "Responses to DEQ's November 22, 2016
informal comments"
Duke Energy Response #3. Duke Energy's response does not address the exclusion of
autocorrelated samples that have been collected too close in time. DEQ's protocol
included a criterion that samples not be collected too close in time and selected 60 days
as a reasonable frequency at which autocorrelation would not be expected. Duke Energy
should acknowledge and address this issue.
Duke Energy Response #11. The summary spreadsheets to which Duke Energy refers
do not, in many cases, include an indication of outliers or autocorrelated data. These
values should be clearly indicated within the summary spreadsheets.
• Non -detect values above the 2L/IMAC standards should be deleted from use in the raw
background dataset.
Soil Background Comments
Part I, Page 4. The memorandum states that "Duke Energy and NCDEQ have agreed
that soil samples can be pooled from multiple depth intervals. A statistical evaluation of
the soil data sets will be performed to confirm the approach is appropriate". The
memorandum references USEPA's Guidance for Comparing Background and Chemical
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Concentrations in Soil far CERCLA Sites (USEPA 2002) but does not specify the
statistical evaluations that are proposed to evaluate the applicability of pooling soil data.
The memorandum should be updated with methodology to:
confine that pooling multiple depth intervals at a site is appropriate.
confirm that pooling soil data from different geologic formations at a site is
appropriate.
statistically evaluate soil results that are below a detection limit when that detection
limit is above the protection of groundwater threshold.
If you have any questions regarding any information provided, please feel free to contact Ted
Campbell (Asheville Regional Office) at 828-296-4683 or Steve Lanter (Central Office) at 919-
807-6444.
Sincerely,
S. Jai erman, P.G., Director
Division of Water Resources
cc: WQROS Regional Office Supervisors
WQROS Central File Copy
HDR (Atte: Mark Filardi) 440 South Church Street, Suite 1000, Charlotte, NC 28202
SynTerra (Attn: Kathy Webb) 148 River Street, Suite 220, Greenville, SC 29601
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