NC0003425_Geochem Memo and Summary_20200320

TECHNICAL MEMO
To:
Scott Davies, PG, Duke Energy
526 South Church Street
Charlotte, North Carolina 28202
From:
Julie K Sueker, PhD, PH, PE (CO)
Margy Gentile, PhD, PE (CA)
Date:
March 20, 2020
Arcadis Project No.:
30012663
AARCADIS Oesign&Consultancy
frnaturaland
built oassets
Subject:
Summary of Geochemical Modeling Approach — Roxboro Steam Electric Plant
Arcadis U.S., Inc.
11400 Parkside Drive
Suite 410
Knoxville
Tennessee 37934
Tel [phone]
Fax [phone]
Duke Energy was required to model potential geochemical effects related to ash basin decanting and ash
basin closure on the transport of constituents of interest (COls) in groundwater at the Roxboro Steam
Electric Plant (Roxboro or site; Figure 1). SynTerra, in collaboration with others, generated the
geochemical model for Roxboro. For Roxboro, geochemical modeling evaluated potential effects of both
the East Ash Basin (EAB) and the West Ash Basin (WAB).
The objectives of the modeling' were to demonstrate an understanding of COI geochemical behavior,
describe source terms in the model, to simulate downgradient concentrations of COI at various stages of
closure, and to provide a basis for translating between detailed geochemical modeling and the sitewide
flow and transport model. Site -specific data incorporated into the modeling included COI concentrations
and trends in ash pore water and groundwater, solid phase mineralogy for estimates of sorption and ion
exchange sites, COI leaching behavior, and hydrogeologic information. Modeling analysis included
overviews of groundwater data, geochemical evaluations of ash leaching data 2, batch PHREEQC3 models
and sorption coefficient derivations, and PHREEQC 1-D advection models.
KEY FINDINGS
The key findings of the geochemical modeling effort associated with the selected closure scenario
(closure -by -excavation for the WAB and a portion of the EAB) are listed below:
The framework was developed through collaboration with NCDEQ, William Deutsch (external reviewer for NCDEQ), and the flow
and transport (F&T) modeling team (CAP Update -Appendix G, Synterra 2019b) over many meetings, presentations, and
conference calls (Duke 2017a, Duke 2017b).
2 Via USEPA Method 3052 (1996) and USEPA LEAF Method 1313 (2012a) and 1316 (2012b).
3 PHREEQC- original acronym pH-REdox-EQuilibrium written in C programming language.
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MEMO
1. Closure activities are anticipated to minimize groundwater flow through the ash basin and maximize
the input of upgradient unaffected groundwater, resulting in decreased downgradient COI
concentrations
2. The pH and redox potential (EH) remain stable to maintain sorption as a dominant attenuation
mechanism for most nonconservative COls.
3. Closure activities that generate extreme pH values (generally less than 4 and greater than 10) may
cause increased mobility locally of certain COls. However, major changes to pH are not predicted
under the closure scenario at the Roxboro site.
4. Increased EH values that may be generated from oxygen infiltration during decanting or other closure
activities, such as closure -by -excavation, will not cause enhanced mobility of most COls. An increase
in EH would make ferrihydrite more stable, resulting in more sorption sites. Potential exceptions might
be enhanced mobility of hexavalent chromium or pentavalent arsenic if EH conditions allow such
species to persist, although these species were not identified as COls for the site.
COI Evaluation
At the Roxboro site, five COls exhibit mean concentrations greater than background threshold
values (BTVs), 02L standards, or interim allowable maximum concentrations (IMACs) with plume
characteristics downgradient of the EAB or WAB at or beyond the ash basin geographic limitation:
boron (B), selenium (Se), strontium (Sr), sulfate (SO4 2), and total dissolved solids. Results from
site -specific partition coefficient (Kd) values evaluations are as follows:
• Nonconservative, reactive COls: Kd values for reactive COI remained high in most cases and are
unlikely to be affected geochemically by remedial actions based on Kd evaluation (values remained
high for tested scenarios in most cases).
• Conservative, nonreactive COI: Kd values for B and SO4 2 were low (less than 1 liter per kilogram)
for all modeled scenarios and will not change significantly due to changes related to closure.
• Variably reactive COls: Kd values for Se and Sr were greatly variable in relation to geochemical
changes and dependent on the pH and EH.
References
Duke Energy. 2017a. DWR-ARO Meeting to Discuss Asheville Models. Asheville, North Carolina: NCDEQ.
August 29.
Duke Energy. 2017b. NCDEQ Meeting to Review Cliffside Models and CSAs. Asheville, North Carolina:
NCDEQ. October 11.
SynTerra. 2019a. CAP Update- Appendix H. Geochemical Model Report. In Corrective Action Plan Update.
Semora, North Carolina.
SynTerra. 2019b. CAP Update- Appendix G. In Corrective Action Plan Update. Semora, North Carolina.
USEPA. 1996. Method 3052: Microwave assisted acid digestion of siliceous and organically based matrices -
Revision 0. SW-846. USEPA. December.
USEPA. 2012a. Method 1313 - Liquid -solid partitioning as a function of extract pH using parallel batch
extraction procedure. Test methods for evaluating solid waste: Physical/chemical methods. SW-846, 3rd.
USEPA. October.
USEPA. 2012b. Method 1316 - Liquid -solid partitioning as a function of liquid -to -solid ratio in solid materials
using a parallel batch procedure. Test methods for evaluating solid waste: Physical/chemical methods.
SW-846, 3rd. USEPA. October.
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NOTES
1_ ALL BOUNDARIES ARE APPROXIMATE_
2. DUKE ENERGY PROPERTY LINES ARE REPRESENTED BASED ON DUKE
ENERGY'S INTERPRETATION OF HISTORICAL DOCUMENTED PROPERTY
BOIINDARIESANO CURRENT PERSON COUNTYGIS.�'
3. 2016 (1SGS TOPOGRAPHIC MAP, OLIVE HILL QUADRANGLE, OBTAINED l } ff C'-
FROM THE 115G5 STORE AT �`--���•��`
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SUMMARY OF FIGURE
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APPROACH - ROXBORO built assets
STEAM ELECTRIC PLANT