NC0005088_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.:
30012654
AARCAD IS Design &Consultancy
fornaturaland
built assets
Subject:
Summary of Geochemical Modeling Approach — Cliffside Steam Station
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 Rogers Energy
Complex - Cliffside Steam Station (Cliffside or site; Figure 1). SynTerra, in collaboration with others,
generated the geochemical model for Cliffside (SynTerra 2019a). The geochemical analysis for
Cliffside focused on the effects of COI related to the Active Ash Basin (AAB) and associated ash
storage area (ASA) and the Unit 5 inactive ash basin (U5 AB).
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 data2, 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) 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 are anticipated to be effective at minimizing groundwater flow through the ash basin and
maximizing the input of upgradient unaffected groundwater, reducing downgradient COI
concentrations.
2. The pH and redox potential (EH) are anticipated to remain stable in all simulations and 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 of COls. In particular, the oxidation of pyrite that can produce low pH
conditions has the potential to mobilize COls. Significant reduction in pH compared to current
conditions is an unlikely scenario due to kinetic limitations on pyrite oxidation.
4. Increased EH values that may be generated from oxygen infiltration during decanting or other
closure activities will not cause enhanced mobility of most Cols. The increased EH will make
ferrihydrite more stable, resulting in more HFO sorption sites. Notable exceptions to this conclusion
might be enhanced mobility of hexavalent chromium, or pentavalent arsenic if EH values are
sufficiently high to allow such species to persist, although hexavalent chromium was not identified as a
COI for management at the site.
COI Evaluation
At the Cliffside site, 11 COls exhibit mean concentrations greater than background threshold
values (BTVs), 02L standards, or interim allowable maximum concentrations (IMACs) with plume
characteristics at or beyond the ash basin geographic limitation: arsenic (As), boron (B), cobalt (Co),
iron (Fe), manganese (Mn), strontium (Sr), sulfate (SO4-2), total dissolved solids, thallium (TI), total
uranium, and vanadium (V), and lithium (Li) are also on the constituent list. Results from site -specific
partition coefficient (Kd) values evaluations for these COI are as follows:
Nonconservative, reactive COls: Kd values for As, Sr, V and other nonconservative, 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, Li and SOa 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 Co, Fe, and Mn, and TI 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. Analysis of Geochemical Phenomena Controlling Mobility of Ions from
the Coal Ash Basins at the Cliffside Steam Station. In Corrective Action Plan Update. Mooresboro, North
Carolina.
SynTerra. (2019b). CAP Update -Appendix G. Updated Groundwater Flow and Transport Modeling Report in
Corrective Action Plan Update. Mooresboro, 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, October). 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.
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NOTES:
1. ALL BOLINDARIES AREAPPROXA4ATE. as;, -
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2. 2016 USGS TOPOGRAPHfC MAP, CHESNEE R BOILIN}- SPRINGS
SOUTH QUADRANGLE, OBTAINED FROM THE USCS STORE AT
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SUMMARY OF
GEOCHEMICAL MODELING
APPROACH — CLIFFSIDE SITE MAP
STEAM STATION
ARCADI„�4 FIGURE
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