NC0038377_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.:
30012660
AARCAD IS Design &Consultancy
fornaturaland
built assets
Subject:
Summary of Geochemical Modeling Approach — Mayo 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 Mayo Steam Electric
Plant (Mayo or site; Figure 1). SynTerra, in collaboration with others, generated the geochemical model for
Mayo (SynTerra 2019a).
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:
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.
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
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 unexpected for Mayo.
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.
5. The geochemical evaluation and modeling informed the corrective action designs for the two areas
identified above. For the low pH and Coal Pile area, removal of low pH source material within, and low pH
soil below the Retired Ash Basin were added in addition to groundwater extraction and clean water
infiltration based on the evaluation.
COI Evaluation
At the Mayo site, five COls exhibit mean concentrations greater than background threshold values (BTVs),
02L standards, or interim allowable maximum concentrations (IMACs) at isolated wells downgradient of the
ash basin near the ash basin geographic limitation: boron (B), manganese (Mn), molybdenum (Mo),
strontium (Sr), and total dissolved solids. However, only B exhibits a discernable plume and was retained
for evaluation in the Corrective Action Plan (CAP). Results from site -specific partition coefficient (Kd) values
evaluations are as follows:
• Nonconservative, reactive COls: Kd values for 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 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 Mn, Mo, and Sr were greatly variable in relation to geochemical
changes and dependent on the pH and EH.
Given the amount of downgradient area available for attenuation of the variable and nonreactive COls at Mayo,
attenuation through sorption should be considered a primary means of controlling the extent of COI migration.
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. Roxboro,
North Carolina.
SynTerra. 2019b. CAP Update- Appendix G. In Corrective Action Plan Update. Roxboro, 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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