NC0004979_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.:
30012651
AARCAD IS Design &Consultancy
fornaturaland
built assets
Subject:
Summary of Geochemical Modeling Approach — Allen 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 Allen Steam Station
(Allen or site; Figure 1). SynTerra, in collaboration with others, generated the geochemical model for Allen.
For Allen, two general areas were considered as part of the geochemical evaluation.
1. The low pH and Coal Pile area
2. The coal ash basins (Retired Ash Basin & Active Ash Basin)
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 be effective at reducing the downgradient concentrations of COls
due to washout (dilution), as groundwater flow through the ash basin is minimized and the input of
upgradient unaffected groundwater is maximized.
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).
s 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 not predicted to
occur beyond the low pH area west of the Coal Pile.
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 Allen site, seven Cols exhibit mean concentrations greater than background threshold values (BTVs),
02L standards, or interim allowable maximum concentrations (IMACs) with plume characteristics downgradient
of the ash basins and/or coal pile area at or beyond the ash basin geographic limitation: boron (B), iron (Fe),
manganese (Mn), sulfate (SO4 2), total dissolved solids, cobalt (Co), and strontium (Sr). Results from
site -specific partition coefficient (Kd) values evaluations are as follows:
Nonconservative, reactive COls: Kd values for Sr 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, 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, 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. Corrective Action Plan Update- Allen Steam Station. Belmont, North Carolina.
SynTerra. 2019b. CAP Update -Appendix G. In Corrective Action Plan Update. Belmont, 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. October.
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GRAPHIC SCALE
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1
ACTIVEA(SH BASW
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ACTIVE A" BASIN
WASTE BOUNDARYz
jW-Sf`. e v
ASH BASIN
GEOGRAPHIC
LIMITATION
NOTE:
WATER FEATURES DEPICTED WITHIN WASTE BOUNDAR&ES
OF THE ASH BASINS ON THE 2016 USGS TOPOGRAPHIC MAP
DO NOT REPRESENT CURRENT CONDITIONS. THE CONDITIONS J
DEPICTED ARE SIMILAR TO THOSE SHOWN ON THE 1968 AND
1973 USGS TOPOGRAPHIC MAPS OF THE AREA [(1968 WEST
CHARLOTTE (1:240CC1)AKO 1973 BELMONT (1:24000)]. 1
SOURCE:
2016 USGS TOPOGRAPHIC MAP, BELMONT & CHARLOTTE VVIEST
QUADRANGLE, OBTAINED FROM THE USGS STORE AT
https:llstorc.usgs.gov/rtvip-Iocatcc. j'' I
SUMMARY OF
GEOCHEMICAL MODELING
APPROACH -ALLEN SITE MAP
STEAM STATION
•
NPDES OIUTFALL 104
NPDES OUTFALL 103
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CAARCADIS FIGURE
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