NC0038377_3a-NCDENR Mayo Work Plan Conditional Approval letter_20150902

AU'A
NCDENR
North Carolina Department of Environment and Natural Resources
Pat McCrory
Governor
March 6, 2015
Mr. Harry Sideris
Senior Vice -President
Environment, Health, and Safety
Duke Energy
526 South Church Street
Mail Code EC3XP
Charlotte, NC 28202
Re: Mayo Stearn Electric Plant
NPDES Permit No. NCO038377 — Person County, North Carolina
Conditional Approval of Revised Groundwater Assessment Work Plan
Dear Mr. Sideris:
Donald R. van der Vaart
Secretary
On December 31, 2014, the Division of Water Resources (Division) received the revised
Groundwater Assessment Plan (GAP) for the above listed facility. The revised GAP was submitted
in response to the DWR's Review of Groundwater Assessment Work Plan letter dated November 4,
2014. A review of the plan has been completed and several deficiencies or items requiring
clarification were noted. Therefore, in order to keep the site assessment activities on a timely
schedule, the Division has approved the revised GAP under the condition that the following deficient
items are addressed in the Groundwater Assessment Report:
• Comment Section 5.3 Hydrogeologic Site Characteristics:
Information needed to develop the initial site conceptual site model was available in the
revised GAP, but for the most part data were not provided in a clear, cohesive manner to
illustrate where data gaps may exist. Duke Energy should incorporate all existing data at the
site and be prepared to collect additional data if the Division determines that additional data
gaps exist. Continued site conceptual model development should follow guidelines similar to
those presented in the American Standards Testing Measures E1689 - 95(2014) Standard
Guide for Developing Conceptual Site Models for Contaminated Sites to direct data
collection, data interpretation, and model development efforts.
The operations and processes related to building materials manufacturing at the nearby
Louisiana Pacific Corporation facility located upgradient of the Mayo Steam Electric Plant
should be considered in conceptual model development as well as groundwater flow and
transport models. Possible groundwater monitoring and water supply well production at this
facility should be documented and incorporated into the site assessment.
1636 Mail Service Center, Raleigh, North Carolina 27699-1636
Phone: [phone] Internet: hllp:llwww ncwater org
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Mayo Steam Plant
March 6, 2015
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Comment 6.1 Compliance Monitoring Well Groundwater Analytical Results and 6.2
Preliminary Statistical Evaluation of Results
Well BG -1 and BG -2 may not be suitable locations for background wells because of their
proximity to plant activities, US Highway 501, former Highway 501, the railroad, and
possible influence from pumping from former production wells DEP 2 and 3. The detections
of constituents of potential concern in these wells, in particular well BG -1, may be related to
anthropogenic impacts rather than naturally occurring conditions. Proposed monitoring well
cluster MW-13S/BR and possibly monitoring well cluster MW-12S/BR should provide
suitable background monitoring locations; however, it should be noted at least 8 rounds of
data are needed to conduct a valid statistical evaluation of background groundwater
conditions.
8 Comment 7.1.1.1 Borings Within the Ash Basin
The Division recommends that wells be installed in the partially weathered rock, or transition
zone, and bedrock at the 2 locations within the ash basin currently planned to characterize
bedrock if feasible and warranted by site conditions. The Division recommends that wells
installed through the ash basins into native materials are doubled cased to protect well
integrity.
• Comment 7.1.1.2 Borings Outside Ash Basin
The Division recommends installing a soil boring to characterize native materials in the
topographic low below the ash basin dam.
• Comment 7.1.2.3 Proposed Monitoring Wells Downgradient or Sidegradient of the Ash
Basin
Figure 5 of the report depicts MW -7S and MW-7BR located inside the compliance boundary.
The well locations should be relocated to an area at the compliance boundary or outside the
compliance boundary but in the same general location proposed in the work plan.
Comment Section 7.1.4 Hydrogeologic Evaluation Testing
Since no hydraulic conductivity data is available for Mayo Plant, hydraulic tests in a subset
of existing wells is recommended. In addition, vertical hydraulic conductivities in the ash and
saprolite will be determined in accordance with ASTM Standard D-5084 from samples
collected in Shelby tubes for geotechnical analysis. The borings where these samples are
collected in Shelby tubes will be based on site-specific conditions encountered during the site
assessment. These test results will provide a more robust dataset to characterize site
conditions and refine the flow and transport models.
• Comment Section 7.2 Ash Pore Water and Groundwater Sampling and Analysis:
Direction provided in the EPA Region 1 Low Stress Purging and Sampling Procedure for the
Collection of Groundwater Samples from Monitoring Wells (2010) should be followed
strictly and any deviations from the procedure must be approved by the Division and
documented accordingly. For example, samples should not be collected until pH is stabilized
within f 0.1 for three consecutive readings rather than f 0.2 written in the GAP.
Temperature and specific conductivity readings should stabilize within 3% for three

Mayo Steam Plant
March 6, 2015
Page 3 of 3
consecutive readings before samples are collected instead of 10% noted in the GAP. Also
note that if the pumping rate is so low that the flow-through-cell/chamber volume cannot be
replaced in a 5 minute interval, the time between measurements should be increased
accordingly.
Speciation of inorganics from groundwater samples should be focused on wells strategically
located along flowpath transects Collection of data along multiple flowpaths will refine the
assessment of water geochemistry and development of flow and transport models.
The Division recommends analyzing samples for radionuclides from wells CW -2 and CW -
2D instead of MW -3. Samples from these two locations will provide data from both the
transition zone and bedrock downgradient of the facility along a flowpath transect.
• Comment 7.9.2 Development of Kd Terms
It is expected that additional solid phase samples will be collected and analyzed for Kd
determination as well as physical properties at strategic locations along flowpath
transects. These data will refine the assessment of water geochemistry and development
of flow and transport models.
In addition, technical direction that will serve as the basis of expectations for completion of the site
assessment is provided at Attachment 1. Failure to address the deficient items stated above will
result in Duke Energy not being in compliance with the stated statutes. Per G.S. 130A -309,209(a)
(3) and (4), you must begin implementation of the revised GAP on March 16, 2015 and the
Groundwater Assessment Report is due on September 2, 2015. It is our understanding that Duke
Energy may have to obtain additional permits to facilitate installation of certain monitoring wells. In
the event permits are needed for this purpose, Duke Energy should take all steps necessary consistent
with the law to avoid delaying completion of the assessment report.
If you have any questions, please contact Eric Rice at [phone].
Sincerely,
S. a erman, P. G.,
Director Division of Water Resources
cc: WQROS — RRO
WQROS Central Files
DENR Secretary - Don van der Vaart
Synterra (Attn: Kathryn Webb) — 148 River Street, Suite 220, Greenville, SC 29601

Attachment 1
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Duke Energy GAP Review Issues
The items identified in this Groundwater Assessment Plan (GAP) review summary are provided for
general discussion for the various parties to agree upon technical direction and content in the revised
GAPs, comprehensive site assessments (CSAs), and corrective action plans (CAPS).
Groundwater Monitoring
1. A schedule for continued groundwater monitoring is mandated by the Coal Ash Management
Act 2014. An interim plan should include.at least two rounds of groundwater samples collected
and analyzed in 2015. The analytical results of the first round of data collected in 2015 would be
included in the CSA report, while the results of the second round would be submitted as a CSA
addendum. After CSA data can be evaluated, a plan for continued groundwater monitoring can
be developed for implementation in 2016.
2. Sites impacted by inorganics are typically managed using a tiered site analysis which includes
four elements as referenced in EPA/600/R-07/139:
• Demonstration of active contaminant removal from groundwater & dissolved plume
stability;
• Determination of the mechanism and rate of attenuation;
• Determination of the long-term capacity for attenuation and stability of immobilized
contaminants, before, during, and after any proposed remedial activities; and
• Design of performance monitoring program, including defining triggers for assessing
the remedial action strategy failure, and establishing a contingency plan.
This reference and the framework described above should be used as applicable to meet
the corrective action requirements found in 15A NCAC 02L .0106.
3. Because of uncertainty concerning the site's ability to attenuate contaminants over the long
term given potentially changing geochemical conditions, there is a need to address the elements
of the tiered site analysis described above and collect appropriate samples as part of the CSA,
CAP development, and continued groundwater monitoring.
4. The Division of Water Resources (Division) Director is responsible for establishing background
levels for COPCs in groundwater. This determination is based on information and data provided
by the responsible party and may include formal statistical testing using background wells with
at least four rounds of data. Wells identified as "background" are subject to periodic review
based on a refined understanding of site chemistry and hydrogeologic conditions. In general,
each facility must have a background well or wells screened or open to each of the dominant
flow systems that occur at the site and are associated with groundwater contamination. Any
questions concerning adequacy of background monitoring locations or conditions at the
facilities should be directed to the Regional offices.

Attachment 1
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5. Delineation of the groundwater contaminant plume associated with coal combustion residuals is
a requirement of the investigation and if off-site monitoring wells are ultimately required to
perform this task, then it is expected that these activities will be completed as part of the
groundwater assessment activities and included in the final report. Documentation of the effort
to gain off-site access, or right of way permits, will be provided if off-site access is denied or
alternate means of assessing the area were not available within the allocated timeframe (such
as within right-of-ways).
Site Assessment
Data Requirements and Sampling Strategy
1. Robust data collection is warranted to support timely completion of site assessments and
subsequent corrective action plans because of the impending deadlines for completion of CSAs
and CAPS, scale and geologic complexity of the sites, the challenges of modeling heterogeneous
systems, and site proximity to potential human and sensitive ecosystem receptors.
2. Robust data collection will be focused along strategically positioned flowpath transect(s) -from
ash pond source to potential receptor—as an efficient approach for model development
(analytical, geochemical, groundwater flow, and transport) in support of risk assessment and
CAP development. Data collected to support evaluation of site conditions along the flowpath
transects should be located along or defensibly proximate to the modeled transects.
3. The dataset developed along proposed flowpath transects will include any information needed
to determine constituent concentrations, conduct Kd tests, and perform batch geochemical
modeling in multiple flow horizons as appropriate. This data will include a) solid phase sample
collection for Kd measurement and batch geochemical modeling, inorganic analysis and
speciation, and other parameters identified in General Comment #4 of the November 4, 2014
GAP comments issued by DWR, b) solution phase sample collection for total and dissolved
inorganic analysis of total concentrations, small pore filtration for dissolved samples, etc., and c)
slug, constant/falling head, and packer testing. The solid phase sample mineralogy, total
concentration results, re-dox measurements, and other geochemical parameters will be used as
input for equilibrium speciation calculations of redox sensitive constituents calculated by
PHREEQC or similar program (EPA/540/5-92/018). This geochemical modeling will be performed
to identify potential mineral phases, estimated species speciation and concentrations, and will
be performed varying key solubility controlling parameters to predict mineral phases,
speciation, and concentrations under varying conditions. Solid samples for Kd tests collected
from along from proposed flowpath transects will be handled and preserved in order to
eliminate exposure to ambient air in the field. Kd samples should be collected in plastic bags
and sealed with a conventional vacuum plastic bag sealer. The samples will be then placed on
ice in a cooler for transport and kept out of direct sunlight. Once received by the analytical
laboratory, the Oxidation -Reduction Potential (ORP) of the sample will be measured using an
ORP probe and meter in accordance with ASTM method G200-19 (Reapproved 2014). Based on
this ORP measurement, either normal or "glove -box" processing of a sample will be applied

Attachment 1
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(EPA/600/R-06/112). An additional sample will be retained, pending confirmation of subsequent
ORP and DO testing. ORP and dissolved oxygen will be measured in the groundwater monitoring
wells subsequently installed at these sample locations. In the event that the groundwater field -
measured ORP and DO reveal reducing conditions, the additionally -retained sample will be
subject to glove box processing for the Kd analyses. Refer to EPA/600/R-07/139 Section III for
the data collection and characterization needed to support the four -tiered analysis discussed
above.
4. Speciations for groundwater and surface water samples should include Fe, Mn, and any COPCs
whose speciation state may affect toxicity or mobility (e.g. As, Cr, Se, or others if applicable).
This speciation will apply for groundwater samples collected at wells located along proposed
flowpath transects and in wells where these constituents exceed 2L groundwater standards as
well as for surface water samples collected within ash impoundments.
5. Solid phase samples shall be analyzed for: minerals present, chemical composition of oxides,
hydrous Fe, Mn, and AL oxides content; moisture content; particle size analysis; plasticity;
specific gravity; porosity; permeability, or other physical properties or analyses needed to
provide input to a chosen model. These analyses for physical properties will be conducted at up
to 15 locations along proposed flowpath transects where Kd samples are collected. Solid phase
samples at up to 15 additional locations will be collected and analyzed for hydrous ferric oxide
(HFO) content. At these additional locations where HFO content is analyzed, analyses for
physical properties will not be performed. Solid phase samples will be analyzed for total organic
content from the same locations where samples are collected for Kd determination. Solid phase
samples will be analyzed for total organic carbonate content from the same locations where
samples are collected for Kd determination only at facilities located in the coastal plain.
6. In addition to conducting the SPLP leachable inorganic compounds analysis for selected ash
samples to evaluate the potential for leaching of constituents to groundwater, the leachable
analysis should also be conducted for some soil samples from locations beneath the ash ponds,
within the plume, and outside the plume to evaluate potential contributions from native soils.
7. In addition to collecting solid phase samples onsite for Kd procedures, soil samples should be
also collected from unaffected soils within groundwater flow pathways to evaluate Kd(s) or
hydrous ferrous oxide.
8. Rock samples for laboratory analyses should be collected as commented in General Comment 4
of the November 4, 2014 GAP comments issued by DWR. This GAP review comment indicated
that the sample(s) collected from bedrock well soil and rock cores shall be analyzed, at a
minimum, for the following: type of material, formation from which it came, minerals present,
chemical composition as oxides, hydrous Fe, Mn, and Al oxides content, surface area, moisture
content, etc.; however, these analyses were not mentioned in the GAP. The Division reserves
the right to request analysis for organic carbon content, organic carbonate content, and ion
exchange capacity if needed to complete the site assessment process.
9. The coal ash and soil analyte lists should match the groundwater analyte lists.
10. Total uranium analysis should be analyzed where total radium is analyzed for groundwater.

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11. If analytical results from a seep sample exceed 2L standards, then the area in the vicinity of the
sample location should be investigated for groundwater contamination. If analytical results
from a surface water sample exceed 2B standards, then the area in the vicinity of the sample
location should be investigated for groundwater contamination.
12. Surface water/seep samples should be collected during baseflow conditions and that the
groundwater monitoring (water levels and sampling) should occur at about the same time.
13. Measurement of streamflow in selected perennial streams is expected as needed in support of
simulation/calibration of flow and transport models; major rivers that serve as groundwater
divides are not included in this expectation.
Conceptual Model Elements
1. In the CSA report, data gaps remaining should be specifically identified and summarized.
2. Site heterogeneities should be identified and described with respect to; a) their nature, b)
their scale and density, c) the extent to which the data collection successfully characterizes
them, d) how the modeling accounts for them, e) and how they affect modeling uncertainty.
3. The impact of data gaps and site heterogeneities should be described in relation to the
elements developed in the Site Hydrogeologic Conceptual Model and Fate and Transport
Model subsections.
4. For sites in the Piedmont or Mountains, the CSA Report should include a subsection within the
Site Geology and Hydrogeology Section titled `Structural Geology'. This section should
describe: a) foliations, b) shear zones, c) fracture trace analysis, and d) other structural
components anticipated to be relevant to flow and contaminant transport at the site.
5. Duke Energy will include a poster -sized sheet(s) (ANSI E) combining tabulated analytical
assessment results (groundwater, surface water, and leachate samples); multiple sheets may
be needed to present the data. This should be provided in addition to the individual analytical
results tables that will be prepared for the CSA reports. Any questions concerning format or
content of the analytical result summaries should be directed to the Regional offices.
Geochemical Modeling
1. The Division agrees that a geochemical model "coupled" to a 3-D fate and transport model is
inappropriate given the size and complexity of the sites and the extremely large amount of data
required to calibrate such a model. Rather, a "batch" geochemical model approach should be
sufficient for successfully completing the site assessment and/or corrective action plan.
2. Samples collected for "batch" geochemical analysis should be focused along or defensibly
proximate to flowpath transects.
3. To support successful batch geochemical modeling, dissolved groundwater samples collected
along a contaminant flowpath transect should be obtained using a 0.1 um filter. This will help
ensure a true dissolved phase sample. Note that the dissolved samples are for assessment
purposes only and may not be used for purposes of compliance monitoring. If there is
uncertainty about which areas/wells will be used in the batch geochemical modeling, the initial
round of assessment sampling at the facility can utilize the 0.45 um filter until the contaminant

Attachment 1
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flow path transects are selected, Once determined, Duke Energy can go back and re -sample the
wells needed for geochemical modeling using the 0.1 um filter. It is recognized that the use of a
0.1 um filter will be difficult for wells with elevated turbidity; in this case, it is recommended
that Duke Energy use two filters in series (the water initially passes through a 0.45 um filter to
remove larger particles prior to passing through the 0,1 um filter). Information for a disposable
0.1um field filter designed specifically for sampling groundwater for metal analysis is provided at
the following link: http_://www.vosstech.com/index.php/products/filters. If field comparisons of
0.1 versus 0.45 micron filtration at several transect wells at a given site show no significant
differences between the two methods, then 0.45 micron filters may be used for evaluating the
dissolved phase concentrations at that site.
4. In support of the objectives of General Comment #2 of the November 4, 2014 GAP comments
issued by DWR, Duke Energy should add a column titled 'relative redox' to the analytical results
tables to record the geochemical conditions for that location/sample date. The redox
determination should be based on observed DO, ORP, and any other relevant measures and
presented for historic and new samples (wells, ash pore water, surface waters, etc.). Relative
redox designations may include "iron reducing", "sulfate reducing", mildly oxidizing, moderately
oxidizing, etc. and should be footnoted with a statement about the degree of confidence in the
designation based on amount and quality of available data.
5. Duke Energy shall also evaluate: a) spatial geochemical trends across the facility and along
selected flow paths, b) temporal geochemical trends where observable (such as for compliance
boundary wells), along with the likely reason for the change (e.g. increase in seasonal recharge,
pond de -watering and subsequent reversal of groundwater flow direction, inundation of well
from river at flood stage, etc.) in support of the CAP. This evaluation step will require a
comparison of geochemical conditions over time with rainfall data, notable ash capping,
dewatering, disposal/removal, or other plant operations, etc. The quality of existing
geochemical data will be evaluated using field notes, calibration records, and consistency in
redox measurements (e.g. eH vs. raw ORP),
Groundwater Models
1. The technical direction for developing the fate and transport modeling will follow guidelines
found in Groundwater Modeling Policy, NCDENR DWO, May 31, 2007, and discussions
conducted between Duke Energy and their consultants with the Division. Ultimate direction for
completion of fate and transport models will be provided by the Division.
2. The CAP Report should include a subsection within Groundwater Modeling Results titled 'Site
Conceptual Model' that succinctly summarizes, for purposes of model construction, the
understanding of the physical and chemical setting of the site and shall include, at a
minimum: a) the site setting (hydrogeology, dominant flow zones, heterogeneities, areas of
pronounced vertical head gradients, areas of recharge and discharge, spatial distribution of
geochemical conditions across the site, and other factors as appropriate), b) source areas and
estimated mass loading history, c) receptors, d) chemical behavior of COPCs, and f) likely

Attachment 1
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retention mechanisms for COPCs and how the mechanisms are expected to respond to changes
in geochemical conditions.
3. Modeling will be included in the Corrective Action Plan (CAP). The four -tiered analysis
previously referenced and appropriate modeling should be conducted, and the mass flux
calculations described in the EPA/600/R-07/139 should be performed.
4. The CAP Report shall provide separate subsections for reporting groundwater flow models and
fate and transport models.
5. The CAP Report should include subsections within Groundwater Modeling Results titled
'Groundwater Model Development' that describes, for each chosen model: a) purpose of model,
built-in assumptions, model extent, grid, layers, boundary conditions, initial conditions, and
others as listed in Division guidance. Include in this section a discussion of heterogeneities and
how the model(s) account for this (e.g. dual porosity modeling, equivalent porous media
approach, etc.). Separate subsections sliould be developed for the groundwater flow model,
fate and transport model, and batch geochemical models, respectively.
6. CAP Reports should include a subsection within Groundwater Modeling Results titled
'Groundwater Model Calibration' that describes, for each model used, the process used to
calibrate the model, the zones of input and calibration variables (for example, hydraulic
conductivities) that were used, the actual (measured) versus modeled results for all key
variables, and others. Separate subsections should be developed for the groundwater flow
model, fate and transport model, and batch geochemical model(s), respectively.
7. CAP Reports should include a subsection within Groundwater Modeling Results titled
'Groundwater Model Sensitivity Analysis' that describes, for each model used, the process used
to evaluate model uncertainty, variable ranges tested, and the key sensitivities. Separate
subsections should be developed for the groundwaterflow model, fate and transport model,
and batch geochemical model(s), respectively.
Development of Kd Terms
1. Kd testing and modeling in support of CAP development should include all COPCs found above
the NCAC 15A 02L .0106(g) standards in ash leachate, ash pore water, or compliance boundary
well groundwater samples.
2. The selected Kd used in transport modeling often will profoundly affect the results. Duke
Energy should acknowledge this concept and document within the transport modeling section(s)
of the CAP all widely recognized limitations inherent in the estimation of the Kd term.
Risk Assessment
1. Provide references for guidance and potential sampling methodology related to conducting a
baseline ecological risk assessment or habitat assessment, if warranted.