NC0001422_Refined Geochemical Report Comments_20170801

Water Resources
Environmental Quality
August 1, 2017
Ed Sullivan
Duke Energy
526 South Church Street
Mail Code EC13K
Charlotte, North Carolina 28202
Subject: Comments regarding the Refined Geochemical Model Report
L.V. Sutton Energy Complex
Dear Mr. Sullivan:
ROY COOPER
Governor
MICHAEL S. REGAN
Secretary
S. JAY ZIMMERMAN
Director
On October 31, 2016, the North Carolina Department of Environmental Quality's Division of
Water Resources (Division) received the Refined Geochemical Modeling Report for the L. V.
Sutton Energy Complex. The review of the subject report by Division staff and subject matter
expert focused on the following components of the document:
The Refined Geochemical Model Report;
Responses prepared by Duke Energy to address comments submitted by the Division on
August 19, 2016 to Duke Energy concerning the Corrective Action Plan (CAP) Part 1
Appendix D and CAP Part 2 Appendix C reports; and
A revised report dated October 25, 2016 titled Analysis of Geochemical Phenomena
Controlling Mobility of Ions from Coal Ash Basins at the Duke Energy L. V. Sutton Energy
Complex (Geochemical Phenomena Report).
These observations and recommendations concerning geochemical model revisions at the Sutton
facility consist of general comments regarding the three documents along with specific comments
and clarification concerning items that should be addressed in revised models. Recent technical
direction regarding inclusion of additional report sections to support model development is
included.
General Comments
1. The Refined Geochemical Model Report describes the current geochemical conditions
(distribution of pH and Eh) along flow transects and how these conditions help explain the
constituent of interest (COI) concentration distribution. The addition of these flow transects at
Sutton to describe the occurrence of COIs such as arsenic and selenium was an important focus of
Division comments on the original geochemical model. The Refined Geochemical Model report
also describes how the geochemical conditions are expected to change after the proposed
corrective action has been implemented.
State of North Carolina I Environmental Quality I Division of Water Resources
Water Quality Regional Operations Section
1636 Mail Service Center I Raleigh, North Carolina 27699-1636
919-707-9129

2. Elevated boron in groundwater from the ash basins is stated to only occur in the upper and lower
surficial aquifer (Section 2.3.2). Available data (see Specific Comment #7 below) suggest that the
elevated boron in the upper Pee Dee aquifer may also be attributed to the ash basin. If this is the
case, this occurrence of boron groundwater contamination should be addressed in the geochemical
model and in the remediation design.
3. The refined geochemical model (Section 2.3.5) acknowledges that the 1984 ash basin is the
likely source of selenium north of this ash basin. This removes one of the data uncertainties that
has existed since elevated selenium was detected in one of the northern monitoring wells.
4. The responses prepared to address Division comments dated August 19, 2016 on the Duke
Energy CAP geochemistry appendices satisfy the majority of the comments. It is noted that the
conversion of Eh to pe was done correctly in the PHREEQC modeling and the document
discussion of this conversion has been corrected. Also, the calculation of gibbsite (HAO)
adsorption site density has been corrected in the report and in the modeling runs. Suggestions for
potential fixture improvements to the geochemical conceptual model and simulations are provided
in the specific comments section below.
5. The revised Analysis of Geochemical Phenomena Controlling Mobility of Ions from Coal Ash
Basins at the Duke Energy L. V. Sutton Energy Complex dated October 14, 2016 describes the
geochemical behavior and subsurface mobility of several constituents of interest (As, B, Cr, Co,
and Se) in the subsurface by considering sorption of the constituent to the aquifer solids,
oxidation/reduction reactions, and precipitation/coprecipitation in mineral phases using the USGS
computer program PHREEQC. The major revision to this report was the addition of descriptions
of the geochemical behavior of the COIs along the three flow transects of particular interest at
Sutton. This satisfies a major comment on the original document. Suggestions for potential fixture
improvements to this document are provided in the specific comments section below.
6. Development of a PHREEQC 1-dimensional fate and transport model along a flow path may be
considered if beneficial with respect to predicting site conditions downgradient of the source areas.
7. A section (or sections) should be added to the report that discuss model construction,
assumptions, sensitivity analysis, and limitations. This is especially necessary when the model is
used to predict responses of the environment to remedial actions and estimate effects on
contaminant concentration and mobility.
8. The iron adsorbent throughout Appendix A is termed ferrihydrate. The correct mineral name
is ferrihydrite.
Specific Comments
1. Refined Geochemical Model. Page 2-9. As water migrates away from the basin, the pH will
decrease toward background concentrations as the Eh increases. As a result, it would be expected
that if Se(VI) is present, it could migrate from the source area under high reduction potential
conditions; thereafter, little ifany Se(VI) will persist.
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Division Comment: What leads to the conclusion that "little if any Se(VI) will persist'?
2. Refined Geochemical Model. Page 2-5. With ash basin closure by excavation, and the
anticipated groundwater extraction system(s), groundwater will return to the lower background
pH and higher Eh conditions, which will tend to drive the geochemistry toward favoring the less
mobile AsM species.
Division Comment: The redox potential is expected to increase after ash removal. This
will favor Se(VI), which is generally more mobile than more reduced selenium species.
Clarify why the future change in groundwater geochemistry will not increase selenium
mobilization Include selenium in future flow and transport modeling for this northern
transect at Sutton.
3. Appendix A — Duke Energy Response to Division General Comment #3. The measured Kd
values for arsenic at Sutton ranged from 8.7 to 500 mL/g. The transport model used a low value
of 9, which results in a retardation factor of 73. This level of retardation allows the arsenic to
migrate only a short distance from the ash basins in the transport simulation, and the simulation
was notable to reproduce the observed arsenic values at MW--21C. The use of a higher value of
Kd in the simulation would further restrict the migration of arsenic. The use of a lower Kd value
did not appear to be justified by the experimental data or geochemical modeling.
Division Comment: Perhaps, the lesson here is that the Kd approach is not appropriate for
modeling arsenic migration at this location. This concept should be considered with
respect to documentation of model limitations and development of alternative approaches
to estimate site conditions and finalize the model.
4. Appendix A — Duke Energy Response to Division Site -Specific Comment #3, Page 8. Most
colloids appear to be associated with ash, with highest concentrations down gradient of the basin
and lower concentrations side gradient of the basin, following groundwater flow from the ash
basin.
Division Comment: The discussion of transport modeling in the CAP reports should
mention the potential importance of colloidal transport because it has been shown to be
associated with the ash basins.
5. Geochemical Phenomena Report. Page 10. This approach uses the total Fe and AI solid phase
concentrations in specific wells to identify how much sorbent is available.
Division Comment: The total Fe and Al solid phase concentrations may vastly
overestimate the amount of available adsorbent, which is only attributable to the
ferrihydrite or HFO portion of the total Fe in the solid phase. See additional discussion of
Fe and Al sorbent mass and capacity in Section 9 of this report, where it also states that
total masses of Fe and Al in the solid phase are used to calculate sorbent concentrations.
Need clarification on how the adsorbent concentration was calculated. If total Fe and Al
were used, then the amount of adsorbent used in the geochemical model may be much
higher than the actual amount available in the solid phase. Because adsorption is the
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primary geochemical process affecting metals migration and natural attenuation in the
aquifer, a correct adsorption capacity is necessary.
6. Geochemical Phenomena Report. Page 44. Therefore, a saturation index of I or greater
indicates that the solution is saturated.
Division Comment: A saturation index of 0 or greater indicates saturation.
7. Geochemical Phenomena Report. Page 52. Figure 5.2 (Sutton, East transect) shows a good
correlation between boron in the Surficial Lower Aquifer and the Pee Dee Upper Aquifer. It
appears possible that the boron in the Pee Dee Upper is associated with the Surficial Aquifer
plume, and should be addressed in the geochemical and transport models as well as consideration
with respect to the vertical delineation of impacts from the ash basins at the Sutton facility.
8. Geochemical Phenomena Report. Page 95. ... it is assumed that the pump and treat system not
impact the pH and Eh of the pore waters and is unlikely to result in enhanced mobilization of a
constituents ofconcern.
Division Comment: In order to test these assumptions, the geochemical model of the site
could be used in a predictive mode to contact fresh groundwater with the impacted aquifer
solids and estimate the actual impact to site geochemistry and contaminant levels in
response to groundwater extraction.
If you have any questions, please feel free to contact Geoff Kegley at the Wilmington Regional
Office at [phone] or Steve Lanter at [phone].
Sincerely,
S. Jay immerman, P.G., Director
Division of Water Resources
cc: Jim Gregson— WIRO Regional Office Supervisor
WQROS Central File Copy
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