NC0005088_Cosler_Expert_Report_Cliffside_FINALw Tables_20160229

Expert Report of
Douglas J. Cosler, Ph.D., P.E.
Chemical Hydrogeologist
Adaptive Groundwater Solutions LLC
Charlotte, North Carolina
Cliffside Steam Station Ash Basins
Mooresboro, North Carolina
February 29, 2016

Introduction
Site Background
The Cliffside Steam Station (CSS) is a coal-fired generating station owned by Duke Energy and located
on a 1,000-acre site in Mooresboro, Rutherford and Cleveland Counties, North Carolina, adjacent to the
Broad River. CSS began operations 1940 with Units 1-4, followed later by Unit 5 (1972) and Unit 6
(2013). Units 5 and 6 are currently operating, but Units 1-4 were retired from service in 2011. An ash
basin system has been historically used to dispose of coal combustion residuals ("coal ash") and other
liquid discharges from the CSS coal combustion process. The ash basin system consists of an active ash
basin (constructed in 1975 and expanded in 1980; used by Units 5 and 6), the Units 1-4 inactive ash
basin (retired in 1977 upon reaching its capacity), and the Unit 5 inactive ash basin (retired at capacity in
1980, but local stormwater collects and infiltrates within its footprint). The active ash basin also contains
an unlined dry ash storage area.
Duke Energy performed voluntary groundwater monitoring around the active ash basin from August 2008
to August 2010 using wells installed in 1995/1996, 2005, and 2007. Compliance groundwater monitoring,
required by a NPDES permit, has been performed by Duke starting in April 2011. Recent groundwater
sampling results at Cliffside have indicated exceedances of 15A NCAC 02L.0202 Groundwater Quality
Standards (2L Standards). In response to this, the North Carolina Department of Environmental Quality
(NC DEQ) required Duke Energy to perform a groundwater assessment at the site and prepare a
Comprehensive Site Assessment (CSA) report. The Coal Ash Management Act of 2014 (CAMA) also
required owners of surface impoundments containing coal combustion residuals (CCR) to conduct
groundwater monitoring and assessment and prepare a CSA report. The recently -completed CSA
(August 2015) prepared by HDR Engineering, Inc. of the Carolinas (HDR) determined that the source and
cause of certain constituent regulatory exceedances at the CSS site is leaching from coal ash contained
in the active and inactive ash basins and the ash storage area into underlying soil and groundwater. The
Cliffside CSA report defined Constituents of Interest (COI) in soil, groundwater, and seeps that are
attributable to coal ash handling and storage.
CAMA also requires the submittal of a Corrective Action Plan (CAP); the CAP for the Cliffside site
consists of two parts. CAP Part 1 (submitted to DEQ in November 2015) provides a summary of CSA
findings, further evaluation and selection of COI, a site conceptual model (SCM), the development of
groundwater flow and chemical transport models of the site, presentation and analysis of the results of
the modeling, and a quantitative analysis of groundwater and surface water interactions. The CAP Part 2
(recently completed and not available for meaningful review) will contain proposed remedial methods for
achieving groundwater quality restoration, conceptual plans for recommended corrective action, proposed
future monitoring plans, and a risk assessment.
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Information Reviewed
My opinions are based upon an analysis and technical review of (i) hydrogeologic and chemical data
collected at the Cliffside site; (ii) the analyses, interpretations, and conclusions presented in site -related
technical documents and reports; (iii) the groundwater flow and chemical transport models constructed
for the site (including model development, calibration, and simulations of remedial alternatives); (iv) the
effectiveness of proposed remedial alternatives to achieve groundwater quality restoration; and (v)
proposed future site monitoring. These opinions are subject to change as new information becomes
available.
As a basis for forming my opinions I reviewed the following documents and associated appendices:
(1) Comprehensive Site Assessment Report, Cliffside Steam Station Ash Basin (August 18, 2015);
(2) Corrective Action Plan, Part 1, Cliffside Steam Station Ash Basin (November 16, 2015);
(3) Miscellaneous historical groundwater and soil concentration data for the Cliffside site collected
prior to the CSA;
(4) Specific references cited in and listed at the end of this report.
Professional Qualifications
I have advanced graduate degrees in Hydrogeology (Ph.D. Degree from The Ohio State University) and
Civil and Environmental Engineering (Civil Engineer Degree from the Massachusetts Institute of
Technology), and M.S. and B.S. degrees from Ohio State in Civil and Environmental Engineering. I have
36 years of experience as a chemical hydrogeologist and environmental engineer investigating and
performing data analyses and computer modeling for a wide variety of projects. These projects include:
investigation, remediation, and regulation of Superfund, RCRA, and other hazardous waste sites involving
overburden and bedrock aquifers; ground water flow and chemical transport model development; natural
attenuation/biodegradation assessments for chlorinated solvent and petroleum contamination sites;
volatile organic compound vapor migration and exposure assessment; exposure modeling for health risk
assessments; hydrologic impact assessment for minerals and coal mining; leachate collection system
modeling and design for mine tailings disposal impoundments; and expert witness testimony and
litigation support. I also develop commercial groundwater flow and chemical transport modeling software
for the environmental industry.
The types of sites I have investigated include: landfills, mining operations, manufactured gas plants,
wood -treating facilities, chemical plants, water supply well fields, gasoline and fuel oil storage/delivery
facilities, nuclear waste disposal sites, hazardous waste incinerators, and various industrial facilities. I
have investigated the following dissolved, nonaqueous-phase (LNAPL/DNAPL), and vapor -phase
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contaminants: chlorinated solvents, various metals, gasoline and fuel oil constituents, wood -treating
products, coal tars, polychlorinated biphenyls, pesticides, dioxins and furans, phenolic compounds, flame
retardants (PBDE), phthalates, radionuclides, and biological constituents.
Summary of Opinions
The following is a brief summary of the opinions developed in my report:
• A total of 62 Compliance Boundary groundwater samples exceeded North Carolina groundwater
standards for these COI: antimony, boron, chromium, cobalt, iron, manganese, sulfate, total
dissolved solids, and vanadium. Of these 62 exceedances, 36 were greater than the proposed
provisional background concentrations by HDR;
• The statistical analyses of shallow background groundwater concentrations at the Cliffside site
(well MW-24D) are invalid due to the characteristically slow rate of COI migration in groundwater;
• There is a significant risk of chemical migration from the ash basin to neighboring private water
supply wells in fractured bedrock;
• Major limitations of the CAP groundwater flow and chemical transport models prevent simulation
and analysis of off -site migration;
• The CAP Closure Scenario simulations greatly underestimate (by factors of 10 or more) the time
frames required to achieve meaningful groundwater concentration reductions in response to
remedial actions;
• For either the Existing Condition or Cap -in -Place Model Scenario groundwater concentrations of
coal -ash constituents much higher than background levels will continue to exceed North Carolina
groundwater standards at the Compliance Boundary because saturated coal -ash material and
secondary sources will remain in place;
• Source -area mass removal included in the Excavation Scenario results in COI concentration
reductions at the Compliance Boundary that are generally two to ten (2 - 10x) times greater
compared to Cap -in -Place, best reduces impacts to surface water, and reduces cleanup times by
factors of two to five (2 - 5x). Additional excavation of secondary sources would further
accelerate concentration reductions; and
• The CAP simulations show that source excavation reduces groundwater concentrations for many
COI below North Carolina groundwater standards (antimony, arsenic, chromium, hexavalent
chromium, cobalt, nickel, thallium, vanadium), but cap -in -place closure does not;
• CSA data show multiple exceedances of groundwater standards in bedrock not only at the
compliance boundary but also inside the CB. However, the CAP Closure Scenarios do not
address either concentration reduction or off -site chemical migration control in the fractured
bedrock aquifer; and
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• Future Compliance Monitoring at the Cliffside site should include much more closely -spaced
Compliance Wells to provide more accurate detection, and groundwater sampling frequency
should be re-evaluated to allow valid statistical analyses of concentration variations.
Hydrogeology of the Cliffside Site
Introduction
The groundwater system at the Cliffside site is an unconfined, connected system consisting of three basic
flow layers: shallow, deep, and fractured bedrock. The shallow and deep layers consist of residual soil,
saprolite (clay and coarser granular material formed by chemical weathering of bedrock), and weathered
fractured rock (regolith). A transition zone at the base of the regolith is also present and consists of
partially-weathered/fractured bedrock and lesser amounts of saprolite. The ash basin system overlies
native soil and was constructed in historical drainage features formed from tributaries that flowed toward
the Broad River using earthen embankment dams and dikes. As described in the CSA report, the active
ash basin was formed by construction of two dams across natural drainages. At the upstream dam, Suck
Creek was diverted through a canal and away from the ash basin to the Broad River, at its present-day
configuration. The active ash basin downstream dam is located near the historical discharge point of
Suck Creek into the Broad River. A large percentage of the coal ash lies below the groundwater table
and is saturated. Groundwater flow through saturated coal ash and downward infiltration of rainwater
through unsaturated coal ash leach COI into the subsurface beneath the basin and via seeps through the
embankments.
As described by HDR, groundwater flow in all three layers within the site boundary is generally from south
to north toward the Broad River. Vertical groundwater flow between the three layers also occurs, and
surface water ponding in the active ash basin effects flow directions locally. The CSA and CAP
investigations assumed that all groundwater north of the ash basin system (overburden and bedrock
aquifers) discharges into the Broad River. However, these studies did not collect hydrogeologic data or
perform data analyses or groundwater flow modeling to support this assumption. The CSA and CAP Part
1 also did not analyze potential changes to site groundwater flow directions, or the risk of off -site
migration of COI in the overburden or bedrock aquifers, caused by groundwater extraction from numerous
private and public water supply wells located close to the site boundaries and near the Broad River.
My report begins with a discussion of significant errors in CSA data analysis and conceptual model
development that contradict HDR's interpretation of three-dimensional groundwater flow patterns at the
Cliffside site. This is followed by a presentation and discussion of measured exceedances of North
Carolina groundwater standards at multiple locations on the ash basin compliance boundary. I then
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address several limitations of the CAP groundwater flow and chemical transport models and identify
various model input data errors. Finally, I present my evaluations of the CAP Closure Scenario
simulations and provide my opinions regarding the effectiveness of various remedial alternatives for
restoring groundwater quality to North Carolina standards.
Errors in Hydraulic Conductivity Test Analyses
Background
Throughout the CSA and CAP reports HDR provides interpretations and conclusions regarding the
horizontal and vertical variations of groundwater flow directions and rates, and the fate and transport of
COI dissolved in groundwater. The most important site -specific parameter that controls these time -
dependent flow and transport mechanisms is the hydraulic conductivity (also referred to as "permeability")
of the underlying soils and fractured bedrock (Bear, 1979). Hydraulic conductivity (length/time) is a
media -specific measure of the rate at which water can flow through a porous (soil) or fractured (bedrock)
porous medium. Groundwater flow and chemical transport rates are directly proportional to the product of
hydraulic conductivity and the hydraulic gradient (hydraulic head difference between two points divided by
the separation distance; e.g., the water table elevation slope at the Cliffside site). Therefore, accurate
measurement of hydraulic conductivity is critical for understanding the current future distributions of COI
in soil and groundwater and for evaluating the effectiveness (e.g., cleanup times) of alternative remedial
measures.
In addition, the contrast in hydraulic conductivity between adjacent hydrogeologic units is the key factor in
determining three-dimensional groundwater flow directions and the ultimate fate of dissolved COI. For
example, at the Cliffside site accurate measurement of hydraulic conductivity is critical in evaluating the
potential for: downward chemical migration into the fractured bedrock unit, off -site COI migration in the
overburden (soil) or fractured bedrock aquifers, groundwater flow and COI transport into or beneath the
Broad River.
A slug test is one of the standard field methods for measuring hydraulic conductivity (K using a soil
boring or installed monitoring well. Slug tests were performed in most of the overburden and bedrock
wells at the Cliffside site. In this test the static water level in the open hole (boring) or well casing is
suddenly increased or decreased and the resulting transient change in water level is recorded. Two
commonly -used techniques for quickly changing the water level are the introduction (increases the water
level) and removal (decreases the water level) of a solid rod, or "slug" into the boring or well casing.
These tests are called "falling -head" and "rising -head" tests, respectively. Higher rates of water -level
recovery correspond to higher values of K. The measurements of water level versus time are analyzed
using mathematical models of the groundwater flow hydraulics and information regarding the well
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installation (e.g., length of the slotted monitoring well screen and well casing diameter) to compute an
estimate of K.
As discussed below, HDR made significant errors in all of their analyses of field slug test data. Their
analysis errors caused the reported (CSA report) slug test hydraulic conductivity values to be as large as
a factor of two (almost 100 percent) smaller than the correct Kvalues. I discuss the impacts of these
analysis errors on HDR's groundwater flow and chemical transport assessments and the CAP modeling
later in my report.
Overburden Slug Tests
HDR analyzed all of the CSA overburden slug tests in shallow and deep wells with the Bouwer-Rice
(1976) method using a vertical anisotropy, Av = Knorizonrat 1 Kvertioat , that is as large as a factor of 100 lower
than the values presented in the CSA report (e.g., compare geometric mean values in CSA Tables 11-10
and 11-11) and used in the CAP modeling (CAP report Appendix C, Table 2), where Kis hydraulic
conductivity. Comparing CSA Tables 11-10 and 11-11, the measured Avforoverburden soil units ranges
from 4 to 50. In the calibrated CAP flow model Av is on the order of 100 for overburden soil. However,
the Bouwer-Rice slug test analyses assumed Av = 1 for every monitoring well (CSA Appendix H). If the
CAP flow model results (Av - 100) are used in the Bouwer-Rice analyses all of the measured overburden
hydraulic conductivity values increase by about 70 percent (factor of 1.7), depending on how the slug -test
radius of influence was computed. Using the Tables 11-10/11-11 measured vertical anisotropies (Av= 4
to 50) increases all of the measured overburden hydraulic conductivity values (CSA Table 11-4) by about
20-60 percent.
Since every reported overburden Kvalue in the CSA report (at least for new shallow and deep wells) is
up to 70 percent too low, the actual average chemical transport rates in overburden soils are up to 70
percent greater than reported. This site -wide data reduction error also affects the CAP flow and transport
model calibrations. For example, the transport model developers significantly reduced laboratory
measurements of the soil -water partition coefficient, Kd, for various COI during the transport model
calibration based on comparisons of observed and simulated chemical migration rates. However, if the
correct (i.e., higher) overburden Kvalues had been used in the model calibration the Kd values would not
have been reduced as much (compared to laboratory values). The reason for this is, assuming linear
equilibrium partitioning of COI with soil, the chemical migration rate is proportional to K/Kd (except for Kd
<< 1). The CAP transport model history matching indicated that the simulated transport rate was too low,
so the model developers reduced the model Kd. In other words, the reductions in calibrated Kd values
would not have been as great if the correct (higher) Kvalues were used in the first place. This is
important because, as discussed below, aquifer cleanup times (i.e., chemical flushing rates) are generally
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proportional to the chemical retardation factor, which is directly proportional to Kd , except when Kd << 1
(Zheng et al., 1991).
Groundwater Flow
Throughout the CSA and CAP reports HDR made several critical assumptions, not supported by data,
regarding the horizontal and vertical groundwater flow directions near the boundaries of the Cliffside site
which impacted their conclusions regarding the ultimate discharge locations for site groundwater and
dissolved COI. Two examples discussed in this section are (i) the relationship between site groundwater
and the Broad River and (ii) groundwater flow directions and the potential for offsite migration of COI.
Broad River and the LeGrand Conceptual Model
Most of the groundwater at the Cliffside site was apparently assumed to discharge into the Broad River
(other than groundwater discharges to small streams such as Suck Creek) according to a generalized
conceptual model (LeGrand, 2004) before actual site -specific hydrogeologic data were analyzed.
Statements to this effect were made at numerous points in the CSA and CAP reports. However, HDR did
not present any site -specific data analyses or groundwater flow modeling that would support this
assumption in either report. In fact, as discussed below, the CAP flow model boundary conditions
effectively forced site groundwater to discharge into the river at the downgradient model boundary.
The LeGrand (2004) guidance document presents a general discussion of groundwater flow patterns that
may occur near streams in the Piedmont and Mountain Region of North Carolina based on ground
surface elevations (i.e., site topography and surface watershed boundaries). However, surface water and
groundwater watersheds commonly do not coincide (Winter et al., 2003). Further, groundwater flow
patterns and rates in bedrock have been found to be poorly related to topographic characteristics (Yin
and Brook, 1992). LeGrand does not present or derive any mathematical equations or quantitative
relationships for groundwater flow near rivers or streams. The author emphasizes that site -specific data
must be collected in order to correctly evaluate river inflow or outflow. In strong contrast to the LeGrand
generalizations, numerous detailed and sophisticated mathematical (analytical and numerical) river -
aquifer models and highly -monitored field studies have been published in the scientific and engineering
literature in the past several decades. What these investigations and applied hydraulic models show is
that the water flow rate into or out of a river or stream and the depth of hydraulic influence within an
underlying aquifer are highly sensitive to several factors, including: the transient river water surface
elevation and slope; river bed topography; bed permeability and thickness; horizontal and vertical
permeability (and thickness) of the different hydrogeologic units underlying the river; transient horizontal
and vertical hydraulic head variations in groundwater beneath and near the river; and groundwater
extraction rates and screen elevations for neighboring pumping wells (e.g., Simon et al. 2015; McDonald
and Harbaugh, 1988; Bear, 1979; Hantush, 1964).
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The CSA investigation did not: measure river bed permeability or thickness; characterize the river
bathymetry; monitor transient water surface elevation variations at more than one location (one average
value was used); collect river bed hydraulic gradient data; measure horizontal or vertical overburden or
bedrock permeability beneath or on the northern side of the river; characterize the geology beneath or
north of the river; measure hydraulic heads in the overburden or bedrock beneath or north of the river; or
consider the hydraulic effects of groundwater extraction from nearby private water supply wells, as
discussed in the following section. With regard to the Cliffside site, much of the data that were collected
in the CSA contradict the LeGrand hypothesis. A strong downward flow component (- 10 feet head
difference) from deep overburden to bedrock was measured at the following locations next to the Broad
River: GWA-21 (near several private bedrock supply wells), GWA-29, IB-3D/GWA-11 BRU, MW-
38D/MW-36BRU, and the entire area between the river and northern portion of the Active Ash Basin as
generally bounded by the 650- to 725-foot bedrock head contours (compare CSA Figures 6-6 and 6-7).
The vertical flow direction from shallow to deep overburden is also downward in this area located
between the Broad River and the Active Ash Basin (compare CSA Figures 6-5 and 6-6). In addition,
downward groundwater flow was measured at several other locations across the site (CSA Table 11-13).
Contour maps of vertical hydraulic gradient variations were not generated for the CSA, and HDR did not
discuss the significance of downward hydraulic gradients next to the Broad River. These downward
groundwater flow measurements are consistent with the hydraulic conductivities of the bedrock and
overburden being of similar magnitude.
The strong measured downward groundwater flow components right next to the Broad River indicate that
site groundwater is entering the deep fractured bedrock unit in these areas and that not all of the site
groundwater discharges into the river as the site Conceptual Model and the CAP flow and transport
models assume. The downward flow into bedrock may also be due in part to groundwater extraction from
private bedrock water supply wells located near the eastern property boundary, but in the CSA and CAP
investigations HDR assumed these factors related to the potential for off -site COI migration beneath the
river were not important and did not evaluate them.
Groundwater Flow Directions
The CSA assumptions and analysis errors discussed above have had a strong effect on: the Conceptual
Model development; the site hydrogeologic and COI transport assessment; the construction/calibration
of the CAP flow and transport models; and the simulations of CAP Close Scenarios. The hydrogeologic
assumptions should have been carefully evaluated and tested during the performance of the CSA and as
part of the CAP groundwater flow model construction and calibration to determine whether they were
valid. Instead, the hypotheses appear to have effectively guided the model development and led to
inaccurate interpretations.
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As an illustration, because the permeability of the weathered bedrock is similar to the overlying soils at
the Cliffside site the CSA and CAP interpretation that the bedrock acts as a lower confining layer for
groundwater flow and chemical transport is incorrect. In addition, the similarity of the overburden and
bedrock aquifer permeability values increases the potential for off -site COI migration toward private water
supply wells. Therefore, the CSA and CAP conclusions that (i) all site groundwater discharges into the
Broad River and (ii) groundwater and dissolved coal -ash constituents are restricted from migrating to
residential water supply wells are not consistent with the data.
The CSA and CAP reports also did not adequately evaluate the three-dimensional groundwater flow field
near and beneath the Broad River. Numerous private water supply wells are located in the following
areas (CSA Figure 4-2): a few hundred feet north of the Broad River and immediately east of the
Compliance Boundary for the Active Ash Basin, less than 1,500 feet from the Active and Unit 5 Inactive
Ash Basins, and less than 1,500 feet from the Active Ash Basin and on the southern shore of the Broad
River (close to the northeastern portion of the Compliance Boundary). Bedrock hydraulic head
measurements (CSA Figure 6-8) for monitoring wells located next to the river (e.g., Wells GWA-32BR,
GWA-11 BRU, GWA-29BR, and GWA-21 BR) indicate a strong easterly bedrock aquifer flow component
from downgradient areas of the site toward these private wells on the southern shoreline. However, CSA
Figure 6-8 does not show these head contours, and the CAP flow model boundary conditions artificially
prevent groundwater from either flowing east or northeast beneath the Broad River (as underflow), or
flowing toward the private wells near the northeast Compliance Boundary. The CSA and CAP reports
also do not address the large measured downward hydraulic gradients in the northern portion of the
Active Ash Basin and near the river, and their potential relationship to offsite groundwater extraction from
the bedrock aquifer. The CAP flow model was not properly constructed to allow evaluation of these
observed three-dimensional flow patterns due to: the model no -flow boundary condition on the eastern
and western sides of the grid; the uniform specified head boundary condition in grid cells underlying the
river (i.e., the sloping, west -to -east water surface elevation in the river was not represented in the model);
and the fact that the CAP flow model did not include the effects of groundwater extraction from off -site
water supply wells.
Exceedances of Groundwater Standards
In this section I compare measured groundwater concentrations in shallow, deep, and bedrock
groundwater samples to North Carolina 2L and IMAC standards and show the following: (i) 60 measured
exceedances for several COI at multiple locations on the Compliance Boundary (CB); (ii) an additional
two CB exceedances based on chemical transport modeling I performed; (iii) 36 of the 62 Compliance
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Boundary exceedances were greater than the proposed provisional background concentrations (PPBC)
by HDR; (iv) 37 of the 62 Compliance Boundary exceedances were greater than the maximum
concentration at any background well from the same hydrogeologic unit (e.g., shallow, deep, or bedrock)
for a particular constituent; (v) 12 more exceedances were measured in wells located on the Broad River;
(vi) 54 additional exceedances were observed in wells screened in the highly -permeable fractured
bedrock unit underlying the ash basin system and located inside the CB; and (vii) the statistical analyses
of groundwater concentrations at shallow monitoring well MW-24D for purposes of defining background
levels were performed incorrectly.
Throughout this report I reference the ash basin compliance boundary and the Duke Energy property
boundary for the Cliffside site as drawn on maps developed by HDR (e.g., CSA Figure 6-2). My reference
to the "compliance boundary" is only for identification purposes and not an opinion that this boundary as
drawn by HDR is accurate or legally correct.
Summary of Exceedances
Table 1 summarizes exceedances of 2L or IMAC standards in shallow, deep, and bedrock groundwater
samples obtained from monitoring wells located: (i) on the Ash Basin Compliance Boundary (CB) as
drawn by HDR; (ii) on the southern shore of the Broad River (RV), which is the downgradient boundary of
the CAP groundwater flow and chemical transport models; (iii) bedrock wells (BR) located inside the CB;
and (iv) modeled Compliance Boundary concentrations (CBM), using modeling techniques described
below. The proposed provisional background concentrations (PPBC) by HDR are also listed in Table 1.
A total of 33 Compliance Boundary groundwater samples exceeded North Carolina 2L standards, and
IMAC standards were exceeded in an additional 27 samples for these COI: antimony, boron, chromium,
cobalt, iron, manganese, sulfate, total dissolved solids, and vanadium. I estimated an additional two CB
exceedances dowgradient from wells MW-11 S and GWA-27D for boron based on chemical transport
modeling and measured upgradient concentrations (designated CBM in Table 1). In addition, 39
exceedances of 2L regulatory limits were observed in wells screened in the highly fractured bedrock unit
located inside the CB. An additional 15 bedrock sample concentrations were greater than IMAC limits.
Ten more 2L (plus two IMAC) exceedances were measured in wells located on the Broad River.
A total of 29 of the 35 Compliance Boundary 2L (and 9 of 25 IMAC) exceedances were greater than the
maximum concentration at any background well (from the same hydrogeologic unit; e.g., shallow or
deep) for a particular constituent. All of the Broad River shoreline "RW exceedances were greater than
background levels. A total of 27 of the 39 bedrock 2L (and 8 of 15 IMAC) exceedances were greater than
the maximum background concentration.
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A total of 36 of the 62 Compliance Boundary exceedances were greater than the proposed provisional
background concentrations (PPBC) by HDR.
Note that the iso-concentration contours in all of the CSA Section 10 figures are not consistent, and are in
many cases misleading, with regard to chemical transport mechanisms in the subsurface. For example,
the iso-concentration contours in Section 10 generally closely encircle a monitoring well and infer no
subsequent transport downgradient from the well location. This contouring problem is especially
prevalent near the southern shore of the Broad River. Figure 10-65 (cobalt) is a good example of this
practice. These closed contours at the downgradient property boundary suggest that COI transport
beyond the farthest downgradient line of monitoring wells does not occur and that no COI migrate north of
the southern shore of the river. However, the simulated (CAP model) "existing conditions" cobalt
concentration contours in CAP Appendix C are 'open" at the Broad River, indicating transport beneath the
river.
Modeled Compliance Boundary Exceedances
I computed Compliance Boundary (CB) concentrations labeled "CBM" with footnote "e" in Table 1 (MW-
11 S and GWA-27D) using a calibrated one-dimensional, analytical chemical transport model (van
Genuchten and Alves, 1982; Equation C5) because the CB at these locations was up to 400 feet
downgradient from the wells and boron is highly mobile in the subsurface. I calibrated the analytical
model to chemical -specific site conditions (i.e., determined model input parameter values) using CAP
transport model simulated concentration versus time curves for "Existing Conditions" (CAP report
Appendix C). The analytical model input parameters in my model were: groundwater pore velocity,
chemical retardation factor, and longitudinal dispersivity. For each constituent, I used the calibrated
analytical model to compute the concentration versus time curve immediately downgradient at the
Compliance Boundary.
Exceedances of Groundwater and Surface Water Standards in Seep Samples
Concentrations in seeps discharging from the active ash basin (upstream toe, adjacent to Suck Creek)
have exceeded North Carolina surface water standards (2B) and 2L and/or IMAC groundwater standards
(e.g., arsenic, chromium, iron, lead, manganese, nickel, selenium, and vanadium; CAP Figures 2-2 and
2-3, CSA Table 7-9). Groundwater discharges to Suck Creek were confirmed by the CAP flow modeling.
Elevated concentrations of boron, calcium, chloride, sulfate, and total dissolved solids were detected in a
surface water sample from Suck Creek (SW-3) collected downgradient from the toe of the active ash
basin upstream dam (page 90 of the CSA report).
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The CSA also identified other continuously -flowing seeps as tributaries of the Broad River [e.g., S-1, S-3,
S-6, and S-8; refer to Table 1 in the Topographic Map and Discharge Assessment Plan(DAP)]. Seep S-3
is apparently part of a stream discharging to the Broad River north of inactive units 1-4 (DAP Figure 2).
Seep S-6 is located downgradient from the downstream dam of the active ash basin and coincides with
historical Suck Creek discharge (CSA Appendix I, Figure 1). Concentrations in samples from seep S-6
have exceeded relevant surface water 2B standards, and 2L and/or IMAC groundwater standards for
boron, cobalt, iron, manganese, and vanadium. Concentrations in samples from seep S-3 have
exceeded relevant surface water 2B standards for cobalt, iron, manganese, sulfate, thallium and total
dissolved solids.
Referring to my Table 1, 122 of the seep samples exceeded North Carolina groundwater standards (84
2L exceedances and 38 IMAC exceedances; CSA Table 7-11) for these COI: arsenic, barium, beryllium,
boron, chromium, cobalt, iron, lead, manganese, nickel, sulfate, total dissolved solids, thallium, and
vanadium. These samples were collected at the active ash basin; inactive ash basins 1-4 and 5; and
the ash storage area.
Statistical Analyses of Background Concentrations
Appendix G of the CSA report presents statistical analyses of historical concentrations from Monitoring
Wells MW-24D and MW-24DR, which HDR described as following methods specified by the U.S.
Environmental Protection Agency (EPA, 2009), in an attempt to establish background groundwater
concentrations for the Cliffside site. As outlined in Sections 3.2.1 and 5.5.2 of the EPA guidance
document these data must be checked to ensure that they are statistically independent and exhibit no
pairwise correlation. Groundwater sampling data can be non -independent (i.e., autocorrelated) if the
sampling frequency is too high (i.e., time interval between sampling events is too small) compared to the
chemical migration rate in the aquifer (groundwater pore velocity divided by chemical retardation factor).
Section 14 of the EPA guidance presents methods for ensuring that the Wells MW-24D and MW-24DR
background data are not autocorrelated, but the analyses in CSA Appendix G did not include evaluations
for statistical independence.
As an illustration, "slow -moving" groundwater combined with high chemical retardation (i.e., large soil -
water partition coefficients, Kd), which is the case at the Cliffside site, can lead to the same general
volume of the chemical plume being repeatedly sampled when the monitoring events are closely spaced.
Examining shallow wells at the Cliffside site, the shallow groundwater pore velocity (Vp) is in the order of
70 ft/yr (CSA Table 11-14), which is representative of the pore velocity near well MW-24D. Note that
shallow pore velocities are as much as a factor of 100 greater in many areas downgradient of the ash
basin system (e.g., the active ash basin) due to much greater hydraulic gradients (- 10x larger) and larger
hydraulic conductivity (- 10x greater) in these areas. In addition, groundwater pore velocities in deep
13

overburden and in fractured bedrock are generally more than a factor of 1,000 greater than velocities in
the shallow overburden (CSA Table 11-14).
The retardation factors, Rd, based on laboratory Kd measurements (Kd - 10 cm3/g, or greater) are on the
order of 100 (or greater) for many of the COI (except conservative parameters such as sulfate and
boron). Therefore, the average shallow chemical migration rate at Cliffside (VP / Rd) is on the order of 0.7
ft/yr many of the non -conservative COI near well MW-241D, assuming linear equilibrium sorption (refer to
discussion below). For quarterly sampling, the chemical migration distance between sampling rounds is
about 0.2 feet for several COI, which is smaller than the sand pack diameter for the monitoring wells.
Therefore, based on either quarterly or annual monitoring the shallow groundwater samples at Cliffside
are basically representative of the same volume of the plume (i.e., the sandpack, depending on the well
purge volume) for many COI, and any measured sample concentration changes are not due to real
chemical transport effects in the aquifer. In this case, this means that the groundwater samples are non -
independent and that the statistical analyses of background concentrations at Wells MW-24D do not
satisfy the key requirements of the analysis method.
CAP Groundwater Flow Model Underestimates Potential for Off -Site Chemical Migration
My discussions in this section focus on limitations of the CAP groundwater flow model. I focus specifically
on model boundary conditions representing the Broad River; the overall size of the model grid and no -
flow boundary conditions on the western, southern, and eastern grid boundaries; groundwater flow in the
fractured bedrock aquifer; and the potential for off -site groundwater flow in relation to groundwater
extraction from numerous private and public water supply wells located close to the model boundaries,
but not incorporated into the flow model
Broad River Boundary Condition
The CAP groundwater flow model forces all Cliffside site groundwater along the northern model boundary
to discharge directly into the Broad River and underestimates the potential for off -site flow and chemical
migration in fractured bedrock. No -flow boundary conditions defined along the entire western, eastern,
and southern model boundaries prevent any off -site groundwater flow and chemical transport in these
areas (refer to Figures 1 and 5 in Appendix C of the CAP Report). The bottom surface (bedrock) of the
flow model is also assumed to be a no -flow boundary even though the hydraulic conductivity data and
measured downward hydraulic gradients at several monitoring well clusters do not support this
assumption. The only locations where groundwater and dissolved constituents are allowed to leave the
model are streams (e.g., Suck Creek and unnamed tributaries to the west), top -layer flood plain cells next
14

to the Broad River, and the vertical array of cells underlying Broad River along the northern grid
boundary; these cells are specified as constant -head boundary conditions in which the head is uniform
with depth.
This hydraulic representation of the Broad River in the flow model is inaccurate for several reasons. First,
the river bottom is assumed to extend all the way through the unconsolidated deposits and the fractured
bedrock unit, which is not the case. Second, groundwater flow beneath and adjacent to the river is
assumed to be horizontal with zero vertical flow component. Because this boundary condition does not
allow groundwater to flow vertically in areas that underlie the river, the CAP model does not represent
actual site hydrologic conditions. Further, groundwater flow at the Cliffside site is not strictly horizontal
and, as discussed above, many of the vertical hydraulic gradient measurements (including next to the
river) are downward. Third, as represented in the CAP model, neither the lower -permeability river bed
sediments nor the smaller vertically hydraulic conductivity of underlying soils restricts the potential flow
rate into or out of the river (i.e., a perfect hydraulic connection exists between the aquifer and the Broad
River). The actual degree of aquifer -river hydraulic connection was not evaluated in the CSA. In
summary, due to all of these factors the potential for site groundwater and dissolved constituents to
migrate off -site northward beyond the Broad River or eastward as underflow beneath the river cannot be
evaluated with the model.
The CAP model should have represented the Broad River using a "leaky -type" (i.e., river) boundary
condition in the top model layer (McDonald and Harbaugh, 1988), and the model grid should have
extended farther north so that the above factors could have been evaluated during model calibration and
sensitivity analyses. The model also should have included groundwater extraction from the private water
supply wells installed at many points close to the river bank. A river boundary condition incorporates the
bed permeability and thickness, the river water surface elevation, and the simulated hydraulic head in the
aquifer (at the base of the river bed) to dynamically specify a flux (flow rate per unit bed area) into or out
of the groundwater model depending on the head difference between the river and aquifer. Typically,
permeability and vertical hydraulic gradient measurements for the river bed (not collected in the CSA) and
flow model calibration (three-dimensional matching of simulated and measured hydraulic head
measurements in the aquifer) are used to determine a best -fit estimate of river bed conductance
(permeability divided by thickness) in the model. Again, this routine analysis was not performed.
Limitations of No -Flow Boundary Conditions and Small Model Domain Size
The limited areal extent and depth of the CAP flow and transport model grids prevent the use of the
models as unbiased computational tools that can be used to evaluate off -site migration of coal -ash
constituents. For example, the model grids should have extended farther north and east to incorporate
groundwater extraction from off -site private water -supply wells and allow three-dimensional groundwater
15

flow patterns to naturally develop. The eastern and western no -flow boundaries in the current CAP
models artificially prevent any off -site flow or transport in either the bedrock or overburden aquifers. The
same is true for the entire northern and southern model boundaries despite the fact that several private
homes are located north and east of the Active Ash Basin, and the bedrock hydraulic head map (CSA
Figure 6-7) exhibits a strong easterly flow component in this area. Some additional private water supply
wells are also located close to the northern shore of the Broad River (CSA Figure 4-2). Artificial
limitations created by the northern Broad River boundary condition are outlined above.
The bottom boundaries of the CAP models should extend much deeper because the hydraulic
conductivity of the fractured bedrock zone is of the same order of magnitude as the overburden soils
based on slug test results. In the present configuration the lower boundary of the model grids is only
about 50 feet below the bedrock surface. Because several bedrock wells were screened to this depth the
bedrock hydraulic conductivity data collected for the CSA demonstrate that imposing an impermeable
model boundary at this depth is incorrect. As discussed above, the strong downward hydraulic gradients
between deep and bedrock wells in the northern portion of the Active Ash Basin also demonstrate that
vertical and horizontal groundwater flow in bedrock is important, and these transport mechanisms need to
be accurately simulated in the CAP models in order to accurately assess the potential for off -site chemical
migration.
Off -Site Groundwater Extraction
The CSA and CAP fail to examine the strong potential for coal -ash constituents from the Cliffside site to
migrate with groundwater to private water supply wells located immediately east and northeast of the
Active Ash Basin. COI may also potentially migrate to private wells located close to the northern Duke
Energy property boundary on the northern side of the Broad River. CSA Figure 4-2 shows the locations
of water supply wells near the site. The basis of my opinion includes the following: hydraulic conductivity
measurements for the overburden and bedrock formations; three-dimensional variations in measured
hydraulic head in the bedrock and overburden units; groundwater concentration data; and calculations of
potential hydraulic head reductions (i.e., drawdown) that could be caused by off -site groundwater
extraction. As discussed throughout my report, neither the CSA nor CAP investigations addressed the
potential for off -site migration.
COI's were detected in several water supply well samples (CSA Appendix B), but the CSA report did not
plot these detections on a map and did not discuss their possible relationship to the Cliffside site.
Appendix B also did not present the well construction details (e.g., well diameter and elevation range of
the well screen or open bedrock interval) so that well dilution effects and potential chemical transport
pathways in the bedrock unit could be evaluated. In addition, the CSA investigations and CAP modeling
did not include these areas east and north of the Cliffside site. The additional hydrogeologic and
16

chemical data collection that CAP Part 1 recommended may address some of these data gaps; however,
the model grid size changes, the boundary condition limitations, and the model input data errors that I
have outlined would need to be corrected in the CAP 2 model before it could be used to accurately
analyze the potential for off -site chemical transport.
Another important model input data error is the bedrock hydraulic conductivity, which is assumed in the
CAP flow model to be about a factor of ten (10x) lower than the overburden aquifer in different areas
(CAP Appendix C, Table 2). The bedrock slug test results show that the mean bedrock permeability is
approximately the same as the overburden permeability. Therefore, downward groundwater flow from the
overburden aquifer into the upper portion of the bedrock unit is not restricted at the Cliffside site, as the
CAP model represents, and the potential for off -site chemical migration is underestimated by the model.
CAP Chemical Transport Modeling
Due to model calibration, model construction, and boundary -condition and input -data errors the CAP
models significantly underestimate remediation time frames. As discussed in this section, reasons for this
include significant underestimation of the chemical mass sorbed to soil, failure to account for slow
chemical desorption rates, inaccurate analyses of water -table lowering due to capping, and flaws in the
transport model calibration.
Soil -Water Partition Coefficients and Model Calibration
The fraction of chemical mass sorbed to soil can be represented by the soil -water partition coefficient, Kd
(Lyman et al., 1982). Kd is an especially important parameter at the Cliffside site because for most of the
COI the bulk of the chemical mass in the soil is associated with the solid phase (i.e., sorbed to soil grains
rather than dissolved in pore water). In effect, the solid fraction of the soil matrix acts as a large "storage
reservoir" for chemical mass when Kd is large [e.g., metals, many chlorinated solvents, and highly -
chlorinated polycyclic aromatic hydrocarbon (PAH) compounds associated with coal tars and wood -
treating fluids]. Kd is also a very important chemical transport parameter which is used to compute the
chemical retardation factor, Rd, assuming linear equilibrium partitioning of mass between the soil (solid)
and pore -water phases (Hemond and Fechner, 1994):
Rd = 1 + pb Kd l ne
where pb is the soil matrix bulk dry density and ne is the effective soil porosity. For example, the
chemical migration rate is directly proportional to hydraulic conductivity and inversely proportional to Rd.
17

The total contaminant mass in an aquifer is also directly proportional to Rd, as well as aquifer cleanup
times once the source is removed (e.g., Zheng et al., 1991).
Accordingly, it is very important to use accurate Kd values in the CAP Closure Scenario modeling.
Specifically, the CAP transport modeling used Kd values that were typically factors of 10 - 100 (i.e., one
to two orders of magnitude) smaller than the measured site -specific Kd's reported in CAP Appendix D.
Further, the CAP model soil -water partition coefficients are significantly smaller than most values
presented in the literature for the COI (e.g., EPRI, 1984; Baes and Sharp, 1983). This means that, using
the actual measured Kd's for the Cliffside site, the times required to reach North Carolina water quality
standards at the Compliance Boundary are more than a factor of 10 longer (see additional discussion
below) than cleanup times predicted by the CAP transport model.
The CAP modeling report (CAP Appendix C; Section 4.8) argues that the major Kd reductions were
needed due to the following:
"The conceptual transport model specifies that COis enter the model from the shallow saturated source
zones in the ash basins. When the measured Kd values are applied in the numerical model to COIs
migrating from the source zones, some COls do not reach the downgradient observation wells where they
were observed in June/July 2015 at the end of the simulation period. The most appropriate method to
calibrate the transport model in this case is to lower the Kd values until an acceptable agreement
between measured and modeled concentrations is achieved. Thus, an effective Kd value results that
likely represents the combined result of intermittent activities over the service life of the ash basin. These
may include pond dredging, dewatering for dike construction, and ash grading and placement. This
approach is expected to produce conservative results, as sorbed constituent mass is released and
transported do wngradient. "
First, considering the approach that was used to develop the chemical transport model (history matching),
it is not true that "the most appropriate method to calibrate the transport model is to lower the Kd values."
The CAP transport model used an incorrect value (2.65 g/cm3) for overburden materials; this value is the
density of a solid mass of mineral (e.g., quartz) with zero porosity. The bulk density should have been
computed using the total porosity (n) values in CSA Table 11-1 using the following formula (e.g., Baes
and Sharp, 1983):
pe = 2.65(1—n)
Based on the Table 11-1 values p,, - 1.0 - 1.9 g/cm3, which means that the CSA model Rd values before
Kd adjustment were as much as a factor of 2.65 (2.65/1.0) too high. Also, as discussed earlier, the
overburden slug test values were about 70 percent too low. Both of these errors correspond to a
modeled transport rate that was up to five times (5x) too low before calibration simply due to data input
errors.
18

At least two other important factors were not considered during the transport model calibration. First, the
groundwater flow model is based on average hydraulic conductivity (K) values within a material zone, but
Kdistributions in aquifers are highly variable (e.g., varying by factors of 3-10, or more, over distances as
small as a few feet: Gelhar, 1984, 1986, 1987; Gelhar and Axness, 1983; Rehfeldt et al., 1992;
Rehfeldt and Gelhar, 1992; Molz, 2015). The Cliffside site hydrogeology certainly qualifies as
"heterogeneous". This is very important to consider for the CAP transport model calibration because the
high -permeability zones and/or layers control the time required (Tt,,,, ) for a constituent to reach a
downgradient observation point, and HDR used differences in observed versus simulated Tt,,l (i.e., time
to travel from sources zones to downgradient monitoring wells) as the justification for lowering measured
Kd values.
Second, the history matching that was performed is very sensitive to the assumed time at which the
source (i.e., coal ash) is "turned on" and the assumed distribution of source concentrations (fixed pore
water concentrations) in source area cells. Section 5.3 of CAP Appendix C explains that the source was
activated 58 years ago in the model:
"The model assumed an initial concentration of 0 within the groundwater system for all COls at the
beginning of operations approximately 58 years ago. A source term matching the pore water
concentrations for each COI was applied within the Units 1-4 inactive ash basin, Unit 5 inactive ash basin,
active ash basin and the ash storage area at the start of the calibration period. The source concentrations
were adjusted to match measured values in the downgradient monitoring wells that had exceedances of
the 2L Standard for each COI in June 2015. "
For several reasons it is a major simplification (and generally inaccurate) to use 2015 ash pore water
concentrations to define year-1957 source zone (fixed concentration) boundary conditions. These
reasons include: coal ash was gradually and nonuniformly distributed (spatially and temporally) in ash
basins throughout the 58-year simulation period (not instantaneously in 1957); it is very difficult (or not
possible) to accurately extrapolate geochemical or ash -water leaching conditions (i.e., predict COI pore -
water concentrations) that existed during the 2015 sampling round to conditions that may have existed in
1957 and thereafter; the actual source -area concentration distributions are highly nonuniform, but it is not
clear from the CAP modeling report how "... source concentrations were adjusted to match measured
values ... ", or if the source area concentrations were nonuniform. All of these uncertainties are further
magnified when using history matching to calibrate a transport model.
Based on the above model input errors and major uncertainties in hydraulic -conductivity variations and
source -term modeling, it is incorrect to simply reduce Kd values by factors of 10 to 100 below site
measurements (and the large database of literature Kd values) based only on the transport model "history
matching" exercises that HDR performed. My additional comments on the CAP 1 transport modeling of
Closure Scenarios are listed in the following section.
19

Simulation of Closure Scenarios
As discussed below, CAP 1 Closure Scenario simulations greatly underestimate (by factors of 10 or
more) the time frames required to achieve meaningful groundwater concentration reductions in response
to remedial actions. Compared to the Cap -in -Place remedial alternative evaluated in the CAP Part 1, the
Excavation Scenario results in COI concentration reductions at the Compliance Boundary that are
generally two to ten times greater compared to Cap -in -Place and best reduces impacts to surface water.
In addition, the time frames to achieve equivalent concentration reductions are at least factors of 2 to 5
shorter for excavation compared to cap -in -place for most of the COI; further, several COI concentrations
reduce below 2L or IMAC standards with excavation but remain significantly higher than the groundwater
standards with cap -in -place.
Source Concentrations for Cap -in -Place Scenario
In this scenario the CAP flow model predicts cap -induced water -table declines equal to approximately 5
feet (relative to the Existing Conditions simulation) within the Units 1-4 inactive ash basins, 12 feet within
the Unit 5 inactive ash basin, and 10 feet within the active ash basin. However, the geologic cross -
sections presented in the CSA show that the saturated coal ash thickness at several borings is as great
as 30-60 feet. This means that under the simulated Cap -In -Place Scenario most of the coal ash, which is
the source of dissolved COI, would remain saturated and continue to leach constituents into groundwater
in several parts of the ash basin system. The CAP simulations ignored this fact and set all source
concentrations equal to zero (i.e., assumed all coal ash was dewatered). Therefore, the simulated Cap-
in -Place concentrations should be much higher than the values presented in the CAP Part 1.
The groundwater flow model simulations also exaggerate the hydraulic effects of the cap (i.e., overstates
water table lowering) because the no -flow boundary conditions along the entire western, southern, and
eastern grid boundaries prevent flow into the Ash Basin System when the laterally inward hydraulic
gradients are created by capping. In addition, the base of the flow model is assumed to be impervious
even though the bedrock aquifer hydraulic conductivity is about the same as the overburden aquifer; this
artificially restricts upward flow from bedrock into the capped area and exaggerates predicted water table
lowering.
In addition, a site -specific distribution of groundwater recharge values should have been developed for
this and the other simulation scenarios to take into account site -specific topography and soil types (e.g.,
runoff estimation) and climate data (precipitation, evapotranspiration, etc.; e.g., using the U.S. Army
Corps of Engineers HELP Model; Schroeder et al., 1994). The CAP 1 flow model uses an assumed
value of 6 inches year uniformly throughout the model domain even though the actual value is highly
variable across the Ash Basin System and site land surface. The predicted water table lowering due to
20

capping is very sensitive to the model recharge value, so more effort should have been made to develop
a site -specific recharge -rate distribution.
Slow and Multirate Nonequilibrium Desorption of COI
Since the 1980's the groundwater industry has learned how difficult it is to achieve water quality
standards at remediation sites without using robust corrective actions such as source removal (Hadley
and Newell, 2012, 2014; Siegel, 2014). Two of the key reasons for this in aqueous -phase contaminated
soil are inherently low groundwater or remediation fluid flushing rates in low -permeability zones and slow,
nonequilibrium chemical desorption from the soil matrix (Culver et al., 1997, 2000; Zheng et al., 2010). A
good example of this is the "tailing effect" (i.e., very slow concentration reduction with time) that is
commonly observed with pump -and -treat, hydraulic containment systems. These factors are also related
to the "rebound effect" in which groundwater concentrations sometimes increase shortly after a
remediation system is turned off (Sudicky and Illman, 2011; Hadley and Newell, 2014; Culver et al.,
1997).
The CAP flow model uses different permeability (K) zones, but the scale of these zones is very large and
within each zone Kis homogeneous even though large hydraulic conductivity variations (e.g., lognormal
distribution) are known to exist at any field site over relatively small length scales ("references").
Moreover, the transport model assumes linear, equilibrium soil -water partitioning which corresponds to
instantaneous COI release into flowing groundwater. The transport code (MT3D) has the capability of
simulating single -rate nonequilibrium sorption, but the Close Scenario simulations did not utilize this
modeling feature. Slow desorption of COI can also be expected at the Allen site because sorption rates
are generally highly variable, and multi -rate (Culver et al., 1997, 2000; Zheng et al., 2010), and Kd
values are nonuniform spatially (Baes and Sharp, 1983; EPRI, 1984; De Wit et al., 1995). The CAP flow
and transport models can be expected to significantly underestimate cleanup times required to meet
groundwater standards at the compliance boundary because they do not incorporate these important
physical mechanisms.
Adequacy of the Kd Model for Transport Simulation
The laboratory column experiment effluent data (CAP Appendix D) generally gave very poor matches with
the analytical (one-dimensional) transport model used to compute Kd values. Since the CAP transport
model solves the same governing equations in three dimensions, the adequacy of the Kd modeling
approach for long-term remedial simulations should have been evaluated in much more detail in the
modeling appendix.
The transport modeling also did not evaluate alternative nonlinear sorption models such as the Freundlich
and Langmuir isotherms (Hemond and Fechner, 1994), which are input options in the MT3D transport
21

code. Several of the batch equilibrium sorption experiments exhibited nonlinear behavior, and such
behavior is commonly observed in other studies (e.g., EPRI, 1984). However, HDR only computed linear
sorption coefficients (i.e., Kd) for the Cliffside site. De Wit et al. (1995) showed that the nonlinear sorption
mechanism is similar in importance to aquifer heterogeneities in extending remediation time frames.
Closure Scenario Time Frames
As outlined in my report, the CAP chemical transport model underestimates the time intervals required to
achieve groundwater concentration reductions (i.e., achieve groundwater quality restoration) by factors
that are at least on the order of 10 to 100. In other words, the transport model significantly overestimates
the rate at which concentrations may reduce in response to remedial actions such as capping or source
removal. This is due to several factors, including major errors in model input data, model calibration
mistakes, field data analysis errors, and oversimplified model representation of field conditions (e.g.,
hydraulic conductivity) and transport mechanisms (e.g., chemical sorption/desorption). These limitations
of transport models for realistically predicting cleanup times have been recognized by the groundwater
industry for the past few decades based on hands-on experience at hundreds of extensively -monitored
remediation sites.
Even if we ignore the factors of 10 or more errors in cleanup time predictions with the CAP model, the
remediation time frames for the Excavation Scenarios are still more than two centuries for several
constituents due to slow groundwater flushing rates from secondary sources (surrounding residual soil)
left in place after excavation and due to high chemical retardation factors for most of the COI. However,
excavation of secondary -source material would further accelerate cleanup rates under this alternative.
The simulated Cap -In -Place concentration reduction rates are much slower, compared to excavation, but
are also incorrect (i.e., overestimated) because the cap -induced water -table lowering is insufficient to
dewater all of the source -area coal ash, as discussed above, and the CAP flow model overestimates cap -
induced water -table lowering due to boundary condition errors. Furthermore, these simulation times are
well beyond the prediction capabilities of any chemical transport model for a complex field site (especially
one that is as geochemically complex as the Cliffside site). The historical model -calibration dataset
(1957-2015) is also significantly smaller than the predictive (remediation) time frames. In addition, the
"history matching" technique used to calibrate the transport model (e.g., major reduction in measured Kd
values) was not performed correctly.
Cap -In -Place versus Excavation Closure Scenarios
Even though the CAP model underestimates remediation time frames, the CAP Closure Scenario
simulations demonstrate several significant advantages of excavation for restoring site groundwater
quality versus cap -in -place. First, predicted COI concentration reductions in groundwater downgradient
from the ash basin system are generally factors of 2-10 greater with excavation compared to cap -in -place
22

(e.g., refer to most of the simulated concentration versus time curves in CAP Appendix C). Further, if the
cap -in -place simulations would have been performed correctly the simulated cap -in -place concentrations
would be much higher because predicted water -table lowering due to the cap would be insufficient to
dewater all of the coal ash. Second, North Carolina 2L or IMAC standards for many COI (antimony,
arsenic, chromium, hexavalent chromium, cobalt, nickel, thallium, vanadium) are not achieved by cap -in -
place but are achieved by excavation (e.g., Appendix C Figures 13, 20, 21, 26, 27, 28, 29, 30, 31, 33, 34,
36, 37, and 39). Third, the time frames to achieve equivalent concentration reductions are at least factors
of 2 to 5 shorter for excavation compared to cap -in -place; further, several COI concentrations reduce
below 2L or IMAC standards with excavation but remain significantly higher than the groundwater
standards with cap -in -place.
The CAP Closure Scenarios do not include hydraulic containment remedial alternatives (e.g., gradient
reversal) for the bedrock aquifer that would address the risk of off -site COI transport. As discussed
above, the CSA data show many exceedances of groundwater standards in bedrock not only at the
compliance boundary but also inside the CB. In addition, strong downward groundwater flow components
from the deep overburden to bedrock aquifers were measured during the CSA at multiple locations
across the site, including the southern shoreline of the Broad River. The cap -in -place alternative does not
address either concentration reduction or off -site chemical migration control in the fractured bedrock
aquifer.
The CAP does not assess whether water quality standards will be achieved in the tributaries and
wetlands between the ash basins and the Broad River [e.g., seep locations S-3 or S-6 (Broad River
tributaries) or the wetland located along Suck Creek downgradient from the upstream dam of the active
ash basin] under any closure scenario. As discussed above, for the cap -in -place scenario a significant
fraction of the source material will remain saturated and dissolved COI will continue to migrate with
groundwater toward these seep locations. Although unaddressed by the model, COI concentration
decreases in groundwater and unsaturated zone pore water due to source removal would also reduce
impacts to tributaries and wetlands that are influenced by the ash basins.
Conclusions
Based on my technical review and analyses of the referenced information for the Cliffside site I have
reached the following conclusions:
• A total of 62 Compliance Boundary groundwater samples exceeded North Carolina groundwater
standards for these COI: antimony, boron, chromium, cobalt, iron, manganese, sulfate, total
23

dissolved solids, and vanadium. Of these 62 exceedances, 36 were greater than the proposed
provisional background concentrations by HDR;
• The statistical analyses of shallow background groundwater concentrations at the Cliffside site
(well MW-24D) are invalid. The time periods between groundwater sample collection from this
well are too small and the concentration data are not independent;
• There is a significant risk of chemical migration from the ash basin to neighboring private water
supply wells in fractured bedrock. The design of the CAP flow and transport models prevents the
potential for off -site migration from being evaluated;
• The limited CAP model domain size; the no -flow boundary conditions along the western,
southern, and eastern boundaries; and incorrect boundary condition representation of the Broad
River prevent simulation and analysis of off -site COI migration;
• The CAP Closure Scenario simulations greatly underestimate (by factors of 10 or more) the time
frames required to achieve meaningful groundwater concentration reductions in response to
remedial actions. This is due to oversimplification of field fate and transport mechanisms in the
CAP model and several model input errors;
• The simulated water table lowering for the Cap -in -Place Scenario is more than a factor of five too
small at several locations in the ash basin system in order to dewater all source material; and the
actual cap -induced water table elevation reduction would be much less than predicted due to the
incorrect no -flow boundary conditions. Therefore, the remediation time frames for this scenario
would be much greater because a large percentage of the source zone would still be active with
the cap installed;
• For either the Existing Condition or Cap -in -Place Model Scenario groundwater concentrations of
coal -ash constituents much higher than background levels will continue to exceed North Carolina
groundwater standards at the Compliance Boundary because saturated coal -ash material and
secondary sources will remain in place;
• Source -area mass removal included in the Excavation Scenario results in COI concentration
reductions at the Compliance Boundary that are generally two to ten (2 - 10x) times greater
compared to Cap -in -Place and best reduces impacts to surface water. In addition, the time
frames to achieve equivalent concentration reductions are factors of two to five (2 - 5x) shorter for
excavation compared to cap -in -place, and source removal reduces the number of COI that will
exceed North Carolina groundwater standards in the future. Additional excavation of secondary
sources would further accelerate concentration reductions; and
• The CAP simulations show that source excavation reduces groundwater concentrations for many
COI below North Carolina groundwater standards (antimony, arsenic, chromium, hexavalent
chromium, cobalt, nickel, thallium, vanadium), but cap -in -place closure does not;
24

• The CAP Closure Scenarios do not include hydraulic containment remedial alternatives for the
bedrock aquifer and do not address the risk of off -site COI transport. CSA data show multiple
exceedances of groundwater standards in bedrock not only at the compliance boundary but also
inside the CB. The cap -in -place alternative does not address either concentration reduction or
off -site chemical migration control in the fractured bedrock aquifer; and
• Future Compliance Monitoring at the site should include much more closely -spaced Compliance
Wells to provide more accurate detection, and the time intervals between sample collection
should be large enough to ensure that the groundwater sample data are statistically independent
to allow accurate interpretation of concentration trends.
My expectation was that the results of the CAP Part 2 data collection and groundwater flow and chemical
transport modeling would be available prior to submitting this report, but this information was not available
for my meaningful review. The recently -completed CAP Part 2 may address some of the data gaps and
limitations of the CSA and CAP Part 1. 1 intend to review the CAP 2 information and, if necessary,
supplement my report at that time.
25

References
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for Use in Assessment Models. Journal of Environmental Quality, Vol. 12, No. 1. 17-28.
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19, No. 6. 1558-1564.
Bear, J. 1979. Hydraulics of Groundwater. New York: McGraw-Hill.
Bouwer, H., and R.C. Rice. 1976. A Slug Test for Determining Hydraulic Conductivity of Unconfined
Aquifers with Completely or Partially Penetrating Wells. Water Resources Research. Vol. 12, No. 3.
423-428.
Culver, T.B., S.P. Hallisey, D. Sahoo, J.J. Deitsch, and J.A. Smith. 1997. Modeling the Desorption of
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28

Table 1. Exceedances of NC Groundwater Standards at Compliance or Property Boundaries and in Bedrock
for 2015 Monitoring Well and Seep Samples
Background
Constituent Sample
Sample
Location b
Concentration
Concentrations°d Standards
Name
Deptha
Measured PPBC 2L IMAC
Antimony MW-42D
Deep
CB
1.4
5 1 -
AS-2D
RV
2.5
1.4 (MW-24D), 0.52 (MW-30D)
ND (BG-2D,MW-32D,BG-1 D,MW-24DR)
AS-7BR
Bedrock
BR
1.4
0.29 (BG-1 BR)
GWA-11 BRU
BR, RV
1.3
ND (MW-32BR)
AB-3BRU
BR
1.3
Arsenic AS-2S
Shallow
RV
11.5
ND (BG-1 S) 10 10 -
0.5 (MW-30S), ND (MW-32S)
U5-2D
Deep
RV
4,680
ND (MW-24DR), 0.18 (BG-1 D)
1.8 (MW-24D), 2.9 (MW-30D)
3.4 (BG-2D), 1.1 (MW-32D)
S-10
Seep
IAB9 1-4
374
S-11
186
CLFSP051
145
CLFTD052
77
CLFTD063
Seep
IAB 5
18
S-14
Seep
Active Ash
45.4
S-15
Basin
19
S-16
17.5

Table 1. Continued
Background
Constituent
Sample
Sample
Location
Concentration'
Concentrationsc,d Standard
Name
Depth'
Measured PPBC 2L IMAC
Barium
GWA-29BR
Bedrock
BR, RV'
2,900
38.4 700 -
AS-6BR
BR
1,000
4.3 (MW-32BR), 39 (BG-1 BR)
S-16
Seep
Active Ash
1,400
Beryllium
CLFTDO05
Seep
IAB9 5
16.5
4 -
S-14
Seep
Active Ash
11.3
S-15
Basin
8.6
S-16
7
Boron
MW-11 S
Shallow
CBMe
850e
50 700 -
ND (BG-1 S,MW-30S,32S)
GWA-27D
Deep
CBMe
1,200e
ND (MW-24D,DR,-30D), 150 (BG-1D)
28 (BG-2D), 54 (MW-32D)
CLFTDO04
Seep
IAB 5
720
CLFSP061
Seep
Active Ash
1,600
CLFFWW057
Basin
830
S-6
940
Chromium
MW-42D
Deep
CB
178
10 10 -
MW-20DR
CB
32.8
0.74 (MW-24D), 2.2 (MW-30D)
GWA-29D
RV
26.1
6.2 (BG-2D), 1.9 (MW-32D)
AS-2D
RV
39.1
0.55 (BG-1 D), 0.18 (MW-24DR)

Table 1. Continued
Constituent
Sample
Name
Sample
Depth'
Location
Concentration'
Background
Concentrationsc,d Standard
Measured PPBC 2L IMAC
Chromium
AS-7BR
BR
95.8
10 10 -
(continued)
AB-3BRU
BR
18.7
GWA-22BRU Bedrock
CB
10.2
4.8 (BG-1 BR)
GWA-32BR
BR
27.6
0.8 (MW-32BR)
CLFTDO05
Seep
IAB9 5
20.1
S-18
14.7
S-14
Seep
Active Ash
566
S-15
Basin
373
S-16
248
Cobalt
MW-34S
Shallow
CB
25.6
10.7 - 1
MW-38S
CB
5.1
MW-40S
CB
2.5
5.4 (BG-1 S)
MW-42S
CB
5.2
2 (MW-30S), 1.2 (MW-32S)
GWA-33S
CB
33
GWA-22S
CB
2.4
GWA-24S
CB
16.3
GWA-21 S
RV
204
GWA-11 S
RV
5.7
MW-42D
Deep
CB
2.1
ND (MW-24DR), 6.1 (BG-1 D)
MW-38D
CB
4.3
0.54 (MW-24D), 0.16 (MW-30D)
MW-25DR
CB
1.8
0.37 (BG-2D), 0.36 (MW-32D)
GWA-23D
CB
4.4
GWA-24D
CB
1.3
MW-22DR
CB
1.7

Table 1. Continued
Constituent
Sample
Name
Sample
Depth'
Location
Concentration'
Background
Concentrationsc,d Standard
Measured PPBC 2L IMAC
AB-5BRU
Bedrock
BR
10.6
10.7 - 1
Cobalt
GWA-12BRU
BR
2.3
(continued)
AS-7BR
BR
2.4
ND (BG-1 BR)
GWA-28BR
BR
1.1
ND (MW-32BR)
GWA-28BRU
BR
6.8
GWA-21 BRU
BR
1.5
S-10
Seep
IAB9 1-4
2
S-11
8.8
S-3
77.8
CLFSP051
4.4
CLFTD052
5
CLFTDO04
Seep
IAB 5
16.6
CLFTDO05
43.6
S-18
126
S-19
29.4
S-20
11.4
S-14
Seep
Active Ash
300
S-15
Basin
76
S-16
95.7
CLFSP061
115
S-9
Seep
Ash Storage
2.1
Iron
MW-36S
Shallow
CB
890
58 (BG-1 S) 3,200 300 -
MW-38S
CB
1,300
265 (MW-30S), 30 (MW-32S)

Table 1. Continued
Constituent Sample
Name
Sample
Depth'
Location
Concentration'
Background
Concentrationsc,d Standard
Measured PPBC 2L IMAC
MW-40S
CB
1,800
3,200 300 -
Iron MW-42S
CB
15,800
(continued) GWA-33S
CB
7,280
GWA-22S
CB
3,710
GWA-24S
CB
1,000
MW-23D
Deep
CB
900
1,100 (MW-24DR), 44 (BG-1 D)
MW-20D
CB
3,000
3,200 (MW-24D), 367 (MW-30D)
MW-22DR
CB
1,500
910 (BG-2D), 700 (MW-32D)
MW-40BRU
Bedrock
BR
3,200
5 (BG-1 BR)
GWA-22BRU
BR
1,170
3,700 (MW-32BR)
MW-21 BR
BR
930
GWA-33BR
BR
1,040
GWA-5BRU
BR
1,200
AB-6BR
BR
3,600
AB-4BR
BR
14,300
GWA-13BR
BR
3,300
GWA-12BRU
BR
610
GWA-11 BRU
BR
720
IB-2BRU
BR
1,680
IB-4BR
BR
2,860
AS-7BR
BR
2,100
S-10
Seep
IAB9 1-4
5,900
S-11
5,200
S-3
3,500
CLFSP051
3,600

Table 1. Continued
Constituent
Sample
Name
Sample
Depth'
Location
Concentration'
Background
Concentrationsc,d
Measured PPBC
Standard
2L IMAC
CLFTD052
1,800
300 -
Iron
(continued)
CLFTDO04
Seep
IAB9 5
6,800
S-2
1,090
S-18
195,000
S-19
32,800
S-20
13,200
S-14
Seep
Active Ash
501,000
S-15
Basin
353,000
S-16
1,070,000
S-6
320
CLFSP061
9,000
CLFSP058
520
S-9
Seep
Ash Storage
640
S-4
Area
1,600
S-12
7,000
CLFSP059
1,600
Lead
GWA-29D
Deep
RV'
30.3
10
15 -
0.52 (BG-2D), 0.43
(MW-32D),
ND (MW-24DR,BG-1
D), 0.34 (MW-24D)
S-14
Seep
Active Ash
457
S-15
Basin
176
S-16
214

Table 1. Continued
Constituent Sample
Name
Sample
Deptha
Location b
Concentration'
Background
Concentrations°d Standard'
Measured PPBC 2L IMAC
Manganese MW-36S
Shallow
CB
1,100
61 50 -
MW-38S
CB
580
MW-40S
CB
130
87 (BG-1 S)
MW-42S
CB
260
34 (MW-30S), 40 (MW-32S)
GWA-33S
CB
740
GWA-22S
CB
64
GWA-24S
CB
1,200
MW-34S
CB
710
MW-23D
Deep
CB
680
56 (MW-24DR), 260 (BG-1 D)
MW-20D
CB
390
44 (MW-24D), 6.3 (MW-30D)
MW-22DR
CB
140
29 (BG-2D), 12 (MW-32D)
MW-38D
CB
2,300
GWA-33BR
Bedrock
CB
230
6.7 (BG-1 BR)
MW-40BRU
CB
110
29 (MW-32BR)
GWA-30BR
BR
83
GWA-5BRU
BR
150
AB-6BR
BR
79
AB-5BRU
BR
700
AB-4BR
BR
220
GWA-21 BR
BR
380
GWA-21 BRU
BR
120
GWA-29BR
BR
110
GWA-13BR
BR
140
GWA-12BRU
BR
58

Table 1. Continued
Background
Constituent Sample Sample Location Concentration' Concentrations,d Standard'
Name Depth' Measured PPBC 2L IMAC
Manganese
(continued)
GWA-11 BRU
IB-2BRU
IB-4BR
AS-7BR
S-10
Seep
S-11
S-3
CLFSP051
CLFTD052
CLFTDO04
Seep
CLFTDO05
CLFTD063
S-2
S-18
S-19
S-20
S-14
Seep
S-15
S-6
S-16
CLFSP061
S-9
Seep
S-4
S-12
CLFSP059
BR
670
BR
83
BR
170
BR
92
IAB9 1-4
1,500
1,700
4,000
1,500
1,300
IAB 5
4,500
1,400
110
1,900
3,100
4,300
4,000
Active Ash
1,200
Basin
3,700
2,000
27,600
7,200
Ash Storage
160
Area
230
490
100
61 50 -

Table 1. Continued
Background
Constituent Sample Sample Location Concentration' Concentrations,d Standard'
Name Depth' Measured PPBC 2L IMAC
Nickel
Sulfate
S-14 Seep
S-15
S-16
GWA-11 S Shallow
MW-23D Deep
MW-38D
AS-7BR Bedrock
S-3
Seep
S-11
CLFSP051
CLFTD052
CLFTDO04
Seep
S-18
CLFSP061
Seep
Active Ash 631
Basin 126
118
RV' 345,000
CB 367,000
CB 284,000
IAB9 1-4 336,000
275,000
270,000
295,000
IAB 5 297,000
353,000
Active Ash 472,000
Total GWA-11 S Shallow RV 711,000
Dissolved
Solids
MW-23D Deep
MW-38D
CB 757,000
CB 543,000
10 100 -
10,000 250,000
ND (MW-30S, -32S, BG-1 S)
9,900 (MW-24DR), 5,900 (BG-1 D)
520 (MW-24D), 20,500 (MW-30D)
46,400 (BG-2D), 20,500 (MW-32D)
35,000 (BG-1 BR), 15,400 (MW-32BR)
48,000 (BG-1 S) 120,000 500,000 -
ND (MW-30S), 46,000 (MW-32S)
115,000 (MW-24DR), 53,000 (BG-1 D)
47,000 (MW-24D), 257,000 (MW-30D)

Table 1. Continued
Constituent Sample
Name
Sample
Depth'
Location
Concentration'
Background
Concentrationsc,d
Measured PPBC
Standard
2L IMAC
MW-42D
CB
710,000
229,000 (BG-2D), 132,000
(MW-32D)
TDS (cont.)
AS-7BR
Bedrock
BR
2,510,000
174,000 (BG-1 BR)
AS-3BRU
BR
503,000
100,000 (MW-32BR)
AS-6BR
BR
2,040,000
S-3
Seep
IAB9 1-4
571,000
500,000 -
S-10
539,000
S-11
626,000
CLFSP051
624,000
CLFTD052
633,000
CLFTD063
Seep
IAB 5
503,000
S-18
600,000
CLFSP061
Seep
Active Ash
755,000
Thallium S-3
Seep
IAB 1-4
0.56
- 0.2
CLFTDO05
Seep
IAB 5
1.2
S-18
1.1
S-19
0.38
S-14
Seep
Active Ash
7.2
S-15
Basin
2.9
S-16
4

Table 1. Continued
Constituent Sample
Name
Sample
Deptha
Location b
Concentration'
Background
Concentrations°d Standard'
Measured PPBC 2L IMAC
Vanadium GWA-22S
Shallow
CB
45.3
5 - 0.3
MW-38S
CB
3
MW-42S
CB
5.1
ND (BG-1 S)
GWA-33S
CB
0.88
0.44 (MW-30S), ND (MW-32S)
GWA-24S
CB
1.4
MW-42D
Deep
CB
11.9
ND (MW-24DR), 0.35 (BG-1 D)
MW-38D
CB
0.72
ND (MW-24D), 12.3 (MW-30D)
GWA-33D
CB
1.2
17.1 (BG-2D), 2.1 (MW-32D)
MW-40BRU
Bedrock
CB
7.5
8 (BG-1 BR)
GWA-22BRU
CB
26.3
ND (MW-32BR)
MW-36BRU
CB
1.4
MW-22BR
CB
5.9
MW-34BRU
CB
0.5
MW-21 BR
CB
2
AB-5BRU
BR
1.3
GWA-29BR
BR
0.89
GWA-1 BRU
BR
1.6
GWA-30BRU
BR
5.1
AB-3BRU
BR
8.8
AS-7BR
BR
12
AS-5BR
BR
0.91
GWA-21 BR
BR
0.94
IB-2BRU
BR
2.2

Table 1. Continued
Background
Constituent Sample Sample Location Concentration' Concentrations,d Standard'
Name Depth' Measured PPBC 2L IMAC
Vanadium S-10 Seep
IABg 1-4
1.1 - 0.3
(cont.) CLFSP051
1.3
CLFTD052
2.2
CLFTDO05 Seep
IABg 5
9.3
S-18
8.4
S-19
26
S-20
6.3
S-14 Seep
Active Ash
546
S-15
Basin
347
S-16
290
S-6
0.36
CLFSP061
2
CLFFWW057
3.6
S-4 Seep
Ash Storage
0.81
S-12
Area
0.37
S-5
0.39
' Refer to the CSA Report for Monitoring Well locations and screen intervals
b CB = Monitoring Well located on the Ash Basin Compliance Boundary; PB = Monitoring Well located on the Duke
Energy Property Boundary; BR = Monitoring Well screened in Bedrock
' All values are total measured concentration in the water samples in units of micrograms per liter

d Refer to the CSA Report for information regarding placement of Background Wells.
PPBC = Proposed Provisional Background Concentration (CAP Report Table 2-3)
e CBM = Projected downgradient concentration at Compliance Boundary based on calibrated one-dimensional, analytical
chemical transport model discussed in this Report
f RV = Monitoring Well is located at the downgradient boundaries of the CAP Groundwater Flow and Chemical Transport
Models which coincide with the southern boundary of the Broad River
5 IAB = Inactive Ash Basin