NC0003468_11. DRSS CAP Part 2_Appx H_FINAL_20160210

Appendix H
Monitored Natural
Attenuation Technical
Memorandum

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Technical Memorandum
To Mark Filardi / HDR Engineering, Inc.

From Gregory P. Miller
Date October 13, 2015

Subject Dan River Steam Station
Corrective Action Plan
Monitored Natural Attenuation – Tier I and II

This technical memorandum presents the Tier I and Tier II evaluation of the attenuation of
compounds dissolved in groundwater at the Dan River Steam Station (DRSS), Eden County,
North Carolina. The memorandum consists of three sections: Background on Monitored Natural
Attenuation (MNA) as a remedial alternative for coal combustion products; the Findings of the
Tier I MNA Demonstration activity, followed by the Findings of the Tier II Demonstration.
Investigative and data evaluation efforts are in progress as of the date of this memorandum.
The findings herein represent the understanding as of this date and are subject to revision as
new information becomes available.
BACKGROUND
Introduction to Monitored Natural Attenuation for Groundwater
MNA is a strategy and a set of procedures used to demonstrate that physiochemical and/or
biological processes in an aquifer will reduce concentrations of undesirable substances to levels
below regulatory concern. It has been broadly applied to releases of petroleum hydrocarbons in
many hydrogeological environments. There has been less application of MNA to the
remediation of inorganic or radiologic substances in groundwater than to organic compounds.
While MNA of organic contaminants is a readily accepted remedy at the State and Federal level,
inorganic attenuation is more complicated and there is limited implementation experience in
industry, science, and government.

Metals, radionuclides, and/or other inorganic compounds are found in all aquifers. The
mechanisms that regulate their release from solids and movement through aquifers are for the
most part the same processes that control movement of inorganics in aquifers impacted by Coal
Combustion Products (CCP) Leachate. These processes attenuate the concentration of
inorganics in groundwater by depositing inorganics on aquifer solids. Unlike organic
compounds that break down from hydrolysis or bacterial action, a reduction in concentration of
dissolved inorganic compounds requires water to be removed from the aquifer, or the
compounds immobilized by conversion to, or adsorption onto, solids. When demonstrated that
the mechanism and permanence of natural processes will result in attenuation of undesirable
compounds to acceptable levels, then remediation can be conducted by verifying that the
remediation proceeds as predicted. MNA relies on natural processes to remove contaminants
of concern from groundwater resulting in lower cost, and less disturbance of ground and
infrastructure.

Active remediation of inorganic compounds in the subsurface may rely on processes that are
similar to or the same as natural attenuation mechanisms. Active remediation often seeks to
improve or hasten these natural processes through chemical or biological augmentation of the
aquifer. There are no clear distinctions between the late phases of conventional groundwater
Geochemical, LLC
PO Box 1468
Socorro, NM 87801 USA
[phone]
www.geochemical.com

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 2 of 25
remediation and MNA, but such distinctions may be unnecessary. Relying on limited or low-
energy active remediation as a preliminary step, then achieving final remedial goals through
MNA is sometimes called “enhanced MNA”, or “enhanced attenuation.” Active remediation
methods for petroleum products often leave residual dissolved contamination that is degraded
slowly, requiring monitoring under regulations allowing MNA. Enhanced MNA is a common
strategy in hydrocarbon remediation and the principle is adaptable to inorganic MNA.
Authorizing Statutes
MNA is a federally recognized remedial technology that can meet Resource Conservation and
Recovery Act (RCRA) Corrective Action requirements and could be included when remedial
alternatives for legacy CCP management sites are evaluated. Implementation of MNA for
inorganics at RCRA facilities is clearly and favorably proposed by the United States
Environmental Protection Agency (EPA) in the 1999 Office of Solid Waste and Emergency
Response (OSWER) Directive 9200.4-7P. In that directive MNA is defined as:

“the reliance on natural attenuation processes (within the context of a carefully
controlled and monitored site cleanup approach) to achieve site-specific remediation
objectives within a time frame that is reasonable compared to that offered by other
more active methods. The ‘natural attenuation processes’ that are at work in such a
remediation approach include a variety of physical, chemical, or biological processes
that, under favorable conditions, act without human intervention to reduce the mass,
toxicity, mobility, volume, or concentration of contaminants in soil or groundwater.
These in-situ processes include biodegradation; dispersion; dilution; sorption;
volatilization; radioactive decay; and chemical or biological stabilization,
transformation, or destruction of contaminants.”
Site characterization and assessment of MNA is different for organic and inorganic strategies.
The focus in organic MNA is on a working understanding of the transformation rates and
daughter products when organic chemicals break down in aquifers. Complex organic
compounds are reduced to water, carbon dioxide, methane, and salts by biotic and abiotic
processes in organic MNA. The science and engineering looks to the dissolved phase
concentrations derived from groundwater samples. From those analytical results and aquifer
dimensions and flow rates, the mass and flux of organic contaminants is evaluated over time.
Meeting a single objective, mass reduction of organic compounds, is all that is needed to
validate the organic MNA remedial approach.

For inorganic contaminants, the scientific and engineering investigations must consider both the
dissolved and solid phase. This is because inorganic compounds are not destroyed (with the
exception of some radioactive decay chains), but become comingled with the aquifer solids.
Inorganic compounds that are removed from the dissolved phase are transferred to the aquifer
solids in inorganic MNA processes. The undesirable inorganic compound is not destroyed; its
location is changed, from dissolved and mobile to immobile via incorporation into/onto a solid.
Because inorganics are left in place, there is stakeholder concern about the stability and
longevity of the remedial action. It is partly this uncertainty regarding inorganic MNA that drives
the need for more in-depth study. While much of the chemistry is abiotic, biotic reactions can be
very important in some inorganic MNA pathways. Inorganic MNA has two focus points:

1. Demonstrate that the attenuation process is reducing contaminant mass in groundwater;
and,
2. Demonstrate that the remediation will meet long-term stability criteria without
intervention.

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 3 of 25
Principles of Inorganic Attenuation
The 1999 OSWER Directive provides limited policy and implementation guidance for inorganic
MNA. EPA indicates that: the site-specific mechanisms of attenuation of inorganic
contaminants should be known; and, the stability of the remedial action should be evaluated
under potential changes in conditions. The OSWER Directive is fairly specific about the classes
of attenuation processes that may be proven effective, stating:

“MNA may, under certain conditions (e.g., through sorption or oxidation-reduction
reactions), effectively reduce the dissolved concentrations and/or toxic forms of
inorganic contaminants in groundwater and soil. Both metals and non-metals
(including radionuclides) may be attenuated by sorption reactions such as
precipitation, adsorption on the surfaces of soil minerals, absorption into the
matrix of soil minerals, or partitioning into organic matter.”
“Oxidation-reduction (redox) reactions can transform the valence states of some
inorganic contaminants to less soluble and thus less mobile forms (e.g.,
hexavalent uranium to tetravalent uranium) and/or to less toxic forms (e.g.,
hexavalent chromium to trivalent chromium). Sorption and redox reactions are
the dominant mechanisms responsible for the reduction of mobility, toxicity, or
bioavailability of inorganic contaminants.”
“It is necessary to know what specific mechanism (type of sorption or redox
reaction) is responsible for the attenuation of inorganics so that the stability of the
mechanism can be evaluated. For example, precipitation reactions and
absorption into a soil’s solid structure (e.g., cesium into specific clay minerals)
are generally stable, whereas surface adsorption (e.g., uranium on iron-oxide
minerals) and organic partitioning (complexation reactions) are more reversible.”
“Complexation of metals or radionuclides with carrier (chelating) agents (e.g.,
trivalent chromium with EDTA) may increase their concentrations in water and
thus enhance their mobility. Changes in a contaminant’s concentration, pH, redox
potential, and chemical speciation may reduce a contaminant’s stability at a site
and release it into the environment. Determining the existence, and
demonstrating the irreversibility, of these mechanisms is important to show that a
MNA remedy is sufficiently protective.”
“Inorganic contaminants persist in the subsurface because, except for radioactive
decay, they are not degraded by the other natural attenuation processes. Often,
however, they may exist in forms that have low mobility, toxicity, or bioavailability
such that they pose a relatively low level of risk. Therefore, natural attenuation of
inorganic contaminants is most applicable to sites where immobilization or
radioactive decay is demonstrated to be in effect and the process/mechanism is
irreversible.”
There have been numerous advances in the understanding of the environmental geochemistry
of inorganic compounds and metals since publication of the Directive in 1999. The inorganic
MNA topics discussed in the Directive are still valid, and form a first step to evaluate inorganic
MNA. Over the period of 2006−2010 EPA has released much more detailed guidance on
implementation of inorganic MNA and the Interstate Technical and Regulatory Council (ITRC)
has published on practical and regulatory aspects of implementing MNA inorganic at the state
regulatory level. These guidance documents are summarized in a following section.

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 4 of 25
Governing Equations
The concentration of a dissolved compound in flowing groundwater changes over time due to
movement, mixing, and chemical reactions. The concentration of COIs in DRSS groundwater is
governed by these same processes. At DRSS, groundwater movement and mixing probably
changes COI concentrations as much or more than chemical reactions do. Following source
control, COI concentrations in DRSS groundwater will decrease due to the combination of these
processes, resulting in COI concentration attenuation.

The change in COI concentration with respect to time and place can be described by the
advection-dispersion equation (ADE). The ADE is used to determine the concentration of COI
in groundwater during remedial scenarios − including MNA. In differential form, including
reversible and irreversible sorption to solids, the ADE is:
Ct
S
x
Cvx
CDt
C b µδ
δ
θ
ρ −−∂
∂−∂
∂=∂
∂
2
2

where:

C = concentration of solute in water
t = time
x = distance from source
D = Dispersivity
v = average groundwater velocity
ρb = solid bulk density
ϴ = solid porosity
S = concentration of solute on solid
µ = rate of solute irreversible sorption

The first and second terms in the ADE describe the change in COI concentration due to mixing
and diffusion in granular aquifers (D − Dispersivity), and the change in concentration from
groundwater flow moving the solute away from the point of interest (v − average groundwater
velocity), respectively.

The third term in the ADE, related to concentration of solutes on solids and reversible sorption
processes, has alternate formulations that are very useful in solute fate and transport modeling.
The relationship between the concentrations of reversibly adsorbed solutes on solids and the
dissolved concentration in groundwater can be stated as a proportionality – Kd, where:

𝐾𝐾𝑑𝑑=𝑆𝑆𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶
where:
SB is for the concentration of COI on solids, and
CB is for the concentration of COI in the water, when COI chemistry is at equilibrium.

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 5 of 25
Kd is a proportionality of concentration between the solids and the liquids in an aquifer. It
commonly has units of volume over mass. In order to use Kd in the ADE formulation it is often
expressed as Retardation – R, where:
𝑅𝑅=1 +𝜌𝜌𝜃𝜃𝐾𝐾𝑑𝑑
In the Retardation equation, we see the reappearance of bulk density and porosity from the
ADE with the surface concentration term contained in the proportionality Kd. The effect of R and
D on solute transport time and concentration can be seen from the figure below for model
transport of retarded and non-retarded solutes with equal Dispersivity. Note that in the retarded
case ‘a’ that velocity is divided by Retardation; an R value of 1 indicates no retardation.
Retardation and Dispersivity are conservative with respect to solute mass.
If COI has a high Kd, it will have a high R. A high R puts higher contaminant mass on the solids
in proportion to the amount dissolved in water. When the water concentrations decrease to
background over time, the solids will release COIs until that mass was exhausted to background
concentrations. If R is low, that return to background concentrations on solids will happen
relatively quickly. The final term in the ADE is related to irreversible sorption processes.

To describe the attenuation of COI at DRSS, the inputs to the ADE are satisfied as follows:

ADE
Variable Data Source
C Concentration measured from groundwater samples.
t Time is determined by the nature of the problem to be investigated, e.g., the time
for COI in groundwater to fall below the 2L Standard.
x Distance is determined by the nature of the problem to be investigated.
D Dispersivity is estimated from published values or determined from solute
concentrations over time using numerical models.
v Average groundwater velocity varies in space and time and is supplied by a
calibrated groundwater flow model.
ρb Bulk density measured from samples.
ϴ Porosity measured from samples.
S Concentration measured from solid samples. Calculated Kd values from laboratory
batch and column testing.
µ Irreversible component of S. Not determined at this time.
0
1
C/
C
o
Increasing Distance →
D and R
D Only
a b
tvxb=

+
=
d
a
K
tvx
θ
ρ1

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 6 of 25
Kd Calculations and Interpretation
There are many different mathematical models to describe sorption, Kd, Langmuir Isotherms,
and Freundlich Isotherms are common methods (USEPA, 1999). All of the models have
advantages and limitations. If you know the solution concentration, and can model or measure
the solid concentration, you can calculate Kd. If confidence in the model is high, the solution
concentration can be assumed and surface concentrations of COI calculated directly (Goldberg
et al., 2000; Dzomback and Morel, 1990).

One limitation to the Kd approach is that it does not describe the effect of variable pH conditions.
The question of pH is important on many levels. We know that COI will take on an electrical
charge based on pH, that the surfaces of minerals that adsorb COI are variably charged, again
dependent on pH, and that the propensity of COI to approach a surface depends on all of these
charges. To handle these complexities, surface complexation theory was developed (Dzomback
and Morel, 1990).

Surface complexation theory states that we can describe these sorption reactions the same way
as we describe chemical reactions, as mass-action equations that have equilibrium constants.
K’s are numerical constants for chemical mass-action equations. They express the equilibrium
relationships between chemicals. Setting an equilibrium chemical reaction with A and B as
reactants, C and D as products, where the variable is concentration:
DCBA+=+,
the mass-action equation takes the form:
BA
DCK=.
K is constant at constant temperature and pressure. Three constants are needed to model the
surface complexation approach, one for the COI adsorption to the surface and two to describe
the charging of the surface. The surface charging constants K- and K+ are determined for pH-
dependent protonation-deprotonation reactions at the oxide surface. The protonation reactions
with the surface, SOH, are described by the two-step reversible process below.

The mass-action relationship for COI (KCOI) can be expressed in a generic form as:

+−+⇔+HCOISHCOIHSOHyxyx
[][]
[][]()
()RT
F
COIHSOH
HCOISH
COI yx
yxK 0expψ−+−=,

where square brackets indicate concentrations (mol/L), F is the Faraday constant (C/mol), Ψ is
the surface potential (V), R is the molar gas constant (J/mol K), T is the absolute temperature
(K), and the exponential terms represent solid-phase activity coefficients that correct for charges
[]
[][]
()
()RT
F
HSOH
SOH
sK 02exp
ψ
+
+=+
[][]
[]
()
()RT
F
SOH
HSO
sK 0exp
ψ−
−
+−=
++⇔+2SOHHSOH SOHHSO⇔++−

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 7 of 25
on the surface complexes. Each KCOI mass-action expression must be constructed from a
balanced equation using the generic form above as an example. Mass-action constants can be
determined from adsorption experiments (Kd determinations) using the code FITEQL (Herbelin
and Westall, 1999) and several surface complexation sub-models are available in PHREEQC.
Irreversible Sorption of COIs
Irreversible sorption of COIs has been observed in/on a range of geomedia for over 100 years.
Here the irreversible sorption for COIs is defined as: COIs that are retained in the structure of
minerals and mineral-like inorganic and organic compounds on a time scale significantly longer
than reversible surface adsorption reactions. In most cases, reversible surface sorption
reactions are instantaneous as compared to groundwater flow rates. Irreversible sorption
processes for COIs are common in the natural environment. COIs will be taken up to form part
of the mineral lattice of clay minerals, hydrous metal oxides, carbonates, and composted
organic material, as examples. Clay minerals and hydrous metal oxides are common at the
site. Release of COI from these materials is slow.

The chemical equilibrium between solutions and solids is a balance between the rate of forward
reactions and the rate of back reactions. Surface complexation adsorption processes are
reversible equilibrium reactions. Irreversible sorption is a process where the forward reaction is
so much faster than the back reaction that the reverse reaction is insignificant on the time scale
of interest. The remobilization mechanism is too slow to be significant. Irreversible sorption is a
COI sink; COI is removed from the flow system. First-order decay is one way to represent
irreversible sorption in a numerical model. First-order decay rates are also used to calculate the
half-life of a compound.
Inverse Modeling of Hydrologic Parameters
Determination of most of the physical properties that effect solute transport in granular aquifers
can be accomplished using laboratory or field tracer tests. Dispersivity, retardation, and
effective porosity can be quantified this way. These are important parameters for modeling of
solute transport in groundwater. Laboratory and field tracer tests are controlled experiments
often involving forward modeling of expected conditions to constrain the experimental design.
Determining the appropriate concentration of tracer to be used and how long to apply it to have
a successful experiment (source term) would be an example of forward modeling. Inverse
modeling is fitting of aquifer parameters to observed concentration/distance/time data.
Laboratory and field breakthrough curves (BTCs) generally trace smooth curves due to frequent
sampling and the ability to rerun experiments.

Data from groundwater monitoring programs can be used for determination of aquifer
parameters in the same manner as laboratory tests, if the data provide a fairly complete record
of changes in concentration of a dissolved solute. Ideally, monitoring would be frequent and the
rising limb and falling tail of the solute BTC would be captured with accuracy and precision.
Forward modeling of field conditions is possible if the source term is known or assumed.

BTC data from groundwater monitoring generally has an imperfect understanding of the
concentration (C) and duration (t) of the contaminant source over time – the source term. The
distance between the source (z) and the monitored well may also be hard to quantify.

A public-domain program for numerical evaluation of BTCs is available. CXTFIT, Version 2.1
(Toride et al., 1999), is useful for determining values for average linear velocity (v), dispersivity
(D), retardation (R), and decay (µ), which is used to account for irreversible adsorption.

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 8 of 25
Independent data for v are available from the calibrated flow model, tests of hydraulic
conductivity, and measurement of gradients. Similarly, R has been determined independently
from laboratory measurements of bulk density (ρb), effective porosity (n), and partition
coefficients (Kd). Laboratory or field determined values for dispersivity (D) or decay (µ) are not
available at this time.
Coal Combustion Product Leachate Characteristics
Inorganic MNA relies on being able to observe, quantify, and in some cases modify subsurface
chemistry. There are major differences in the chemical characteristics of leachate across the
major groups of CCP (fly ash, bottom ash, pollution control residue), and complexities
introduced by co-disposal of different CCPs. The composition of leachate from CCP is highly
variable. Controls on leachate chemistry are numerous and are strongly influenced by site-
specific factors. Leachate chemistry is influenced to variable and unequal degrees by:

• The type of coal;
• The basin, sub basin, mine, or seam it is produced from;
• Variability of coal composition within the seam;
• Coal cleaning and pretreatment processes;
• Combustion conditions;
• Pollution control operations;
• CCP handling;
• CCP management strategies – impoundments, landfills or combined methods; and,
• Climate and environmental factors – such as hydrology, geology, and weather.

The preceding list of factors, that ultimately control the success of inorganic MNA, is imperfect
and not meant to be exhaustive. There is a large knowledge base on the geochemistry of CCP
and CCP leachate. That information is very helpful to constrain remedial strategies for legacy
CCP sites, but does not replace the detailed site-specific assessment of ash and aquifer
geochemistry required for inorganic MNA.

The Electric Power Research Institute (EPRI) has conducted extensive research on the
occurrence, concentration, and mobility of CCP leachate components. The selection criteria
EPRI used to identify constituents of concern (COCs) included prevalence, mobility, and risk as
determined by comparison to water quality standards. EPRI’s assessment identifies five
leachate constituents that are probably present in pore water at all legacy CCP management
sites; arsenic, boron, chromium, selenium, and sulfate. Each of these substances has triggered
at least one remedial action at a legacy CCP site. While these constituents are the primary
focus of concern due to their prevalence and mobility, several other inorganic constituents may
be of concern at legacy CCP management sites. From EPRI’s 2006 effort, the following
constituents are noted:

“The leachate data indicate that concentrations of antimony, arsenic, cadmium,
chromium, selenium, and thallium were higher than health-based MCLs in at least 10
percent of the samples. In addition, the 90th percentile concentrations of boron,
lithium, manganese, molybdenum, sodium, sulfate, and vanadium were higher than
alternative drinking water criteria. These constituents are more likely to trigger a
remedial action in the event of a leachate release than constituents that typically
have leachate concentrations lower than drinking water standards.”

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 9 of 25
The EPRI-listed constituents above are primary candidates for MNA because of their
occurrence, mobility, and propensity to exceed drinking water standards at legacy CCP sites.
Not all of these constituents are amenable to remediation through MNA processes because of
their chemical properties. Table 1 compares the leachate chemistry as identified by the
previously listed EPA and EPRI sources, with the compounds suitable for inorganic MNA as
found in the EPA, EPRI, and ITRC guidance documents.

The EPA sources cited identify the following thirteen COCs based on the potential for human
health and/or ecological impacts using a screening risk assessment: aluminum, arsenic,
antimony, barium, boron, cadmium, cobalt, chromium, lead, mercury, molybdenum, selenium,
and thallium. One of the findings of the 2009 EPA report was nine of the thirteen COCs listed
above were found in CCP leachate at levels of concern for groundwater protection, as indicated
in Table 1.
Published Guidance on Inorganic Monitored Natural Attenuation
United States Environmental Protection Agency Guidance
The EPA is the primary source of guidance on MNA. The EPA has published their inorganic
MNA guidance in three volumes. Volume 1 is a procedural and technical guide for
implementing MNA; Volume 2 is a detailed exploration of issues associated with inorganic MNA
for nine compounds; and Volume 3 contains discussion of MNA issues unique to radionuclides
and a detailed exploration of inorganic MNA for ten radionuclides. Volumes 1 and 2 were
published in 2006 and Volume 3 in 2010. The EPA guidance documents cover inorganic MNA
at a high level of scientific detail. The EPA guidance, while not specific to CCP, is a technical
resource on evaluation of the feasibility of inorganic MNA for compounds as listed in Table 1.

Volume 1 introduces EPA’s “Tiered Analysis Approach to Site Characterization”; the tiers being
four demonstrations or findings to be accomplished when implementing inorganic MNA.
Volume 1 provides specific technical guidance on implementing inorganic MNA using the four-
tiered approach. Volume 2 expands on the technical guidance to include specific strategies and
chemical mechanisms for arsenic, cadmium, chromium, copper, lead, nickel, nitrate,
perchlorate, and selenium inorganic MNA. Volume 3 presents the evaluation of MNA for
radionuclides.

Table 2 presents a simplification of the four-tiered approach found in the EPA guidance. CCP
inorganic MNA is not considered by EPA to be a viable remedial alternative for groundwater
unless the leachate additions to groundwater have been controlled. Developing a site-specific
understanding of the mechanism by which inorganic MNA may attenuate (Tier II) and the
capacity of the aquifer to do so (Tier III) is a complex and iterative task. Volume 1 provides the
technical basis for inorganic MNA evaluations to accomplish Tier II and III goals. Monitoring of
inorganic MNA is necessarily more detailed than for many active remediation strategies and is
discussed in Volume 1.
Interstate Technology Research Council Guidance
In late 2010, ITRC published a decision framework for inorganic MNA. The document contains
a summary of the three-volume EPA guidance, expanded to decision framework for determining
MNA feasibility. Case studies are included and compared to the EPA four tiered approach. The
MNA implementation at the state level was surveyed and reported. The ITRC document
summarizes the topic of inorganic MNA, presents an introduction to the science, and presents a
strategy for training and implementation of inorganic MNA under state laws and regulations.

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 10 of 25
This guidance is not specific to CCP. As with the EPA guidance, it is a technical resource on
evaluation of the feasibility of inorganic MNA for compounds in Table 1.

Table 1. Compounds in CCP leachate as compared to MNA guidance.
EPA CCP EPRI CCP EPA MNA ITRC MNA EPRI MNA
Aluminum (Al) X XX
Antimony (Sb) XX XX
Arsenic (As) XXX XXX X X X
Barium (Ba) XXX X
Beryllium (Be) X X
Boron (B) XX XXX X
Cadmium (Cd) XX XX X X
Calcium (Ca) X X
Chromium (Cr) XXX XXX X X X
Cobalt (Co) X X X
Copper (Cu) X X X
Fluorine (F) X X
Iron (Fe) X X
Lithium (Li) XX
Lead (Pb) X X X X
Magnesium (Mg) X
Manganese (Mn) X XX
Mercury (Hg) X X
Molybdenum (Mo) XX XX
Nickel (Ni) X X X X
Potassium X X
Selenium (Se) XXX XXX X
Silicon (Si) X
Silver (Ag) X X
Sodium (Na) X X
Strontium (Sr) X X
Thallium (Th) XX XX
Vanadium (V) X XX
Zinc (Zn) X X X
pH X X
Sulfate (SO4) X XXX X
Dissolved Solids X XX
Radionuclides X X
EPA CCP: X=mentioned, XX= of concern, XXX=exceeds toxicity criteria
EPRI CCP: X= evaluated, XX= of concern, XXX=of concern and common

Table 2. Summary of USEPA four-tiered feasibility evaluation for inorganic MNA.
Tier I Source Control Is the contaminant mass in the plume decreasing?
Tier II Attenuation Mechanism Is the chemical mechanism well understood?
Tier III Attenuation Capacity Is the capacity and permanence of the mechanism
sufficient?
Tier IV Monitoring & Contingency How will monitoring be conducted? What actions will be
taken if monitoring indicates attenuation is lacking?

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 11 of 25
TIER I DEMONSTRATION FOR THE DAN RIVER STEAM STATION
MNA is a candidate remedial technology for Constituents of Interest (COIs) exceeding 2L
Standards in groundwater at the DRSS. Attenuation processes reduce the concentration or
toxicity of compounds in groundwater primarily by chemical interaction with aquifer solids.
When attenuation processes result in a reduction of contaminant concentrations in a
timeframe similar to other remedial technologies, attenuation can be an acceptable remedial
alternative (USEPA, 2007). This section documents that the first milestone in the Four-tiered
MNA process has been achieved; attenuation of certain COIs has been observed. This
successful Tier I demonstration is based on the methods contained in EPA guidance (USEPA,
2007). The MNA demonstration for certain COIs will now be advanced to Tier II/III.

A work plan for MNA evaluation was submitted, reviewed, revised and approved by the North
Carolina Department of Environment and Environment Resources (DENR). All proposed
activities were accomplished with minor deviations from the plan. Data were collected on the
distribution of COIs in groundwater, porewater, and soil in three dimensions to evaluate if
attenuation was occurring. A strong, positive correlation between COI concentration in water
and solid pairs indicates attenuation (USEPA, 2007) and is the first step (Tier I) in evaluation of
MNA as a remedial technology.

EPA guidance defines Tier I (USEPA, 2007, page 6) as:

“Demonstration that the ground-water plume is not expanding and that sorption of the
contaminant onto aquifer solids is occurring where immobilization is the predominant
attenuation process.”

Determination of plume stability or non-expansion generally follows source control. Quantitative
analysis of plume stability assuming source control was conducted as part of the numerical
modeling effort supporting the CAP. Fate and transport modeling used the site-specific
partitioning coefficients developed as part of a laboratory testing program to determine site-
specific attenuation capacities (Kd) for COIs at DRSS.

Tier I objectives were accomplished by chemical evaluation of collocated aquifer solids and
groundwater (i.e., solid/water pairs) as described in the EPA guidance (USEPA, 2007, page 6):

“Determination of contaminant sorption onto aquifer solids could be supported through
the collection of aquifer cores coincident with the locations of ground-water data
collection and analysis of contaminant concentrations on the retrieved aquifer solids.”

MNA requires additional testing and data interpretation beyond Tier I to allow reasonable
comparison to other remedial alternatives with respect to efficacy, cost, stakeholder acceptance,
and time to meet remedial goals. This will be accomplished for COIs by completing Tiers II and
III of MNA Demonstration.

The groundwater COI’s for DRSS are arsenic, boron, iron, manganese, sulfate, and TDS with
localized 2L exceedances for antimony, chromium, cobalt, selenium, and thallium. Cobalt, iron,
manganese, and vanadium occur naturally in regional groundwater. Sulfate and TDS are
generally not attenuated by reactions with solids, but are reduced in concentration by diffusion,
mechanical mixing, or dilution. Here we consider water-solid pairs and site-specific Kd data as
evidence for attenuation for antimony, arsenic, boron, chromium, cobalt, selenium, and thallium.

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 12 of 25
Water and solid chemistry data from the CSA, and previous sampling by Duke consultants,
were used to conduct the Tier I analysis. A single extraction method was used on solids,
USEPA SW-846 Method 3051A. When a water sample could be matched with more than one
solid sample in the screened section of the well, multiple solid-water pairs were created. Non-
detection (U values, below detection limit) were not used in the comparison or were substitute
values used. J values (presence indicated, below quantification limit.) were used as reported.
As reflected in Table 3, a total of 21 solid-water pairs were used for the DRSS Tier I
Demonstration.
Tier I Analysis
Tier I analysis uses two lines of evidence for attenuation. Solid-water pair comparison of COI
concentrations are performed, a mutually rising relationship indicating attenuation (EPA 2007);
and, laboratory determination of the solid-water partitioning coefficient or Kd value (EPA 1999) is
used as measure of the propensity of COIs to adsorb to site-specific solids and be attenuated.
If a solid-water pair comparison chart is not presented, there was insufficient data for Tier I
analysis for that element.

Laboratory determination of Kd was performed by the University of North Carolina Charlotte
(UNCC) under the supervision of Dr. William Langley on 12 site-specific samples of soil, or
partially weathered rock (saprolite) from the transition zone (Table 4). Solid samples were batch
equilibrated and/or tested in flow through columns to measure the adsorption of COIs at varying
concentrations. These multiple data points for each COI and sample are evaluated to
determine if the observed data can be fit to an adsorption isotherm. If fitting to an isotherm is
supported, a Kd is calculated; blanks in Table 4 indicate that a reasonable isotherm was not
observed in that test. Tests are conducted in duplicate or triplicate to evaluate error. Table 4
also presents calculated median batch-test Kd values for each COI averaged across all tests,
the minimum and maximum values, and the ratio of the maximum to the minimum value as an
indication of the variance in the results. The same information is presented for the column tests.
There were 22 batch tests and 14 column tests conducted on Dan River samples.

Column testing was not successful at producing isotherms for antimony, chromium, cobalt, iron,
and manganese. Iron and manganese Kd determinations in batch or column were difficult due
to the high concentrations of poorly crystalline iron and manganese oxides and oxyhydroxides
present in the Piedmont soils and saprolite. Inspection of the comparison of batch and column
tests in Table 4 reveals that column Kd values are significantly less than batch values with
exception of boron and cadmium, where column Kd values are slightly higher for cadmium and
approximately four times higher for boron.

The variability in Kd measurement across the 12 samples is dependent on the COI. Kd values
vary from thousands of ml/g to single digits. The variation in the ratios of maximum/minimum
(max/min) Kd is not correlated to the calculated medians of Kd. We observe that the highest
median Kd value (iron, Kd 3899, max/min 13.5) does not have a max/min ratio much greater
than smallest Kd (boron, Kd 2.5, max/min 3.5) when compared to the observed max/min ratios of
1.7 (chromium) to 621.7 (manganese). Combined, this seems to indicate that we can use the
max/min ratio as a subjective indicator of the potential for COI to have a Kd that is variable
across geomedia. The relative strength in binding based on the batch Kd data is: Iron > Arsenic
> Thallium > Cobalt > Selenium > Molybdenum > Vanadium > Cadmium > Antimony >
Chromium > Manganese > Boron. The potential for a variable Kd across the site is in the order:
Manganese > Molybdenum > Selenium > Antimony > Arsenic > Vanadium > Iron > Cobalt >
Thallium > Cadmium > Boron > Chromium.

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 13 of 25
Antimony
Antimony attenuation using solid-water pairs was not observed. There were less than three
data pairs, preventing analysis. The calculations of a median Kd of 120 ml/g for antimony
indicated that it should be moderately to strongly attenuated at the site. The max/min ratio of
81.5 suggests that some variability in Kd across the site is to be expected. Natural attenuation
of antimony should be observed and the COI should be carried through to Tier II.
Arsenic
From Table 3 the solid-water pair concentrations of arsenic are plotted in Figure 1. The rising
concentrations in groundwater are matched by rising concentrations in solids, indicating that the
solids are attenuating arsenic concentrations in groundwater. Kd determinations for arsenic
support the solid-water pair observation with a calculated median Kd of 1416 ml/g. Variability in
observed Kd should be moderate with a max/min ration of 31.1. Natural attenuation of arsenic
has been observed, and the COI should be carried through to Tier II.

Figure 1

Boron
Figure 2 depicts the solid-water pair analysis for boron. Attenuation is not observed in this plot.
Calculated site-specific boron Kd values are low (2.5 – 12 ml/g) but are consistent with Kd values
observed for boron uptake in soils (Goldberg, 2000). Boron attenuation is often controlled by
clay mineralogy. The large amounts of illite clay observed, boron’s favored sorption by illite, and
observations of partitioning and attenuation in controlled laboratory conditions suggest that
boron attenuation is taking place, albeit in smaller amounts than other COIs. The attenuation of
boron has been observed and it should be carried through to a Tier II evaluation.
0
100
200
300
400
500
600
700
800
0 20 40 60 80
Ar
s
e
n
i
c
,

d
i
s
s
o
l
v
e
d

(
µ
g
/
L
)

Arsenic (mg/kg)

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 14 of 25
Figure 2

Chromium
Figure 3 depicts the solid-water pair analysis for chromium. Attenuation is not observed in this
plot. Calculated site-specific chromium Kd values are low (27 ml/g) but are not absent. It was
difficult to fit isotherms for many samples. While Tier I identified limited evidence for
attenuation, inclusion of chromium in initial Tier II analysis may reveal more certain
observations. The attenuation of chromium is suggested by limited successful determinations of
Kd on site-specific geomedia and chromium should be carried through to a Tier II evaluation
based on limited potential.

Figure 3

Cobalt
Cobalt attenuation using solid-water pairs was not observed. There were less than three data
pairs, which prevented analysis at this time. The calculations of a median Kd of 1041 ml/g for
cobalt indicated that it should be strongly to very strongly attenuated at the site. The Max/Min
ratio of 12.5 suggests that limited variation in Kd across the site is to be expected. Natural
0
200
400
600
800
1000
1200
1400
0 10 20 30 40 50
Bo
r
o
n
,

d
i
s
s
o
l
v
e
d

(
µ
g
/
L
)

Boron (mg/kg)
0
1
2
3
4
5
6
7
0 20 40 60 80
Ch
r
o
m
i
u
m
,

d
i
s
s
o
l
v
e
d

(
µ
g
/
L
)

Chromium (mg/kg)

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 15 of 25
attenuation of cobalt should be observed and the COI should be carried through to Tier II on the
basis of the high Kd value observations.
Selenium
Selenium attenuation using solid-water pairs was not observed. There were less than three
data pairs, which prevented analysis at this time. The calculations of a median Kd of 624 ml/g
for selenium (batch) and 125 ml/g in column tests indicated that it should be moderately to
strongly attenuated at the site. The Max/Min ratio of 82.5 (26.7 column) suggests that moderate
variation in Kd across the site is to be expected. Natural attenuation of selenium should be
observed and the COI should be carried through to Tier II on the basis of the high Kd value
observations in batch and column tests.
Thallium
Thallium attenuation using solid-water pairs was not observed. There were less than three data
pairs, which prevented analysis at this time. The calculations of a median Kd of 1085 ml/g for
thallium (batch) and 408 ml/g in column tests indicated that it should be moderately to very
strongly attenuated at the site. The Max/Min ratio of 10.5 (13.6 column) suggests that little
variation in Kd across the site is to be expected. Natural attenuation of thallium should be
observed and the COI should be carried through to Tier II on the basis of the high Kd value
observations in batch and column tests.
Vanadium
From Table 1 the solid-water pair concentrations of vanadium are plotted in Figure 4. The rising
concentrations in groundwater are matched by rising concentrations in solids, indicating that the
solids are attenuating vanadium concentrations in groundwater. The calculations of a median
Kd of 425 ml/g for vanadium (batch) and 283 ml/g in column tests indicated that vanadium
should be moderately to strongly attenuated at the site. The Max/Min ratio of 20.6 (31.7
column) suggests that little variation in Kd across the site is to be expected. Natural attenuation
of vanadium should be observed and the COI should be carried through to Tier II on the basis of
the high Kd values in batch and column tests.

Figure 4

Tier I Findings
Tier I analysis indicates that antimony, arsenic, boron, chromium, cobalt, selenium and thallium
should be carried through to Tier II determinations of mechanism. Vanadium, although related
0
5
10
15
20
25
0 20 40 60 80
Va
n
a
d
i
u
m
,

d
i
s
s
o
l
v
e
d

(
µ
g
/
L
)

Vanadium (mg/kg)

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 16 of 25
to background concentrations, should be carried through to Tier II to improve the understanding
of its site-specific occurrence and mobility.
TIER II DEMONSTRATION FOR THE DAN RIVER STEAM STATION
MNA is a candidate remedial technology for the groundwater COIs originating on DRSS
property. A strong positive correlation between COI concentrations in solid-water pairs
indicates attenuation. Correlation was demonstrated as documented in the Tier I
demonstration. This section covers Tier II demonstration of mechanism and rate for COI
attenuation.

Attenuation processes reduce the concentration or toxicity of compounds in groundwater
primarily by advection, dispersion, and chemical interaction with aquifer solids. When
attenuation processes result in a reduction of contaminant concentrations in a timeframe similar
to other remedial technologies, attenuation can be an acceptable remedial alternative (USEPA,
2007). In completion, the Tier I, II, and III MNA demonstrations indicate that MNA is operable
on a timescale comparable to active remedial technologies (e.g. pump and treat) that have been
screened for the DRSS groundwater portion of the CAP. MNA is being considered in the CAP
for DRSS groundwater as a remedial technology. A work plan for MNA evaluation was
submitted, reviewed, revised and approved by DENR. All proposed activities were
accomplished with minor deviations from the plan. Data were collected on the distribution of
COI in groundwater and geomedia in three dimensions to evaluate if attenuation was occurring.
The first use of the data was to evaluate the premise of COI attenuation (Tier I) using a subset
of results from the approved data collection program.

The groundwater COIs for DRSS are arsenic, boron, iron, manganese, sulfate, and TDS with
localized 2L exceedance for antimony, chromium, cobalt, selenium and thallium. Tier I analysis
indicates that antimony, arsenic, boron, chromium, cobalt, selenium and thallium should be
carried through to Tier II determinations of mechanism. Vanadium, although related to
background concentrations, should be carried through to Tier II to improve the understanding of
its site-specific occurrence and mobility.

The process was intended to be flexible based on findings (the observational approach) or in
response to agency comments or new scientific opportunities that may arise. EPA guidance
defines Tier II (USEPA, 2007, page 6) as;
“Determination of the mechanism and rate of the attenuation process.”
and, continues on page 7 with further clarification on rate:
“The objective under Tier II analysis would be to eliminate sites where further
analysis shows that attenuation rates are insufficient for attaining cleanup
objectives established for the site within a timeframe that is reasonable
compared to other remedial alternatives.”
MNA tasks require an observational approach and the reliance on predictive models increases
over the duration of the project (Tier I to III). The MNA demonstration process results in
increasing levels of confidence in the reliability of MNA as a remedial solution. There are two
technical paths that contribute to the same site conceptual model. There is a technical path
primarily related to calculations and modeling of groundwater flow, contaminant transport, and
how fate and transport of the COIs is affected by remedial alternatives, attenuation, or no action.
Supporting those calculations are geochemical activities primarily related to technical evaluation
of COI attenuation capacity, permanence, and remedial effectiveness over time. The technical

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 17 of 25
activities proposed for the CSA relied on our pre-Tier I best understanding of COI attenuation
processes in DRSS groundwater and were subject to change based on observation, study
objectives, or Tier goal attainment by alternate methods.
Site Conceptual Model for Attenuation
Lithologic Controls on Water Quality
The USGS and NCDENR recently completed regional studies (USGS 2009, USGS 2013) of the
chemical quality of groundwater in the Blue Ridge and Piedmont Provinces. DRSS is located in
the Mesozoic Geozone of USGS 2009, a small province occupying only about five percent of
the surface area of the Blue Ridge and Piedmont Provinces in NC. The Mesozoic Basin in the
vicinity of the DRSS is composed of siliciclastic (high-quartz and feldspar arkose) sediments
with significant intervals of fine-grained material (siltstones and mudstones). The carbonate
rock type often found in Mesozoic basins is absent at DRSS. While waters from the siliciclastic
aquifers have different water quality profiles from crystalline rock and carbonate aquifers,
comparative data was not meaningful in that both aquifer types have overlapping water quality
criteria. A notable consideration from the two recent USGS papers is that is it possible to
roughly rank the potential for natural background occurrence of heavy metals (Sb, As, Ba, Co,
Cr, V, and Zn as examples) by rock type as Mafic Crystalline > Felsic Crystalline > Siliciclastic >
Carbonate > Recent Alluvium. Mafic material (igneous-origin rock that contains larger
percentages of Fe and Mn silicates than quartz or feldspars) has a link to background levels of
heavy metals at DRSS. Mafic minerals present in sedimentary units have the same connection.
The Dan River surface geologic mapping indicates a diabase dike cross-cuts the property,
which is a mafic rock unit that can contribute to observable trace metals in groundwater.
Mineralogical Controls on Attenuation
COIs are adsorbed to organic matter, oxide minerals, and clay minerals. These are the
predominant sources and sinks for COIs in groundwater, soil, or sediment. All of these solids
are present in subsurface materials at DRSS. COIs adsorbed on these solids equilibrates with
solution COI concentrations. Some COIs adsorbed to clays migrates from clay edges to clay
inter-layer spaces.

The strength of partitioning of COIs from groundwater to solids ranges from loosely bound by
ion exchange reactions to irreversibly bound in clay structures or chemical precipitates. By
example, through the method of selective or partial extractions (Tessier et al., 1979) mineral
phases and sediment fractions associated with higher concentrations of COIs through
attenuation can be identified. By mineral or size separation and chemical extraction, the solids
that contribute to attenuation can be classified and segregated for degree of effect. The degree
of COI partitioning (Kd and µ) over a range of concentrations can be determined by testing co-
located solids and groundwater, or the examination of COI concentration over time in
groundwater at a fixed location for the purpose of solving the ADE. This and other information
is used to calculate the amount of COI removal and time to achievement of remedial objectives
using a MNA remedial approach.
Role of Site Lithology
Typically, geologic materials are sampled off site and away from potential contamination to
establish baseline values for comparison to those analyzed from study sites. In lieu of detailed
offsite sampling, published values for average crustal abundances can be utilized to evaluate
whether indigenous materials contribute to COI concentrations in an area. For this report,
crustal abundance values from Wedepohl (1978) are used for comparison to samples analyzed
at the Dan River site. Table 5 presents average crustal abundances for comparison.

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 18 of 25
Site Geologic Formation Influence on COI Occurrence
Arkose - The Pine Hall Formation is the dominant lithology underlying the site and consists
predominantly of arkosic sandstones and conglomerates. The clasts in the unit are dominated
by metapelites derived from the Carolina Slate Belt. These units are interpreted to be
fanglomerate deposits near the locus of basin sedimentation during the Triassic (Thayer et al.,
1970). Accordingly, these immature units contain nearly sub-equal proportions of quartz,
feldspar, and clays (predominantly illite/muscovite) (Table 6-1, DRSS CSA Report). Whole rock
chemical analyses show abundances of antimony, arsenic, boron, chromium, cobalt, selenium,
thallium, and vanadium within typical crustal abundances for sandstones and conglomerates of
the region. Accordingly, weathering of the Pine Hall Formation contributes very little to any
elevated values for COIs.

Shale - The Cow Branch Formation is a subordinate lithology of limited exposure in the study
area. It consists of shales, mudstones, pyritic-shale, and carbonaceous shales formed in
lacustrine environments within a rift basin (Thayer et al., 1970). Samples with muscovite/illite
concentrations over 50 percent probably represent this particular lithology. Similar to the Pine
Hall Formation, the abundances of COIs in this unit are typically low with respect to average
crustal abundance.

Diabase - A diabase dike bisects the site and represents a fairly unique lithology with respect to
the dominant types. Mafic rocks typically contain higher V and Co concentrations on average
compared to sedimentary rocks and could cause elevated values in zones of significant
weathering. Especially Co which tends to concentrate during the weathering process in residual
soil profiles as illustrated at the site (Fig. 5-4, DRSS CSA Report). However, no sampling
appears to have been attempted in proximity to the diabase unit.

Biotite Gneiss – Outcroppings of biotite gneiss occur down gradient and across the Dan River,
adjacent to the study site. Although not immediately connected to the hydrologic system at the
site, the proximity of these rock units as a potential source of COIs should be considered.
However, based on the crustal abundances of biotite gneiss it is not anticipated that any
significant elevation in COIs could be derived from the gneisses.
Soil Development Influence on COI Occurrence
Soil formation typically results in the loss of common soluble cations and the accumulation of
quartz and clay. Feldspars are hydrolyzed to clays. Concentrations of COIs liberated during the
bedrock weathering and soil development on the lithologies noted above are negligible other
than for a potential increase in vanadium and cobalt from diabase weathering. Soil chemistry
results do not show marked deviation from normal crustal abundances at the site (Tables 6-2
and 6-3, DRSS CSA Report). Accordingly, the indigenous soils do not appear to contribute
significantly to the COI abundances in the soils.
Mechanisms for Natural Attenuation at DRSS
Active precipitation of COI secondary minerals at the site appears unlikely, with exception of
iron and manganese oxides and oxyhydroxides. The very low abundances of the other elements
suggest saturation of these elements and precipitation as secondary oxides, arsenates, or
carbonates is not attained.

The attenuation of chemical contamination by reaction with existing natural materials may play a
significant role at the site. The abundant clay content of the soils and host rock lithologies
suggests much of the COI concentrations in the ash basin and ash storage may be attenuated
by these materials. Harder (1970) and Perry (1972) showed in pioneering studies that boron is

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 19 of 25
adsorbed and incorporated into illite and chloritic clays respectively. The low content of boron in
most rivers is attributed to the same processes. Thallium is also documented to be adsorbed to
clays, especially potassium and rubidium types, as well as to iron and manganese oxides.
Vanadium is also adsorbed and incorporated into clay structures and oxide coatings (Butler,
1953). Krauskopf (1956) suggests vanadium displays a preference for adsorption in decreasing
order from Fe oxides > Mn oxides > montmorillonite > organic matter. In addition to the reported
abundance of clays at the site, there is significant potential for more clay to be present as an
alteration product on the surfaces of the abundant feldspars reported in the soils and rocks.

Adsorption to iron oxides and hydroxides has long been demonstrated for As, B, Ba, Cd, Co, Cr,
Co, Fe(II), Hg, Mn, Ni, Pb, Sb, Se, SO4-2, V and Zn (Dzomback and Morel, 1990). Soil
chemistry results show abundant Fe2O3 and MnO values in soils from the site (Table 6-2, DRSS
CSA Report) and a strong potential for adsorption. High Al2O3 (aluminum hydroxide) content is
an indicator of clay minerals and also can indicate the presence common in soil-forming
aluminum oxyhydroxides gibbsite, boehmite and diaspore. Adsorption to most-common gibbsite
has been demonstrated for As, B, Cd, Co, Cr, Co, Fe(II), Hg, Mn, Ni, Pb, Se, SO4-2, and Zn
(Karamalidis and Dzomback, 2010). The high amorphous content, 18 to 30 percent in the soil
mineralogy from the site (Table 6-4, DRSS CSA Report), suggests a strong potential for COI
adsorption in the site soils on amorphous Fe-Mn-Al oxide-hydroxides or on amorphous organic
materials.
Tier II Discussion & Conclusions
Following successful completion of a Tier I demonstration that antimony, arsenic, boron,
chromium, cobalt, selenium, and thallium are attenuating in groundwater at the site, a
conceptual model for COI attenuation involving reversible and irreversible interaction with clay
minerals, metal oxides, and organic matter is proposed. A Tier II demonstration based on that
conceptual model was partially executed. The findings follow:

1. The sampling obtained geomedia representative of the material that the COI plume will
traverse.
2. Clay minerals and iron oxides were found in all samples. Organic matter is probably not
a significant sink for COI at the site.
3. Chemical extractions identified that COIs were concentrated in samples exposed to
groundwater containing higher concentrations of COIs, validating the attenuation
conceptual model.
4. Chemical extractions were used to determine a probable range of Kd values that suggest
attenuation is taking place for antimony, arsenic, boron, chromium, cobalt, selenium,
thallium, and vanadium.
5. Additional data collection is necessary to complete the Tier II assessment with respect to
specific attenuation mechanisms for each COI, and quantification of the magnitude of
that attenuation by specific geomedia to support numerical modeling.
Data Gaps and Recommendations
Tier II demonstrations address the mechanism and rate of attenuation. The mineralogical and
physical characterization of solids with respect to type and adsorbed COI concentrations leads
to predictions of COI attenuation expected in the future. Selective extraction (wet chemical
analysis of solids), hydrostratigraphy, and broad water quality data collection provide part of the
information needed to compare MNA to other remedial alternatives in the Tier II evaluation.

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 20 of 25
The data collected to date has provided critical information on using MNA at DRSS and
supports Tier II evaluations. To continue to Tier II and advance to Tier III, additional data
collection is necessary. The data evaluation and analysis process for MNA is complex and
iterative. The following table proposes some specific geochemical analysis approaches to
completing Tier II evaluation of mechanism and support Tier III determinations of attenuation
capacity of COIs.

Many of these methods can be performed on archived materials. It was noted in the Tier I
evaluation that additional solid-water pair data is needed from areas of higher concentration to
improve the fit to these field adsorption isotherms (Field Kd). This will provide opportunity for
additional data collection on samples that may be unavailable in archive. Field testing to
determine dispersivity would also require redeployment to the site for testing activities.

Analysis
Group
Action Method Purpose
Wet
Chemical
Analysis

Partial
Extractions of
Sediments
Chemical extractions
with differing degrees
of action.
Chemical analysis of extracts provides
information on what solid materials the
COI is associated with and the COI
concentration on those materials.
Total Analysis Complete digestion
of sediment. Method
3052.
HNO3 + HF digestion provides a total
COI analysis of solid matrix to compare
with partial extractions. More sensitive
for trace metals than oxide analysis.
Physical
Analysis
Field
Sedimentology
Field logging and
classification by
geologist.
Record used for field selection of
samples for preservation and analysis,
selection of screen elevations for well
installation, and correlation of
stratigraphy between borings.
Office
Petrography
Mineralogy by visual
examination by
geologist. Bulk
samples and thin
section. Optical and
electron microscopy.
Knowledge of the aquifer mineralogy and
relative percentages of minerals is
needed to evaluate observed
groundwater chemistry and potential for
COI attenuation by reaction with aquifer
solids.
XRD
Reitveld
Mineral identification
by X-Ray Diffraction.
Used to identify clay minerals and
confirm/ support visual determinations of
mineralogy.
EDS Energy Dispersive
Spectroscopy
Provides information on the location and
relative concentration of trace elements
on mineral surfaces and polished
sections.
Hydrologic
Parameters
Tracer tests and
break-through curve
analysis.

Site-specific determinations of
dispersivity are needed
Surface
Chemistry
Surface area of
sediments.
Determination of reactive area of the
aquifer material. Needed to scale the
results of chemical determination.

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 21 of 25
REFERENCES AND CITATIONS

Butler, J.R., 1953, The geochemistry and mineralogy of rock weathering. I. The Lizard area:
Geochimica et Cosmochimica Acta, v. 4, p. 157.
Dzomback and Morel, 1990. Dzomback, D.A., Morel, F.M.M., 1990. Surface Complexation
Modeling: Hydrous Ferric Oxide. John Wylie and Sons, New York.
EPRI, 2005. Chemical Constituents in Coal Combustion Product Leachate: Boron, EPRI, Palo
Alto, CA. 2005. 1005258.
EPRI, 2006. Groundwater Remediation of Inorganic Constituents at Coal Combustion Product
Management Sites: Overview of Technologies, Focusing on Permeable Reactive Barriers.
EPRI, Palo Alto, CA: 2006. 1012584.
Goldberg et al., 2000. Goldberg, S., Lesch, S. M., and D. L. Suarez. Predicting Boron
Adsorption by Soils Using Soil Chemical Parameters in the Constant Capacitance Model.
Soil Sci. Soc. Am. J., 64(5): 1356-1363, 2000.
Harder, H., 1970, Boron content of sediments as a tool in facies analysis: Sedimentary Geology,
v. 4, p. 153.
Herbelin and Westall, 1999. A. L. Herbelin and J. C. Westall. FITEQL - A Computer Program
for Determination of Chemical Equilibrium Constants from Experimental Data. Report 99-
01, Department of Chemistry, Oregon State University, Corvallis, OR 97331. 1999.
ITRC, 2010. A Decision Framework for Applying Monitored Natural Attenuation Processes to
Metals and Radionuclides in Groundwater. APMR-1. 2010. Interstate Technology &
Regulatory Council
Karamalidis and Dzomback, 2010. Karamalidis, A. K., Dzomback, D.A., 2010. Surface
Complexation Modeling: Gibbsite. John Wylie and Sons, New York.

Krauskopf, K., 1955, Sedimentary deposits of rare metals: Economic Geology, v. 50, p. 411.
Perry, E.A. Jr., 1972, Diagenesis and the validity of the boron paleosalinity technique: American
Journal of Science, v. 272, p. 150.
Tessier 1979. Tessier, A., Campbell, P. G. C., and M. Bisson. Sequential Extraction Procedure
for the Speciation of Particulate Trace Metals. Analytical Chem., 51(7)844-850.
Thayer et al., 1970. Thayer, P.A., Kirstein, D.S., and Ingram, R.L., 1970, Stratigraphy,
sedimentology, and economic geology of Dan River Basin, North Carolina: Carolina
Geological Society Guidebook, 29p.
Toride et al., 1999. Toride, N., F.J. Leij, and M.Th. van Genuchten, The CXTFIT Code for
Estimating Transport Parameters from Laboratory or Field Tracer Experiments: Version 2.1.
US Department of Agriculture, Agricultural Research Service, US Salinity Laboratory, April,
1999. Research Report No. 137

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 22 of 25
USEPA, 1999. Understanding Variation in Partition Coefficient, Kd, Values. Volume I: The Kd
Model Of Measurement, And Application Of Chemical Reaction Codes. EPA 402-R-99-
004A. United States Environmental Protection Agency, Office of Air and Radiation, August
1999.
USEPA, 2007. Monitored Natural Attenuation of Inorganic Contaminants in Groundwater, Vol. 1:
Technical Basis for Assessment. 2007. US EPA, EPA/600/R-07/139.
USGS, 2009. Harden, S L., M J. Chapman, and D A. Harned. Characterization of Groundwater
Quality Based on Regional Geologic Setting in the Piedmont and Blue Ridge
Physiographic Provinces, North Carolina. U.S. Geological Survey Scientific Investigations
Report 2009–5149.

USGS, 2013. Chapman, M.J., C. A. Cravotta III, Z. Szabo, and B.D. Lindsey. Naturally
Occurring Contaminants in the Piedmont and Blue Ridge Crystalline-Rock Aquifers and
Piedmont Early Mesozoic Basin Siliciclastic-Rock Aquifers, Eastern United States. 1994–
2008. U.S. Geological Survey Scientific Investigations Report 2013–5072.

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 23 of 25
Table 3. Solid-water pairs used for Tier I demonstration - DRSS

Soil Aluminum Aluminum Antimony Antimony Arsenic Arsenic Barium Barium Boron Boron Beryllium Beryllium Cadmium Cadmium Chromium Chromium Cobalt Cobalt Copper Copper Iron Iron Lead Lead Mangane
se
Mangane
se Mercury Mercury Molybden
um
Molybden
um Nickel Nickel Selenium Selenium Strontium Strontium Thallium Thallium Vanadium Vanadium Zinc Zinc
mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L mg/kg µg/L
AS-12S(15-16.5)18600 13.1 0.24 8.2 0.48 98.1 33 1.5 1000 0.2 34.1 0.22 17.5 2 40.3 0.4 43200 82.6 21.5 479 420 0.02 4.9 2.8 39.4 9.6 69.6 600 32.6 0.29 86.9
AS-12S(20-21.5)22700 13.1 0.24 8.5 0.48 62.4 33 1.2 1000 0.2 36.1 0.22 26.2 2 70.1 0.4 44800 82.6 15.5 396 420 0.012 4.6 2.8 41.9 9.6 5.6 56.5 600 33.2 0.29 99.8
GWA-12S(13-15)32900 21 30.6 200 53 2 60.1 48.7 23.9 67.5 1 58700 21.5 698 102 0.009 5.6 41.9 2.14 72.5 140 67.2 119 6
GWA-6S(9-11)2610 2.4 31.9 200 0.16 620 8.9 0.9 3.5 2.5 4.4 5260 25300 3.8 97.9 3900 0.0072 13.7 3.2 1.4 3.6 860 0.14 12.1 11.9 2.7
GWA-6S(13.0-14.0)10700 2.4 110 200 0.61 620 24.7 0.9 13.6 2.5 16 20300 25300 7.9 237 3900 0.018 13.7 11 1.4 21 860 0.14 52.4 34.3 2.7
GWA-7S(16-17)10100 420 131 25 0.96 38 4.7 0.46 21.9 0.95 9.6 19.2 15 1 17600 16.2 659 1000 0.0074 12 51.1 0.62 21.2 780 28.1 37.8 560
GWA-7S(22-23)12900 420 3.2 195 25 2.3 38 4.7 0.46 17.9 0.95 16.3 19.2 13.6 1 25600 20.2 567 1000 23.6 51.1 6.1 0.62 32.8 780 22.8 74.2 560
GWA-10D(31.8-32.8)4620 9 52.1 55 1.4 172 0.38 8.2 6 2.11 13.5 7.79 23700 146 4.6 1.04 579 453 11.8 6.43 24.5 381 6.7 0.324 35.1 13
MW-318D(42.5-43.42 2420 21 1 14.9 35 0.24 1250 1 1 2.4 1 5.4 12.9 1.5 1.38 5130 588 8.2 1 210 607 0.011 1 5.1 12.4 1 11.1 589 0.2 3.9 0.3 17.3 78
OW-301D 10.4 10.8 162 157 186 3.4 23.6 4.5 11.9 2.3 39,400 3,190 246 159 1.2 61.2 3.6 3.6
OW-302D 5.2 83.6 16.6 44.8 3.1 0.67 21.5 2.2 40 14.4 34,800 188 338 337 1.7 2.9 30.6
OW-307D 8.3 180 13.8 27.1 15.4 7.7 3.6 14,400 359 522 55 2.8 5 17.6
OW-308D 0.86 76 543 577 39.4 11.4 407 17.3 0.48 37.3 6,840 26,000 66.3 1,310 1.4 3.1 3.6 0.41 47.2
OW-308D 543 106 39.4 407 0.65 17.4 15 37.3 25,800 26,000 182 1,310 0.075 3.1 0.61 0.35 26.8
OW-309D 2.6 53.1 386 440 489 45.3 466 20.1 6.6 4.9 3.5 5,430 2,470 52.9 553 1.7 16.4 2.1 1.5 5.5 72.9 20.9
OW-309D 1.1 27.6 386 269 489 6.6 466 8.1 6.6 1.1 3.5 5,380 2,470 75.2 553 0.86 16.4 2.9 5.5 26.4 20.9
OW-310D 13.4 713 197 433 921 1.2 65.5 5.1 1.1 8.7 28,600 7,080 354 1,880 2.3 30.1 1.7 9.6 70.6 14
OW-312D 2.8 102 77.5 502 0.82 15.4 5 10.8 26.2 31,500 20,800 665 3,040 0.046 4.2 0.65 27.8 9
OW-312D 90.6 77.5 502 22.3 5 2.3 26.2 13,800 20,800 156 3,040 4.2 0.87 0.41 34.3 9
OW-312D 68.8 77.5 502 13.5 5 3.6 26.2 7,110 20,800 114 3,040 0.054 4.2 0.68 0.37 20.8 9
OW-315D 228 72.1 114 1,290 4.6 15.4 22 16.3 52,000 10,700 326 542 0.046 16.5 1.1 52.4

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 24 of 25
Table 4. Kd data used for Tier I evaluation - DRSS
Dan River
DRAFT
Batch Tests AB-30BR AB-30BR AB-30BR AB-30BR AB-10SL AB-10SL AS-2D AS-2D GWA-10 GWA-11 GWA-5BR GWA-5BR GWA-4D GWA-4D GWA-11D GWA-11D GWA-12D GWA-12D GWA-1S GWA-1S GWA-3S GWA-3S
Kd ml/g 32-34 FT 32-34 FT 43-44.1 FT 43-44.1 FT 48-50 FT 48-50 FT 47-50 FT 47-50 FT 102-104 FT 102-104 FT 8-12 FT 8-12 FT 38 FT 38 FT 23-25 FT 23-25 FT 20-21 FT 20-21 FT 33-35 FT 33-35 FT 25-27 FT 25-27 FT
Arsenic 2563 2476 1975 1929 1868 1781 478 527 83 84 548 522 1007 1062 1750 1649 725 670 1744 1714 1339 1494
Boron 2 2 1 2 1 4 4 3 3 3 2 3 3
Cadmium 194 202 350 452 650 724 560 794 154 164 244 255 125 126 463 477
Chromium 20 34
Cobalt 365 430 225 2752 2815 2464 2378 748 717 558 1370 1334
Iron 536 7263
Manganese 18 17 13 14 182 187 79 78 4 4 0 0 6 6 64 66 15 9 58 57 40 40
Molybdenum 352 384 10 1486 1524 2257 2258 445 535
Antimony 936 1125 210 195 28 39 14 15 46 45 187 190 136 123 24 25 772 824 117 103
Selenium 1239 1324 180 176 115 125 58 60 135 121 1067 1165 2869 2703 55 50 3830 4082 1313 1553
Thallium 576 585 3142 3268 3790 3915 718 726 379 382 1100 1070 3980 3949 547 551 2139 2767
Vanadium 583 546 409 436 65 67 429 383 425 342 1345
Column Tests AB-30BR AB-30BR AB-30BR AB-30BR AB-10SL AS-2D AS-10D GWA-11 GWA-5BR GWA-4D GWA-11D GWA-12D GWA-1S GWA-3S
Kd ml/g 32-34 FT 43-44.1 FT 43-44.1 FT 43-44.1 FT 48-50 FT 47-50 FT 10-11 FT 102-104 FT 8-12 FT 38 FT 23-25 FT 20-21 FT 33-35 FT 25-27 FT Median Min Max Max/Min
Arsenic 370 420 400 450 350 210 290 30 30 700 475 40 370 150 360 30 700 23.3
Boron 12 25 20 15 12 10 12 8 8 8 12 8 15 9 12 8 25 3.1
Cadmium 360 530 500 460 390 290 135 370 175 290 650 340 400 315 365 135 650 4.8
Chromium
Cobalt
Iron
Manganese
Molybdenum 180 130 50 50 18 15 130 6 10 130 210 9 180 70 60 6 210 35.0
Antimony
Selenium 285 230 110 120 100 60 160 15 18 210 400 20 285 130 125 15 400 26.7
Thallium 375 600 680 620 390 280 190 50 425 300 650 340 425 575 408 50 680 13.6
Vanadium 350 390 275 370 340 80 210 15 30 290 475 40 325 140 283 15 475 31.7
Kd ml/g Batch
Median
Column
Median
Batch
Max/Min
Column
Max/Min
Arsenic 1416 360 31.1 23.3
Boron 2.5 12 3.5 3.1
Cadmium 302 365 6.4 4.8
Chromium 27 1.7
Cobalt 1041 12.5
Iron 3899 13.5
Manganese 18 621.7
Molybdenum 535 60 237.7 35.0
Antimony 120 81.5
Selenium 624 125 82.5 26.7
Thallium 1085 408 10.5 13.6
Vanadium 425 283 20.6 31.7

M. Filardi
DRSS MNA Tier I & II
October 12, 2015
Page 25 of 25

Table 5. Average crustal abundances for lithologies at the Dan River Steam Station.
(ppm) B V Cr Co As Se Sb Tl
Arkose (Pine Hall Fm) 30 43 38 5 1.2 0.2 1 1
Shale (Cow Branch Fm) 130 110 62 19 12 0.3 2 2
Diabase 5 300 260 48 1.5 0.1 0.1 0.05
Biotite Gneiss 4 56 35 11 2 0.1 0.3 0.3

Summary of Statistically Derived Kd Values – Dan River Steam Station
Variable Num Obs # Missing Minimum Maximum Mean SD SEM MAD/0.675 Skewness Kurtosis CV
Arsenic 20 52 4 1440 404.4 383.6 85.78 209.8 1.644 2.969 0.949
Boron 22 47 0 19 4.136 4.063 0.866 2.224 2.657 8.561 0.982
Cadmium 31 41 25 6110 981.9 1192 214 607.9 2.966 11.2 1.214
Iron 4 35 24 25 24.25 0.5 0.25 0 2 4 0.020
6
Manganese 36 36 6 2247 170.7 414.9 69.15 50.41 4.148 18.92 2.431
Molybdenum 17 55 5 2057 208.2 529.4 128.4 8.895 3.176 10.29 2.543
Selenium 30 42 31 8721 1044 1996 364.5 177.9 2.941 8.837 1.912
Thallium 16 53 289 18354 2403 5166 1291 219.4 2.724 6.704 2.15
Vanadium 22 50 12 794 308.6 218.1 46.49 228.3 0.0783 -0.614 0.707
Cobalt 4 53 534 1196 899.8 322.4 161.2 349.1 -0.273 -4.211 0.358
Antimony 38 34 12 1007 139.8 255 41.36 37.81 2.637 5.715 1.824
Note: Prepared by Geochemical, LLC based on UNCC laboratory results.
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