NC0038377_Mayo Appendix C - Methodology_20150902

Duke Energy Progress — Mayo Steam Electric Plant
Groundwater Assessment Report
Comprehensive Site Assessment Report
Appendix C - Methodology
The approach to conducting the Comprehensive Site Assessment (CSA) at the Mayo Plant was
described in the Groundwater Assessment Work Plan (Revision 1, December 2014). The objective
was to collect and analyze samples of soil, ash, groundwater, surface water, and sediment to
more accurately determine the vertical and horizontal concentrations of Constituents of Interest
(COIs). A COI is defined as a parameter detected at a concentration greater than
NCDENR/DWR Title 15, Subchapter 2L and Interim Maximum Allowable Concentrations
(IMAC). This section describes equipment and methods employed to collect and preserve
appropriate samples and obtain representative analytical results.
1. Subsurface Investigation
Characterization of subsurface material was conducted by collecting ash, soil, sediment, ash
pore water, groundwater, seep water, and surface water samples for analysis. Ash, soil, ash
pore water, and groundwater samples were obtained through completion of drilled borings and
monitoring wells. The approach for subsurface sample acquisition and analysis follows.
1.1 Drilling Methods
Three drilling methods were employed to collect subsurface samples. The selection of
drilling technology was dependent on soil conditions, sample depth, and accessibility.
Drilling tools (sonic core barrels and casings, GeoprobeTM samplers and drill rods, and
hand auger buckets) were thoroughly decontaminated prior to starting a boring. Daily
equipment rinse samples were collected to confirm the effectiveness of decontamination.
Water used for drilling operations was obtained from the Mayo Plant water supply.
Water is pumped from Mayo Lake, treated using a combination of techniques including
reverse osmosis (RO), electro -deionization (EDI), and sand -bed filtering. A sample of
this source water was analyzed for the full set of GAP parameters (Appendix D).
1.1.1 Sonic Coring
The primary method used for drilling soil borings and monitoring wells was
rotary sonic drilling. The advantages provided by this method include less
disturbance of the borehole wall and minimized groundwater sample turbidity.
1.1.2 Direct Push Technology
A GeoprobeTM direct push sampling rig was used to acquire sample cores in the
ash basin. The primary advantage of this drilling method was the small size and
weight of the rig.
1.1.3 Hand Auger
A hand auger was used at locations where sample depth was shallow or site
conditions precluded the use of mechanized drilling equipment.
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Duke Energy Progress - Mayo Steam Electric Plant
Groundwater Assessment Report
Comprehensive Site Assessment Report
Appendix C - Methodology
1.2 Monitoring Well Installation
Each monitoring well was constructed by North Carolina -licensed well drillers using
sonic and direct push drilling techniques and in accordance with 15A NCAC 02C (Well
Construction Standards). Drilling equipment was decontaminated prior to use at each
location.
Monitoring wells were constructed of 2 -inch ID, National Sanitation Foundation (NSF)
grade polyvinyl chloride (PVC) (ASTM D-1785-12) schedule 40 flush -joint threaded
casing and 0.010 -inch machine -slotted pre -packed screens. Well construction also
included the use of pre -packed screens with additional sand in the annular space, to
minimize sample turbidity. Packed well screens for each well were filled with clean,
well-rounded, washed high grade No. 1A silica sand. No.1A sand is a 12/40 mesh filter
sand comparable to the 20/40 mesh sand referenced in the Work Plan and is commonly
used in monitoring well construction. Monitoring wells installed in ash pore water
utilized additional precautions against fly ash fines incursion by using an extra fine
"Geothermal" silica sand in the annular space and No. 1A silica sand in the pre -packed
well screens. The filter pack was placed approximately two feet above the top of the
pre -packed screen and then a minimum two -foot pelletized bentonite seal was placed
above the filter pack. The remainder of the annular space was filled with a neat cement
grout from the top of the upper bentonite seal to near ground surface.
For some wells completed in the bedrock aquifer, double -cased wells were constructed
as a precautionary measure to prevent COI migration from overlying material along
annular space of the borehole. Surface casings were employed where saturated
conditions existed in the saprolite or transition zone, and the necessity was determined
based on observations of continuous cores recovered during drilling. Protective outer
casing was installed using sonic drilling equipment with a 10 -inch core barrel into the
top of the bedrock which was determined based on observation of continuous cores
recovered during drilling. A permanent 6 -inch diameter schedule 40 PVC protective
outer casing was then installed and grouted in place. After the grout had sufficient time
to set (minimum 24 hours), drilling was advanced through the outer casing using a 5 -
inch diameter core barrel and through the bedrock to the depth of water -bearing
fractures (determined based on observation of continuous cores). If water -bearing
fractures were not encountered in the shallow bedrock, drilling continued to a
maximum depth of 50 feet beneath the top of bedrock. Wells were then installed in a
similar approach as shallow monitoring wells as described above.
Monitoring wells were completed with either steel above ground protective casings or
flush -mounted vaults with locking caps, locking expansion caps, and well tags.
Protective covers were secured and completed in a concrete collar and a minimum two-
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Duke Energy Progress — Mayo Steam Electric Plant
Groundwater Assessment Report
Comprehensive Site Assessment Report
Appendix C - Methodology
foot square concrete pad. Vaults were installed in high traffic areas or other areas
designated by Mayo Plant personnel.
1.2.1 Monitoring Well Development
Following installation, monitoring wells were developed to remove drill fluids,
clay, silt, sand, and other fines which may have been introduced into the
formation or sand pack during drilling and well installation, in addition to
establishing connectivity of the well with the aquifer. Well development was
performed using a portable submersible pump that was repeatedly moved up
and down the well screen interval until the water was relatively clear. Some
wells were initially developed with a bailer to remove the most turbid water and
were later completed by developing with a submersible pump or a peristaltic
pump. Development continued by sustained pumping until monitoring
parameters (e.g., conductivity, pH, DO, and temperature) were generally
stabilized, estimated quantities of drilling fluids, if used, were removed, and
turbidity decreased to acceptable levels (approximately 10 NTUs). Wells were
developed no sooner than 24 hours after well installation to allow for an
adequate grout cure time.
1.3 Sample Collection and Analytical Methods
Methods used for the collection and preservation of samples for various analyses are
described in this section. Samples were collected in accordance with the quality
assurance and quality control procedures outlined in the Work Plan.
1.3.1 Soils and Ash Sampling and Analysis
Borings were logged and ash/soil samples were photographed, described, and
visually classified in the field for origin, consistency/relative density, color, and
soil type in accordance with the Unified Soil Classification System (ASTM
D2487/D2488).
Rinse blanks from soil sample collection equipment were collected for each soil
boring/well installation location. At times, drilling for one location took more
than one day and the rinse blank was collected on the first day. Rinse blanks for
soil samples were collected by pouring deionized water through the sonic drill
bit or through the hand auger bit. These pieces of equipment are normally the
first introduced into the subsurface for a soil boring. Laboratory results for the
rinse blanks are provided in Appendix D.
Soil and ash samples were collected wearing nitrile gloves and prepared and
analyzed using the following methods:
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Groundwater Assessment Report
Comprehensive Site Assessment Report
Appendix C - Methodology
1.3.1.1 Metals
Soil and ash samples were placed in amber glass bottles and stored on ice
for shipment for total metals analysis. Concentrations of metals present
in soils and ash were determined using analytical parameters presented
on Table 6-2.
1.3.1.2 Organic Constituents
Soil and ash samples were placed in amber glass bottles and stored on ice
for shipment for total organic carbon (TOC) analysis. TOC content of
soils and ash were determined in accordance with EPA Method 9060.
1.3.1.3 Leaching Characteristics
Select soil and ash samples were placed in amber glass bottles and stored
on ice for shipment for the mobility of inorganic analytes present using
the Synthetic Precipitation Leaching Procedure (SPLP) following U.S.
EPA Method 1312.
1.3.1.4 Bulk Chemistry
Select soil and ash samples were stored and shipped in sealable plastic
bags for bulk chemistry analysis. Identification and relative
concentration of bulk chemistry was determined by American Assay
Laboratories. Samples were dried at 80 °C overnight and pulverized to -
150 mesh and analyzed as follows:
XRF-PP - a known sample amount was combined with binder,
ground finer, and pressed into a disk. The disk was then analyzed
by X-ray fluorescence (XRF).
ICP-D4A - A sample pulp was digested with a combination of HF,
HC104, HCl, and HNO3 for a near-total digestion. The solution
was then analyzed by Inductively Coupled Plasma (ICP) -Atomic
Emission Spectroscopy (AES) and ICP -Mass Spectroscopy (MS).
ICP -NF - A sample pulp was fused with Na202 and digested with
HCl. The solution was then analyzed by ICP -AES and ICP -MS.
Eltra Carbon & Sul {ur - A sample pulp was combined with
tungsten and iron accelerator and combusted in Eltra furnace for
analysis of carbon and sulfur.
Loss of Ignition (LOD - A sample pulp was gradually heated in a
gravimetric furnace to 1000° C while sample loss was calculated.
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Duke Energy Progress — Mayo Steam Electric Plant
Groundwater Assessment Report
Comprehensive Site Assessment Report
Appendix C - Methodology
1.3.1.5 Mineralogy
Select samples were stored and shipped in sealable plastic bags for
mineralogical analysis. Identification and relative concentration of
mineral types were determined by X-ray diffraction (XRD). The original
sample was dried at 80'C (no pulp) and combined with water and
disaggregated in an ultrasonic bath. The sample was then wet sieved at
63 micrometers to separate and quantify the sand fraction. The slurry
was centrifuged at a calculated rotation per minute and time to separate
the clay particles from the silt. The clay solution was then decanted off.
Sand and silt fractions were dried, quantified, and analyzed using XRD as
random mounts (XRD is a process by which X-rays are scattered by atoms
that comprise the crystal structure of a given mineral, creating a pattern).
Clay particles are deposited on slides forming an oriented mount for
XRD. After the analysis, the clay particles were placed in an ethylene
glycol environment overnight to test for expanding clays. When
necessary, the clay particles are then heated to 400° C and re -analyzed for
collapsing layers.
1.3.1.6 Development of Kd Terms
To determine the sorption capacity of site soils, select samples were
collected along proposed flow path transects. Samples were collected,
handled, and preserved in order to eliminate impacts of ambient air on
the oxidation-reduction potential (ORP) and hydrous ferrous oxide
(HFO) on sampled materials. Samples were collected in plastics bags and
sealed with a conventional vacuum sealer. The samples were stored on
ice for shipment and kept out of direct sunlight.
Samples were prepared and analyzed by the Civil and Environmental
Engineering Department of the University of North Carolina at Charlotte
(UNCC). Prior to sorption determinations, soil samples were dried at
room temperature and periodically mixed throughout the drying process
to prevent grain aggregation. Once dry, samples were sieved using a No.
10 U.S. standard sieve (10 mm) with 0.0787 inch openings. To quantify
soil partition coefficients (Kd), column and batch tests were performed.
Effluent samples collected from these tests were analyzed by inductively
coupled plasma -mass spectroscopy and ion chromatography. To provide
a basis for estimating COI source terms, leaching tests were performed on
ash samples. Details of the analytical procedures/methods are briefly
discussed below with more detail included in the analytical reports.
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Comprehensive Site Assessment Report
Appendix C - Methodology
1.3.1.6.1 Column Tests
Column tests were conducted by compacting soil and ash samples
into 8 inch long (20.3 cm) polyethylene tubes (with dimensions
0.675 in. (16 mm) I.D. by 0.75 in. (19 mm) O.D) and plugged with
two polypropylene end caps. Using groundwater and ash pore
water analytical results from the site, feed solutions amended with
all COIs found above the NCAC 15A 02L .0106(g) standards were
produced and pumped into the columns. Analyses and
equipment used are provided in the table below:
Analyte
Method
Trace metals (Sb, As, B, Cd, Cr, Fe,
Mn, Pb, Tl)
EPA 200.8
Sulfate
EPA 300.0
pH
Standard Method 4500 B
Conductivity
Standard Method 2510
Oxidation-reduction potential (ORP)
ASTM method G200-19
1.3.1.6.2 Batch Tests
Batch tests were conducted in accordance with U.S.
Environmental Protection Agency Technical Resource Document
EPA/530/SW-87/006-F. COI -amended feed solution (described
above in 1.3.1.6.1) and soil samples were mixed across a range of
soil -to -solution ratios, followed by shaking until chemical
equilibrium was achieved. Once equilibrium was achieved,
solutions were drawn and analyzed as described in the above
table.
1.3.1.6.3 Hydrous Ferrous Oxides Analysis
The method for HFO determination in soil and ash samples was
adapted from Chao and Zhou (1983). Following this method, soil
samples were extracted using a 0.25M NH2OH•HCl-0.25M HCl
combined solution.
1.3.1.6.4 Ash Leaching Tests
Ash leach tests were performed to provide a basis for estimating
COI source terms to develop the Kd terms. Ash samples were
prepared and analyzed using EPA Method 1313 [2] (Liquid -Solid
Partitioning as a Function of Extract pH Using a Parallel Batch
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Duke Energy Progress — Mayo Steam Electric Plant
Groundwater Assessment Report
Comprehensive Site Assessment Report
Appendix C - Methodology
Extraction Procedure) and EPA Method and EPA Method 1316 [3]
(Liquid -Solid Partitioning as a Function of Liquid -Solid Ratio
using a Parallel Batch Extraction Procedure). Method 1313
provides COI concentration as a function of pH (the test was
conducted only at natural pH). Method 1316 provides COI
concentration as a function of liquid to solid ratio.
1.3.1.7 Index Property Sampling and Analysis
Select soil and ash samples were collected for laboratory analysis of
physical properties to provide data for use in groundwater modeling.
Samples were collected at selected locations for the following analyses
using the described methods:
10 Natural Moisture Content Determination, in accordance with
ASTM D-2216
10 Grain size with hydrometer determination, in accordance with
ASTM Standard D-422
In addition, undisturbed samples were collected using thin-walled tubes
("Shelby" tubes) advanced in ash and soil at select locations. Undisturbed
samples were tested for the following:
y Natural Moisture Content Determination (ASTM D-2216)
y Grain size with hydrometer determination (ASTM
Standard D-422)
y Hydraulic Conductivity Determination (ASTM Standard D-5084)
1610 Specific Gravity of Soils (ASTM Standard D-854)
Sample porosity was calculated from parameters measured by these tests.
1.3.2 Ash Pore Water and Groundwater Sampling and
Analysis
New and existing wells were sampled using low -flow sampling techniques in
accordance with the USEPA Region 1 Purging and Sampling Procedure for the
Collection of Groundwater Samples from Monitoring Wells (revised January 19, 2010),
the Groundwater Monitoring Program Sampling, Analysis and Reporting Plan, Mayo
Steam Electric Plant (SynTerra, October 2014), updated by the Low Flow Sampling
Plan, Duke Energy Facilities, Ash Basin Groundwater Assessment Program, North
Carolina, June 10, 2015. NCDENR conditionally approved the Low Flow Sampling
Plan in a June 11, 2015 email with an attachment summarizing their approval
conditions.
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Comprehensive Site Assessment Report
Appendix C - Methodology
Equipment blanks for groundwater sampling were collected daily. The sample
was collected from a laboratory -supplied container of deionized water into
laboratory -supplied bottleware. The equipment consisted of the pump to be used
for sample collection that day and tubing was pulled from the supplies planned
for the day.
Ash pore water and groundwater samples were analyzed in the field and
laboratory. The analytical parameters and associated analytical methods are
summarized on Table 10-2. Only select samples were analyzed for radionuclides
and metals speciation. For speciation analysis, select ash pore water or
groundwater samples were collected with peristaltic or Engineered Plastic Mega
Monsoon pump as described above but with acid -washed tubing and following a
condensed version of EPA Method 1669.
2. Hydrogeologic Testing
Hydrogeologic tests were performed and measurements were made to characterize site-specific
hydrogeologic conditions. These activities included measuring water levels and conducting
slug tests and packer tests.
2.1 Water Level Measurements
During groundwater sampling activities, water level measurements were recorded at
existing site monitoring wells, observation wells, piezometers, and newly installed
wells. The measurements were made using electrical tapes that indicate the contact with
water. A tape was lowered into a well and a graduated scale on the tape was used to
measure the depth of the water level below the top of the casing. The uncertainty in this
measurement is approximately +/- 0.01 ft, which is the resolution of the scale on the tape.
The elevation of a reference point on the top of each casing was determined by a
licensed surveyor, and we estimate the accuracy of these elevations to be +/- 0.01 ft
Elevations were measured relative to a common datum, which was then referenced to
mean sea level. Depth to water and the elevation of the top of casing were used to
calculate the elevation of the water level in the well. This elevation was assumed to be
the hydraulic head at the mid -point of the screen on the well.
The hydraulic heads measured in shallow wells were contoured using standard linear
interpolation methods to generate water level maps for shallow, water table conditions.
These maps were used to infer approximate directions of horizontal groundwater flow
within the constraints that the hydraulic conductivity was isotropic in the horizontal
direction. Measurements from clusters of piezometers were used to estimate vertical
hydraulic head gradients. Water levels in wells in the ash basins were used to estimate
the elevation of saturated conditions, and multiple measurements provided
approximate estimates of the directions of ground water flow. Water level
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Duke Energy Progress — Mayo Steam Electric Plant
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Appendix C - Methodology
measurements used for preparation of hydraulic head maps were collected during a
single 24-hour period and prior to purging for sampling.
2.2 Slug Tests
2.2.1 Field Methods
After the development and sampling of monitoring wells, hydraulic conductivity
tests (rising head and/or falling head slug tests) were conducted on each of the
new wells. Slug tests were performed in general accordance with ASTM D4044-
96 Standard Test Method (Field Procedure) for Instantaneous Change in Head (Slug)
Tests for Determining Hydraulic Properties of Aquifers and NCDENR Performance and
Analysis of Aquifer Slug Test and Pumping Test Policy, dated May 31, 2007.
A slug test involves the injection or withdrawal of a known volume (slug) while
measuring the resulting fluctuation of the groundwater level. Site slug tests
employed both the solid cylinder (PVC slug) and the water withdrawal (bailer)
methods. When a PVC slug was utilized, both a falling -head (slug -in) and a
rising -head (slug -out) test was performed. However, when a bailer was used,
only a rising -head slug test was performed.
Prior to the performance of each slug test, static water level was measured and
recorded. A Solinst Model 30010 Edge electronic pressure transducer or
equivalent was placed in a well at least one foot beneath the lowest level that
would be reached by the submerged slug. This ensured that the cross-sectional
area of the well bore remained uniform during the slug test, which facilitates
analysis of the results. The Levelogger® was connected to a field laptop and
programmed with the well identification, date, and time. The pressure
transducer was then allowed to equilibrate to the conditions in the well for at
least 10 minutes.
Failing -Head Slug Test
Falling -head slug tests were conducted by rapidly lowering a slug into the well
and positioning the top of the slug approximately one foot below the initial static
water level. This caused the water level to rise, and then the water level fell and
the resulting pressure change was measured and recorded electronically using
the pressure transducer. The water level in the well was allowed to recover to
between 60 and 80 percent of the initial water level. After falling -head slug tests
were completed, sufficient time was allowed for the water level to reach
equilibrium before conducting the rising -head slug test.
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Groundwater Assessment Report
Comprehensive Site Assessment Report
Appendix C - Methodology
Rising -Head Slug Test
A rising -head slug test, also known as a slug -out test, was conducted by rapidly
withdrawing a submerged slug, dropping the water level in the well. Most
rising -head slug tests were performed until the water level in the well recovered
to 60 to 80 percent of the pre-test water level. Slug tests in wells completed in
low permeability formations were terminated after 70% recovery or one hour,
whichever occurred first.
When using a bailer to withdrawal a slug of water, the rising -head procedure
was followed, first allowing time for the well to equilibrate with the bailer in the
water before removing the slug of water.
Hydraulic well tests were conducted with packers in some bedrock borings. Packer tests
utilized a double packer system arranged to straddle an interval of the borehole between
5 feet and 10 feet long. The straddle packer system was used to isolate and characterize
depth intervals identified from observations of the rock core. Typically, depth intervals
of the core containing fractures that were thought to be open in the subsurface were
selected for packer testing. This might bias the hydraulic conductivity determined from
packer tests, but it is justified because the objective of the testing was to characterize the
depth intervals that controlled groundwater flow. The U.S. Bureau of Reclamation test
method and calculation procedures as described in Chapter 17 of their Engineering
Geology Field Manual (2nd Edition, 2001) were utilized.
Slug tests were conducted with packers using a solid cylinder as described in Section 2.2.
Packers were also used during a purge -recovery investigation. The purge -recovery
investigation involved purging the well and monitoring the recovery of water level in
the packed -off zone. Purge -recovery results were used as an indicator of the location of
water bearing zones to and determine well screen placement.
2.2.2 Parameters
Slug test data were analyzed using AQTESOLVTM for Windows, version 4.5
(AQTESOLVTM). This involved using pressure data from transducers, aquifer
data thickness data from cross-sections, and boring logs and well completion
data (such as screen length, inside radius of well casing, and radius of well
obtained) from CSA boring logs.
2.2.3 Analytical Methods
In selecting a method for analyzing the slug tests, the formations intersected by
the wells were assumed to isotropic and uniform over the scale influenced by the
well. Furthermore, it was assumed that Darcy's Law was valid in all the
formations, and that the wells sampled a scale that was large enough so that flow
through fractures could be characterized using an equivalent hydraulic
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Duke Energy Progress - Mayo Steam Electric Plant
Groundwater Assessment Report
Comprehensive Site Assessment Report
Appendix C - Methodology
conductivity. This is an accepted approach for characterizing flow through
porous and fractured media.
Anisotropy was assumed when analyzing the slug tests because slug tests are
only weakly sensitive to anisotropy and the resulting large uncertainty in
parameters would not warrant pursuing the interpretation of anisotropy using
slug tests.
Wells were recently developed prior to slug testing, and it was assumed that
development was sufficient to remove well skin. As a result, effects of well skin
were ignored in the analyses.
Slug tests are weakly sensitive to storage properties in aquifers. Methods of
analysis are available to estimate storativity from slug tests, but the uncertainty is
typically large because of the weak sensitivity. Pressure transients were of minor
importance compared to steady state conditions during this investigation.
Storativity is unnecessary for steady state analyses, so methods of analyzing slug
tests that provided estimates of storativity were ignored.
Slug tests were analyzed using the Hvorslev method. This method is consistent
with the assumptions outlined above, and it is the most widely used method of
analyzing slug tests among practicing hydrogeologists.
AQTESOLVTM was used to input parameters listed above and plotted
normalized head vs. time. Hvorslev method was chosen in AQTESOLVTM and
visual line matching used for the Hvorslev equation was used to calculate the
transmissivity. The Hvorslev method involves plotting the log of the normalized
head as a function of time and fitting the data with a straight line (Butler, 1998).
The slope of the straight line is used in a calculation to determine the
transmissivity of the aquifer. The length of the well screen was assumed to be
the characteristic thickness of the aquifer, and this length was used to calculate
the hydraulic conductivity from the transmissivity.
Three Hvorslev equations were used while using AQTESOLVTM under three
different conditions: partially penetrating wells within confined aquifer; a well
abutting a confined aquifer; and a fully penetrating well within a confined or
unconfined aquifer.
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Appendix C - Methodology
The equations are listed below:
(1) Hvorslev's equation for a partially penetrating well not in contact with an impermeable
boundary.
In(HO) — In(H) _
r21n( L + 1 2K,.Lt + ( L )2
2rwe 2rwe
rwe=rw V KlKr
(2) Hvorslev's equation for a well abutting a confining unit.
r
In(HO) — In(H) = 2KLt2
r ln(200)
(3) Hvorslev's equation for a fully penetrating well in an unconfined or confined aquifer.
where:
ln(HO) — In(H) =
2K,Lt
rzln( L + 1+( L )z
2rwe 2rwe
167
Ho
= initial displacement at t=0 [L]
47
H
= displacement at time t [L]
0
K,.
= radial (horizontal) hydraulic conductivity [L/T]
41'
L =
screen length [L]
161'
t =
elapsed time since initiation of the test [T]
47
r, = nominal casing radius [L]
'67
r,
= well radius [L]
47
KZ
= vertical hydraulic conductivity [L/1']
The hydraulic head data obtained during slug tests from this investigation could
always be approximated by a straight line, but the degree of approximation
varied between the tests. A concave -upward curvature of the slug test data was
common, and this could result from storage processes in the aquifer (Butler,
1998)—in general, storage is ignored in the Hvorslev analysis. In such cases, the
two most common approaches to fit a straight line to the plotted curve for the
Hvorslev method were exercised (Butler, 1998). The most common approach
matches the early time data of the slug test while ignoring the late time data
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Comprehensive Site Assessment Report
Appendix C - Methodology
where the curvature is typically manifested. Another approach breaks up the
curve into two separate line segments and the analysis uses the slope of the
second linear segment. This approach was applied when the slug test was
affected by storage in a well sand pack, which affected the early response of the
test.
3. Screening Level Risk Assessments
To support the groundwater assessment, potential risks to human health and the environment
were assessed in accordance with applicable federal and state guidance. Screening level human
health and ecological risk assessments were conducted that included development of
conceptual exposure models (CEM) to serve as the foundation for evaluating potential risks to
human and ecological receptors. The purpose of the human health and ecological CEMs was to
identify potentially complete exposure pathways to environmental media associated with the
site and to specify the types of exposure scenarios relevant to include in the risk analysis.
Potential exposure pathways were considered complete when all of the following elements
applied: 1) a constituent source; 2) mechanisms of constituent release and transport from the
source area to an environmental medium; and 3) feasible routes of potential exposure at the
point of contact (e.g., ingestion, inhalation, dermal or ambient contact). Maximum constituent
concentrations were compared to appropriate risk-based screening values as a preliminary step
in evaluating potential for risks to human and ecological receptors. Based on results of the
screening level risk assessments, a refinement of COPCs will be conducted and more definitive
risk characterization will be performed as part of the corrective action process if needed.
3.1 Human Health Risk Assessment
The screening level human health risk assessment process involved comparison of
constituent concentrations in various media to the following risk-based screening
criteria:
y Soil analytical results collected from the 0 to 2 foot depth interval compared to US
EPA residential and industrial soil Regional Screening Levels (RSLs) (US EPA, June
2015);
0 Groundwater results compared to NCDENR Title 15A, Subchapter 2L Standards
(NCDENR, 2006);
0 Surface water analytical results compared to North Carolina surface water standards
(Subchapter 2B) and US EPA national recommended water quality criteria
(NCDENR, 2007; US EPA, 2006).
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Duke Energy Progress — Mayo Steam Electric Plant
Groundwater Assessment Report
Comprehensive Site Assessment Report
Appendix C - Methodology
y The surface water classification as it pertains to drinking water supply, aquatic life,
high/exceptional quality designations and other requirements for other activities
(e.g., landfill permits, NPDES wastewater discharges) were noted;
0 Sediment results compared to US EPA residential and industrial soil RSLs (US EPA,
October 2014 or latest update); and
h Sediment, soil and ground water results compared to available local, regional and
national background sediment, soil and ground water data, as available.
,611 If warranted as part of corrective action decisions, site and media specific risk-based
remediation standards can be calculated in accordance with the Eligibility
Requirements and Procedures for Risk -Based Remediation of Industrial Sites
Pursuant to N.C.G.S. 130A-310.65 to 310.77, North Carolina Department of
Environment and Natural Resources, Division of Waste Management, 29 July 2011.
3.2 Ecological Risk Assessment
The screening level ecological risk assessment (SLERA) for the site included a
description of the ecological setting and development of the ecological CEM specific to
the ecological communities and receptors potentially exposed to site -related COPCs. A
list of potential ecological receptors (e.g., plants, benthic invertebrates, fish, mammals,
birds, etc.) was compiled, as well as identification of sensitive ecological populations and
critical habitat based on information from the North Carolina Natural Heritage Program,
and U.S. Fish and Wildlife Service.
Step 1 of the SLERA consisted of completion of an ecological checklist as required by
Guidelines for Performing Screening Level Ecological Risk Assessment within North
Carolina (NCDENR, 2003).
Step 2 of the SLERA consisted of performing screening level exposure estimates and risk
calculations. This involved comparison of maximum detected concentrations or
maximum detection limits for non-detected constituents to applicable ecological
screening values (ESVs) intended to be protective of ecological receptors. If exposure
concentrations exceeded ESVs, potential ecological impacts could not be ruled out.
The following ESV sources were used in the SLERA:
0 US EPA Ecological Soil Screening Levels;
0 US EPA Region 4 Recommended Ecological Screening Values; and
y US EPA National Recommended Water Quality Criteria and North Carolina
Standards.
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Duke Energy Progress - Mayo Steam Electric Plant
Groundwater Assessment Report
Comprehensive Site Assessment Report
Appendix C - Methodology
Constituents were identified as a Step 2 COPCs as follows:
0 Category 1- Constituents whose maximum detection exceeded the media -specific
ESV;
y Category 2 - Constituents that generated a laboratory sample quantitation limit that
exceeded the US EPA Region IV media -specific ESV;
h Category 3 - Constituents with no US EPA Region IV media -specific ESV but were
detected above the laboratory sample quantitation limit;
4? Category 4 - Constituents that were not detected above the laboratory sample
quantitation limit and had no US EPA Region IV media -specific ESV; and
0 Category 5 - Constituents with a sample quantitation limit or maximum detection
that exceeded the North Carolina Surface Water Quality Standards.
3.3 Surface Investigation
Samples were collected at the ground or water surface to support the screening level risk
assessment. Samples were collected wearing nitrile gloves.
3.3.1 Surface Water Sampling
Surface water samples were collected to assess groundwater to surface water
pathways and evaluate surface water quality. Sample jars that did not contain a
preservative were used to collect the water directly from the source. The water
was decanted from the sample jars into the jars that required preservative.
Surface water samples were analyzed for parameters listed in Table 10-2, except
for radionuclides and metals speciation parameters. Stream flow measurements
were recorded at the time of sampling with the exception of measurements
within major waterways.
3.3.2 Sediment Sampling
Sediment samples were collected from the bed surface and co -located with
surface and seep samples to evaluate sediment quality and provide data to be
used in the screening level risk assessment. Where possible, samples were
collected directly into sample jars. If surface water was too deep to safely collect
sediment samples directly, sediment was obtained using a sampling dredge.
Sediment samples were analyzed for parameters listed in Table 6-2.
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Duke Energy Progress — Mayo Steam Electric Plant
Groundwater Assessment Report
Comprehensive Site Assessment Report
Appendix C - Methodology
3.3.3 Seep Sampling
Seep samples were collected wearing nitrile gloves to assess groundwater to
surface water pathways at the site and support the human health and ecological
risk assessment. Seep samples were analyzed for parameters listed in Table 10-2,
except for radionuclides and metals speciation parameters. Seep flow
measurements were recorded at the time of sampling.
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