Response to SELC comments - Mayo - 12 12 2015

MEMORANDUM
Date: December 15, 2015 File: 1026.105
To: Ed Sullivan, Duke Energy
From: Kathy Webb, Jerry Wylie
Subject: Duke Energy Groundwater Assessment Program — Comprehensive Site Assessment
Report — Comments by Southern Environmental Law Center for Mayo Steam Station
dated October 8, 2015
On October 8, 2015, the Southern Environmental Law Center (SELC) provided comments to the
North Carolina Department of Environmental Quality (NCDEQ) (formerly the North Carolina
Department of Environment and Natural Resources [NCDENR]) pertaining to Duke Energy's
Comprehensive Site Assessment (CSA) report for the Mayo Steam Electric Plant (referred to by
SELC as "Mayo Steam Station"). SynTerra has considered these comments and offers the
following responses.
SELC Assertion: The CSA report is "inconsistent" and thereby 'fails to document the extent of
pollution at the site and its likely impacts to public waters, including Mayo Lake."
SynTerra Response: The CSA report provides substantive investigation and sampling results
that support the conclusions presented to NCDEQ and the public. Based on the results of the
assessment, SynTerra identified areas where supplemental investigation would be helpful to
further evaluate the site and presented those areas as data gaps in the CSA report. NCDEQ,
Duke Energy, and SynTerra understood and discussed the possibility of developing additional
data as a result of the Site investigation process.
The purpose of the CSA was to characterize the extent of impact resulting from historical
production and storage of coal ash; to evaluate the chemical and physical characteristics of the
constituents; to investigate the geology and hydrogeology of the Site, including factors relating
to contaminant transport; and to examine risk to potential receptors and exposure pathways.
SynTerra is confident that the CSA does determine the source -related constituents; provides
substantive (but not conclusive) information regarding the underlying geology and
hydrogeology of the Site; and correctly interprets groundwater flow at the Site and the potential
risk to public and private drinking water wells and surface water. For findings that were
considered incomplete, SynTerra is implementing, in collaboration with NCDEQ and Duke
Energy, a supplemental data collection and evaluation effort to refine the delineation of the
vertical extent of impact beneath the ash basin and to more completely characterize
hydrogeologic conditions at the Site.
SELC Comment: Section 6. 1, Site Geology and Hydrogeology - Site-specific geology on the east side of
the ash basin near the Flue Gas Desulfurization (FGD) ponds is inadequately defined. The Site Layout

SELC Response to Comments (Oct 8 2015) — Mayo Steam Electric Plant December 15, 2015
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Map (Figure 2-1) and Water Level Map (Figure 6-9) each show the FGD ponds located inside the waste
boundary, apparently indicating that the FGD ponds were constructed over disposed ash. However,
cross-sections identified as C -C' (Figures 6-3, 6-6, 8-3, 11 -1 b, 11-2b. 1 1-3b, 1 14b, 1 1-5b, 11-6b, 11-7b,
11-8b, 11-9b, and 11-1 Ob) each show coal ash waste boundary stopping short of the FGD ponds. Instead,
the FGD ponds are shown in these figures as being located above soil/fill, rather than coal ash. The
accurate location of the waste boundary must be determined and figures must be consistent throughout
the report. This is important for at least two reasons.
First, it appears that coal ash extends underneath the FGD ponds and is saturated by groundwater. Any
closure plan must account for how to separate this coal ash from the groundwater so it does not continue
to be a source of contamination.
Second, the location of the waste boundary is important in evaluating the hydraulic gradient and
groundwater flow velocity between the ash basin and Mayo Lake. As shown in the cross sections,
groundwater will remain level across the extent of the ash deposits before sloping downward to the lake,
so if the waste boundary extends closer to the lake, the gradient will be steeper and the velocity of
groundwater flow will be greater. It appears this groundwater flow may carry contaminants from the
edge of the FGD ponds east-southeast to Mayo Lake.
SynTerra Response: The waste boundary is accurately located based on the current
understanding of Site conditions at the time that the CSA report was prepared. Future
submittals, including a planned supplement to the CSA report, will reflect updated
interpretations and graphic representations of new information, as appropriate. Closure plans
will incorporate available site-specific data and observations.
The CSA report provides an interpretation that groundwater flow is to the northwest (towards
the ash basin) on the west side of the railroad tracks that bisect the property, in the vicinity of
the FGD ponds. To the east of the railroad tracks in this area, the flow is towards Mayo Lake.
The interpretation of groundwater flow direction is consistent with the "slope -aquifer system"
model by LeGrand (documented in the CSA). A review of analytical data from compliance
wells CW -01 and CW -01D, located on the east side of the railroad across from the FGD ponds,
indicates that no constituent concentrations associated with coal ash have been detected greater
than the 2L. Vanadium has been detected greater than the IMAC at this location; however, the
concentrations are less than background for the Site.
SELC Comment. Section 6.3.1, Groundwater Flow Direction - The direction of groundwater flow in the
vicinity of the FGD ponds is inadequately defined. The CSA report describes Mayo Lake as acting as a
groundwater discharge area on the east side of the Plant. Each of the C -C' cross-sections (listed
previously) show the water table declining to the east between the ash basin and wells CW -111D with
flow toward the east beneath the area of the FGD ponds. But the Water Level Map (Figure 6-9) shows the
hydraulic gradient flowing toward the north from a high beneath the electric plant toward the ash basin
in this same location. In other words, one set of figures shows groundwater flowing into the ash basin,
and another shows the groundwater flowing out of the ash basin in the same location. An additional well

SELC Response to Comments (Oct 8 2015) – Mayo Steam Electric Plant December 15, 2015
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cluster located just to the north-east of the FGD ponds, in the rectangular open area between the FGD
ponds and the discharge canal, is needed to provide additional information on geology and groundwater
flow direction, velocity and chemistry in the area of the FGD ponds. Again, it appears contaminated
groundwater may be flowing from the coal ash basin to Mayo Lake in this location.
SynTerra Response: Hydrogeologic conditions in the vicinity of the FGD ponds will be more
completely evaluated during forthcoming (early 2016) assessment activities planned to comply
with requirements of the federal Final Rule - Disposal of CCR from Electric Utilities (April
2015). As part of that assessment work, additional borings and monitoring wells will be drilled
and installed around the FGD ponds. These data will be useful in better defining the
groundwater flow regime in the area east of the ash basin/FGD ponds and west of Mayo Lake.
SELC Comment: Section 6.4, Hydrogeologic Site Conceptual Model— The CSA report (p. 38) states
that "there are no substantive differences in water level among wells completed in the different flow
zones. " However, an examination of water level data provided on the Water Level Map (Figure 6-9)
shows the following water level differences at clustered well locations:
Wells (Water Elevation)
Water Level Elevation Difference and
Vertical Gradient Direction
MW -08S (437.02) MW-08BR (432.54)
4.48 ft. Downward
MW -07D (444.46) MW-07BR (445.94)
1.48 ft. Upward
MW -16S (366.92) MW-16BR (365.41)
1.51 ft. Downward
CW -05 (501.53) MW-05BR (501.94)
0.41 ft. Upward
ABMW-02 (481.56) ABMW-02BR (482.75)
1.19ft. Upward
ABMW-04 (485.11) ABMW-04BR (483.42)
1.69 t. Downward
Based on these data, it is inappropriate for the conceptual site model to indicate that there are no
significant differences in water level between the different flow zones. The data show that significant
water level differences are present and variable in direction. The conceptual site model must indicate that
bedrock discharges into overlying units in some locations and is recharged from overlying units in other
locations. The amount of groundwater that discharges into overlying units from bedrock or from
overlying units to bedrock must be accurately incorporated into any valid groundwater model for Duke
Energy's upcoming corrective action plan, and will likely become a critical component of any subsequent
evaluation of remediation strategy. DENR must require Duke Energy to incorporate the vertical
components of groundwater flow into its current and future analyses.
SynTerra Response: Groundwater modeling performed and reported in the CAP Part 1
utilized the actual measured water levels and thus, the vertical gradients across the Site have
been incorporated into corrective action plan evaluations. A refined Site Conceptual Model
being developed with the Part Cap 2 will take the varying hydraulic gradients into
consideration.

SELC Response to Comments (Oct 8 2015) – Mayo Steam Electric Plant December 15, 2015
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SELC Comment. Section 6.4, Hydrogeologic Site Conceptual Model— The CSA report (p. 38) states
Section 11.2 Hydrostratigraphic Layer Properties - Cross-sections (Figures 11-1a through 11-1 ON
presented in this section show concentration contours for various constituents based on concentrations
detected in monitoring wells. Many of these figures are misleading because they do not incorporate the
probability that contaminants have migrated downgradient beneath the ash basin. Concentrations of
boron, manganese and vanadium have been detected in ash basin water in concentrations above the 2L
standard. Infiltration of this water through the underlying ash and into the saprolite and bedrock will
undoubtedly result in elevated contaminant levels below the impounded water. The cross-sections should
incorporate the surface water data contained in the CSA and should indicate that the extent of
contamination for extends downgradient beneath the ash basin, as appropriate.
SynTerra Response: All data available at the time the CSA report was submitted are included
on the cross-sectional figures.
Groundwater data for the bedrock wells completed in the ash basin indicates no apparent
impact to bedrock beneath the ash basin from boron, the most prevalent coal ash constituent
observed in Mayo Plant groundwater. Further, other constituents (e.g., manganese and
vanadium) in the bedrock aquifer were detected at concentrations at/near background
concentrations (as detailed in the Corrective Action Plan Part 1). Regardless, NCDEQ and Duke
Energy have agreed that an additional groundwater monitoring well will be installed at a
deeper elevation to verify that the vertical extent of impact has been defined. Well installation
is planned for January 2016.