Duke Ash Basin SELC- Roxboro - Comment Summary rev 2_20160418

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Memo
Date: December 11, 2015
Project: Duke Energy Groundwater Assessment Program
Comprehensive Site Assessment Reports
To: Ed Sullivan, Duke Energy
From: Kathy Webb, Perry Waldrep, SynTerra
Subject: Roxboro Southern Environmental Law Center Letter dated October 27, 2015
SynTerra Comments
On October 27, 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]) concerning Duke Energy's Comprehensive Site Assessment (CSA)
report for the Roxboro Steam Plant. In summary, the SELC asserts that Duke
Energy (and by extension its consultant, SynTerra):
(1) Concluded incorrectly that the ash basins are separated from
groundwater; they appear to be deeply submerged up to 70 feet deep
into groundwater.
(2) The groundwater flow direction in the regolith/saprolite and bedrock
were not all mapped.
(3) There is insufficient data to support that the transition zone under the
ash basins are dry and that the ash pore water is perched.
(4) The vertical hydraulic gradient is downward from the ash basins to the
underlying bedrock.
(5) GMW-8 is referred to as an upgradient well.
(6) There appears to be an erroneous hydraulic conductivity value reported for
a soil sample below the ash basin.
(7) Concentration contours on cross -sections appear to be misleading.
SynTerra has considered these comments and offers the following responses.

Ed Sullivan
December 2015
SELC Assertion 1: The ash basins are not separated from groundwater
and ash is submerged up to 70 feet in depth.
SynTerra Response: Field observations in more than one location appeared to
indicate the transition zone beneath the ash basin(s) was dry and therefore the
pore water appeared to be perched above the water table in the underlying
bedrock. This field observation during drilling was not subsequently supported by
water levels collected from monitoring wells.
Concern: Modeling, closure and corrective action plans should take into account
that the present dam configurations create significant thicknesses of saturated
ash. Regional groundwater flow toward the basins may result in ash remaining
saturated following closure (assuming cap in place) without engineering controls.
Therefore: The groundwater modeling conducted in CAP Part 1 utilized the
observed water levels. The refined Site Conceptual Model to be provided in the
CAP Part 2 will provide clarity on the observations and the data. Closure and
corrective action modeling will utilize the actual water level measurements to
support the evaluations. Associated figures will be revised accordingly in future
submittals.
SELC Assertion 2: The groundwater flow direction in the regolith/saprolite and
bedrock were not all mapped
SynTerra Response: Due to the dry saprolite and transition zone in areas of the site,
water level maps for these zones were not prepared. The groundwater flow zones in
these units are not continuous and therefore mapping the units is challenging. The water
level data from these units will be presented on maps in future submittals.
SELC Assertion 3: There is insufficient data to support that the transition zone
under the ash basins are dry and that the ash pore water is perched.
SynTerra Response: Based upon field observations, some areas under the ash basins
appeared to be dry while others, particularly where alluvial deposits were encountered,
were wet. The alluvial deposits under the ash basins will be preferential flow zones above
bedrock. These areas are known from historical topographic maps. These features will
be more fully discussed in the refined Site Conceptual Model for CAP Part 2. As
previously discussed, the modeling for corrective action uses actual water level
measurements.
E

Ed Sullivan
December 2015
SELC Assertion 4: The vertical hydraulic gradient is downward from the
ash basins to the underlying bedrock.
SynTerra Response: The vertical hydraulic gradient is variable across the site.
The modeling is based upon actual water level measurements in the ash pore
water and groundwater. Generalizations that may be misleading will be avoided.
SELC Assertion 5: GMW-8 is referred to as an upgradient well
SynTerra Response: GMW-8 was originally installed as an upgradient background well.
However, it is understood that the well is located downgradient of the current CCR unit
configuration. The data from the well has not been used for background groundwater
quality determination purposes. The `upgradient' description of the well was not intended
to infer `background' conditions. In the future the description for the well location will be
more clearly defined.
SELC Assertion 6: There appears to be an erroneous hydraulic conductivity
value reported for a soil sample below the ash basin.
SynTerra Response: The laboratory data will be checked and the error corrected
in future reports if necessary. The CSA incorrectly cited the sample in question as
an ash sample when it was in fact a soil sample collected below the ash. The
hydraulic conductivity value reported is comparable to others for soil below the ash
in the east basin.
SELC Assertion 7: Concentration contours on cross -sections appear to be
misleading.
SynTerra Response: The vertical extent of fractured bedrock and the constituent
concentrations have been inferred based upon available data from monitoring
wells. It is anticipated that the alluvial deposit preferential flow zones combined
with low permeability silts and clays in other areas have reduced the potential for
infiltration into the underlying bedrock. Care will continue to be taken to avoid
misleading representations of the data. Further understanding of the influence of
the alluvial deposits and the low permeability silts and clays will be provided in the
refined Site Conceptual Model.
SELC Conclusion: The Roxboro CSA misrepresents the relationship
between the groundwater and the porewater within the ash basins at the site.
SynTerra Response: The hydrogeological properties of the site will be more fully
evaluated and clarity will be provided in the refined Site Conceptual Model. The
groundwater modeling for closure and groundwater corrective action evaluations
utilize actual field measurements. The modeling conducted for the CAP Part 1 will
be used in preparation of the refined Site Conceptual Model for the CAP Part 2.
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