WQ0000020_Response to comments from letter dated Feb. 18, 2014_20140618

AIZGI-K�AWA
NCDENR
North Carolina Department of Environment and
Pat McCrory
Governor
FILES
Natural Resources
John E. Skvarla, III
Secretary
John R. Toepfer June 16, 2014
RECEIVED/DENR/DWR
Duke Energy Progress, Inc. JUN 1$ 2014
410 S. Wilmington Street/NC14 Water Quality Regional
Raleigh, NC 27601 Operations Section
RE: Response to comments from Duke Energy Progress dated February 18, 2014
concerning the Asheville Airport engineered fill project - Area 3 development
Dear Mr. Toepfer,
The Asheville Regional Office of the Division of Water Resources (DWR) acknowledges
receiving a response to our review comments dated October 23, 2013. Additional comments
and questions are given in the bulleted items below:
• In your letter, you state that all future water quality data pertaining to leachate or CCB
contact water within the open cells will be submitted beginning with the April 2014
groundwater monitoring results. Please submit all past and future water quality data
pertaining to the contact water within CCB structural fill areas 3 and 4.
In your letter, you state that it is well documented in the scientific literature that
chloride can be responsible for a false positive result for thallium. Based on
conversations with our own DWR laboratory, chloride levels must be significantly
elevated before any polyatomic interference can take place. Do you have additional
evidence that suggests chloride concentrations as low as 3 mg/1 may be a source of
analytical interference with regards to thallium? Please submit any pertinent scientific
literature that supports your position.
According to 15A NCAC 02T.1204 (d) (2), new and expanding [CCB] structural fill sites
must describe and explain site specific engineering or institutional controls proposed
to prevent adverse impacts to public health and the environment. The composite liner
system for CCB areas 3 and 4 is designed to significantly restrict infiltrating
precipitation and limit the release of CCB leachate. The composite liner system, while
restrictive, cannot be considered entirely impervious. Therefore, limited solute
transport modeling was conducted by GTI to demonstrate the performance of the
composite liner system and its ability to protect groundwater quality at nearby
regulatory and water supply receptors.
The ARO previously provided comments regarding Geotrack Technologies Inc. (GTI)
groundwater modeling results where the 90% concentrations for several analytes
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Duke Energy Progress - Asheville
June 16, 2014
Page 2 of 3
were higher at the compliance boundary than the review boundary for three different
modeled geometries. The ARO requested a more detailed description of the IWEM
model's "prediction error" or alternatively, further analysis that includes a greater
number of Monte Carlo realizations. GTI responded by performing additional ground
water modeling (January 30, 2014) to reflect conditions more specific to the area. The
modeled geometry included receptors at the review boundary (62 feet), compliance
boundary (165 feet), and an offsite water supply well (240 feet). Please note that the
distance given for the modeled review boundary is not midway between the waste and
compliance boundaries as defined by 15A NCAC 02L.0108.
The conceptual model of groundwater flow in the Blue Ridge and Piedmont regions of
North Carolina is essentially a two-part system consisting of regolith or weathered
rock and the underlying fractured bedrock. In many locations, the regolith includes a
transition zone above the fractured bedrock consisting of coarse fragments of partially
weathered bedrock. According to technical documents, the IWEM model is designed for
relatively simple groundwater flow systems in which flow is dominated by a regional
gradient. The model may not be appropriate for sites associated with fractured
bedrock, heterogeneous aquifers, non-uniform thickness of water bearing zones, or
sites with significant seasonal variations in water table elevation. Please provide a
detailed discussion as to why the IWEM model is appropriate for this location.
In the "no liner" scenario, GTI does not adequately address potential problems with the
model's performance. In one instance, there is a slight increase in modeled
concentrations for arsenic and barium from the compliance boundary to the water
supply receptor. GTI responds by saying the no liner scenario is unrealistic for this site
and that predicted arsenic and barium concentrations follow a more logical reduction
in the other two liner scenarios. The suggestion of ignoring one set of modeling results
for another is not a viable answer. Another observation is an almost complete lack of
attenuation for any constituent in the "no liner' scenario. It is our understanding that
the greatest influx of CCB leachate to the unsaturated/saturated zone modules occurs
in the "no liner" scenario. It seems unrealistic that little to no depletion of the leachate
constituents occur as a result of dispersion, diffusion, or sorption processes.
In the "clay liner" scenario, the modeled concentration for chromium increases from
the review to the compliance boundary. Similarly, the modeled concentrations for
boron, iron, and sulfate increase from the review to the compliance boundary in the
"composite liner" scenario. In both scenarios, GTI ignores problematic trends based
solely on the fact that the predicted concentrations are below analytical detection.
After it has been demonstrated that the model is producing consistently reliable
results, it is suggested that sensitivity analyses be conducted to determine which
variables are the most critical to the modeled solute transport.
An attempt was made by GTI to address performance issues with the IWEM model.
They suggest that default infiltration rates combined with the relatively low subsurface
hydraulic conductivities result in theoretical ground water mounding. According to

Duke Energy Progress - Asheville
June 16, 2014
Page 3 of 3
their interpretation, this mounding dilutes modeled concentrations near the CCB fill
and less so as the mounding dissipates with distance. According to technical
documents, the unsaturated zone (1-D) and saturated zone (3-D) modules are separate
domains computationally linked through continuity of flow and constituent
concentration across the water table directly underneath the waste management unit
(WMU). Groundwater mounding due to high infiltration rates may occur in the
unsaturated zone module and is represented by increased head values at the top of the
aquifer. The actual saturated zone module is based on the assumption of a constant
saturated thickness and the only direct effect of ground -water mounding is to increase
simulated ground -water velocities. In other words, the module does not simulate the
actual physical increase in the thickness of the saturated zone.
GTI concludes by saying, "while the results indicate [for the composite liner scenario]
small increases in concentrations of certain compounds with distance from the fill, all
of the computed concentrations can be considered essentially zero" and "thus, we
conclude that the modeling demonstrates that the proposed liner system is protective
of the environment." According to technical documents, the mass of constituents
leached from the WMU is directly proportional to the infiltration rate of the various
liner scenarios. There is no discussion as to the fate of leachate mass once it enters the
unsaturated/saturated zone modules.
Modeling is an important component of the project given the close proximity of
receptors to the CCB waste boundary. To this end, the Division would like to see a more
robust conceptual model, a model that is producing consistently reliable results, and a
sensitivity analysis of key variable affecting the predictive results. Along with this, the
Division would like to see a description of the model limitations and any associated
implications regarding its use for the Asheville Airport CCB structural fill project.
Please feel free to contact me if you have any questions regarding our review comments. I can
be reached at [phone] or [e-mail].
Sincerely
L
Br t Lav
Water Quality Regional Operations Section
Asheville Regional Office
Cc: file
Jon Risgaard- DWR Raleigh