NC0038377_Mayo_Appendix B_20191231

Appendix B Corrective Action Plan Update December 2019
Mayo Steam Electric Plant
APPENDIX B
SynTerra
COMPREHENSIVE SITE ASSESSMENT UPDATE
(SYNTERRA, 2017)
NCDEQ COMMENTS AND RESPONSES
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2017 COMPREHENSIVE SITE ASSESSMENT
UPDATE COMMENTS
On October 31, 2017, Duke Energy submitted a CSA Update to NCDEQ. In a letter to
Duke Energy dated May 7, 2018, NCDEQ stated that sufficient information had been
provided in the CSA Update to allow preparation of a CAP. The letter also provided
CSA-related comments and items required to be addressed prior to or as part of the
CAP submittal (CAP Update, Appendix A).
On June 7, 2018, NCDEQ Raleigh Regional Office (RRO) submitted an email with the
subject: "Draft comprehensive review comments for Mayo and Roxboro". The email
provided an attachment with additional draft CSA Update comments (CAP Update,
Appendix A).
Duke Energy responses to both sets of CSA-related comments are provided below. The
comments were originally provided in paragraph form, some with bulleted lists. For
ease of reference, the comments and provided responses have been sequentially
numbered.
CSA Update report deficiencies (May 7, 2018 letter)
Comment 1 - The report contents are presented in a data summary format, exhibiting a
lack of conclusive data analysis and interpretation of site conditions.
Response — The CSA Update report was presented in data summary format
to clearly provide the facts regarding site characterization. Duke Energy
disagrees with the contention that data analysis conclusions and interpretation of
site conditions were not provided. The CSA Update report was provided to
supplement the previously submitted CSA Report. Data previously provided to
NCDEQ was intentionally not included so that the Update could focus on
additional site assessment. Duke Energy now recognizes that NCDEQ desires
updates to include previously reported data and historical information directly
rather than by reference.
Comment 2 - The report fails to fully integrate and evaluate data collected from
previous versions of the CSA reports for the facility.
Response — As indicated in the response to Comment 1, all historical and
current CSA data (as of the date of report submittal) was provided in tabular and
graphical form for succinct definition of site conditions. Previous CSA and CAP
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submittals provided detailed information that was not deemed necessary for
repetition in the CSA Update report.
Comment 3 - The distribution of constituents of interest related to coal ash sources
presented in the report often fail, for at least some areas of the site, to fully and clearly
acknowledge and delineate exceedances of the 15A NCAC 2L or 2B standards above
background levels.
Response — Discussion regarding 02L exceedances are defined in more detail
in the CAP Update, Section 6.1. Additional sampling for groundwater (02L) to
surface water (02B) evaluation has been conducted and is provided in this CAP
Update, Section 6.2.1 and Appendix I).The evaluations concluded that
discharging groundwater does not currently cause and is not predicted to cause
concentrations in surface water greater than the 02B standard.
Comment 4 - Groundwater: The characterization of other primary and secondary
sources other than impoundments that contribute to the groundwater plumes is inadequate.
Response — An overview of primary and secondary sources, along with
NCDEQ consensus, is presented in Section 3.0 of the CAP Update.
Comment 5 - As detailed more fully in the attached document, additional data gaps
remain concerning delineation of impacts from coal ash at the facility.
Response — Additional data gaps as presented in the Mayo site -specific CSA
comments (below) are addressed in this CAP Update.
COMMENTS FOR MAYO CSA UPDATE
(ATTACHMENT TO MAY 71 2018 LETTER)
Soils
Comment 6 - The SCSA lacks a comprehensive summation of the extent of soil impacts
beyond the compliance boundary and in the areas beyond the waste boundary.
Response — There is no impact to soil beyond the ash basin waste boundary;
therefore, there is no ash basin -related impact to soils beyond the compliance
boundary. On January 25, 2019 (revised February 11, 2019) an additional soil
assessment technical memorandum was submitted to NCDEQ. The technical
memorandum focused on assessment of potential impact to soils surrounding
the periphery of the Mayo ash basin - including downslope of the ash basin dam.
On March 7, 2019 NCDEQ (Raleigh Regional Office (RRO)) approved the
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perimeter soil assessment approach which has been completed. The results of
that assessment, including analytical data, conclude there is no impact to soil
beyond the ash basin waste boundary and are included in Section 6.1.2 of this
CAP Update.
Comment 7 - The majority of the ash basin footprint where native soils are expected to
be present was not investigated.
Response — Unsaturated soils were not encountered within the footprint of
the accessible portions of the ash basin. Saturated soil and rock is considered a
component of the groundwater flow system, which has been extensively
evaluated, and can serve as a source for groundwater constituents at the Site. The
potential leaching and sorption of constituents in the saturated zone is included
in the flow and transport and geochemical model evaluations by continuously
tracking the constituent concentrations over time in the surficial, transition zone,
and bedrock materials throughout the models. Historical transport models
simulate the migration of constituents through the soil and rock from the ash
basin, and these results are used as the starting concentrations for the predictive
simulations.
Comment 8 - Detailed maps which depict soil impacts were not submitted with the
update.
Response — Figure 7-1 was provided in the CSA Update indicating the
limited nature of impact to soil beneath the ash basin. Since submittal of the CSA
Update, additional soil samples have been collected. Figure 6-9 in the CAP
Update shows soil sample locations with depths for all unsaturated soil samples
collected. Any exceedances of PSRG POGs and/or background values are noted.
Comment 9 - RRO currently does not agree with Duke's PBTV soil "background"
values for Mn, Fe, or As.
Response — Soil BTVs for manganese, iron, and arsenic were approved by
NCDEQ in a letter dated May 23, 2019 (CAP Update, Appendix A).
Sediment
Comment 10 - Sediment sample locations S-3, S-4 and S-8 demonstrate some presence
of CCR/COI in greater concentrations than either the PSRG or PBTV soil values.
Response — Locations S-4 and S-8 are associated with non -constructed seeps
inside the ash basin compliance boundary. These locations are predicted to be
eliminated by decanting/closure. Location S-3 is in Crutchfield Branch
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immediately downstream of a constructed seep (toe drain). COI concentrations
for sediment associated with location S-3 are within the range of site background
values except for chromium, which is not a ash basin -sourced constituent.
Additional information about sediment impacts is discussed in the Section 6.1.2
of this CAP Update.
Comment 11 - Sediment samples for antimony, arsenic, chloride, nitrate, sulfate,
selenium and thallium did not have detection limits that comply with either the PBTV or
PSRG POG and so cannot be evaluated for possible CCR constituent exceedances by this
criterion.
Response — Sediment samples collected after submittal of the CSA Update,
as part of the evaluation of potential groundwater impacts to surface water
(current conditions) in April 2018, were analyzed using reporting limits less than
the BTV and/or PSRG POGs values for antimony, arsenic, chloride, nitrate,
sulfate, selenium, and thallium (Appendix C, Table 5 of this CAP Update).
Sediment impacts are discussed in Section 6.1.2 of this CAP Update.
Comment 12 - Additional investigation and corrective action may be necessary. Also,
an effort to compare the results of the sediment results to potential background sediment
conditions will be necessary to discern impacts of CCR constituents.
Response — Based on the sediment evaluation, there are no COIs in sediment
that require corrective action. No sediment sample from Crutchfield Branch, at or
beyond the compliance boundary had sediment constituent concentrations
greater than the range of sediment background values. Table 4-5 provides the
range of sediment background values.
Sediment concentrations from one constructed seep (west toe drain, S-1) and
three non -constructed seeps (S-3, S-4, and S-8) are greater than the range of
sediment background values; however, corrective action for these sediments is
not required as discussed in this CAP Update (Section 6.1.2). Sediment data is
provided in Appendix C, Table 5.
Surface Water
Comment 13 - The collected sampling data indicates that CCR constituents (boron as
an example) has been detected at various locations below the dam such as daylight seepage in
the dam structure, near the point of compliance for the NPDES permit and off site on private
property near the Virginia border. Surface water 2B standard exceedances were identified in
the CSA Update in Crutchfield Branch for copper and dissolved oxygen. Additional
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sampling of surface waters and a determination if there are 2B standard violations will be
necessary to support corrective action.
Response — A detailed evaluation for potential impacts from groundwater to
surface water under current conditions, including exceedances of 02B standards,
was conducted in April/May 2018 in accordance with NCDEQ protocols. The
results were provided to NCDEQ on March 21, 2019 (CAP Update Appendix I).
The evaluation concluded that no verified exceedances of 02B standards are
caused by groundwater discharging to Crutchfield Branch. In addition, an
evaluation of potential groundwater to surface water impacts under future
conditions (ash basin closure) is presented in this CAP Update (Appendix J). The
future conditions evaluation predicts no exceedances of 02B standards caused by
discharging groundwater.
Comment 14 - Detection limits for some constituents were too elevated to assess for 2B
standards such as cadmium and lead (etc.).
Response — According to the surface water evaluation to assess 15A NCAC
02B .0200 compliance for implementation of corrective action under 15A NCAC
02L .0106 (k) and (1) (CAP Update, Appendix I), this was true for cadmium,
silver, and lead. Silver and lead are not COIs for the Mayo ash basin so any
concentration greater than 02B standards would not be related to the ash basin.
Cadmium low level analyses were conducted as part of the surface water future
conditions evaluation and cadmium was not detected
(CAP Update, Appendix I).
Groundwater
Comment 15 - Ash basin wells installed on the southern end of the basin have detected
the presence of CCR constituents greater that the 2L groundwater standardslIMAC or
PBTVs. The extent of CCR constituents at this location has not currently been defined to
precisely meet the 2L standardslIMAC or PBTVs.
Response — At ash pore water well ABMW-1, deeper wells into native
material (saprolite, transition zone, or bedrock) were not possible due to ash
stability and worker safety concerns. A drill rig or other heavy, mechanized
equipment is not able to be driven onto the ash basin at that location; therefore,
wells beneath the ash basin are not possible at that location.
A well was completed in the surficial material at ABMW-3S and in the transition
zone at ABMW-4D. Both wells indicated exceedances of COIs in the respective
flow zones at these locations. A bedrock well was not attempted at ABMW-3
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because bedrock wells were successfully installed at the ABMW-2 and ABMW-4
locations.
At well cluster ABMW-2, surficial material was not encountered beneath the ash
at that location; however, both a bedrock well and later, a second deeper bedrock
well, were installed during original and subsequent CSA assessment field work.
Groundwater is unimpacted in the bedrock flow zone beneath the ash basin as
indicated by groundwater analytical data from ABMW-2BR, ABMW-2BRL, and
ABMW-4BR.
In addition, as described in this CAP Update (Section 5.0), the ash basin was
constructed within a former perennial stream valley. The ash basin's physical
setting is a horizontal flow -through water system with groundwater migration
into the upgradient end, flowing north through the middle regions, and
migrating downward near the dam. Generally, the physical setting of the ash
basin within a former perennial stream valley limits the horizontal and vertical
migration of constituents to areas near and directly downgradient of the basin's
dam.
A localized effect to the overall flow -through water system occurs just south of
the ponded water in the ash basin. A review of historical aerial photographs
shows that in early 2006, sluiced ash in the built-up ash "delta" area (formed
from historic sluicing) had begun to be mechanically moved and stacked in the
area of exposed ash south of the present-day ponded water. The ash was
"stacked" for dewatering prior to transport for beneficial reuse. The effect of this
ash "stack out" out area is a localized one that results in a slight variation of the
horizontal flow -through concept. In this area of the ash basin, within the ash
basin boundary, there is slight downward vertical migration of groundwater
within the overall ash basin flow -through system. Downward flow is limited to
the saprolite and transition zone as bedrock upward vertical gradients prevent
downward flow below the transition zone. Based on direct groundwater data
and the revised understanding of the CSM, vertical delineation has been
achieved.
Comment 16 - For Orange County NC "background groundwater quality data"
included in the SCSA to be considered relevant to the investigation and factored into
correction action plan activities, a detailed discussion should be included in the CAP.
Response — The referenced information was not used in developing this CAP
Update; therefore, the reference is not included.
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Comment 17 - RRO currently does not agree with Duke's PBTV groundwater
"background" values for Hexavalent Chromium in bedrock and Sodium in the Transition
Zone.
Response — Groundwater BTVs were approved by NCDEQ on May 14, 2018
(CAP Update, Appendix A) with exceptions for hexavalent chromium in
bedrock and sodium in bedrock. Additional detailed information concerning the
four values that were not initially approved by RRO were addressed in a
conference call with DWR, Duke Energy, and SynTerra on July 10, 2019 and
summarized in a technical memorandum and associated statistical evaluation
files provided to RRO via email on July 11, 2018 (CAP Update, Appendix A).
Comment 18 - It may be necessary to install additional background monitoring wells
for the surficial flow regime given that the PBTV established for strontium in this unit is
based on data from one well.
Response — Flow and transport modeling has demonstrated that when
Crutchfield Branch is not free flowing and is impounded due to the presence of
beaver dams, locally losing stream conditions occur. When these conditions
occur, surface water recharges the shallow alluvium in which MW-16S is
screened. Under these conditions, detected groundwater concentrations of boron
in MW-16S reflect surface water concentrations of boron indicating that ash
basin -related constituents present in MW-16S are related to surface water rather
than a groundwater plume. Furthermore, strontium in the surficial flow zone
downgradient of the ash basin is not considered a concern. The detected
concentrations of strontium in MW-16S are within the Mayo surficial BTV
approved by NCDEQ RRO on May 14, 2019, in addition to NC Piedmont
regional background ranges presented in Table 4-3. Installation of background
wells screened in the surficial zone was attempted at multiple locations
throughout the Site over multiple assessment efforts. The occurrence of saturated
surficial materials is limited at Mayo; therefore, the surficial zone at those
locations was either not present or did not yield usable sample data (e.g., high
turbidity).
Comment 19 - Sampling results from monitoring well MW-5BR detected various
constituents (iron and manganese) greater than the PBTVs. Elevated concentrations greater
than PBTVs were not detected in the shallow well at this location.
Response — Elevated iron and manganese detected at MW-5BR is not
influenced by the ash basin; the presence of elevated iron and manganese is not
an indication of ash basin impacts because no boron is detected in MW-5BR or
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CW-5. Further, multiple lines of evidence demonstrate that flow is away from the
MW-5BR location. The wells at this location are located upgradient of the ash
basin (supported by flow and transport modeling which indicates groundwater
from the ash basin does not flow toward MW-5BR (CAP Update, Appendix G),
field water level measurements (Table 5-1), potentiometric maps (Figure 5-4a
through Figure 5-4c), and velocity vector maps (Figure 5-5a through 5-5c).
Comment 20 - Additional sampling from monitoring wells MW-18BR, MW-19D and
MW-19BR should be performed as the SCSA sampling data has revealed the presence of
various CCR constituents greater than the PBTVs.
Response Summary — Elevated iron, manganese, and/or vanadium
detected at MW-18BR, MW-19D, and MW-19BR are not influenced by the Mayo
ash basin. Additional sampling and analysis at those locations have been
included in this CAP Update. In addition, the presence of elevated constituents is
not an indication of ash basin impact because (1) no boron was detected in MW-
18D, MW-18BR, MW-19D, and MW-19BR; (2) concentrations are only outside of
the referenced background ranges for manganese (MW-18BR, MW-19D, and
MW-19BR), iron (MW-19D and MW-19BR), and TDS (MW-19D and MW-19BR);
and (3) these wells are not downgradient of the ash basin as supported by flow
and transport modeling (CAP Update, Appendix G), potentiometric maps
(Figure 5-4a through Figure 5-4c), and the velocity vector maps (Figure 5-5a
through Figure 5-5c).
The occurrence of inorganic constituents in groundwater of the Piedmont
Physiographic Province is well documented in literature. The distribution of
iron, manganese, and vanadium in groundwater does not exhibit a discernable
plume associated with the ash basin.
Site Conceptual Model
Comment 21 - The update to the SCM in the SCSA was lacking in detail. It was
anticipated and agreed that the latest submittal would combine the information collected in
the various reports to summarize a picture of how known site conditions relate to the initial
SCM and how the current understanding of the model provides a basis that can be beneficial
for closure/corrective action purposes. It will be necessary to enhance and refine the SCM for
CAP submittal.
Response — Data obtained through 2Q2019 has been used to enhance and
refine the CSM. The revised/updated CSM is included in this CAP Update
(Section 5.0).
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NCDEQ RALEIGH REGIONAL OFFICE - DRAFT 2017 CSA UPDATE
COMMENTS (JUNE 712018)
Soils
Comment 1 - The soils investigation in the Comprehensive Site Assessment (CSA) is
not considered sufficient regarding the assessment of the extent of soil impacts from coal
combustion residuals (CCR). The CSA does not contain a sufficient dataset to determine
whether there has been an impact to soil chemistry by coal ash residuals. The CSA lacks a
comprehensive summation of the extent of soil impacts beyond the compliance boundary and
in the areas beyond the waste boundary. The majority of the ash basin footprint where native
soils are expected to be present was not investigated. These are areas of the basin that have
saturated conditions (free water) present as well as areas with unsaturated conditions that
could not be investigated due to the instability of the ash (i.e. unsafe conditions for personnel
and assessment equipment). Soil environments in these areas would be expected to have
predominantly saturated conditions. Detailed maps which depict soil impacts were not
submitted with the update. There was no recommendation for the collection of additional
soils data or a presentation of where data gaps exist. Overall there is insufficient soils data to
determine if ash basin contents have impacted areas of regolith around the margins of the
basin including areas below the dam where Crutchfield Branch discharges and fluvial
deposits are present.
Response — Additional assessment of soil in the vicinity of the Mayo ash
basin concluded there is no impact to soil beyond the ash basin waste boundary;
therefore, there is no ash basin -related impact to soils beyond the compliance
boundary. On January 25, 2019 (revised February 11, 2019) an additional soil
assessment technical memo was submitted to NCDEQ (SynTerra, 2019). The
technical memo focused on assessment of potential impact to soils surrounding
the periphery of the Mayo ash basin - including downslope of the ash basin dam.
On March 7, 2019 NCDEQ (DWR - Raleigh Regional Office) approved the
perimeter soil assessment approach which has been completed. The results of
that assessment (CAP Update Section 6.1.2), including analytical data, conclude
there is no impact to soil beyond the ash basin waste boundary.
Unsaturated soils were not encountered within the footprint of the accessible
portions of the ash basin. Saturated soil and rock is considered a component of
the groundwater flow system, which has been extensively evaluated, and can
serve as a source for groundwater constituents at the Site. The potential leaching
and sorption of constituents in the saturated zone is included in the flow and
transport and geochemical model evaluations by continuously tracking the
constituent concentrations over time in the surficial, transition zone, and bedrock
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materials throughout the models. Historical transport models simulate the
migration of constituents through the soil and rock from the ash basin, and these
results are used as the starting concentrations for the predictive simulations.
A map of soil sample locations with depths for unsaturated soil samples is
included in this CAP Update (Figure 6-9). Two exceedances (chromium and
manganese) of PSRG POGs and background values are identified in two
samples. The sample locations are NCDEQ-approved background soil locations;
therefore, the chromium and manganese detections at these locations are not an
indication of impact from the ash basin.
Comment 2 - It is anticipated that further investigation of soils will be necessary
whether the approved corrective action is excavation, cap in place, active remediation, other
type or a combination of corrective actions. A more robust dataset will be important to the
groundwater modeling efforts to support the corrective action/s. The scope of the additional
investigation may be influenced by the chosen corrective action and/or basin closure. If, for
instance, excavation is the method of corrective action, excavation activities will allow for the
opportunity to investigate contaminated soils around the margins of the basin as excavation
activities progress including the areas downgradient below the dam. If the correction action
consists of a cap in place, it is recommended that soils around the margins of the ash basin
cap are evaluated for various environmental impacts as well as the area below the current ash
basin dam, including the FGD pond, NPDES Outfall 002, including impacted native soils to
be left in place that currently underlie the basin. It is suggested that additional soils
investigation take place in conjunction with corrective action or closure activities as
appropriate.
Response — As noted in the response to Comment 1 (above); additional
assessment of soil in the vicinity of the Mayo ash basin concluded there is no
impact to soil beyond the ash basin waste boundary; therefore, there is no ash
basin -related impact to soils beyond the compliance boundary. On January 25,
2019 (revised February 11, 2019) an additional soil assessment technical memo
was submitted to NCDEQ. The technical memo focused on assessment of
potential impact to soils surrounding the periphery of the Mayo ash basin -
including downslope of the ash basin dam. On March 7, 2019 NCDEQ (DWR -
Raleigh Regional Office) approved the perimeter soil assessment approach which
has been completed. The results of that assessment (CAP Update Section 6.1.2),
including analytical data, conclude there is no impact to soil beyond the ash
basin waste boundary.
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Comment 3 - The constituents listed below had discrepancies between Duke calculated
PBTV and DWR values. The description listed indicates if the recalculation resolved the
discrepancies. Recalculation by Duke resolved a portion of the PBTV proposals:
• Copper- 97.5 mg/kg Acceptable. RRO agreeable with revised PBTV in CSA update
• Iron- 51501 mg/kg Acceptable. RRO agreeable with revised PBTV in CSA update
• Manganese-1856 mg/kg Not Acceptable. RRO calculated value 1268 mg/kg. Duke
previous calculation was 3144 mg/kg.
• Sodium-1111 mg/kg Not Acceptable. RRO calculated value 691 mg/kg. Duke
proposal same value as previous PBTV.
• Thallium- 0.406 mg/kg Acceptable. RRO agreeable with revised PBTV in CSA
update.
Response — Soil BTVs for manganese and sodium were approved by NCDEQ
in a letter dated May 23, 2019 (CAP Update, Appendix A).
Comment 4 - The following constituents were revised in the CSA Update
without an explanation to the revision and/or what new data was included in the
dataset to establish the new value:
Arsenic, Barium, Cadmium, Calcium, Chloride, Copper, Iron, Lead,
Magnesium, Manganese, Nickel, Nitrate (as N), Potassium, Strontium,
Sulfate, Thallium, Zinc
Response — Soil BTVs for all constituents except manganese, sodium, and
thallium were approved by NCDEQ and documented in a table submitted by the
RRO titled Mayo Steam Electric Plant - Soil Background Threshold Values (May 14,
2018). Manganese, sodium, and thallium were later approved in a letter dated
May 23, 2019 (CAP Update, Appendix A).
Comment 5 - All of the preceding constituents were revised "down" to a more
stringent value with the exception of Arsenic which had a previous approved PBTV
of 4.54 mg/kg. The proposed PBTV in the CSA Update is 5.8 mg/kg. It will be
necessary for Duke Progress to provide an explanation for the revised soil PBTVs.
➢ The CSA Update PBTV Soil Tables with comments are included with this
submission to CO
Response — The arsenic 5.8 mg/kg BTV was approved by NCDEQ and
documented in a table submitted by the RRO titled Mayo Steam Electric Plant -
Soil Background Threshold Values (May 14, 2018; CAP Update, Appendix A).
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Sediment
Comment 6 - Fluvial/alluvium sediment samples were collected from various
locations at the site. The CSA data compares the results to the PBTV or PSRG POG.
Also, the field collection of sediment samples has not been consistent. The
description of sample locations in the CSA states that the conditions vary from a
soil/sediment sample in an area of wetness water with flowing water to a sediment
sample collected from the bottom of a stream channel (Section 8.1). Specific
examples of sample locations were S-3 (collected from a channel bottom) then S-4
and S-8 (surface water/area of wetness) demonstrate some presence of CCR/COI in
greater concentrations than either the PSRG or PBTV soil values.
Response — Field collection of sediment samples was conducted in
accordance with the approved Proposed Groundwater Assessment Work Plan,
Revision No. 1 (SynTerra, December 2014). Details for the field sample collection
process were provided in the CSA Update, Appendix G. The varied
environments at Mayo from which sediment samples were collected necessitated
various sample collection approaches (e.g., stainless steel scoops for soil material
over which seeps were flowing, hand augers for sampling shallow stream
sediments, etc.). All sediment collection methods employed at Mayo are
industry -accepted practices that yield valid sample data.
Locations S-4 and S-8 are associated with non -constructed seeps inside the ash
basin compliance boundary. These locations are predicted to be eliminated by
decanting/closure. Location S-3 is in Crutchfield Branch immediately
downstream of a constructed seep (toe drain). COI concentrations for sediment
associated with location S-3 are within the range of site background values
except for chromium, which is not a ash basin -sourced constituent. Additional
information about sediment impacts is discussed in the Section 6.1.2 of this CAP
Update.
Comment 7 - Sediments samples for antimony, arsenic, chloride, nitrate, sulfate,
selenium and thallium did not have detection limits that comply with either the
PBTV or PSRG POG and so cannot be evaluated for possible CCR constituent
exceedances by this criterion. Elevated concentrations of arsenic, chromium, iron
and manganese were present in one or more of the sediment samples below the ash
basin dam that exceed either the PSRG POG or PBTV.
Response — Sediment samples collected after submittal of the CSA Update,
as part of the evaluation of groundwater impacts to surface water (current
conditions) in April 2018, were analyzed using reporting limits less than the
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PSRG POGs/background values (CAP Update, Appendix C, Table 5). Sediment
impacts are discussed in Section 6.1.2 of this CAP Update.
Comment 8 - The extent of the sediments sampling in the CSA would best be
referred to as "spot checks" of possible impacts of CCR constituents in various
locations but not in the effort to define impacts. Additional investigation and
corrective action may be necessary dependent upon the regulatory standards
applied to the results as well as the factoring in of the compliance boundary. Also,
an effort to compare the results of the sediment results to potential background
sediment conditions may be necessary to discern impacts of CCR constituents.
Response — Based on the sediment evaluation, there are no COIs in sediment
that require corrective action. No sediment sample from Crutchfield Branch, at or
beyond the compliance boundary had sediment constituent concentrations
greater than the range of sediment background values. Table 4-5 provides the
range of sediment background values.
Sediment concentrations from one constructed seep (west toe drain, S-1) and
three non -constructed seeps (S-3, S-4, and S-8) are greater than the range of
sediment background values; however, corrective action for these sediments is
not required as discussed in this CAP Update (Section 6.1.2). Sediment data is
provided in CAP Update, Appendix C, Table 5.
Surface Water
Comment 9 - Surface Water - Sufficient sampling efforts have been completed to
provide a general evaluation of geochemical conditions in the areas below the ash
basin dam for impact by CCR constituents for assessment purposes. Although there
are data gaps associated with this dataset. Detection limits for some constituents
were too elevated to assess for 2B standards such as cadmium and lead (etc.). The
collected sampling data indicates that CCR constituents (boron as an example) has
been detected at various locations below the dam such as daylight seepage in the
dam structure, near the point of compliance for the NPDES permit and off site on
private property near the Virginia border. Surface water 2B standard exceedances
were identified in the CSA Update in Crutchfield Branch for copper and dissolved
oxygen. Additional sampling of surface waters will be necessary to support
corrective action.
Response — A detailed evaluation for potential impacts from groundwater
to surface water under current conditions, including exceedances of 02B
standards, was conducted in April/May 2018 based on a DEQ-approved work
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plan. The results were provided to NCDEQ on March 21, 2019 (CAP Update,
Appendix I). The evaluation concluded that no exceedances of 02B standards are
caused by groundwater discharging to Crutchfield Branch. In addition, an
evaluation of potential groundwater to surface water impacts under future
conditions (ash basin closure) is presented in this CAP Update (Appendix I). The
future conditions evaluation predicts no exceedances of 02B standards caused by
discharging groundwater.
The 02B exceedances observed in Crutchfield Branch include:
Two detections of copper, which are anomalous, are attributed to field
sampling error. Therefore, they are unrelated to site conditions and
should be disregarded. Laboratory testing determined that residual
copper was present on filters and pre -flushing of the filters with deionized
water is required prior to sample collection; analytical results for
dissolved copper was much higher than the total copper concentration in
the two sample results indicating influence from the filters.
Three concentrations of low dissolved oxygen were detected from the
initial three surface water samples from a single location, beaver dam
impounded SW-CB2. These detections have been identified as outliers
from the current larger sampling data set.
Additionally, Duke Energy now owns the property north of the northern Mayo
property line to the North CarolinaNirginia State border which encompasses
Crutchfield Branch to the North Carolina state line.
Comment 10 - The section summary for surface water sampling implies that the
observed surface water chemistry may be consistent with samples collected at
locations upgradient of the coal ash impoundment where CCR impacts are not
expected to be present and so are more indicative of background conditions.
Additional efforts will be necessary to evaluate surface water conditions outside of
the ash basin dam to establish whether background geochemistry has a role in the
detected concentrations of constituents.
Response — As noted in the response to Comment 9 above, a detailed
evaluation for potential impacts from groundwater to surface water under
current conditions, including exceedances of 02B standards, was conducted in
April/May 2018 based on a DEQ-approved work plan. The results were provided
to NCDEQ on March 21, 2019 (CAP Update, Appendix I). The evaluation
concluded that no exceedances of 02B standards are caused by groundwater
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discharging to Crutchfield Branch. In addition, an evaluation of potential
groundwater to surface water impacts under future conditions (ash basin
closure) is presented in this CAP Update (Appendix I). The future conditions
evaluation predicts no exceedances of 02B standards caused by discharging
groundwater.
Comment 11 - Additional investigation/sampling is necessary for surface waters
to complete the assessment picture so that the results can be integrated with the
proposed corrective action/s.
Response — As noted in the response to Comment 9 above, a detailed
evaluation for potential impacts from groundwater to surface water under
current conditions, including exceedances of 02B standards, was conducted in
April/May 2018 based on a DEQ-approved work plan. The results were provided
to NCDEQ on March 21, 2019 (CAP Update, Appendix I). The evaluation
concluded that no exceedances of 02B standards are caused by groundwater
discharging to Crutchfield Branch. In addition, an evaluation of potential
groundwater to surface water impacts under future conditions (ash basin
closure) is presented in this CAP Update (Appendix I). The future conditions
evaluation predicts no exceedances of 02B standards caused by discharging
groundwater.
Groundwater
Comment 12 - The CSA Update focused on the following constituents of interest
(COI) regarding groundwater:
• Arsenic, Barium, Boron, Chromium (hexavalent), Chromium (total), Cobalt,
Iron, Manganese, Molybdenum, pH, Strontium, Sulfate, Total Dissolved
Solids (TDS), Vanadium
Section 10.3 states that site specific COIs were developed by evaluating groundwater
sampling results with respect to PBTVs, applicable regulatory standards, and
additional regulatory input/requirements. This office found the criteria used to
establish COI's for the facility to be vague in description.
Response — A constituent management process was developed by Duke
Energy at the request and acceptance of NCDEQ (NCDEQ letter dated October
24, 2019; CAP Update, Appendix A), to gain a thorough understanding of
constituent behavior and distribution in site groundwater and to aid in
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identifying unit -specific COIs. The constituent management process consists of
three steps:
1. Perform a detailed review of the applicable regulatory requirements
under NCAC, Title 15A, Subchapter 02L
2. Understand the potential mobility of unit -related constituents in
groundwater based on Site hydrogeology and geochemical conditions
3. Determine the constituent distribution at the unit under current and
predicted future conditions.
The COI management process is supported by multiple lines of evidence
including empirical data collected at the Site, geochemical modeling, and
groundwater flow and transport modeling. The management process uses a
matrix evaluation to identify those constituents that have migrated
downgradient of the source unit, in the direction of groundwater flow at
concentrations greater than 02L/IMAC/background value with a discernable
plume. This approach has been used to understand and predict COI behavior in
the subsurface related to the ash basin or COIs that are naturally occurring.
Details on the COI management approach are presented in Section 6.1.3 of this
CAP Update.
Comment 13 - A number of previously approved PBTVs varied from the
approved values as illustrated in Table 10-2. An explanation was provided in the
table notes that values have been updated based on ongoing statistical evaluations.
Values that varied from the Department approved values were:
• Barium- TZ No action recommended. Note that 2L standard is more than an
order of magnitude greater than the PBTV
• Iron TZ Note that difference is not considered statistically significant from the
PBTV approved previous.
• Copper- TZ No action recommended. The 2L standard is several orders of
magnitude greater than the PBTV
• Hexavalent Chromium- BR Not acceptable. RRO calculated value is 0.3 ug/L.
Duke value is 0.4 ug/L. Request Hexavalent Chromium dataset used in
calculation.
• Methane- TZ Note that difference is not considered statistically significant
from the PBTV approved previous.
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Sodium- TZ Not acceptable. RRO calculated value is 60 mg/L. Duke value is
72.6 mg/L. Request Sodium dataset used in calculation.
Zinc- TZ Note that difference is not considered statistically significant from
the PBTV approved previous.
➢ For all other constituents where values varied from the approved PBTV
the values listed in the CSA update were less than (<) the Department
approved values and so more stringent than the previous approved
values. ProUCL recalculations were completed where revised PBTV were
less stringent than prior approved values. This was not a straightforward
task given all groundwater data is lumped together in the CSA Update
Tables.
The CSA Update does not provide a clear description of the new data that was
included in the datasets for updated PBTV calculations. It is further complicated
because a number of the results from identical wells have two sample designations
such as "BG-01" and "BG-01 CAMA" which would have been collected on the same
date. The duplicate groundwater samples collected from the same well have some
variations in results for constituents. It is unclear what data Duke Energy utilized in
the datasets for updated PBTVs. For RRO purposes the more stringent value was
used in the ProUCL recalculations.
Response — Groundwater BTVs were approved by NCDEQ on May 14, 2018
(CAP Update, Appendix A) with only four exceptions (hexavalent chromium in
transition zone and bedrock; sodium in bedrock; and vanadium in bedrock).
Additional detailed information concerning the four values that were not
initially approved by RRO were addressed in a conference call with DWR, Duke
Energy, and SynTerra on July 10, 2019 and summarized in a technical
memorandum and associated statistical evaluation files provided to RRO via
email on July 11, 2018 (Appendix A).
Comment 14 - Section 10.1.2 is a presentation using some of the groundwater
sampling results in piper diagrams to explain site geochemistry and background
conditions. The section predominantly discusses the general use for piper diagrams
and cites Orange County, N.C. data as a comparison to the conditions at the Mayo
plant. The discussion contains little detail regarding an interpretation of the data
and does not go into any specifics to draw a conclusion on the data. For this
information to be considered relevant to the investigation and factored into
correction action plan activities, a detailed discussion should be included in the
CAP.
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Response — The referenced information was not used in developing this CAP
Update; therefore, the reference is not included.
Comment 15 - The current groundwater monitoring well network has provided
a basis for a general understanding of the extent of impacts. The collected dataset
should be sufficient to develop a corrective action plan. Due to the size and scope of
the investigation, it is assumed that additional investigation of the ash basin will be
necessary as natural conditions change and the basin and possibly the surrounding
area are modified to complete corrective action requirements. Locations that are
anticipated to require additional groundwater investigation and/or monitoring,
including the margins of the basin (outside the waste boundary), will be assessed at
some point for potential peripheral impacts of ash in both a solid and water phase.
Response — Additional soil, sediment, surface water, and groundwater data
collection and evaluation has been conducted since the submittal of the CSA
Update in October 2017. Data through 2Q2019 and corresponding evaluations
are included in this CAP Update in addition to recommendations for evaluation
and reporting to monitor Site conditions during and after basin closure
(CAP Update, Appendix J).
Comment 16 - Groundwater Vertical and Horizontal Extent of Contamination -
Ash basin wells installed on the southern end of the basin have detected the
presence of CCR constituents greater that the 2L groundwater standards/IMAC or
PBTV for a number of sampling events. The extent of CCR constituents at these
locations have not currently been defined to precisely meet the 2L standards/IMAC
or PBTVs due to the assessment limitations stated previously (soils section).
However, the investigated extent of CCR impacts is considered to be adequate to
create a general picture of the nature of coal ash impacts in the flow regimes (ash,
soils, bedrock) below the impoundment. This data along with data collected outside
the basin are considered sufficient so that a corrective action plan can be developed.
It is anticipated that as ash basin dewatering activities progress and other areas of
the unit become available for investigation, additional soil and groundwater data for
modeling, closure purposes and/or corrective action refinement will be necessary.
Response — At ash pore water well ABMW-1, deeper wells into native
material (saprolite, transition zone, or bedrock) were not possible due to ash
stability and worker safety concerns. A drill rig or other heavy, mechanized
equipment is not able to be driven onto the ash basin at that location; therefore,
wells beneath the ash basin are not possible at that location.
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A well was completed in the surficial material at ABMW-3S and in the transition
zone at ABMW-4D. Both wells indicated exceedances of COIs in the respective
flow zones at these locations. A bedrock well was not attempted at ABMW-3
because bedrock wells were successfully installed at the ABMW-2 and ABMW-4
locations.
At well cluster ABMW-2, surficial material was not encountered beneath the ash
at that location; however, both a bedrock well and later, a second deeper bedrock
well, were installed during original and subsequent CSA assessment field work.
Groundwater is unimpacted in the bedrock flow zone beneath the ash basin as
indicated by groundwater analytical data from ABMW-2BR, ABMW-2BRL, and
ABMW-4BR.
In addition, as described in this CAP Update (Section 5.0), the ash basin was
constructed within a former perennial stream valley. The ash basin's physical
setting is a horizontal flow -through water system with groundwater migration
into the upgradient end, flowing north through the middle regions, and
migrating downward near the dam. Generally, the physical setting of the ash
basin within a former perennial stream valley limits the horizontal and vertical
migration of constituents to areas near and directly downgradient of the basin's
dam.
A localized effect to the overall flow -through water system occurs just south of
the ponded water in the ash basin. A review of historical aerial photographs
shows that in early 2006, sluiced ash in the built-up ash "delta' area (formed
from historic sluicing) had begun to be mechanically moved and stacked in the
area of exposed ash south of the present-day ponded water. The ash was
"stacked" for dewatering prior to transport for beneficial reuse. The effect of this
ash "stack out" out area is a localized one that results in a slight variation of the
horizontal flow -through concept. In this area of the ash basin, within the ash
basin boundary, there is slight downward vertical migration of groundwater
within the overall ash basin flow -through system. Downward flow is limited to
the saprolite and transition zone as bedrock upward vertical gradients prevent
downward flow below the transition zone. Based on direct groundwater data
and the revised understanding of the CSM, vertical delineation has been
achieved.
Comment 17 - Various Constituents of Focus - Boron- Defined for the most part
in the vertical and horizontal. Monitoring well CW-2 has boron that exceeds the 2L
standard which would place at to near the compliance boundary in the transition
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zone. Additional attention may be necessary for boron given figure 14-42 indicates
an increasing trend for boron.
Response — The CAP Update provides detailed information concerning the
occurrence and distribution of boron with respect to the ash basin and the
compliance boundary. At Mayo, boron is the only COI with a discernable plume
related to the ash basin resulting in the only unit specific groundwater COI as
defined in the COI management process (Section 6.1.3). Boron is the focus of this
CAP Update..
Analytical data obtained over one year of quarterly monitoring indicate boron
concentrations have been less than the 02L standard in the groundwater at the
compliance boundary (CW-2 and CW-21)). Additional evaluation of the boron
plume detailed in this CAP Update (Section 6.1.4) indicates that concentrations
of boron are stable or decreasing. Further, flow and transport model simulations
demonstrate that boron concentrations in any flow zone are not currently
greater, nor predicted to be greater, than the 02L standard beyond the
compliance boundary for the closure -by -excavation or closure -in -place scenarios.
Predicted modeling demonstrates that the boron plume will contract towards the
south (CAP Update, Appendix G).
Comment 18 - Calcium- MW-16S has detection of calcium at approximately four
times the PBTV. Calcium does not exceed PBTV in the TZ or bedrock at this location.
No explanation is provided for the presence of elevated calcium in the shallow flow
regime. An explanation is necessary to provide a basis for the presence of calcium.
Response — The PBTV used in the CSA Update for calcium in the surficial
flow zone is approximately 20 times lower than the PBTV for calcium in the
transition zone and approximately 30 times lower than the PBTV for calcium in
the bedrock flow zone. The calcium PBTV in the surficial zone is likely artificially
low and the result of the limited number of usable background data points for
the surficial flow zone available to calculate a robust value. Calcium
concentrations in MW-16S are less than adjacent wells MW-16D and MW-16BR.
The transition and bedrock flow zones are not confined. Calcium concentrations
in ash pore water are similar to concentrations in unimpacted transition zone and
bedrock groundwater; therefore, calcium does not appear to be an indicator of
ash basin -related impact.
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Comment 19 — Hexavalent Chromium- Present in the ash basin greater than the
PBTV, but impacts beyond the basin, are unclear. 4.4 parts per billion detected in
upgradient well MW-12S. Recommend no additional work to define the extent until
a larger dataset exist to evaluate its presence.
Response — The distribution of hexavalent chromium in groundwater does
not exhibit a discernable plume associated with the Mayo ash basin. Continued
groundwater sampling and analysis has generated a more robust dataset for
hexavalent chromium evaluation in the CAP Update). The highest concentrations
of hexavalent chromium are found in background and upgradient monitoring
wells. An analysis of the presence and distribution of each COI, including
hexavalent chromium, is included in this CAP Update Section 6.1.3.
Comment 20 - Iron- Elevated pockets of iron greater than the PBTV are present
at MW-19D in the transition zone and MW-5BR and MW-13BR for bedrock.
Recommend Duke provide a detailed explanation why present at these locations or
conduct additional investigation as necessary.
Response — Elevated iron detected at the referenced wells are not influenced
by the Mayo ash basin. Additional sampling and analysis at those locations have
been included in this CAP Update. In addition, the presence of elevated iron is
not an indication of ash basin impact because (1) no boron was detected in these
wells and (2) these wells are not downgradient of the ash basin as supported by
flow and transport modeling (CAP Update, Appendix G) and groundwater
velocity vector maps (CAP Update, Figure 5-4a through Figure 5-4c and
Figure 5-5a).
The occurrence of inorganic constituents in groundwater of the Piedmont
Physiographic Province is well documented in literature. The distribution of iron
in groundwater does not exhibit a discernable plume associated with the ash
basin.
Comment 21 - Manganese- Elevated concentrations that exceed PBTV at CW-2D
at the compliance boundary and exceedance at CW-6 outside compliance boundary.
The report implies that the CW-6 concentration is an isolated pocket. Boron was not
detected in CW-6 which provides some support for the position. Additional
investigation of manganese is recommended to draw a complete picture of the
detections at CW-6.
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Response — Elevated manganese detected at CW-6 is not influenced by the
Mayo ash basin. Additional sampling and analysis has been included in this CAP
Update. In addition, the presence of elevated manganese is not an indication of
ash basin impact because (1) no boron was detected in CW-6 and (2) CW-6 is not
downgradient of the ash basin as supported by flow and transport modeling
(CAP Update, Appendix G), potentiometric maps (Figure 5-4a through
Figure 5-4c), and the velocity vector maps (Figure 5-5a through Figure 5-5c).
The occurrence of inorganic constituents in groundwater of the Piedmont
Physiographic Province is well documented in literature. The distribution of
manganese in groundwater does not exhibit a discernable plume associated with
the ash basin.
Comment 22 - Molybdenum- Defined for the most part but an isolated pocket is
present downgradient at MW-16D and MW-16BR. Upgradient wells do not
demonstrate a clear connection to the ash basin. Also, present greater than the PBTV
in CW-1D and CW-1BR. Recommend Duke provide a detailed explanation why
present at these locations or conduct additional investigation as necessary.
Response — Molybdenum mean concentrations that are greater than Site
background values near or beyond the compliance boundary are observed only
in wells MW-16D and MW-16BR. Molybdenum is not observed in concentrations
above background immediately downgradient of the ash basin and its
distribution does not exhibit a discernable plume. The mean concentrations of
molybdenum observed in MW-16D and MW-16BR are not attributed to
groundwater migration from the ash basin.
There are no PBTV exceedances in groundwater from the transition zone well
CW-1. Exceedances of molybdenum in bedrock well CW-1D (there is no "CW-
1BR" at Mayo) are attributed to natural variation. There is a hydrologic divide
between the ash basin and CW-1D; therefore, there is no groundwater flow
pathway from the ash basin to CW-1D (Figure 5-4a through Figure 5-4c and
Figure 5-5a).
Comment 23 - Strontium-Surficial flow regime does not appear to be defined
beyond the compliance boundary at MW-16S. Vertical definition appears to be
adequate down gradient of the ash dam. Vertical definition not defined in the
general vicinity of the ash basin monitoring wells. It may be necessary to install
additional background monitoring wells for the surficial flow regime given that the
PBTV established for strontium in this unit is based on data from one well. The
alluvium environment downgradient of the dam which would be expected to have
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shallow hydraulic gradients could have diffuse migration patterns that have not
been investigated. Additional supportive information/ investigation is expected to
be necessary to resolve this issue.
Response — Flow and transport modeling has demonstrated that when
Crutchfield Branch is not free flowing and is impounded due to the presence of
beaver dams, locally losing stream conditions occur. When these conditions
occur, surface water recharges the shallow alluvium in which MW-16S is
screened. Under these conditions, detected groundwater concentrations of boron
in MW-16S reflect surface water concentrations of boron indicating that ash
basin -related constituents present in MW-16S are related to surface water rather
than a groundwater plume. Furthermore, strontium in the surficial flow zone
downgradient of the ash basin is not considered a concern. The detected
concentrations of strontium in MW-16S are within the Mayo surficial BTV
approved by NCDEQ RRO on May 14, 2019, in addition to NC Piedmont
regional background ranges presented in Table 4-3. Installation of background
wells screened in the surficial zone was attempted at multiple locations
throughout the Site over multiple assessment efforts. The occurrence of saturated
surficial materials is limited at Mayo; therefore, the surficial zone at those
locations was either not present or did not yield usable sample data (e.g., high
turbidity).
Comment 24 - Sampling results from monitoring well MW-5BR detected
various constituents (iron and manganese) greater than the PBTVs. Elevated
concentrations greater than PBTVs were not detected in the shallow well at this
location. It is unclear whether the elevated detections are associated with the coal
ash impoundment. It does not appear that, if the constituents are related to the coal
ash impoundment, these constituents have migrated west of MW-5BR based on the
results of MW-14BR. Given that there are active water supply wells near this area, it
is recommended that this location should be investigated further or additional
evidence provided to explain the specific source/s of the elevated constituents in
conjunction with ongoing groundwater monitoring.
Response — Elevated iron and manganese detected at MW-5BR is not
influenced by the ash basin; the presence of elevated iron and manganese is not
an indication of ash basin impacts because no boron is detected in MW-5BR or
CW-5. Further, multiple lines of evidence demonstrate that flow is away from the
MW-5BR location. The wells at this location are located upgradient of the ash
basin (supported by flow and transport modeling which indicates groundwater
from the ash basin does not flow toward MW-5BR (CAP Update, Appendix G),
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field water level measurements (Table 5-1), potentiometric maps (Figure 5-4a
through Figure 5-4c), and the velocity vector maps (Figure 5-5a through
Figure 5-5c).
Comment 25 - The sampling from monitoring well MW-18BR has detected the
presence of various CCR constituents (manganese and strontium) greater than the
PBTVs. There does not appear to be a clear association with the ash basin such as the
presence of boron in the well or nearby well network. The groundwater sample from
the shallow well at this location (most recent event) did not detect iron and
manganese at greater than the PBTVs. It should be noted that NPDES Outfall 002,
the FGD ponds and the designated rail car storage area are in the general vicinity of
well MW-18BR. It is recommended that groundwater monitoring continue so that a
more robust dataset is available to evaluate this area. A larger dataset in
combination with closure activities and the investigation of secondary sources will
help to determine origin of elevated concentrations and the necessary path forward.
Response — The presence of manganese and strontium at MW-18BR is not an
indication of impact from a CCR related source. Strontium detected at this
location has been within the range of the approved background values approved
by NCDEQ on May 14, 2018 (CAP Update, Appendix A) for bedrock. Additional
hydrogeologic evaluation, including groundwater flow and transport modeling,
has confirmed that the MW-18 well pair is not impacted by the ash basin. Duke
Energy concurs that additional groundwater monitoring and the ongoing
investigation of secondary sources, combined with closure activities, is an
appropriate approach for evaluating groundwater concentrations in the MW-18
well pair.
Comment 26 - It is unclear whether the area in the vicinity of monitoring wells
MW-19D and MW-19BR are influenced by CCR constituents (MW19D-iron, MW-
19BR- iron and manganese >PBTV's). Boron has not been detected in this locale
providing some evidence that it is not ash basin related. Potential sources (other
than the basin) that may have some influence include the NPDES Outfall or
activities associated with the plant facility. This area should be investigated further
or evidence provided to explain what are the specific source/s of the elevated
constituents.
Response — Elevated iron, manganese, and/or vanadium detected at MW-
18BR, MW-19D, and MW-19BR are not influenced by the Mayo ash basin.
Additional sampling and analysis at those locations have been included in this
CAP Update. In addition, the presence of elevated constituents is not an
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indication of ash basin impact because (1) no boron was detected in MW-18D,
MW-18BR, MW-19D, and MW-19BR; (2) concentrations are only outside of the
referenced background ranges for manganese (MW-18BR, MW-19D, and MW-
19BR), iron (MW-19D and MW-19BR), and TDS (MW-19D and MW-19BR); and
(3) these wells are not downgradient of the ash basin as supported by flow and
transport modeling (CAP Update, Appendix G), potentiometric maps (Figure 5-
4a through Figure 5-4c), and the velocity vector maps (Figure 5-5a through
Figure 5-5c).
The occurrence of inorganic constituents in groundwater of the Piedmont
Physiographic Province is well documented in literature. The distribution of
iron, manganese, and vanadium in groundwater does not exhibit a discernable
plume associated with the ash basin.
Comment 27 - The CSA Update report indicates that two general areas are
designated to be in separate groundwater flow regimes from the ash basin. The first
area has monitoring well locations identified as MW-7D, MW-7BR, MW-8BR, MW-
9BR, and MW-9BRL which are positioned on the Northeast side of the basin and in a
down gradient direction from the basin. The report indicates that piezometers
installed for the future FGD basin confirm the hydrogeologic environment for the
first area but without the supporting information or a detailed summary. The
second location includes MW-18D, MW-18BR, MW-19D, and MW-19BR. This area is
adjacent to the railroad spur and northeast of the power plant in a location between
Mayo Lake and the ash basin. For the second area, the general statement is made
about the hydrogeologic environment with no supporting information. A summary
of the position is necessary with more detailed evidence provided to confirm the
hydrogeologic features so that this can be included as a part of the CAP.
Response — The FGD Settling Basin/Wastewater Treatment Pond area (in the
vicinity of MW-7D, MW-7BR, MW-8BR, MW-9BR, and MW-9BRL) and west of
the power plant (in the vicinity of MW-18D, MW-18BR, MW-19D, and MW-
19BR) are not hydrologically connected to the ash basin. Multiple lines of
evidence in the CAP Update confirm the descriptions provided in the CSA
Update which include: (1) additional groundwater monitoring wells installed in
the FGD Settling Basin/Wastewater Treatment Pond area (25 wells were installed
to comply with the USEPA CCR Rule requirements) and additional water level
data for the area west of the power plant provide field water level information
that is used to refine water levels in the areas (CAP Update, Figure 5-4a through
Figure 5-4c); (2) the wells at the specified locations are not downgradient of the
ash basin (supported by flow and transport modeling); (3) groundwater from the
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ash basin does not flow toward the FGD Basin Wastewater Treatment Pond or
west of the power plant areas as indicated on potentiometric maps (Figure 5-4a
through Figure 5-4c) and velocity vector maps (Figure 5-5a through 5-5c); and (4)
the CSM has been refined, including discussion about the hydrogeologic nature
of the Site (CAP Update Section 5.0).
Comment 28 - Section 11.2 Pending Investigation - The CSA Update indicates
that additional investigation will be conducted to collected data for metal
oxyhydroxide iron (HFO) and aluminum (HAO) to refine the geochemical model for
the CAP. Either previously collected data or new data will be collected to refine the
model. Also, to support a (k) or (1) CAP regarding 2B standards, it is anticipated that
additional surface water samples will be collected with DEQ guidance.
Response — A revised and updated Geochemical Modeling Report that
incorporates various geochemical data types and datasets collected throughout
the Mayo ash basin assessment since 2015 is included in this CAP Update
(Appendix H). The report provides conclusions concerning partitioning
properties that affect constituent migration, conclusions concerning constituent
mobility, and evaluation of source control options on current and future Site
geochemical characteristics.
A detailed evaluation of any impacts from groundwater to surface water under
current conditions, including exceedances of 213 standards, was conducted in
April/May 2018 and the results provided to NCDEQ in March 2019 (Appendix J).
The evaluation determined that no exceedances of 213 standards were caused by
groundwater discharging to Crutchfield Branch. In addition, an evaluation of
potential groundwater to surface water impacts under future conditions is
presented in this CAP Update (Appendix J). The future conditions evaluation
predicts no exceedances of 2B standards caused by discharging groundwater.
Groundwater Modeling
Comment 29 - Groundwater Modeling - The section on groundwater modeling
provided for some discussion on groundwater flow, transport and geochemical
models that are being developed for the future CAP submittal. Three scenarios will
be evaluated- no change, cap in place and ash removal. CAP Part 1 evaluated all
three options where CAP Part 2 only evaluated the no change and cap in place
corrective actions. The future updated CAP will contain a groundwater fate and
transport model that will consider arsenic, boron and possibly other COI that are
hydraulically driven. The section describes in some detail model parameters and
construction efforts for the CAP submittal.
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Response — An updated Flow and Transport Modeling report is provided in
this CAP Update (Appendix G) that provides simulations of boron transport
under pre -decanting, closure -by -excavation, closure -in -place scenarios, and
future (post -closure) conditions.
Comment 30 - Site Conceptual Model - The update to the SCM was lacking in
detail. It was anticipated that the latest submittal would combine the information
collected in the various reports to summarize a picture of how known site conditions
relate to the initial SCM and how the current understanding of the model provides a
basis that can be beneficial for closure/corrective action purposes. It will be
necessary to enhance and refine the SCM for CAP submittal.
Response — Data obtained through 2Q2019 has been used to enhance and
refine the CSM. The revised/updated CSM is included in this CAP Update
(Section 5.0).
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