26015_Sawmill Hollow Methane Assessment Memo_20240515

161 S Lexington Ave Asheville, NC 28801 www.geosyntec.com
GN9110/CAR240046 May 2024

Memorandum
Date: May 15, 2024
To: Mr. Bill Schmithorst, North Carolina Department of Environmental
Quality, Division of Waste Management, Brownfields Redevelopment
Mr. Luke Fowler, Northwestern Housing Enterprises, Incorporated
Mr. William Clarke, Roberts & Stevens, P.A.
(NC)
(NC)
Site: Sawmill Village Brownfields Redevelopment
Sawmill Hollow Road, Burnsville, North Carolina 28714
NCDEQ BRS Project No. 26015-22-100
On behalf of Northwestern Housing Enterprises, Incorporated (NHE; the Prospective Developer),
Geosyntec Consultants of NC, P.C. (Geosyntec) is pleased to submit the Summary of Methane
Assessment Memorandum (Memo) for the above-referenced property (Site) for your review and
approval. The Site is the 3.86-acre parcel at the intersection of Sawmill Hollow Road and US
Highway 19E in Burnsville, North Carolina identified under Yancey County Parcel Number
083003244675000 and is shown in Figure 1. A Letter of Eligibility (LOE) was received for the
property on November 10, 2022, indicating that the Site was eligible for entry into the North
Carolina Department of Environmental Quality (NCDEQ) Brownfields Redevelopment Section
(BRS) and for eventual finalization of a Brownfields Agreement (BFA). As of this letter a BFA
has not been finalized. This Memo provides an initial summary of woody debris removal and
presents the results of the subsequent methane assessment performed in early 2024.
INTRODUCTION
During supplemental Site assessment work in September 2022, Geosyntec installed, screened, and
sampled 10 soil gas probes (SGPs). During soil gas screening, methane was detected between 0.3
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GN9110/CAR240046
percent (%) by volume (%v) and 5.1%v at 5 of the probes, and greater than 1.25%v at two SGPs
(SG-04 and SG-05) (Figure 2). The source of this methane appeared to be woody debris that was
observed above and below grade during the Site assessment and subsequent test pitting.
The BRS required that the woody debris be removed from Site prior to redevelopment and that a
subsequent methane assessment be conducted to evaluate methane concentrations in soil gas in the
area where methane was detected above 1.25%v. The goal of the woody debris removal and
methane assessment was to demonstrate that excavating woody debris would remove the source
of methane production and that methane concentrations in soil gas would subside in response. If
this relationship was observed during methane assessment, proactive methane mitigation in
finished buildings would not be required and redevelopment could commence.
This Memo summarizes the methane assessment conducted at the Site from February through May
2024. The methane screening results were evaluated using the NCDEQ Brownfields Program
Threshold Criteria for Methane Site Development (December 2020).
METHANE ASSESSMENT ACTIVITIES
Descriptions of the field activities completed from January 5, 2024 to May 1, 2024 in support of
the methane assessment are provided in the following subsections.
Woody Debris Excavation
Excavation and export of the woody debris occurred between January 5 and February 9, 2024 and
Geosyntec provided limited oversight of the excavation and confirmed that excavations were being
completed in general accordance with the Site’s approved Environmental Management Plan
(EMP)1. On January 5, 11, and 23, 2024, Geosyntec mobilized to the Site to observe the excavation
and to periodically field screen the material being exported for volatile organic compounds (VOCs)
using a handheld photoionization detector (PID). No staining, odors, or high PID readings were
noted that could indicate previously unidentified impacts were identified during Geosyntec’s
limited excavation oversight. On excavation and export days when Geosyntec did not mobilize to
the Site, a representative of NHE’s general contractor (ALH General Contractor Inc.), completed
an EMP oversight checklist to document that work had progressed in general accordance with the
Site’s approved EMP. NHE’s grading contractor, Young & McQueen Grading Company, removed
approximately 1,600 tons of woody debris from the Site and hauled the materials to Foothills

1 North Carolina Brownfields Redevelopment Section Environmental Management Plan (Geosyntec 2023)
Summary of Methane Assessment May 15, 2024
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GN9110/CAR240046
Environmental Landfill in Lenoir, North Carolina. The materials’ export will be formally
documented in the next EMP Redevelopment Summary Report.
On February 12, 2024, Geosyntec met with NHE’s general contractor to perform a final inspection
of the woody debris excavation. Geosyntec concurred that the woody debris previously delineated
for export was removed from the Site.
Soil Gas Probe Installation
On February 22, 2024, approximately two weeks after NHE completed the woody debris removal,
Geosyntec oversaw the installation of three SGPs (SG-11 through SG-13) by M&W Drilling, LLC
(M&W) in areas where mulch and buried woody debris had been removed. Soil gas probe locations
are shown on Figure 3. During installation of SGPs, M&W advanced a soil boring to 6 feet (ft)
below land surface (BLS) at each location and Geosyntec logged soils and evaluated SGP screen
intervals. Each SGP was constructed with ¼-inch diameter Nylaflow® tubing connected to a
pre-fabricated 6-inch, stainless steel soil gas sampling probe (¼-inch diameter). The 6-inch probe
was placed in the middle of a 1-ft #2 sand filter pack, which was placed from 5 to 6 ft BLS at each
location. A 1-ft granular bentonite seal was placed above the filter pack and Portland cement grout
was placed up to the ground surface. A new, ¼-inch stainless steel ball-valve was connected to the
portion of each SGP tubing exposed at the ground surface and remained in the closed position
while SGPs were not actively being sampled. Each SGP was completed at land surface with a flush
mount well vault installed within a 2-ft by 2-ft concrete pad.
Boring logs and SGP construction logs are provided in Appendix A.
Methane Screening Frequency
Geosyntec completed three methane screening events as part of the methane assessment, which
are summarized below:
• Baseline: Geosyntec collected two rounds of methane screening results as part of the
baseline screening event. The first baseline methane screening round was completed on
February 22, 2024, at least 2 hours after each SGP was installed. The second baseline
screening round was completed on February 26, 2024 (per the approved Methane
Assessment Work Plan2).
• Month 1: Geosyntec screened SGPs for methane on March 29, 2024.

2Methane Assessment Work Plan (Geosyntec 2024)
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GN9110/CAR240046
• Month 2: Geosyntec screened SGPs for methane on May 1, 2024.
In total, Geosyntec collected four rounds of methane screening data. During each screening event,
Geosyntec screened soil gas from SGPs SG-11 through SG-13. Field methods are described in the
following section.
Methane Screening Procedure
Prior to methane screening, Geosyntec connected Nylaflow® tubing to the SGP using an air-tight
fitting without opening the pre-installed valve. Geosyntec then opened the valve and measured the
static pressure of soil gas relative to the atmosphere using a Landtec GEM 5000 and a hand-held
The Energy Conservatory (TEC) DG-700 micromanometer for redundancy, since the DG-700’s
primary function is to measure differential pressure. Static pressure was reported in Pascals and
inches of water (in. H2O).
The static pressures measured by the GEM 5000 during the baseline event are believed to be
inaccurate because of the large difference between the measurements taken by the GEM 5000 and
DG-700. The cause of this inaccuracy is likely due to equipment malfunction or improper use.
These issues were corrected during subsequent events, and measurements between the two pieces
of equipment were comparable. Based on the presumed inaccuracy of the baseline GEM 5000
static pressure measurements, only static pressure measurements collected using the DG-700 are
evaluated in the Methane Assessment Results Section.
Once the static pressure was measured, Geosyntec assembled a methane screening assembly
consisting of two valves (one of which was the pre-installed valve) connected via Nylaflow tubing.
Geosyntec performed a shut-in leak check in the methane screening assembly by closing the pre-
installed valve on the SGP and generating a vacuum in the methane screening assembly using a
lung box. The second valve near the lung box was then closed, and if the vacuum did not dissipate
in the methane screening assembly after at least 30 seconds, the shut-in test was considered
successful. If the shut-in test was unsuccessful, valve and tubing connections were checked and
tightened if necessary, and another shut-in test was completed.
Following a successful shut-in test, Geosyntec purged and screened soil gas. Helium tracer testing
was conducted during the purging at each location to evaluate if atmospheric air was entering the
methane screening assembly. Geosyntec introduced helium into a shroud that encompassed the
SGP manhole and the methane screening assembly and then collected soil gas in a 1-liter Tedlar®
bag. Geosyntec screened soil gas for: (i) carbon dioxide, methane, and oxygen (using a calibrated
Landfill GEM 5000 multi-gas meter); and (ii) helium (using an MGD-2002 multi-gas leak
detector). At least three times the volume of the SGP and methane screening assembly were purged
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GN9110/CAR240046
prior to collecting and recording final soil gas measurements. The screening logs are provided in
Appendix A.
METHANE ASSESSMENT RESULTS
Methane screening results and initial static pressures measured at each SGP prior to purging during
the baseline, Month 1, and Month 2 screening events are provided in Table 1 and on Figure 3.
The data collected during all screening events was compared to the threshold of 1.25%v
established by the NCDEQ Brownfields Program Threshold Criteria for Methane Site
Development (December 2020). A summary of the results is provided below:
• Initial static pressures measured at each SGP using a DG-700 micromanometer ranged
from -0.134 to 0.075 in H2O. Initial static pressure measurements indicate that no
appreciable pressure above atmospheric is present in the subsurface.
• Methane concentrations were above 1.25%v at two locations during baseline screening and
declined at all locations throughout screening (Figure 3). By Month 1, methane was not
detected above 1.25%v at any location.
METHANE ASSESSMENT CONCLUSIONS
Based on the methane screening results collected during the methane assessment, removing woody
debris from the Site led to the desired reduction of methane concentrations in soil gas to below
1.25%v. Since methane concentrations in soil gas have decreased continuously since woody debris
removal and methane was not detected in soil gas above 1.25%v by the Month 1 methane screening
event, further methane screening is not required prior to construction.
As discussed with the BRS, methane screening will be conducted during construction and after
occupancy to evaluate whether residual methane remains on Site and accumulates beneath building
slabs. The results from these pre-occupancy and post-occupancy methane screening events will be
evaluated to decide whether methane mitigation components need to be installed as part of the
vapor intrusion mitigation system (VIMS). Further details of these methane screening events and
mitigation evaluations will be presented in the forthcoming VIMS Design Report.
CLOSING
We appreciate BRS’ review of this methane assessment and approval of being conducted in
accordance with NCDEQ Program Methane Assessment Protocol (Rev. December 2020). Should
you have questions regarding this Memo or need additional information, please do not hesitate to
contact Mike Burcham at [phone] or [e-mail].
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GN9110/CAR240046
* * * * *
Attachments Table 1: 2024 Methane Assessment Results
Figure 1: Site Location Map
Figure 2: Supplemental Site Assessment Methane Screening Results
Figure 3: Methane Screening Results
Appendix A: Field Forms (Boring Logs, SGP Construction Logs, and Methane Screening Logs)
CERTIFICATION PAGE
SUMMARY OF METHANE ASSESSMENT
SAWMILL HOLLOW
SAWMILL HOLLOW ROAD,BURNSVILLE,NORTH CAROLINA
BROWNFIELDS PROJECT NO.26015-22-100
PROFESSIONAL ENGINEER SIGNATURE
I,MichaelS.Burcham,a Licensed Professional Engineer for Geosyntec Consultants of NC,P.C.,do certify that
the information in this report is correct and accurate to the best of my knowledge.
GeosyntecConsultants of NC,P.C,is licensedto practiceengineering in North Carolina.The certification
number (Fim's License Number)is C-3500.
H ROLKRTHSS
SEAL
050312SIS/24
NEER,
s.BURCHA
Michael S.Burcham,PE(Nc,T)
Senior Engineer
North Carolina P.E.License No.0503 12
Expiration Date:Deccmber 3 1,2024
GeosyntecConsultants of NC,P.C.
Telephone:[phone]
2501 Blue Ridge Road Suite 430
Raleigh,NC 27607

TABLE
Table 1
2024 Methane Assessment Results
Sawmill Village Brownfields Redevelopment
Burnsville, NC
DG GEM
02/22/2024 27.08 0 1.4 20.3 2.4 0.031 11.36*
02/26/2024 27.36 0 0.8 20.7 1.2 0.025 11.47*
03/29/2024 27.35 0 0.0 16.5 0.8 0.032 0.02
05/01/2024 27.41 0 0.0 16.6 2.7 0.009 0.02
02/22/2024 27.07 0 2.2 3.5 3.7 0.023 11.35*
02/26/2024 27.32 0 1.8 3.6 4.9 0.073 11.31*
03/29/2024 27.35 0 1.1 13.5 2.6 0.026 0.02
05/01/2024 27.41 0 0.3 17.1 0.9 0.041
05/01/2024 27.41 0 0.0 0.18
% bv: percent by volume
ft BLS: feet below land surface
in Hg: inches of mercury
in H2O: inches of water column
ppm: parts per million
Hydrogen
Sulfide (ppm)
Barometric
Pressure (in Hg)
SG-11 5 to 6
SG-12 5 to 6
SG-13 5 to 6
Probe Depth
(ft BLS)
Initial Static Pressure
(in H2O)Soil Gas
Probe ID Date Screened Methane
(% bv)
Carbon Dioxide
(% bv)
Oxygen
(% bv)
1 of 1

FIGURES
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0 1,000Feet
³Sawmill Hollow Brownfields PropertyBrownfields Project No: 26015-22-100Burnsville, North Carolina
Site Location Map
Figure
1Asheville, NC May 2024
Legend
Notes:1.Base map from USGS National Topographic Map.2.Parcel Boundary Data from Yancey County GIS.
Site Location
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Site Location
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Sawmill Hollow Road
US Highway 19E
Fair Haven Drive
TP-07
TP-12
TP-10
TP-03
TP-09
TP-11
TP-06
TP-08
TP-05
TP-01
SB-06
SB-07
SB-05
SB-04
SB-03
SB-02
SB-01
SB-08
SB-09
SB-10
SB-01-01
SB-01-02SB-01-03
SG-04
SG-05
SG-03
SG-02
SG-01
SG-08
SG-06
SG-07
SG-09
SG-10
VP-3
VP-4
VP-2
VP-5
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Sawmill Hollow Brownfields Property
Brownfields Project No: 26015-22-100
Burnsville, North Carolina
Supplemental Site Assessment Methane
Screening Results
0 50
Feet
³Figure
2
Asheville, NC May 2024
Legend
Site Boundary
Former Building
Proposed Community Center
Proposed Housing Unit
Proposed Road
Previous Extent of Obvious Mulch
Pile
Previous Extent of Buried Woody
Debris
"")Test Pit with Woody Debris
")Test Pit without Woody Debris
!(Soil Boring Location
!(Soil Gas Sample Location
&>Historical Soil Vapor ProbeNotes:
1. Parcel boundary from Yancey County GIS.
2. Static pressure measurements collected using a Dwyer Series 475-3-FM manometer prior to purging. Methane
screening results collected using a GEM 5000 landfill gas meter in September 2023.
3. "H2O indicates inches of water column.
4. Basemap from Source: Esri, Maxar, Earthstar Geographics, and the GIS User Community.
Sample
Location
Static Pressure
("H2O)
Methane
(% volume)
SG-01 0.0 0.0
Sample
Location
Static Pressure
("H2O)
Methane
(% volume)
SG-02 0.0 0.0
Sample
Location
Static Pressure
("H2O)
Methane
(% volume)
SG-03 0.0 0.0
Sample
Location
Static Pressure
("H2O)
Methane
(% volume)
SG-04 0.0 1.3
Sample
Location
Static Pressure
("H2O)
Methane
(% volume)
SG-05 0.0 5.1
Sample
Location
Static Pressure
("H2O)
Methane
(% volume)
SG-06 0.0 1.2
Sample
Location
Static Pressure
("H2O)
Methane
(% volume)
SG-07 0.0 0.3
Sample
Location
Static Pressure
("H2O)
Methane
(% volume)
SG-08 0.0 0.0
Sample
Location
Static Pressure
("H2O)
Methane
(% volume)
SG-09 0.0 0.4
Sample
Location
Static Pressure
("H2O)
Methane
(% volume)
SG-10 0.0 0.0
!(
!(
!(
!(
!(
!(
!(
!(
!(
!(
!.!.
!.
Sawmill Hollow Road
US Highway 19E
Fair Haven Drive
SG-13
SG-12
SG-11
SG-04
SG-05
SG-03
SG-02
SG-01
SG-08
SG-06
SG-07
SG-09
SG-10
\\Raleigh-01\PRJ\N\Northwestern Housing Enterprises\Sawmill Village\7.0 GIS\MXDs\2020404_Methane_Assessment\F3_Methane_Screening_Results.mxd 5/3/2024 3:05:24 PM; last edited by EHoward
Sawmill Hollow Brownfields Property
Brownfields Project No: 26015-22-100
Burnsville, North Carolina
Methane Screening Results
0 50
Feet
³Figure
3
Asheville, NC May 2024
Legend
Site Boundary
Former Building
Proposed Community Center
Proposed Housing Unit
Proposed Road
Prior Extent of Obvious Mulch Pile
Prior Extent of Buried Woody
Debris
!(Soil Gas Sample Location
!.Soil Gas Probe for Methane
Screening
Notes:
1. Parcel boundary from Yancey County GIS.
2. Methane screening results collected using a GEM 5000 landfill gas meter. Initial static pressure measurements
collected using a DG-700 micromanometer prior to purging.
3. "H2O indicates inches of water column.
4. Basemap from Source: Esri, Maxar, Earthstar Geographics, and the GIS User Community.
Sample Location Date Methane (% by volume) Static Pressure ("H2O)
02/22/2024 0.8 0.0747
02/26/2024 0.6 0.014
03/29/2024 0.4 -0.134
05/01/2024 0.0 0.030
SG-13
Sample Location Date Methane (% by volume) Static Pressure ("H2O)
02/22/2024 2.2 0.023
02/26/2024 1.8 0.0731
03/29/2024 1.1 0.026
05/01/2024 0.3 0.0405
SG-12
Sample Location Date Methane (% by volume) Static Pressure ("H2O)
02/22/2024 1.4 0.031
02/26/2024 0.8 0.025
03/29/2024 0.0 0.0322
05/01/2024 0.0 0.0092
SG-11

APPENDIX A