09058_Midtown Mall_Soil_Gas_Sampling_Work_Plan_R1_20231017

October 17, 2023
Carolyn Minnich, Project Manager
Brownfields Redevelopment Section
North Carolina Department of Environmental Quality
1646 Mail Service Center
Raleigh, North Carolina 27699-1646
Subject: Soil Gas Sampling Work Plan – Revision 01
Midtown Mall Brownfields Project
1200 Metropolitan Avenue, Charlotte, Mecklenburg County, NC
Brownfields Project Number 09058-05-060
Dear Ms. Minnich:
Mid-Atlantic submits for your review our Soil Gas Sampling Work Plan for the above
project. The purpose of this Work Plan is to document the collection of additional soil gas
samples to aid in Vapor Intrusion Mitigation System (VIMS) design for the planned
redevelopment of the site. The site is located south of Metropolitan Avenue, west of S.
Kings Drive and north of Midtown Park. The site encompasses approximately 1.17 acres
as is part of the current Metropolitan development. The site is currently vacant and lightly
vegetated.

Soil Gas Sampling

The soil gas implants will be advanced using hand auguring techniques. Mid-Atlantic
personnel will manually advance a stainless steel hand auger to a minimum depth 6-feet
below ground surface (bgs). The hand auger will be decontaminated between borings.
Mid-Atlantic will not install the soil gas implants within five (5) feet of the perceived water
table and final depth will be adjusted based on any water conditions observed. Based on
our prior experience, we do not anticipate encountering groundwater.

Mid-Atlantic will advance 4 hand auger borings, designated as SG-1 through SG-4. One
duplicate sample will be collected at location SG-1, as shown on the attached Drawing
1. These locations were strategically selected based on both historical contamination and
the new construction. As such, the aerial photograph base of Drawing 1 has been
overlain with the foundation plan to show the locations of soil gas sampling relative to the
Soil Gas Sampling Work Plan-Revision-01 October 17, 2023 1200 Metropolitan Avenue, Charlotte, NC Page 2 Mid-Atlantic Project: H1824.00

planned development. Mid-Atlantic will advance each boring to a depth of 6-feet and fill
the bottom approximate 0.5-foot of the boring with a sand filter pack. The soil gas implants
will be installed using 6-inch stainless steel soil vapor probes which will be inserted to the
bottom of each boring and attached to 1/8-inch diameter dedicated Nylaflow® tubing to
extend above the ground surface for sample collection. A sand filter pack will be installed
in the annulus of the boring to cover 6-inches above the top of the soil gas probe followed
by a hydrated bentonite seal in the remainder of the borehole to the ground surface.
Following installation of the soil gas implants, Mid-Atlantic will allow the soil gas probes
to equilibrate for at least 24 hours prior to conducting a shut-in test, leak check, and
collecting the soil gas samples. Soil boring logs will be recorded for each soil vapor implant
boring. Soils from the hand auger borings will be visually inspected for evidence of
contamination. If contamination is suspected based on field observations, the material will
be containerized for testing, otherwise the soil will be spread thinly on site. Soil vapor
implants will be abandoned after soil gas sampling is completed.

Mid-Atlantic will conduct leak checks at each soil gas sampling location by utilizing helium
as the tracer compound, which will be introduced through a shroud over the probe and
entire sampling train. For the sampling location chosen for the duplicate sample, an
additional T-connection will be added to the sampling train to provide a dedicated sampling
train for the duplicate sample. This sampling train will be leak checked concurrent with the
primary sampling train. The actual sample identification will be logged into Mid-Atlantic’s
field notes, but a separate designation will be made on the summa canister tag and chain-
of-custody to make the sample unidentifiable to the laboratory. This designation will be
cross- referenced in Mid-Atlantic’s field notes. The shroud will be placed over the soil gas
implant location, the sampling train, and canister. A 3-way valve will be placed on the
probe termination and connected to the sampling apparatus. Helium will be introduced
into the shroud, which will encompass the entire sampling train, to a minimum
concentration of 50% as measured using a LandTech Gem™ 5000 helium detector (or
similar device). With the canister valve closed, Mid-Atlantic will conduct a leak check
sample using a helium detector to check for the presence of helium from air collected
in the Tedlar® bag using the specified helium detector. Using a 60-milliliter syringe
connected to the tubing, Mid-Atlantic will collect three purge volumes (including tubing,
annular space, and sampling train volume) from each sampling implant into the Tedlar®
bag and field screen each Tedlar® bag for integrity (absence of Helium) prior to opening
the sampling train between the Tedlar® bag and the Summa canister. A helium
concentration of less than 10% in the shroud will be considered a valid leak check.
Helium concentrations in the shroud, along with concentrations from each purge volume
Soil Gas Sampling Work Plan-Revision-01 October 17, 2023 1200 Metropolitan Avenue, Charlotte, NC Page 3 Mid-Atlantic Project: H1824.00

will be recorded in the field notes. A shut-in test will also be conducted on the sampling
process lines to ensure an air-tight connection by applying vacuum pressure to the lines
and observing for vacuum pressure loss. Soil gas vapor implants will be allowed to
equilibrate a minimum of 20 minutes after leak checks/shut-in tests before the sample is
collected.

Following successful shut-in tests and leak check to verify sample integrity, the soil
gas samples will be collected using laboratory-supplied 1-Liter batch-certified Summa
canisters with calibrated flow regulators set to 100 milliliters per minute (mL/min) to avoid
sample short circuiting and to further ensure sample integrity. Per DWM VI Guidance,
canisters that have lost greater than 10% volume from the initial lab recorded volumes
will not be used. Prior to sampling, vacuum readings, Summa identification and flow
controller identification for each sample will be recorded in the field notes. Once the field
documentation is complete, the sample train will be opened to the Summa flow controller
and the Summa canister will be opened to allow collection of soil gas volume. Sampling
will be deemed complete when the flow controller exhibits between -3 and -5 inches of
mercury. Following sample collection, the Summa canisters will be closed, and the final
vacuum reading and sampling time intervals will be recorded on the canister tag and on
the laboratory chain-of-custody form. Each soil gas sample will be submitted to Pace
Labs of East Longmeadow, Massachusetts, under chain-of-custody protocol for analysis
using EPA Compendium Method TO-15 plus naphthalene. The laboratory will be
instructed to omit acrolein from the TO-15 analyte list and provide a Level II QA/QC data
package including “J” flags and method detection limits (MDLs) at or below NCDEQ
screening levels. The laboratory will provide final canister pressures at the time of receipt.
Analytical data will be input into the DWM Risk Calculator to evaluate potential risk
exposure for future site occupants. Results of these analyses and risk evaluation will be
incorporated into Mid-Atlantic’s VIMS design.

We anticipate soil gas sampling to require one day to complete. Laboratory analytical
results will be available 10 business days after receipt by the laboratory. Once received,
we will provide a copy of the laboratory data sheets, data table and copies of our Risk
Calculator results. A formal written report will be provided as part of our Vapor Intrusion
Mitigation Plan. This plan will be signed by a North Carolina licensed Professional
Engineer or Geologist and include supporting documentation including boring logs, field
notes, laboratory data sheets and any other supporting documentation developed as part
of the project.

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DRAWN BY:
GIS NO.: 07G-H1824.00-01
DRAWING NO.: 1
LEGEND
Subject Property
Parcel Boundaries
Streets
#*Proposed Soil-GasSample Locations
0 70 14035Feet
1:600SCALE =
GDI
DHN
SG-1SG-2
SG-3
SG-4
10/18/2023