25008_North Hills Expansion_VIMWP_20230525

May 25, 2023

Claire Sorrell
Brownfields Redevelopment Section
NC Department of Environmental Quality
Division of Waste Management
Mail Service Center 1646
Raleigh, North Carolina 27699-1646

Subject: ADDENDUM 1
VAPOR INTRUSION MITIGATION SYSTEM DESIGN SUBMITTAL
NORTH HILLS EXPANSION - NHX
LASSITER MILL ROAD
RALEIGH, NORTH CAROLINA
BROWNFIELDS PROJECT NO.: 25008-21-092
MID-ATLANTIC PE LICENSE: F-0967
REVISION NO.: 3
MID-ATLANTIC JOB NO. 000R3746.02

Dear Ms. Sorrell:

On behalf of North Hills Owner, LP, Mid-Atlantic Associates, Inc. (Mid-Atlantic) has
prepared this addendum to the Design Submittal for the Vapor Intrusion Mitigation System
Revision 3 (VIMS Plan). The VIMS Plan is associated with the Brownfields
Redevelopment of North Hills Mall located on Lassiter Mill Road in Raleigh, North
Carolina. This addendum outlines the procedures by which indoor air sampling will be
conducted.

INTRODUCTION

A Vapor Intrusion Mitigation System (VIMS) is under construction at the subject site in
conjunction with redevelopment of the subject site. The VIMS is being constructed in
accordance with the Vapor Intrusion Mitigation System Design Submittal, Revision 3 dated
Addendum 1 VIMS Design Submittal May 25, 2023
NHX Page 2
Raleigh, North Carolina

December 21, 2021. This VIMS Plan was approved by the Brownfields Program (now
known as Brownfields Redevelopment Section (BRS)) on December 29, 2021. Section 4
of the VIMS Plan outlines the Post-Construction/Pre-Occupation System Effectiveness
Testing requirements. The VIMS Plan outlines a sequential sampling approach where the
initial sampling is limited to sampling the 20 sub-slab monitoring points included in the
VIMS design (there are 21 monitoring points, but the DEQ-approved VIMS Plans does not
require sampling of MP-14 located in the fire service access elevators (FSAE) room).
Under this BRS-approved VIMS Plan, indoor air sampling would only be conducted
if/where the sub-slab soil gas testing results indicated it was warranted. Subsequent to the
approval of the VIMS Plan, and in consultation with the BRS, the Prospective Developer
has elected to include indoor air sampling to the initial pre-occupancy testing regime.

As construction has progressed, design changes to the buildings have resulted in changes
to the monitoring point network. The following changes have occurred:

Monitoring Point Location Change Reason
MP-3 North-Central Stair Eliminated Stair is on a 5-foot-thick mat footing with
slab floor placed directly on the footing
MP-4 Central Elevator Eliminated Elevator removed from building
MP-6 West-Central Elevator Eliminated Elevator removed from building
MP-7 Eastern Storage Room Eliminated Storage removed from building
MP-8 East-Central Stair Eliminated Stair is an outdoor stair

In addition, occupancy for the northeastern portion of the development, primarily Building
W, is scheduled approximately 12 months after occupancy of Buildings, T, U and Y (which
is scheduled for August 2023). Therefore, sampling associated with Monitoring Points MP-
1, 2, 19, 20, 21 and 22 will be conducted in 2024. Currently, pre-occupancy sampling
associated Monitoring Points MP-5, 9, 10, 11, 12, 13, 15, 16, 17 and 18 is scheduled for
June 2023.

Although sub-slab sampling procedures were detailed in the VIMS Plan, indoor air
sampling procedures were not included in the VIMS Plan. This addendum was developed
to detail the sampling procedures for the collection of indoor air samples.

INDOOR AIR SAMPLING PLAN

There are 16 sub-slab monitoring points located at the subject site at locations illustrated
in the attached figure (adjusted from the VIMS Plan based on changes in construction).
Addendum 1 VIMS Design Submittal May 25, 2023
NHX Page 3
Raleigh, North Carolina

Indoor air samples will be collected at each monitoring point, except MP-14, creating
‘paired’ samples with the sub-slab soil gas samples collected at each monitoring point. The
indoor air samples will be collected initially with the fans for the VIMS operational. Once
indoor air sample collection is completed, the VIMS fans will be deactivated. In accordance
with the approved VIMS Plan, the sub-slab soil gas samples will be collected no sooner
than 48 hours after the VIMS fans have been deactivated. Aside from the 16 paired indoor
air samples, one outdoor air and one duplicate sample will be collected for each sampling
event (one conducted in June 2023 and the other in 2024).

Pre-occupancy sampling will take place, to the extent feasible, under conditions
representative of when the building will be occupied. Where present, active VIMS will be
operational, concrete slab floors will be in place and the building envelope will be
complete. All windows and doors will be closed during indoor air sampling. Since building
construction is not complete, temporary surrogate doors and windows constructed of tape-
sealed sheet plastic may need to be installed to simulate closed doors and windows. If
operational, the HVAC system will be set to maintain ‘as occupied’ temperature and
humidity conditions in the space being tested.

At least four days before each sampling event, a chemical survey of the building will be
conducted to check for potential interference compounds. An Indoor Air Survey and
Sampling Form (taken from the Vapor Intrusion Guidance) will be used to log the results of
the survey in the building. Potential interference compounds will be removed from the
space a minimum of three days before the sampling event. The condition of the HVAC
system in the space will be recorded during the chemical inventory and will remain
consistent through the sampling event. Since this sampling will be conducted prior to
building occupancy, access to the building during sampling will be controlled.

INDOOR AIR SAMPLING PROCEDURES

Each indoor air sample will be collected from approximately 3-5 feet above the floor in the
lowest level of the building. One ambient outdoor air sample will also be collected at an
upwind location during each indoor air sampling event. The 24-hour indoor air samples will
be collected using individually certified, 6-liter, stainless steel Summa canisters at a
maximum pre-determined flow rate of 8-10 milliliters per minute (ml/min) by using the
laboratory-provided flow regulator. The initial vacuum displayed on the laboratory-provided
vacuum gauge on the Summa/flow regulator will be recorded. Mid-Atlantic will fill the
laboratory-provided Summa canister such that the final vacuum in the canister is at least 5
inches of mercury and the final reading will be recorded on the chain of custody. The

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TABLE
Monitoring
Point ID
Indoor Air
Sample
Location
Sample Date Depressurization
Area?
Building P
North Elevators and Lobby MP-1 IA-1 2024 N
North Elevator Pit MP-2 IA-2 2024 N
North-Central Stair MP-3 IA-3 None N
Central Elevator MP-4 IA-4 None N
Building U/T Elevators & Lobby MP-5 IA-5 June 2023 N
West-Central Elevator MP-6 IA-6 None N
Eastern Storage MP-7 IA-7 None N
East-central Stair MP-8 IA-8 None N
Electrical Room MP-9 IA-9 June 2023 N
Southern Storage MP-10 IA-10 June 2023 N
Sprinkler Riser MP-11 IA-11 June 2023 N
Building Y
Maintenance Shop MP-12 IA-12 June 2023 Y
East Stair MP-13 IA-13 June 2023 N
FSAE MP-14 IA-14 Not Required* Y
Women's Locker Room MP-15 IA-15 June 2023 Y
Training Room MP-16 IA-16 June 2023 Y
West Stair MP-17 IA-17 June 2023 N
Dock Office MP-18 IA-18 June 2023 Y
Building W, slab-on-grade portion
Retail MP-19 IA-19 2024 Y
Retail MP-20 IA-20 2024 Y
Retail Stair MP-21 IA-21 2024 Y
Leasing MP-22 IA-22 2024 Y
Outdoor Air --- OA-1 June 2023 ---
Outdoor Air --- OA-2 2024 ---
Note: Monitoring Points MP-3, 4, 6, 7 and 8 were not constructed due to building design changes
*DEQ-Approved VIMS Plan does not requires sampling at MP-14 (Fire Service Access Elevators)
BROWNFIELD PROJECT NO. 25008-21-92
MID-ATLANTIC JOB NO. 000R3746.02
Location
TABLE
SUB-SLAB SOIL GAS MONITORING POINT AND INDOOR AIR SAMPLE LOCATIONS
NORTH HILLS EXPANSION - NHX
LASSITER MILL ROAD
RALEIGH, NORTH CAROLINA