Vapor Intrusion Mitigation / Subslab Depressurization

Soil Vapor Intrusion

The phrase “soil vapor intrusion” refers to the process by which volatile chemicals migrate from a subsurface source, through soil vapor and into the indoor air of a building where an adverse exposure is possible. Soil vapor, also referred to as soil gas, is the air found in the pore spaces between soil particles (Figure 1). Soil vapor can enter a building through cracks or perforations in slabs or basement floors and walls.  Additional points of entry may include openings around sump pumps or where pipes and electrical wires go through the foundation.  Migration occurs due to differences between interior (within the structure) and exterior (within the soil) pressures.  Specifically, if the contaminant vapor migrates into the radius of influence of an existing building, then typical building pressure differentials related to heating or cooling systems can draw the contaminant vapors into or below the structure where it could accumulate.

Generalized Diagram - Soil Vapor Intrusion

Since no two sites are exactly alike, the approach to evaluating soil vapor intrusion is
Dependent upon site-specific conditions. When determining what actions, if any, are appropriate to mitigate current or prevent future human exposures, PES considers all available information known about a site.  A "whole picture" approach is taken because each site presents its own unique set of circumstances. Every PES investigation plan, is developed based on a thorough understanding of the site, including its history of use, characteristics (e.g., geology, geography, identified environmental contamination, etc.) and potentially exposed populations. 

Methodology used by PES to identify structure specific soil vapor intrusion pathways, involves the recommended multiple lines of evidence approach.   Representative samples of soil vapor are collected from sub‐slab, indoor and outdoor locations.  In addition, groundwater and soil data is often collected to augment the soil vapor information.   PES technical staff is fully versed and experienced in collecting these samples in accordance with pertinent state and federal protocols and regulations.  PES only utilizes laboratories that are certified by the New York State – Department of Health through the Environmental Laboratory Approval Program (ELAP).

Soil Vapor Point Installation (external to structure)

Soil vapor points are typically installed by PES drill crews using a variety of subsurface investigative methods. The chosen method is based on the particulars of the site as well as the desired result.  Consideration is given not only to the effective collection of pertinent data, but to the impact to the property.  PES carefully determined the method that is least disruptive to the property while providing the required information needed for an accurate characterization of potential contaminant impact.  Methods employed by PES include but are not limited to: hand tooling; geoprobe sampling, conventional split spoon sampling and/or small diameter casing. The installation method generally depends on site specific information regarding depth of impacts and geology.  All geologic factors including soil lithology (type), occurrence of groundwater and stratigraphic variation are documented and incorporated into the investigative plan.  Construction of groundwater monitoring, vapor assessment and remedial recovery (groundwater and/or soil vapor) wells are constructed using generally accepted engineering practices. The necessary wellhead protection is then installed.

Geoprobe – Vapor Point Installation

Sub-slab Vapor Point Installation (internal to structure)

Sub‐slab vapor assessment points are generally installed using limited access drilling equipment such as Hefty and/or geoprobe systems.  The utilized equipment advances soil sampling equipment through the concrete slab and into the underlying geologic material.   Soil vapor sensing probes are installed under the direction of a PES geologist or environmental scientist.  Construction of the assessment points follows accepted/recommended industry engineering standards.  Each sample assessment point uses dedicated inert sampling materials that ensure the integrity of the sample results.  Quality Assurance/ Quality Control (QA/QC) methods such as helium tracer tests are implemented prior to sampling each sub‐slab vapor sampling point.

Indoor and Outdoor Air Sampling (Ambient External Control and Interior Air Space)

As part of the QA/QC program – PES performs internal and external evaluations to identify potential sources of cross‐contamination.  Identified sources are evaluated, inventoried, and either removed from the premises or sealed and stored appropriately.  Potential vapor migration pathways are investigated/identified by PES technical staff.  Zones/routes of exposure are identified, which is typically the breathing zone.  Summa air sampling canisters, supplied by ELAP laboratories, are strategically installed to sample for exposure potential in each of the zones of exposure.   NES DOH procedures are strictly adhered to including specific duration timeframes for sample collection.  Site specifics are included in the formulation of sampling procedures.  Outdoor air sampling is generally utilized as a data base control. Similar protocol to indoor air sampling is used during outdoor air sampling.  External air sampling data is evaluated to establish potential adverse contribution of ambient occurring contaminant to the structure’s internal air quality.

Vapor Mitigation – Remedial Corrective Action

Case Study

Pilot Test & Soil Vapor Extraction/Sub-Slab Depressurization

Project Name: Capital District Plastic Product Manufacturer
Project Duration: 2005 - Present

Impact Characterization:   Historic releases of chlorinated solvents, resulting in an adverse impact to the subsurface soil and groundwater regimes beneath a Plastics manufacturing building.

Technical Overview:   The goal at this site was to reduce chlorinated solvent vapors and to create a vacuum within the sub-slab soils as a vapor intrusion control focusing on extracting vapors via soil vapor extraction wells to be installed through the facility’s concrete slab floor at eight locations.

Remediation pilot test equipment was supplied and operated in December 2005 by PES, to obtain site-specific information regarding the function and effectiveness of soil vapor extraction (SVE) wells.

PES installed eight 4” diameter, shallow, low vacuum, soil vapor extraction wells within the subsurface contaminant plume areas as defined by the client.  Two remedial system enclosures were provided and installed to house each of   two skid-mounted remedial systems.   Each remedial system enclosure houses a soil vapor extraction blower, moisture knockout drum, and an integrated control panel.  Significant coordination was required to properly deal with issues arising from site specifics such as performance of trenching and drilling within the floor and working overhead on man-lifts within an active manufacturing facility. 

The SVE wells were connected to the remedial enclosures via below and above ground steel and schedule 80 PVC piping.  Schedule 80 PVC piping was installed in the trenches and in the vertical runs from above the ten-foot steel interval (grade to ten feet) to the ceiling mounted headers. Piping trenches were backfilled with pea-stone and capped with 4,000-PSI concrete.  The replacement concrete consists of a separate monolith pour, which was finished to correspond to the existing floor height. 

Each SVE well is equipped with an airflow gauge and a vacuum gauge to allow monitoring of remedial system performance at each of the eight extraction wells.  Low vacuum applied to each SVE well will induce vacuum to the geologic formation beneath the concrete floor slab to enhance vapor migration to the SVE well and to induce a vacuum to the sub-slab.  The sub-slab vacuum is required to prevent vapor intrusion from the subsurface into the Plastics manufacturing facility.  Vacuum and airflow may be modified at each SVE well via valve adjustments.

Moisture within the extracted vapors is separated at the remedial system via moisture separator tank.  The liquid-phase fraction is stored within the moisture tank pending proper disposal. PES performed the required work, even with several change order situations, within the time and budget framework for this site.

Site Progression Specifics:  SVE pilot testing at this site was initiated in December 2005. The two remedial system installations were completed in June 2006 and the systems were activated on June 23, 2006.  The NYSDEC and NYSDOH mandated systems have remained active and have functioned as designed since activation with greater than 99% operating efficiency.