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.
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.
The 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 are 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 on 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 the 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.
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 database control. A similar protocol to indoor air sampling is used during outdoor air sampling. External air sampling data is evaluated to establish the potential adverse contribution of ambient occurring contaminant to the structure’s internal air quality.
Vapor Mitigation – Remedial Corrective Action
Subsequent to the completion of the soil vapor intrusion study, PES engineering and technical staff analyze collected data to ascertain the need for additional investigation and/or soil vapor impact mitigation. PES staff have performed numerous successful vapor abatement actions, involving private residences, commercial buildings, and large industrial facilities. The PES staff is well versed in the design process to allow for the development of effective and cost-efficient remedial mitigation work plans. Corrective action can be as simple as wind-driven negative air ventilation of the structures sub-slab or large-scale power ventilation (in the case of an industrial application).