Vapor Intrusion

Vapor intrusion is the migration of volatile chemicals (in vapor form) from the subsurface into overlying or adjacent buildings. Volatile chemicals can be found in buried wastes, contaminated soils, and/or contaminated groundwater and can emit vapors that may migrate through subsurface soils into buildings. In extreme cases, the vapors may accumulate in buildings to levels that may pose near-term safety hazards (e.g., explosion), acute health effects, or aesthetic problems (e.g., odors). Typically,
however, if vapors do migrate into buildings, the levels are considerably lower in comparison and health concerns, if any, relate to chronic effects based on long term exposure to low chemical concentrations.

In the past, vapor intrusion was considered to be a phenomenon caused by soil vapors emanating from a source of volatile chemicals (separate-phase or sorbed) located adjacent to or directly beneath the foundation of an occupied building. Investigation of potential human exposure to these volatile chemicals generally involved soil gas surveys and indoor air sampling. If an off-site dissolved contaminant plume flowing beneath a home or business was deep, the assumption was that the concentrations of any vapors entering buildings above would be so low by the time it reached the basement level that it would not represent an indoor air concern. However, based on more recent studies by the scientific community and experience, this assumption appears to underestimate the potential for vapor intrusion. The result is that additional work may be required to investigate or remediate sites that are in the operational or monitoring phase or are even closed.

The following links should provide guidance for identifying and addressing existing and potential human exposures to contaminated subsurface vapors.

NYSDEC vapor intrusion guidance docs

NYSDOH vapor intrusion guidance docs

Demarcation Barriers

Demarcation barriers are permeable filter fabrics and geomembranes that are placed between a contaminated media and a cap. The demarcation barriers typically contain a printed message that will lead a would-be digger to a remedial program manager (e.g. government official or property designee). Such barriers are examples of both an engineering and institutional control.

Vapor Barriers

Contaminant Vapor Barriers are specially designed impermeable materials that are placed to prevent the migration of contaminants in the vapor phase. Typically, vapor barriers are implemented for the vertical migration of contaminants upward through a concrete slab into a building. Such barriers are constructed of high-density polyethylene (HDPE) sheeting installed immediately below subsurface slabs. Placement of the HDPE sheeting and the mix design of the overlying concrete are integral to vapor barrier performance. An improper barrier and/or slab placement can concentrate vapor off-gasses and/or create chronic mold problems within the building interior. Many vapor barriers are installed coincidently with passive or active sub slab depressurization systems. Vapor barriers are a key aspect of restoring property value to contaminated sites.

High-Density Polyethylene (HDPE) Sheeting

Impact Environmental has employed a variety of high-density polyethylene (HDPE) sheeting materials to provide protection from the migration of contaminant vapors to building interiors. Often, these barriers are placed as a component of an integrated remedial approach that can include sub slab depressurization, dewatering (groundwater pump and treatment), soil vapor extraction and/or groundwater sparging. When selecting the components of such systems, it is important that the barrier material, thickness and installation methods be matched to the contaminants of concern and the underlying soil characteristics.