Vapor intrusion is relatively new to the facility regulatory scene, but its effects which can be harmful to human health due to exposure to volatile organic compounds (VOCs), have become a concern when acquiring or changing the use of property.
What is vapor intrusion?
Vapor intrusion is the transfer of a volatile contaminant(s) in its gaseous form through the subsurface soil into a structure via cracks in walls and floors as well as other penetrations such as sewer lines and floor drains. Vapor intrusion assessments aim to identify suspected risks and help lead to the development of mitigation plans to address issues.
Terracon performs vapor intrusion assessments at sites with suspected impacts by collecting soil gas samples outside the footprint of or beneath a building slab primarily via two methods, hydraulic push-probes into site soil or through a Vapor Pin® installed through a floor slab. Samples are commonly collected into Summa canisters which are under vacuum and opened once connected to the probe or Vapor Pin®. Samples are analyzed for VOCs and the results are compared to the state or federal risk-based screening criteria. These criteria have been established to assess risk based on the use of a site (i.e. industrial or residential).
Recognized Environmental Conditions (RECs) originating from past or threatened releases of petroleum products, solvents and/or other hazardous substances can be identified during a Phase I Environmental Site Assessment (ESA) performed at a Site. A Phase II or Limited Site Investigation might be conducted to assess potential impacts to surface or subsurface media (soil, groundwater, and soil gas). Impacts identified during a Phase II or LSI can create potential exposure issues through vapor intrusion.
When elevated concentrations of VOCs are detected beneath a building at concentrations that warrant some form of mitigation, there are several methods used to address the vapor risk for both new construction and existing buildings. Mitigation systems can include passive barriers consisting of a rolled or spray-applied membrane, barriers with passive venting, or active sub-slab depressurization. The active systems are much like radon mitigation systems using a blower to exhaust vapors from beneath the slab to the atmosphere. These methods vary in cost and are largely dependent on building use, design, and size.
• Passive Barriers: Relatively low cost and easy installation. These barriers are common in new builds where vapor intrusion is a low-risk and generally consist of a 10- to 20-mil thick sheet material or spray-applied barrier placed beneath the floor slab. Penetrations are taped and/or mastic sealed.
• Passive Ventilation: Barriers with passive ventilation include perforated PVC pipe beneath the barrier and is vented above the building roof-line. This is generally installed at a site where risk from vapor intrusion is elevated but does not warrant active venting.
•Active Ventilation: Active ventilation includes a fan or blower connected to the perforated piping system to assist with movement of air beneath the slab and exhaust impacted air above the roofline. The system is often installed with a series of sumps to depressurize the sub-slab.
Case Study: Design and oversight during installation of a sub-slab depressurization system
A client was in the process of selling a portfolio of multi-family apartment sites which included two, 93-year-old, three-story apartment buildings located in Minneapolis, Minn. As required by the buyer and the lender, a Phase I ESA was performed at the site resulting in the identification of RECs. The RECs included historical use of underground storage tanks containing fuel oil for heating at the site as well as the potential for impacts from up-gradient area-wide sources including dry cleaning and the associated solvent use, which led to a Limited Site Investigation (LSI).
During the LSI, Terracon collected soil gas samples from the building footprints outside and from beneath the building slabs to evaluate whether impacts were present in soil vapor and if an exposure potential existed from vapor intrusion. The identified concentrations of volatile organic compounds (VOCs) in soil gas exceeded regulatory risk thresholds, thus requiring immediate action to prevent exposure to the buildings’ occupants.
Project challenges included: lack of existing building drawings, unknown footing design and layout, variability in floor slab condition and thicknesses, limited access to tenant occupied spaces, identification and abatement of asbestos containing materials, presence of lead-based paint, and the ongoing use of the site as an apartment complex.
With these challenges in mind, a Response Action Plan (RAP) was prepared for approval by the regulatory agency, which included an initial design and a phased pilot testing plan for a sub-slab depressurization (mitigation) system. A phased approach was necessary given the buildings’ ongoing occupancy and variable sub-surface conditions.
Pilot testing allowed for an assessment of soil conditions beneath the slabs and identification of the radius of influence of vapor removal to identify areas where vapor collection sumps could be installed. The pilot test data was used to begin a mitigation system design that considered the site limitations noted above.
Our phased approach to the pilot testing revealed the need for additional suction points in occupied spaces to effectively cover the entire buildings’ footprints. The final mitigation system design included 11 suction points in the lower level of each building connected through a network of piping leading to exhaust fans installed in each of the buildings’ unused coal rooms. Vented exhaust was discharged above the third story roofline.
Post-mitigation diagnostic testing and sampling confirmed effective sub-slab air flow, decreased concentrations of VOCs beneath the building slabs, and indoor air concentrations meeting regulatory standards. Based upon these results, the site was issued a No Further Action (NFA) letter from the Minnesota Pollution Control Agency. The system design and performance allowed our client to meet post-closing obligations as well as provide a protective environment for the occupants.
Jason Gelling is a senior geologist in Terracon’s Minneapolis office. He has 20+ years of environmental consulting experience related to environmental assessment, tanks, spills, investigations of petroleum and non-petroleum impacted sites, and management of hazardous materials in various regions of the U.S. For more information on due diligence or vapor intrusion you can contact
Jason at email@example.com
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