Effective Methods for Active Vapor Intrusion Mitigation
I got a call from an exacerbated client who was rightly concerned about a letter from the county. “The county letter said my neighbor has finished cleanup at their dry cleaner and they were implementing active vapor intrusion mitigation, “ she told me. We were helping this client navigate the issues, concerns and requirements regarding investigation and cleanup at a property adjacent to her own. “What is active vapor intrusion mitigation and what does it mean for our property?” she asked.
“Good question,” I told her. “I know we’ve already talked about vapor intrusion as it relates to your property, but let's go over it briefly, then we can talk about active vapor intrusion mitigation."
What is Vapor Intrusion?
“When we first spoke about investigation and cleanup at the dry cleaner next door, I told you there was a possibility that groundwater contaminated with tetrachloroethylene (PCE) traveled from beneath your neighbor's property to below your property,” I recalled. “I told you that PCE in the groundwater could contaminate soil vapor and contaminated soil vapor could enter the building on your property. Contaminated soil vapor traveling from an underground source into a building or building basement is vapor intrusion.”
“At the time, I let you know that soil vapor samples collected on your property showed there was no risk of vapor intrusion,” I said. “If we had found there was even a potential risk of PCE-contaminated vapor entering your building, we would have recommended active vapor intrusion mitigation (VIM) to keep vapors from entering into your building and to protect occupants from exposure.”
“While cleanup has occurred next door,” I continued, “it wasn’t enough to eliminate the vapor intrusion threat posed by the remaining contamination. So, to allow safe occupancy of the building, your neighbor decided to use active VIM until cleanup goals are reached.”
“Okay, sounds reasonable,” my client said. “So what is active VIM?”
Active Vapor Intrusion Mitigation (VIM) Methods
Simply put, active VIM methods create a pressure difference between air on the inside of a building and the soil vapor under a building in a way that keeps vapors out of the building. Active VIM methods tend to be more effective than passive methods, but more expensive. These methods can be used on new and existing buildings, they have a successful track record of performance, they can be applied to a wide variety of site conditions, and simple pressure gauges show they work.
VIM methods require periodic maintenance and they have long-term energy and maintenance costs. Let’s consider three active VIM methods: sub-slab depressurization, sub-membrane depressurization, and building overpressurization.
1) Sub-slab Depressurization
Sub-slab depressurization systems are designed to provide continuous pressure reduction beneath a building’s floor and foundation. This method is for new or existing slab-on-grade foundations. Sumps, drain tiles and block wall foundations can also be depressurized. Depressurization refers to using a fan or blower to bring the air pressure in the sub-slab venting layer down below the air pressure in the building. In this method, a blower or fan pulls contaminated vapor from the venting layer and discharges it above the roof line into the atmosphere. An air discharge permit may be necessary to allow discharge to the atmosphere.
For new construction, a vent layer made from sand or pea gravel is placed below the slab foundation. Soil vapor collection piping and a common header are installed in the vent layer to direct contaminated soil vapor to a discharge stack that ends above the roofline. The blower/fan connects the common header with the discharge stack. Regulatory guidelines suggest using a sub-slab liner above the vent layer to provide added protection if the fan fails.
For an existing slab-on-grade building, installation of a sub-slab depressurization system entails cutting holes or trenches in the building slab, removing soil and building venting pits or trenches in the space left behind. The vent pits or trenches are arranged to provide depressurization coverage over the entire building footprint. Collection pipes are installed in the pits or trenches after which they are filled with sand or pea gravel and covered with slab material. A common header connects the collection pipes with a blower or fan. Extracted soil vapor is routed to a discharge stack.
2) Sub-Membrane Depressurization
Sub-membrane depressurization operates like the sub-slab variety but refers to creating a venting layer beneath a membrane that is installed in crawl spaces over bare earth. It can be used in new and existing buildings with crawl spaces. The membrane covers the exposed dirt surface of a crawl space, creating a venting layer between the membrane and the dirt floor. The rest of the system is similar to a sub-slab depressurization system.
The edges of the foundation and any pass-through piping need to be well sealed and the membrane should be loose enough to prevent tearing. Routine inspection of the membrane is necessary to check for the seals and membrane for damage. An air discharge permit may be necessary to allow discharge to the atmosphere.
3) Building Overpressure
Building overpressure involves using a building’s heating, venting, and air conditioning (HVAC) systems or a new system to maintain positive pressure in a building relative to the pressure beneath the building floor. This method is typically used for commercial buildings and can be inexpensive for buildings where the HVAC system maintains a positive pressure.
We wrapped up our conversation about active vapor intrusion mitigation (VIM) methods and turned to the question of how VIM might impact her property. I told my client that VIM at the neighbor’s property means her property won’t be exposed to soil vapor contamination from her neighbor’s property. I let her know that as long as VIM is underway, the state and local environmental health agencies will oversee the operation and that VIM will operate until cleanup goals for her neighbor’s property are reached. It was good news for my client.
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