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In this month’s edition of Fundamentals of Vapor Intrusion, we’ll get a little wonky on you, and discuss soil-gas entry and mixing with indoor air. Like most things in vapor intrusion, there are a lot of unknowns, but we understand enough about this process that we can make reasonable assumptions about what can and can’t happen when vapors migrate from beneath the floor (sub-slab) to indoor air.
In the standard conceptual site model (CSM) for vapor intrusion, the migration of vapors from source to receptor can be broken down into three main components, from deep to shallow:
• Vapor partitioning from groundwater (or soil) to soil gas
• Vapor migration upward through the vadose zone
• Soil-gas entry and mixing with indoor air
Vapor concentrations are lessened or “attenuated” in each of these processes. Because of the complications that arise when sampling indoor air, we typically collect subsurface samples first, and estimate how much the vapors will be attenuated by the time they mix with indoor air. If subsurface concentrations are low enough, we might be able to avoid sampling indoor air.
Today’s discussion focuses on the third component, soil-gas entry and mixing with indoor air. The only thing that happens at this stage is dilution, so while there are still quite a few unknowns, chemistry isn’t one of them, and we can put reasonable limits on how much vapor intrusion could actually occur.
Using the default residential settings from EPA’s advanced soil gas model, we’ll assume the following conditions:
• Building length: 10 meters (32.8 feet)
• Building width: 10 meters (32.8 feet)
• Ceiling height: 2.44 meters (8 feet) – (Total building volume 244 cubic meters)
• Soil-Gas Entry Rate: 5 liters/minute (0.3 cubic meters/hour)
• Outdoor-to-Indoor Air Exchange Rate: 0.25/hour (once every 4 hours)
The Air Exchange Rate indicates how often the air in a building is replaced with new outdoor air. Higher Air Exchange Rates, and lower soil-gas entry rates, result in more vapor dilution and less vapor intrusion.
Under the default residential settings, we assume that every 4 hours, 1.2 cubic meters of soil gas mixes with 244 cubic meters of fresh air, so soil gas is diluted by a factor of 200. (Expressed as an attenuation factor, 1/200 dilution equals sub-slab attenuation of 0.005). Actual attenuation is typically greater. EPA’s Exposure Factors Handbook indicates a median air exchange rate for residences of 0.45/hour, instead of 0.25/hour, so the soil gas will be diluted almost twice as much as we calculated. Commercial/industrial (CI) buildings often have twice the ceiling height (16 feet), and 4 times the air exchange rate (1/hour), which increases soil-gas dilution by 8 times more than in the residential setting. Additionally, the default soil-gas entry rate of 0.3 cubic meters/hour is higher than average, so the default settings exaggerate vapor intrusion in multiple ways.
In fairness, EPA has good reasons to use conservative default attenuation factors – to a point. EPA’s report on the Attenuation Factor Database indicates a median sub-slab attenuation factor in residential settings of 0.003, which is fairly close to the 0.005 value we calculated. Their decision to base soil-gas screening levels on the more conservative attenuation factor of 0.03 provides a safety factor, which makes sense, but one shouldn’t expect indoor air concentrations to actually equal soil gas concentrations multiplied x 0.03, even in residential settings. Our volumetric calculations show that in CI settings, soil gas concentrations are probably less than 0.0003 x soil gas concentrations, and in our experience, they’re often much lower. What’s more, we normally base our calculations on the maximum observed soil-gas vapor concentrations, but the actual soil gas entering a building is a mixture of higher and lower concentrations.
EPA is unlikely to change their default attenuation factors or the soil-gas screening levels derived from them. But in view of the conservativeness of the default attenuation factor of 0.03, especially in CI settings, it might be appropriate to relax the vapor-intrusion response actions based on soil gas data. As we discussed in the September 2016 issue of Focus on the Environment, Ohio EPA and other agencies, have recently recommended urgent responses for high vapor concentrations, especially trichloroethene (TCE). Ohio’s Urgent Response Level – the highest category – for TCE in soil gas in a CI setting, is 880 micrograms/cubic meter (ug/m3). An understanding of soil-gas entry rates and air exchange rates tells us that 880 ug/m3 of TCE in soil gas is very unlikely to result in indoor air concentrations above the target level of 8.8 ug/m3. Soil-gas concentrations above soil-gas screening levels merit further investigation, including indoor-air sampling, but a panic response is not appropriate or helpful.