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Because vapor intrusion is all about soil gas entering buildings, vapor-intrusion assessments include lots of soil-gas sampling. We discussed some of the differences between deep and shallow soil gas in the April issue of Focus on the Environment, and in this issue, we’ll discuss some of the practical differences when sampling them.
There are exceptions, but most shallow soil-gas samples are collected from the subslab – directly beneath a building’s lowest floor. Deep soil gas can be sampled anywhere you can take a drilling rig, providing that the soil is permeable enough to yield soil gas. Unfortunately, in many areas, including glaciated terrains, deep soil is often too tight to yield soil gas, and one can easily spend hundreds of dollars finding that out. Subslab sampling is limited to structures or areas covered with concrete or pavement. But because buildings and pavement are normally constructed over gravel or other permeable fill, in over 500 subslab sample points that I’ve installed, only two or three encountered tight soil. What’s more, subslab soil gas points are installed in several minutes, using a hand-held hammer drill.
More importantly, subslab soil gas is better than deep soil gas for predicting vapor intrusion in indoor air. The graph below, from EPA’s Vapor Intrusion Database, compares vapor concentrations in subslab soil gas, deep soil gas, and groundwater, to concentrations in indoor air. The bar on the right indicates that subslab soil gas does not predict indoor vapor concentrations very precisely. The ratios of subslab soil-gas to indoor-air concentrations vary by a factor of 81, even ignoring the highest and lowest 5% of data. But the precision of deep soil gas for predicting indoor air is lower – lots lower! The 90-percentile data vary by a factor of 3,300 – 40 times as much as subslab soil gas. This shouldn’t be surprising. Vapor intrusion has to pass through subslab before entering indoor air, and additional variables from migration through deep soil can only complicate the picture.
One question arises from this data: Why is the variability in groundwater data lower than in deep soil-gas data? Groundwater adds even more factors which logically should cause more variability. I believe the answer is that soil moisture is more variable in deep soil. Because vapor diffuses 10,000 times faster through soil gas than through water, soil moisture is extremely important to the attenuation, or lessening, of vapor concentrations. There is always moisture in the capillary zone directly above water level, but the amount of moisture in deep soil away from groundwater is extremely variable. And unfortunately, deep soil gas collected outside of a building might be nothing like deep soil gas beneath a building, where vapor intrusion really counts.
Subslab is the last stop before indoor air, and subslab soil-gas samples are best for predicting indoor air. In all, the lower cost, more consistent permeability, and greater representativeness, make subslab soil gas superior to deep soil gas for predicting vapor intrusion.