What is the primary reason that soil-gas sampling for vapor intrusion differs from soil-gas sampling for typical site assessment?
The difference is in how low a concentration you have to measure. For site-assessment applications, we typically worry about contaminant concentrations above 1 g/L. For vapor-intrusion applications, we measure down to levels as low as 1 g/m3, fully 1,000 to 10,000 times lower. This means that we need to be much more careful in how we collect and analyze samples. Field and analytical techniques that are suitable for higher concentrations are often not suitable for these ultra-low concentrations. Small contaminant blanks from equipment, fingers, clothing, the working surface (e.g., the tailgate of your pick-up), even the ambient air can be enough to fail acceptable risk levels.
