Vapor clouds refer to situations where there is subsurface contamination of the soil vapor with little or no coincident soil or groundwater contamination, hence the term 'cloud.' They arise from leaking vapors, not from contaminated soil or groundwater. Common sources for vapor clouds are sites that contain surface sources of chlorinated solvents (e.g., vapor degreasers, dry cleaners, clarifiers), where the dense chlorinated vapors enter the vadose zone from above, or where vapors are leaking out of USTs.
You should care about vapor clouds for a number of reasons. First, unlike groundwater, vapors can move in all directions, regardless of the groundwater gradient, and move quickly-approximately 25 feet/year by molecular diffusion alone. So, a vapor cloud from a dry-cleaning washer unit can move laterally underneath adjoining businesses in a strip mall within one year and represent an upward migration threat to nearby residences within a few years.
Vapors leaking from an UST can move downward through the vadose zone to the groundwater and represent a groundwater contamination threat. (See 'The Downward Migration of Vapors,' LUSTLine #29 and 'The Great Escape from the UST,' LUSTLine #30 for discussions of this pathway.)
When the J-E model under predicts the measured risk, or indoor air results don't match with groundwater patterns, or when vertical profiles of the soil gas don't show increasing concentrations with depth, vapor clouds should be suspected and soil-gas data, not soil or groundwater data, must be collected to adequately assess the upward vapor risk pathway.