The LIZ is the theoretical reasonable maximum distance that vapors have traveled beyond the source, so there are multiple situations that may benefit from a reduced LIZ, including reducing the number of properties or occupied buildings where further investigation is warranted. In these circumstances appropriately placed and monitored soil gas samples allow for an evaluation of the actual extent of vapor migration above site specific VIAC.
Soil gas samples should be collected vertically throughout the vadose zone beyond the vapor source and include samples below any potential or known buildings as well as laterally adjacent to the vapor source. To reduce the need for long-term obligations, the number of sampling events should follow Table C.3-4 and should meet appropriate quality assurance and quality control standards.