Soil Vapor Intrusion

The phrase “soil vapor intrusion” refers to the process by which contaminants migrate from a subsurface source, through soil vapor and into the indoor air of a building where an anthropogenic exposure is possible. Soil vapor, also referred to as soil gas, is the air found in the pore spaces between soil particles. After the contaminant partitions from the source and into “vapor” it travels through the pore spaces of vadose or unsaturated soil via diffusion or advection. If the contaminant vapor migrates into the radius of influence of an existing building, then typical building pressure differentials related to heating or cooling systems can draw the contaminant vapors into or below the structure where it could accumulate. In order to properly evaluate the soil vapor intrusion pathway, Aztech uses the recommended multiple lines of evidence approach where samples are collected from soil vapor, sub‐slab vapor, indoor air, outdoor air, groundwater and soil. Aztech geologists are fully versed and experienced in collecting these samples in accordance with pertinent state and federal protocols and regulations. All soil vapor and/or air sampling is done using summa canisters set to specific sampling durations. The sampling durations are dependent on site conditions. After the samples are collected they are sent to a certified laboratory for analyses.

Soil Vapor Point Installation

Soil vapor points are typically installed by Aztech drill crews using: hand tooling; geoprobe sampling tooling; split spoon sampling tooling; or, small diameter casing. The installation method generally depends on site specific information regarding depth of impacts and geology. If an aqueous groundwater plume is suspected as a potential soil vapor intrusion source, then the soil vapor point is installed to approximately one to two feet above the groundwater table to evaluate the soil vapor concentrations just above the area of contaminant partitioning. Then a six‐inch stainless steel screen is installed and connected to either Teflon or nylon tubing, which extends up to grade. The annular space between the screen and borehole wall is then filled with a certified clean material (i.e., glass beads or silica sand) and the borehole is sealed above the “sand” pack with bentonite slurry to grade. The necessary wellhead protection is then installed.

Sub‐slab Vapor Point Installation

Typically, a sub‐slab vapor point is installed by advancing a drill bit through the concrete slab. After the concrete slab is penetrated either Teflon or nylon tubing is installed just below the bottom of the concrete slab and the penetration through the concrete is sealed with a combination of beeswax and/or bentonite slurry. Prior to sampling the seal of each sub‐slab vapor sampling point is evaluated for adequacy using standard protocol for a helium tracer test. The sub‐slab vapor point is then purged accordingly and sampled to evaluate the accumulation of contaminated soil vapors beneath the concrete slab. After the sampling is completed, the seal is tested a second time for adequacy for quality control and assurance.

Indoor and Outdoor Air Sampling

Prior to sampling indoor air all potential cross‐contamination sources are evaluated, inventoried, and either removed from the premises or sealed and stored appropriately. After the zone of exposure is identified, which is typically the breathing zone, a summa canister is installed to sample that zone for a specified duration of time which is dependent on site conditions. Outdoor air sampling generally provides a control. Similar protocol to indoor air sampling is used during outdoor air sampling, however the location of the outdoor air sampling point is specified to evaluate the potential ambient sources of impact, if any.