Developing a Conceptual Site Model
When conducting a vapor intrusion investigation, at some point in the development of a conceptual site model with existing site data, it may be necessary to collect additional samples. The EPA, agencies and many states have published guidance to assist in the decision making process. Some have included, or at the very least, reference published guidance documents or standards on how to collect these data.
Most sites investigated already have data which may include their historical use, chemicals present, soil and/or groundwater data from spills or earlier investigations. Always be cognisant that early data may not meet the current regulatory needs. However, they may help to direct the environmental professional in the investigative process. Vapor intrusion are bottom up investigations. The samples collected should be taken to confirm what is already known, if there are questions concerning existing data, while filling data gaps to complete the decision making process.
Understanding Site Conditions for subsurface vapor sample collection.
Knowledge of site conditions, building construction, physical conditions, temperature, wind velocity, and barometric pressure can all affect the subsurface vapor concentrations. Studies and experience have shown that for relatively short periods, the concentration of VOCs in the vadose zone does not change significantly unless there are unusual site specific conditions. Some States consider a sub-slab samples a “grab sample” which can be collected at a rate below 200mls/minute. It is not always practical to monitor sub-slab to indoor air differential pressure prior to sample collection. However, this data may be helpful in confirming surface exchange rates. All these factors should be considered when developing a Sampling and Analysis Plan.
A key quality goal when collecting these vapor samples is that they are representative of the subsurface vapor composition at the time of collection. The sample collection does not change the concentrations of the volatile chemicals during collection.
To this end, critical steps in collecting samples are:
Collection media is certified clean
Use materials and supplies that are VOC free
Ensure that there is sufficient flow at the vapor sampling point
Check the integrity of the sampling assembly (Shut-in Check)
Check the annular seal integrity at the surface (Leak Check)
As always, contact me, Will Elcoate, with your questions at 800-624-9220 or email .Image credit: Migration of Soil Vapors to Indoor Air, Source https://www.epa.gov/vaporintrusion/what-vapor-intrusion