Toxic Vapor Migration in Subsurface Soils: Will California’s New Vapor Intrusion Guidance Affect Your Property Transaction?

Sep 11, 2020

Environmental Enlightenment #316

The opening image depicts a case where toxic vapors migrated in the soil from a source point to an appartment complex.  The next image is a case where soil-vapor sampling around source area raised a possibility that the volatile toxics might have reached under neighboring residences.

water droplets on glass panel
water droplets on glass panel

The operative questions are: (1) at what distance from source point will migrating, toxic soil vapors reduce to safe levels, and (2) what levels are safe?

There are four routes by which vapor travels. Here are illustrations taken from a video by California EPA

Route 1:  Sewer lines

Route 2:  Soil

Route 3: Groundwater

Route 4: Ambient air  

And then, there is the distance from the source to the structures of concern (receptors).

The leaking drum is a source point, the discharged fluids go straight down.

The migration path of least resistance is the sewer line that crosses the leaking plume. Fluids enter the sewer line, vaporize, and travel along as vapor.

Sewer lines are far from being tight. They provide ample accessibility for soil vapor. The diagram below and the pictures that follow depict this liability:

The lateral migration of vapors in the soil is generally by diffusion where the vapors move from spaces of high density to clearer zones. Take a room with still air, blow smoke at one end, and it will move to equalize throughout. This is diffusion.

Credit Pixabay

And then there is the movement through groundwater. The water carries the contamination with the flow, and then it evaporates (partitions itself) from the water into the soil above and comes under the structures.


Where groundwater is stagnant or close to being stagnant, the contamination will spread in all directions by diffusion.


Credit: Ami Adini

But whether the vapors rise up from the groundwater or migrate laterally through the soil; they must be in the soil under and around a building to become a threat of intrusion indoors. Therefore, our first step in evaluating the risk of intrusion is to collect samples of soil vapor around and under the structures.

The diagram on the left depicts a way of collecting soil-vapor samples, the photo on the right shows the probe and the vacuum pump, and the one down below shows the technician pulling a sample with a syringe.


With the sample results at hand, we evaluate the possible situation indoors.

A conventional unit of measurement for vapors in soil and indoors is micrograms of the chemical in one cubic meter of air (mcg/m3).  

Now, it used to be that 1000 units of vapor in the soil would be evaluated to predict 1 unit of vapor indoors (ratio of 1:1000, or attenuation factor 0.001). Experiments in recent years by US EPA have demonstrated that the ratio was more like 1000 in soil to 30 indoors (attenuation factor 0.030); that is, 30X stronger!


In real estate transaction, in the Phase I Environmental Site Assessment, when we suspect a possibility of toxic vapor intrusion into buildings, either from on-site spills or by encroachment from outside sources, we recommend a Phase II Environmental Site Assessment where soil vapor samples are taken around and under the structures.

Here is a project where we sampled 4 points inside the building, through the floor slabs, and two points in the parking lot.

And here is another property with two slab-on-grade buildings (SV1 and SV6) and two buildings with crawl spaces. SV1 and SV6 were taken through the floor slabs and other samples taken in the planters and parking lot.

If the Phase II results predict risky levels of vapors inside the structures, the next step is sampling the vapor indoors and outdoors. Technologies for indoor air sampling keep evolving into greater, more meaningful accuracies.

With the soil-vapor attenuation factor now having been increased 30X, the prospects for commercial properties to enter into extended investigation of vapor intrusion have increased, and with that also increased the prospects for high cost remediation or mitigation.

Call me any time, and I shall walk you through the entire search-and-discovery process.

And feel comfortable at exploring our knowledge free of charge.


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