Vapor Intrusion Fundamentals: Part 11 – Spatial Variation

Author: Martin (Mort) Schmidt, CPG & Craig A. Cox, CPG

Little PictureAs discussed in last month’s Focus on the Environment, changes in vapor concentrations over time (temporal variability) complicate the interpretation of vapor intrusion. But a much greater concern is, or should be, spatial variability, which is the difference in vapor concentrations from place to place. A great deal of ink is spent extolling the virtues of some sampling techniques and condemning others, but your efforts are wasted if you don’t allow for spatial variation.

The first time Cox-Colvin used soil gas to locate volatile organic compounds (VOCs) in soil was at a large manufacturing facility in Ohio. The site had several groundwater plumes of chlorinated VOCs, but the plumes’ locations didn’t fit suspected sources, such as degreasing pits and underground storage tanks. Our earlier stabs at finding contamination through soil sampling were unsuccessful, due to the inaccurate information we received about previous history, and because soil heterogeneity (variability) is even greater than soil-vapor heterogeneity. Our revised approach was to collect soil gas beneath and around the building at a depth of 5 feet with a Geoprobe drilling rig, along a grid with 40-feet spacing between samples. Long story short, we found high levels of vapor contamination in unexpected places, and once we found and remediated contaminated soil, the groundwater plumes greatly diminished. We’ve since invented the Vapor Pin™, which lets us quickly collect far more soil-vapor samples for the same or lower cost, and we’ve nailed down VOC sources at a number of facilities in a matter of days. The large number of samples that we can cost-effectively collect provides us with a good understanding of spatial variabililty in soil gas.

There are important implications for vapor intrusion. The map below shows trichloroethene (TCE) concentrations in soil gas we found beneath the same building. Concentrations go from 75,000 parts per billion (400,000 micrograms/cubic meter) to 19 ppb (100 ug/m3) in the space of 40 feet.


If you remember the radar plots from the February issue of Focus on the Environment, it’s immediately obvious that the plot for soil gas taken from multiple places beneath the building (below, left) is far more variable than the plot for indoor air (below, right), which also came from multiple locations within a building.

Radar Plot - Soil GasRadar Plot - Indoor Air

Numerous presentations and reports have been devoted to the advantages and disadvantages of various types of sample tubing, sample containers, purge rates, and other things, but these pale in comparison to variations from spatial heterogeneity. The only remedy is to collect more samples, or to collect samples that represent larger areas.

In the next issue of Focus on the Environment, we’ll discuss when to monitor, and when to mitigate vapor intrusion.

<Published in Cox-Colvin’s March 2015 Focus on the Environment newsletter>

Mort Schmidt is a Senior Scientist with Cox-Colvin & Associates, Inc. He received his BS and MS degrees in Geology and Mineralogy from The Ohio State University, and has been a Cox-Colvin & Associates employee since 1997. His areas of expertise include vapor intrusion and contaminant investigation and analysis, and he currently serves as Cox-Colvin’s Practice Leader – Vapor Intrusion Services. Mort is a Certified Professional Geologist with AIPG and is a registered Geologist in Indiana. Craig Cox is a principal and co-founder of Cox-Colvin & Associates, Inc., and holds degrees in geology and mineralogy from the Ohio State University and hydrogeology from the Colorado School of Mines. Mr. Cox has over 30 years of experience managing large environmental project implemented under CERCLA and state voluntary action programs. In addition, Mr. Cox has developed a variety of software products including Data Inspector, an internet-enabled environmental database application. Mr. Cox is a Certified Professional Geologist (CPG) with AIPG and is a Certified Professional (CP) under Ohio EPA’s Voluntary Action Program.