The first time I used a Geoprobe® machine was in 1988 on a soil gas investigation somewhere in Iowa. My first field project was to track volatile organic contaminant (VOC) plumes in a suburban area and try to locate the potential sources. The area reminded me a little bit of the Twilight Zone but it wasn’t! A Ford van housed the 8M probe unit and boxes of sample bottles, sample coolers, personal protective gear, and of course duct tape and garbage bags. In those days, Geoprobe® and direct push was a brand new concept and most folks were using drill rigs for soil gas investigations or NPT pipe and a slam bar if low-tech and low cost was the need. After using a slam bar and a modified car jack for the extraction process, the Geoprobe® seemed like a dream come true! We pulled more soil gas samples in one day than they used to get in 3 or 4 days with the slam bar!

Generalized description of vapor intrusion in a residential setting from groundwater and subsurface soil sources. (Source: USEPA Region 3)

In the past we had used soil gas sampling and field analysis as a cost-effective way to track and map VOC contaminant plumes and find the source areas. That was much less expensive than augering to install monitoring wells and collecting groundwater (or soil gas) samples to send to the CLP laboratory for analyses (that arrived weeks later). However, as we’ve learned more about the possible health risks from VOCs by subsurface migration and intrusion into our homes, schools, and other buildings, we see that contaminated soil gas can be a much greater concern than was initially anticipated.
As our understanding of this vapor intrusion (VI) pathway has grown, we’ve learned that it may be a substantial health and safety risk. The immediate risk is when combustible vapors build up in a basement or crawl space and exceed the lower explosive limit. One spark from the hot water heater and, in technical terms ... KABOOM!! However, less obvious, but potentially just as dangerous, is the persistent presence of low concentrations of VOCs in the ambient air in

An “artist’s” rendering showing the relationship of the Vadose Zone and the Saturated Zone. The exploded view of the Vadose Zone shows the primary components of the soil matrix. VOCs equilibrate with the solid, liquid, and gas phases of the soil matrix. Free product may also be present in the void space of the soil matrix.

your house or building. While there may be no threat of kaboom, there is the health threat of long-term exposure to some of these contaminants. Several of the most common VOCs have been classified as potential or probable carcinogens by the U.S. EPA (e.g. benzene, trichloroethylene, perchloroethylene, etc.). These and other VOCs also may have other serious health effects, such as organ damage to kidneys, liver, central nervous system, etc., if long-term exposure to elevated levels of these compounds occurs.

Because vapor intrusion has been found to be a real health risk, the U.S. EPA and some state environmental agencies are developing regulations and guidance documents to address this problem. Obviously one of the best ways to determine if vapor intrusion is a potential threat in the vicinity of VOC contamination is to sample and analyze soil gas. If there are no VOCs in the soil gas in an area then there can be no intrusion of the vapors into homes or other buildings. While indoor air sampling may be conducted, it is much more intrusive and is particularly prone to background problems (did Mr. Smith use solvent-X to clean his power tools in the basement the night before indoor air sampling was conducted?).

Sub-slab sampling, beneath the basement floor, is also an option. But again, it’s a relatively intrusive activity and permission to drill holes in someone’s basement floor may be a bit tougher to obtain than permission to put a small hole in the ground in Mr Smith’s backyard. Often, soil gas sampling can be accomplished on public right-of-ways so residential access and request/denial can be minimized.

Assembling an expendable point into the point holder for advancement into the subsurface for soil gas sampling. The Geoprobe® 1.25 in. probe rod used here has a slightly larger OD than the expendable point which gives a good seal above the sample interval to provide a good integrity soil gas sample.

In the late 1980’s, most of us were doing soil gas sampling directly through the probe rods. But we quickly learned that leaky rod joints (no O-rings then) and rusty steel rods could cause sample quality problems (now we use iron filings for remediation of many VOCs !). Geoprobe Systems® developed the PRT soil gas sampling system which eliminated many of these concerns and provided for higher sample integrity, as well as reducing the high intensity rod decon required for direct sampling. The PRT adapter let you connect an inexpensive polyethylene tube to the back of the expendable point holder and so obtain a good seal, and the gas sample would not contact the ID of the iron rods.

While the PRT system still works very nicely for grab sampling of soil gas, there were and are occasions when long-term monitoring of soil gas is required. To make soil gas monitoring possible by direct push methods with small diameter drive rods, small diameter screen implants were developed. This allowed the field team to install a stainless steel screen at the desired depth for long-term monitoring of soil gas contaminants at a relatively inexpensive cost.

Reading pressures using a Portable Vacuum/Volume System with a Geoprobe® Model 54LT.

These previous methods were developed primarily for plume tracking/mapping, source identification, and landfill or remediation monitoring. While these methods work very nicely and cost effectively, they may not meet the higher data quality objectives (DQOs) for soil gas sampling and monitoring for the vapor intrusion (VI) pathway. Why don’t these earlier methods meet the DQOs for the VI pathway you might ask? Evaluation of the VI pathway involves human health risk assessment. The sample integrity, sample quality, and analytical requirements are significantly more stringent for a human health risk assessment than for simple plume tracking and source identification.

Stainless steel implants from Geoprobe Systems® are constructed with either a porous polyethylene lining, used for air sparging (AT9638), or without a porous poly lining, used for soil gas and groundwater sampling.