The risk of volatile organic compounds (VOC), vapourising, migrating, and potentially entering inhabited spaces can pose a significant threat to people unknowingly exposed to, and inhaling, the contaminated vapours.
Some, like the migrating vapours of chlorinated volatile organic compounds (CVOC) have been classified as human carcinogens (a substance capable of causing cancer) and are thus proven to cause severe long-term and short-term health risks.
Another threat is the volatile components of petroleum fuels, which can, in extreme cases, create a risk of fire and/or explosion. Having been widely used and stored all over the world, both are currently posing a great challenge in environmental remediation.
"When you've got a structure, or are looking to build a structure, on a site where you have known contamination in soil and groundwater, there'll be a requirement to do measurements. In some cases, if the concentration in the ground and soil are below certain levels, it might be possible to just use regulatory adopted screening models, and it may not be required to do vapour intrusion monitoring or mitigation," explains Eliot Cooper, vice president technology and business development at Cascade Environmental.
"If not, one thing you can do is to clean up the source of contamination so that you eliminate the chemicals that are tied up in the unsaturated soil matrix or in groundwater either through in situ treatment or excavation and disposal. Or, in some cases, you might not do either, but instead, put up some sort of vapour mitigation system above ground so that any vapours that migrate up, are removed before they enter the breathing space of occupants or workers."
Regulations as to what are safe amounts differ from country to country as does regulation on how risks are to be managed. In some states in the US, for instance, landowners have to prove that the source of the soil gas has been cleaned up to furthest possible extent before they are allowed to install a vapour mitigation system above ground.
Where does the risk come from?
When it comes to the CVOC trichloroethylene (TCE), it has been widely used as a cleaning solvent for industrial processes like degreasing or cleaning steel. Hence, it usually occurs in sites where parts were previously manufactured, or machines repaired or rebuilt. Another common CVOC perchloroethylene (PCE) was widely used as a dry-cleaning solvent, while the CVOC carbon tetrachloride was used as a grain fumigant in grain elevators and bins.
Preliminary environmental investigations on, or nearby, sites where such activities might have taken place are thus likely to warrant further sampling and assessment. Wesley McCall, Geologist at Geoprobe, a US-based manufacturer and designer of tools and machines for environmental investigation, explains. "Usually the impact to the environment occurred when these CVOC were improperly stored, disposed of or leaked or spilt on the ground. This results in soil contamination. And since these chlorinated compounds are heavier than water, the liquid solvent will move down the soil column and into the groundwater resulting in groundwater contamination. The CVOC are often carried along with the groundwater flow and can spread several kilometres from the original spill or release site."
From wherever they end up, these compounds can if near a migration pathway, volatilise from the contaminated soil or groundwater and migrate up through the soil as vapour and enter homes or workplaces.
Another significant source of potential vapour intrusion are the volatile components (e.g. benzene) of petroleum fuels like gasoline or diesel fuel, which are stored in underground storage tanks (UST) at petrol stations. "Unfortunately, almost all of the UST in the world have rusted and leaked fuel into the soil and groundwater," says McCall. "When gasoline or diesel fuel are leaked from UST or the piping to the pump islands it can form pools of light nonaqueous phase liquid (LNAPL) and can spread in the soil and be dissolved in the groundwater."
Again, the vapours from the spilt product can migrate through the soil pores and enter buildings. In extreme cases, the fuel vapour concentration can be high enough to ignite and explode, for example when a natural-gas-fired hot water heater comes on in the basement of a home.
How to detect it
To determine if any of these contaminants are present in the soil or groundwater, site investigators may take soil samples for testing via a direct push technology whereby a pipe is driven into the ground and soil samples are recovered. But there are also more advanced tools which make it possible to give an indication of contamination from below the surface.
"Direct sensing tools and high-resolution site characterisation tools give a better idea of what's going on before you put in a well or take samples," explains Cooper. "It's the optimised way of finding out where the contamination is - like taking an X-ray before doing surgery."
To do this investigative work a Geoprobe drill rig is often used to run logs with the membrane interface probe (MIP), a logging tool that measures volatile hydrocarbon and solvent contamination along with soil electrical conductance and permeability. The MIP can detect both CVOC and lighter petroleum fuels in the vadose zone or groundwater in unconsolidated formations and enables the investigator to map out the general area of contamination.
Obtaining an understanding of soil conductance and permeability along with the contaminant information allows the user to know if the VOC contamination is near to prominent coarse-grained soil migration pathways or bound up in a fine-grained soil storage zone. Knowing this can aid in how the investigator should approach site remedial efforts.
If fuel LNAPLs are present at a facility, Geoprobe's Optical Image Profiler (OIP) logging system may be used to located and define the extent of the free product in the subsurface. "The OIP uses an ultraviolet light-emitting diode (UV LED) to induce fluorescence of the petroleum fuel. The UV LED illuminates the formation through a small sapphire window on the side of the probe. Then a small downhole camera mounted behind the window captures images of the fuel fluorescence," McCall explains.
To track and delineate a soil gas plume directly, Geoprobe's PRT soil gas sampling system provides a quick and effective method. Small diameter rods are advanced to the selected depth with an expendable point. Then a polymer tube with adapter is lowered through the rods and attached to the back of the point holder. The rods are retracted to open a zone in the formation to sample the soil vapour. The soil vapour sample is sent to an onsite or offsite lab for quantitative analysis of soil gas.
One of the essential characteristics of CVOC is that they can form dense nonaqueous phase liquids (DNAPL) in the subsurface. As the liquid solvent flows down through the soil column and groundwater it will pool on top of an impermeable layer (e.g. clay layer) forming a DNAPL pool. From there, the DNAPL can continue to release dissolved contaminants to surrounding media for an extended period of time. Like LNAPLs, it will thus form a source area for on-going groundwater and vapour intrusion concerns.
The fuel vapour concentration can be high enough to ignite and explode
Consequently, cleaning up LNAPLs and DNAPLs constitutes one of the most significant challenges in environmental remediation.
Currently, different technologies that either treat or aim to remove the DNALP are being tested and used; for instance, forms of in situ surfactant flushing, electrical thermal remediation or specialized in situ amendments like the emulsified zero-valent iron developed by NASA for its groundwater challenges at the Kennedy Space Station.
Thermal technology is combined with soil vapour extraction, a technology which involves applying a vacuum to the soil usually through boreholes or trenches to induce a flow of volatised contaminants through the vadose soil. The extracted air containing the contaminants can then be treated to recover or destroy the contaminants.
"Ideally, when a source of possible vapour intrusion is tracked and delineated, you clean all that up and then a potential vapour mitigation system is a second line of defence for making sure everything is sustainable," stresses Cooper.