Environmental Site Investigation
In the unsaturated soil zone, contaminants are distributed in three phases: absorbed on the soil, dissolved in the soil water and as a gas dissolved in the soil gas. The distribution between the three phases depends on the physical and chemical properties of the contaminants. A greater proportion of soil contamination might be gaseous due to organic volatile compounds. As a result, it can be often advantageous to perform soil gas measurements when investigating soil contamination.
Soil gas measurements are generally used to obtain vapor phase data at sites potentially affected by Volatile Organic Compounds (VOC’s) as for example chlorinated solvents. Among other things, the data can be used to identify the source and determine the spatial distribution of VOC’s contamination at the site, which can help in order to localize point sources of soil and groundwater contamination, or to estimate indoor air concentration for risk assessment purposes and to determine most suitable locations for drilling of soil borings.
Soil gas investigation points will depend on previous investigation results, but they might be distributed homogenously within the whole study area, serving as a preliminary contamination investigation.
Soil gas measurements are performed by drilling boreholes using direct push or hollow-stem auger drilling equipment or hand-driven using a rotary hammer or a hand auger. Nevertheless, if applicable, before any drilling activities, utility clearance for the installation area is required.
Boreholes are advanced to the target sampling depth and soil gas is pumped up through a probe, which has been rammed down to a given depth in the unsaturated soil zone, where typical sampling depth is 1-5 m depending on the objectives, geology and expected contamination.
Sample probes consist of a probe tip through which the soil gas sample is collected, and probe tubing extends from the probe tip to the ground surface. The materials of the sample probes should not react or interact with the target compounds, suggesting nylon, polyethylene, copper, PVC (PolyVinyl Chloride) or stainless steel materials.
Boreholes should be grouted to the surface with hydrated bentonite. Adequately sealing soil gas sampling probes is very important to minimize the exchange of atmospheric air with the soil gas and to maximize the representativness of the sample.
Once soil gas samples are pumped up from the probe, they are collected for subsequent analysis, for example, on adsorption tubes. If samples are collected at multiple depths within the same borehole, the borehole is initially advanced to the deepest sampling point. Generally, soil gas samples must be collected at a minimum of two discrete depths at each sampling point. Where contamination or groundwater level is very shallow (less than approximately 1 meter below the ground surface), one sample from a single depth might be sufficient.
Generally, soil gas sample depths should be chosen to minimize the effects of changes in barometric pressure, temperature, or breakthrough of ambient air from the surface, and to ensure that consistent and representative samples are collected. Soil gas sampling depth must be consistent from sampling point to sampling point.
After collecting soil gas samples, subsequent analyses can be either conducted as field measurements using PID instruments (Photo-Ionization Detectors), portable gas cromatographs, etc., or they may be conducted in the laboratory. Selection of the analysis method will depend on the application of soil gas measurements.