A short comparison of soil gas-sampling systems
Sampling of soil air is applied in petroleum exploration and contamination studies. Techniques used range from active collection where soil gas is gathered at the bottom of a shallow borehole to passive collection on buried reactive carbon or other such materials when exposed to soil air over the course of usually 10 days or more.
Each of those sampling methods has disadvantages. Active sampling by pumping soil air into toddler bags draws soil gas from a large undefined space near the sampling interval. This results in preference to gas flow in higher permeable systems. Thus the results do not reflect the true and undisturbed gas content at the sampling interval. A second method of active sampling by pushing a syringe into the bottom soil of short boreholes only records the contents in unsaturated soil close to the surface.
Passive sampling methods usually collect gases by adsorption. Analyses using such sampling methods determine only relative gas concentrations as no direct gas sample is taken.
The unique passive gas sampling system GASSYS has been designed to overcome all of the above disadvantages. It takes true and reproducible gas samples from the unsaturated soil and, since the EVA-tube is permeable to gas but not to water, directly from the groundwater up to a depth of 30 m or more. Up to four sealed sampling intervals can be installed within an EVA-tube at different depth. The samples are collected by diffusion of gas from the immediate surrounding space. Gas collected is transferred from each segment by means of fine stainless steel tubes and calibrated syringes into a headspace vial and directly subjected to laboratory analyses.
Laboratory test data established the diffusion/time characteristics for a mixture of methane through propane with complete equilibrium times of approximately 10 hours and 6.5 hours respectively. Gases moving into the EVA-tube can be detected after less than 0.5 hours upon installation.
Case histories from German sites collected gases from volatile and less volatile hydrocarbons into the EVA-tubing for approximately 20 years.