WDA Consultants Inc. - GASSYS

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.


  • Weyer, K. U., M. Hamann, and H. Kaiser. 2001. GASSYS: A unique in-situ and passive gas sampling system from unsaturated soil and from groundwater. Paper presented at the International Oil and Gas Conference, Zadar, Croatia, October 2-5, 2001, 11 p.

  • H. Kaiser and C. Schillinger, 1999. Passives Gassammelsystem für horizontbezogene Gasprobennahme in Böden und Grundwasser (A passive gas collection system for stratified sampling in soil and groundwater). TerraTech, v. 6, p. 19-21, Mainz

  • Institut für Umweltgeologie und Altlasten der Landesgewerbeanstalt Bayern (LGA)und KaiserGeoconsult, Januar 1999. Erprobung eines segmentierten Gassammelsystems für horizontbezogene Gasbeprobenahme in Wasser und Böden von bereits untersuchten Arealen (Test of a segmented gas collection system for stratified sampling in groundwater and soil of areas with known contamination). Research Report to Bavarian Ministry for Environment.

  • Tetra Tech Inc., 1998. Passive Soil Gas Sampler, W.L.Gore & Associates, Inc., Gore-Sorber® Screening Survey. EPA-Environmental Technology Verification Report. EPA/600/R-98-095, August 1998, 68 p.

  • M. Hübner (Siemens AG), 1998. Leckageerkennungs- und Ortungssystem LEOS (The leakage detection system LEOS). Verein Deutscher Ingenieure (VDI); Meeting "Modern Technologies in the protection of the Environment", Dresden, March 12, 1998.

  • E. Faber, A. Hollerbach, M. Hübner, H. Kaiser, J. Poggenburg , W. Stahl and H. J. Tobschall, 1998. Permeable Membranen - Ein Werkzeug zur vereinfachten Probenahme von Kohlenwasserstoff-Gasen? (Application of permeable membranes for sampling of hydrocarbon gases) Deutsche Wissenschaftliche Gesellschaft für Erdöl, Erdgas und Kohle E.V. (DGMK): DGMK-Tagungsbericht, Spring Meeting 1998. P. 401-410.

  • F. Kern and W. Blumhofer, 1996. Vergleichsmessungen mit aktiven und passiven Bodenluftuntersuchungsmethoden in Lichtenau (Field comparison of passive and active collection systems for soil air at the Lichtenau site). Landesanstalt für Umweltschutz Baden-Württemberg, Texte und Berichte zur Altlastenbearbeitung, v.26, 1996, 53 p.