FRTR Field Sampling and Analysis Technologies Reference Guide

4 COLLECTION TOOLS

4.7 GAS/AIR COLLECTION METHODS

4.7.1 Soil Gas Sampling (Static)

Use:

Detecting VOCs in the unsaturated zone.

Description:

Sampling of soil gases (volatile contaminants such as methane and carbon dioxide, which are indicators of increased microbial activity resulting from organic contaminants) is gaining acceptance as a method for preliminary mapping of contaminant plumes in ground water and monitoring underground storage tanks. This is achieved either by passive (static) sampling, where absorbent collectors are buried for a period of time and retrieved for laboratory analysis, or by using gas-sampling probes.

Static sampling can be done two ways:

An in-situ adsorbent (usually an activated charcoal rod) is buried in the soil for a period of days to weeks. The adsorbent is retrieved and analyzed at a laboratory for VOCs.
Samples are collected from containers placed in the surface soil and analyzed using portable analytical instruments.

Concentrations in soil gas are affected by dissolution, adsorption, and partitioning. Partitioning refers to the ratio of component found in a saturated vapor above an aqueous solution to the amount in the solution. Contaminants can also be adsorbed onto inorganic soil components or "dissolved" in organic components. These factors can result in a lowering of the partitioning coefficient. Soil "tightness," or amount of void space in the soil matrix, will affect the rate of recharging of gas into the soil gas well. Existence of a high, or perched, water table, or of an impermeable underlying layer (such as a clay lens or layer of buried slag) may interfere with sampling of the soil gas. Knowledge of site geology is useful in such situations and can prevent inaccurate sampling.

Analytes:

1. Non-Halogenated VOCs

3. Halogenated VOCs

Media:

Soil:	Ground Water:	Surface Water:	Gas/Air:
Not Applicable	Not Applicable	Not Applicable	MINIMUM

Maximum Depth:	Up to 25 feet.
Production Rate:	Sample is available after a short amount of time.
Investigation Derived Waste Volume:	Small volume of waste.
Technology Status:	Commercially available and routinely used field technology.
Certification/Verification:	Technology has not participated in CalEPA certification and/or CSCT verification program. Note - two soil gas sampling devices were demonstrated during fiscal year 1997.
Relative Cost per Sample:	Least expensive.

Limitations:

Insufficient exposure might result in a false negative.
Overexposure might mask relative differences in soil gas contamination at different sampling locations.
Vertical profiles of soil gas concentrations are more difficult to obtain than with soil probes.
Can not detect contaminant, which does not have a vapor phase such as heavy metals.
Capability to detect compounds that adhere to soils such as PCBs and pesticides is reduced.

ASTM Standards:

D 5314 - 92

Soil Gas Monitoring in the Vadose Zone.

EPA Guidance:

EPA 600/8-87/036

Soil Gas Sensors for Detection and Mapping of Volatile Organics.

Sample Access/Collection Matrix

Sample Analysis Matrix

Home

Areas of Interest