Releases or discharges of pollutants to subsurface soils commonly occur at various industrial and commercial facilities that utilize and store petroleum products, or that produce industrial waste. Such facilities may include aerospace facilities, dry cleaners, manufacturing facilities, active and former gasoline service stations, metal plating and finishing shops, and oil field sites. Volatile organic compounds (VOCs), such as gasoline or chlorinated solvent compounds, may be released into the subsurface from various source areas within these facilities, which may include aboveground or underground storage tanks, clarifiers, piping, drum storage areas, cleaning/wash areas, etc.
Within the subsurface, contaminants may exist in the subsurface in the following phases: as a solid phase by adsorbing onto soil, dissolved phase in groundwater, liquid phase within the soil pore spaces, and/or in the gaseous phase within soil pore spaces. Although full characterization of each phase is essential for most investigations, from a vapor intrusion standpoint, it is particularly important to assess for VOCs in the gaseous phase. Vapor intrusion into residential or commercial structures occurs when VOCs in the soil gas migrate into building structures through cracks or openings in the concrete slab.
Soil gas investigations are normally conducted in accordance with the Advisory – Active Soil Gas Investigations (Advisory), prepared jointly by the Department of Toxic Substances Control, the Los Angeles Regional Water Quality Control Board (LARWQCB), and the San Francisco Regional Water Quality Control Board, dated July 2015. The Advisory provides technically defensible and consistent approaches for the collection and analysis of soil gas samples, and applies to both subsurface soil gas samples collected outside building perimeters and beneath sub-slab areas within buildings. The Advisory is not a regulation, instead it provides technical framework and reference for addressing soil gas sample collection and analysis.
Due to recent developments in the field of soil gas collection, the 2012 version of the Advisory was revised in July 2015. The July 2015 version also supersedes previous versions produced in 2003 and 1997. The 2015 Advisory provides detailed procedures to determine locations, spacing, and depths of soil gas probes, and procedures for the installation, design, and sampling of soil gas probes. Procedures for laboratory analysis of soil gas samples, including QA/QC protocols, are also included.
A typical soil gas probe design consists multiple probes installed in a single borehole. The probes are located at or near contaminant source areas, and on a predetermined grid pattern to delineate the lateral extent of VOC plumes. The borings are initially advanced with the use of a direct push or hollow stem auger drilling rig, or by hand augering methods. Soil gas probes, consisting of Nylaflow®, polyetheretherketone (PEEK), or Teflon® tubing, are then installed at multiple depths (such as 5, 10, and 15 feet below ground surface, or deeper depending on depth of the source area) to vertically delineate VOC plumes. A probe tip, usually 6 inches in length, is also attached to the end of each tubing. The annular space surrounding the probe tips are backfilled with a sand filter, followed by dry granular bentonite and hydrated bentonite or neat cement grout between probes. Soil gas samples are then typically collected and analyzed for VOCs with the use of a mobile laboratory, in accordance with EPA 8260B procedures.
Notable revisions to the 2015 Advisory (from the 2012 Advisory) include the following:
The updated Advisory can be found at:
For questions, please contact Steve Ridenour (email@example.com) or Mike Cassidy (firstname.lastname@example.org). We can also be reached by phone at 562-495-5777.