SOIL-GAS SAMPLING (ACTIVE)
Soil-gas sampling measures the gas contained in the interstitial spaces of the soil, as opposed to directly sampling the soil matrix, above the water table.
Active soil-gas collection methods (as opposed to passive soil-gas sampling) involve ÒpullingÓ a vapor sample from a temporary or permanent probe inserted in the soil into a collection or analytical device. Samples are then transported to a laboratory, or in some cases they are analyzed on site.
Exterior soil-gas sampling is a screening tool used to rapidly and cost-effectively identify and delineate certain volatile contaminants in the subsurface. It is often used to ascertain the source, extent, and movement of pollutants, but it is not a substitute for groundwater sampling.
Sub-slab soil-gas sampling, where samples are taken though holes in buildingsÕ concrete slabs, is used to determine the potential for vapor intrusion. In some cases sub-slab samples are taken to determine whether to test indoor air, while in others it is conducted concurrently with indoor-air sampling to evaluate whether the subsurface is the source of indoor contaminants.
There are several types of soil-gas sampling instruments, including:
á Air-tight syringe. This is used to pull vapors from the soil matrix. The syringe is used to withdraw a soil-gas sample from a probe and inject it directly into an analytical instrument for on-site analysis. Some regulatory agencies require that samples collected by this method be analyzed within a few hours.
á Tedlar¨ bag. This requires a pump to pull the vapors from the soil. Regulatory agencies may require that samples collected by this method be analyzed within 28 to 48 hours. Bagged samples can also be drawn into sorbent tubes, which in turn undergo laboratory analysis.
á Glass bulb (or tube). This has openings at each end, with one end having a valve where samples can be withdrawn with a syringe. The vapor sample is collected by connecting one end of the bulb to the probe and the other to a pump. The advantage of glass bulbs is that the glass is inert; also they are easy to use. The limitations of the glass bulbs are that they break easily and can leak contaminants through the valves. Sample holding times for glass bulbs are usually no more than 24 hours.
á SUMMA¨ canister. Under a vacuum, the canister pulls vapors out of the soil when the valve is opened. Each canister has a flow regulator and vacuum gauge between the vapor well and the canister. Regulatory agencies may require that samples collected by this method samples be analyzed within 30 days. The Summa canisters used for soil-gas sampling have a one to six liter sample capacity. (It is suggested that a one-liter canister be used if the sample is less than 5 feet below ground surface.)
á Hand-held direct measurement. This includes the Photo Ionization Detector (PID). PIDs provide immediate results, but they do not differentiate among contaminants or detect low levels of contaminants. More precise hand-held devices are under development.
á Flux chamber. This consists of an enclosed chamber that is placed on the surface for a specific period of time. This method yields both concentration data in the chamber and flux data (mass/area-time). Flux chambers are the least common soil-vapor survey method, and they are typically used to measure direct vapor migration from the subsurface to the surface.
Tedlar bags and Summa canisters need to be attached to a soil-gas wells or port. To install a soil-gas port, a hole is driven into the ground to a depth of four to five feet and a stainless steel or other non-reactive steel probe is inserted into the hole. The hole is then sealed around the top of the probe using modeling clay. (Some people recommend sealing the probe above the sampling zone with a bentonite slurry for a minimum distance of 3 feet.) Soil-gas wells may be installed using a variety of drilling methods, such as direct push methods or augers.
Soil-gas samples should be sufficiently deep to minimize the effects of changes in barometric pressure (either ambient or within an overlying building due to the operation of heating, ventilation, and air conditioning systems), temperature, or breakthrough of ambient air from the surface.
The procedures for collecting sub-slab soil-gas samples are the same as for collecting sub-surface soil-gas samples, except that a slower flow rate and lower vacuum should be utilized to prevent ambient air from being drawn into the samples. Soil-gas sample collection techniques for vapor intrusion applications require much greater care than techniques historically used for typical site assessment applications because risk-based concentration levels for vapor intrusion scenarios are very low.
For a typical single family residential dwelling (approximately 1500 ft2), one vapor probe installed near the center of the slab is typically used to document the chemical composition of the sub-slab soil gas. Significantly larger dwellings (or other unique conditions in the subfloor or construction of the foundation) may require additional vapor probes.
Limitations and Concerns
Prior to selecting sample locations, locations of underground utility corridors should be well understood.
Heterogeneous soil conditions across a site under investigation can lead to poor delineation and misinterpretation of site contaminants. Data from areas of horizontal low permeability zones within the vadose zone may be misinterpreted as being an area of low contamination, and data from an area of high permeability in an otherwise low permeability area may be misinterpreted as an area of high contamination. High porosity areas such as sewer and utility trenches can serve as conduits for rapid vapor or gas migration, elevating gas readings some distance from the contaminated groundwater.
Recent research suggests that there may be substantial variation in soil gas concentrations beneath the slab. In some cases, where pressure differentials beneath the subsurface and indoors fluctuate between positive and negative, some soil gas contamination under the slab may be attributable to indoor contamination.
In vapor intrusion investigations, sub-slab soil-gas sampling is generally preferred over near-slab soil-gas sampling (within 10 feet horizontally of a building's foundation), and in general exterior soil-gas sampling (beyond 10 feet from the building footprint) is not acceptable as the primary line of evidence in the assessment of the vapor intrusion pathway.
Soil vapor samples collected under high vacuum conditions or under a continuous vacuum may contain contaminants that are "desorbed" and removed from the sorbed soil matrix and pulled from the dissolved phase into soil gas, rather than contaminants present in the undisturbed soil vapor. For collection systems employing vacuum pumps, the vacuum applied to the probe should be kept to a minimum necessary to collect the sample. To minimize the potential desorption of contaminants from the soil, Summa canisters should be filled at a rate less than half a liter per minute.
Also, because Summa canisters generally are under high vacuum, extra care should be exercised during sample collection to ensure that air from the surface is not being inadvertently sampled. The possibility of breakthrough from the surface increases as samples are collected closer to the surface (less than 5 feet below grade). To minimize the potential of surface breakthrough, the probe rod should be sealed at the surface.
Samples should not be exposed to light or extreme temperatures. Syringes and glass bulbs should be kept in cool dark locations. Samples should be covered or wrapped with foil and placed into an insulated container (cool but without ice).
The use of gas-tight glass syringes with Teflon¨ seals is preferred. The use of plastic syringes is discouraged because of the interaction between the plastic (or rubber) of syringes and some target analytes.
Soil-gas samples in Tedlar bags should not be subjected to changes in ambient pressure, because that could adversely affect the integrity of the bags. Increases in pressure may collapse the bag and decreases in pressure may expand the bag.
In general, soil-gas sampling events should be avoided after sizeable rainfall or even irrigation, because sampling in moist soil is unreliable. If no-flow or low-flow conditions are caused by wet soils or if water is drawn into a probe, sampling should be halted.
Other weather conditions may also hamper soil-vapor sampling. For example, condensation in the sample tubing may be encountered during winter sampling due to low outdoor air temperatures. Devices such as tube warmers may be used to address these conditions.
Using sub-slab soil gas sampling is questionable where the water table is high—that is, near (less than 2 feet below) the base of the sub-floor. Typically, vapors migrate through the most coarse and/or driest material. Depending on the analytical method, high moisture content in the soil-gas sample can "mask" results. Additionally, reduced permeability of the soil in the capillary fringe area may limit the movement of soil gas.
A common problem with this sampling method is soil probe clogging. A clogged probe can be identified by using an in-line vacuum gauge or by listening for the sound of the pump laboring.
In vapor intrusion evaluations, soil-gas sampling depth should be dependent on the depth of the contaminants as well as the vertical profile of the buildings, including basements and elevator shafts. Soil-gas samples should be obtained at appropriate depths so that the risk of human exposure can be adequately estimated. Some studies suggest that soil-gas samples collected at depths of 10 to 15 feet are a better indicator of vapor intrusion potential than samples collected at 5 feet where the source depth is greater than 15 feet beneath ground surface.
Exterior and near slab soil-gas samples should be collected at a minimum depth of 5 feet below the ground surface. In situations where the ground water table is less than 5 feet, alternative sampling protocols may have to be employed.
The results of the soil gas samples should not be averaged.
Residents are often reluctant to allow someone to drill a hole in their slab, especially if it's covered by a finished floor. To avoid less accurate near-slab sampling, investigators often seek to drill holes in closets or under carpets.
In buildings with earthen basement floors (instead of concrete), soil-gas sampling may not be appropriate.
Soil-gas sampling methods are used to screen of soil gas for Volatile Organic Compounds (VOCs) and Semi-Volatile Organic Compounds (SVOCs), as well as for gaseous mercury (metals) and radon (radionuclides). Data from exterior soil-gas surveys can be used to establish the extent of contamination at a site and to guide well placement and soil-boring programs. Soil-gas sampling is a commonly used to identify and evaluate contaminant movement and species. Sub-slab soil-gas sampling is a standard technique used to investigate potential vapor intrusion.
Technology Development Status
These techniques are well developed and commercially available.
Other Resources and Demonstrations
See http://serdp-estcp.org/Program-Areas/Envapor intrusionronmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-200423/ER-200423 for the detailed investigation of Vapor intrusion.
For articles on soil gas collection procedures, see:
Also see http://www.nj.gov/dep/srp/guidance/vaporintrusion/rutgers2005/soil_gas_investig.pdf for a related presentation.
See http://www.dnr.mo.gov/env/hwp/tanks/docs/soil-gas-protocol-2005-04-21.pdf for Missouri soil sampling protocols around petroleum tanks.
See http://www.dtsc.ca.gov/lawsregspolicies/policies/sitecleanup/upload/SMBR_adv_activesoilgasinvst.pdf for CaliforniaÕs advisory on active soil gas investigations. See http://www.dtsc.ca.gov/AssessingRisk/upload/Ettinger.pdf for comments on the advisory.
See http://www.airtoxics.com/literature/AirToxicsLtdSamplingGuide.pdf for a comparison between Tedlar Bags and metal canisters.
See the following for the Vapor Pin http://www.youtube.com/watch?v=PvF2m-4zVg4&feature=youtu.be