{"id":9574,"date":"2018-07-18T10:15:50","date_gmt":"2018-07-18T02:15:50","guid":{"rendered":"https:\/\/www.envguide.com\/active-soil-gas-sampling\/"},"modified":"2018-07-24T01:51:44","modified_gmt":"2018-07-23T17:51:44","slug":"active-soil-gas-sampling","status":"publish","type":"post","link":"https:\/\/us.envguide.com\/active-soil-gas-sampling\/","title":{"rendered":"Active Soil Gas Sampling"},"content":{"rendered":"

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\nAbs: Active soil gas methods consist of the withdrawal and analysis of the soil gas from the subsurface. These methods give concentration data (for example, \u03bcg\/m3) for COCs, which can be directly compared to risk-based screening levels or used in predictive models.<\/em><\/p>\n

Active soil gas methods consist of the withdrawal and analysis of the soil gas from the subsurface. These methods give concentration data (for example, \u03bcg\/m3) for COCs, which can be directly compared to risk-based screening levels or used in predictive models. Two techniques are most commonly used to install soil gas probes to collect external active soil gas samples \u2013 driven probe rod and burial of soil gas sampling tubes.<\/p>\n

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Driven Probe Rod<\/p>\n\t\t\t\t\t<\/div>\n

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Figure 1: driven probe rod<\/p><\/div>\n

This method consists of the insertion of a hard rod (probe) driven to a target depth, collection of soil gas through the rod while it is in the ground, and subsequent removal of the rod. Typically, probes are constructed of hollow steel rods with an external diameter ranging between 12.5 mm and 50 mm (0.5 inches and 2 inches). Small diameter inert, replaceable tubing runs down the center of the drive rod to eliminate potential contamination from the inside of the rods. The probes can be driven by hand methods, direct-push systems, or with larger drill rigs using a wire-line hammer. Probe installation can be difficult in over-consolidated or coarse-grained soils. A surface seal is usually used, but this seal does not prevent cross-flow at greater depths, so driven probes are most applicable in relatively uniform moderate to high permeability materials (generally not in low permeability soils).<\/p>\n

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Burial of Soil Gas Sampling Tubes<\/p>\n\t\t\t\t\t<\/div>\n

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Figure 2: typical configuration of nested well<\/p><\/div>\n

This method consists of the burial of a small diameter (typically \u215b inch to 1 inch outer diameter) inert tube or pipe (stainless steel, Teflon, polyvinyl chloride, high density polyethylene, polyether ether ketone, Nylaflow, or similar) to a target depth with subsequent sampling of the soil gas after a period of time. Tubing may be buried in holes created with hand driven rods, direct-push systems, hand-augers, drills (for sub-foundation samples) or drill rigs for deeper samples. Clean sand is used as backfill around the tip, and the remainder of the borehole annulus is sealed, usually with a bentonite and water slurry. This method offers significant advantages when repeated sampling events are needed, or where the geology is not conducive to driven probes. Multiple tubes can be \u201cnested\u201d in the same borehole, if the seals between intervals are tight, and are often referred to as multilevel soil gas wells or nested probes.<\/p>\n

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Soil Vapor Probe Materials\/Construction<\/p>\n\t\t\t\t\t<\/div>\n

It is important that the correct soil vapor probe materials are used and the probes constructed properly. Following are recommended materials and construction issues for soil vapor probes.<\/p>\n

Use tubing material that does not adsorb or off-gas volatile hydrocarbons. nylon, Teflon, and stainless steel all give comparable results for typical PHCs. For heavier molecular weight compounds, stainless steel shows the least adsorption, but may be impractical to use. Nylon is recommended over Teflon tubing, because nylon tubing is less expensive and the compression fittings are easier to seal. Polyethylene tubing, commonly used by direct-push firms for groundwater sampling, should not be used for soil vapor samples because the polyethylene tubing has been shown to adsorb hydrocarbons.<\/p>\n

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Figure 3: Vapor sample materials, including \u215b-inch outer diameter tubing, three types of tips (ceramic, aluminum, and braided steel) and two types of surface terminations (stopcock and Swagelok fitting).<\/p><\/div>\n

Nominally \u215b-inch or \u00bc-inch outer diameter tubing is recommended. Stainless steel, aluminum, ceramic, or plastic (choice depends upon project specifications) are used for probe tip material. Swagelok fittings or plastic valves (two-way plastic valves or stop cocks) are best for sealing tubing that will remain in the ground for an extended time. Options for surface termination include flush mounts on the floor\/surface, belowground termination (with or without a locking cover), and various aboveground completions that are commercially available.<\/p>\n

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Equipment Blanks<\/p>\n\t\t\t\t\t<\/div>\n

Collection of an equipment blank is recommended for all VI investigations, especially if metal probe tips are used. Zero-grade air or nitrogen should be drawn through the probe tubing, probe tip, and the sampling train at the start of the field program. The collected sample should be analyzed for the same compounds as the soil vapor samples. This practice confirms that the metal probe tips and other probe parts are clean before putting them in the ground.<\/p>\n

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Equilibration Time<\/p>\n\t\t\t\t\t<\/div>\n

When probes are installed, the in situ soil vapor can be displaced and a period of time is required for the soil vapor to re-equilibrate to their pre-disturbed values. The following equilibration times were required to reach 80% of the final value, assumed to represent the pre-disturbed value in fine-grained soil:<\/p>\n\t \n\t\t

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