Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 7.9
ACTIVE SOIL VAPOR PROBE INSTALLATION

7.9 ACTIVE SOIL VAPOR PROBE INSTALLATION

Figure-7-14

Figure 7-14: Typical Temporary Soil Vapor Probe Typical temporary soil vapor probe, designed to be driven by a direct-push drill rig. The probe tip components (from left to right in the lower left photo) include a disposable drop-off point, probe tip (threaded to attach the tip to a steel drill rod), inert tubing connecting the probe tip to a sampling pump on the surface, and a steel drill rod. A fine mesh screen is located inside the probe tip.



Figure-7-15

Figure 7-15: Installing a Temporary Soil Vapor Probe Using a Direct-Push Drill Rig After the probe is driven to the desired sampling depth, the steel rod is retracted approximately 1 to 6 inches, allowing the drop-off point to remain at the bottom of the boring, and creating a cavity in the soil that provides access to the soil vapor at the desired depth for sampling. The photograph on the right shows a temporary soil vapor sampling probe in place.



Figure-7-16

Figure 7-16 Vapor Point Completions Left: Surface completion of flush-mounted well with valve installed. Right: Flush mounted nested well with Swagelok fittings; well in background being purged using an electric pump set at a draw rate of 200 ml/minute.



Figure-7-17

Figure 7-17 Typical Nested Permanent Soil Vapor Sampling Probes



Figure-7-18

Figure 7-18 Installation of a Permanent Soil Vapor ProbeUpper left photo: Hand-augering the borehole. Upper right photo: Preparation of soil vapor sampling point. Bottom left photo: Hydrating the bentonite seal. Tape used to measure depth of borehole, sand pack, and bentonite. Bottom right photo: Purging the completed vapor probe. Surface completion is a 9-inch length of 3-inch diameter Schedule 40 PVC pipe placed into upper 6 inches of the borehole around the probe tubing. A slip cap is placed over the PVC when not purging/sampling.



Figure-7-19

Figure 7-19 Schematic of Typical Sub-Slab Soil Vapor Sampling Probe (see also Figure 7-20 & 7-21).



Figure-7-20

Figure 7-20 Sub-Slab Soil Vapor Sampling Probes Upper left photo: Drilling hole with hand-held rotary hammer drill. Upper right photo: Temporary probe tip. Middle sequence: Inserting probe assembly into hole, pouring granular bentonite, hydrating bentonite seal. Bottom Left: Temporary probe completion. Bottom Middle and Right: Vapor probe with Swagelok termination fitted to hole (note larger diameter hole near surface), final completion with Swagelok fitting on tubing cemented in place (see Figure 7-17).



Figure-7-21

Figure 7-21 Sub-Slab Soil Vapor Sampling Probes Left: Example dual Swagelock setup for connection of vapor point to collection device (see Figure 7-17 and Figure 7-20) Right: Tubing from vapor point and collection device connected with a union joint.



Figure-7-22

Figure 7-22: Installation of a Vapor Pin™ with a silicon sleeve directly into slab for collection of subslab vapor samples (screw-on protective cap shown in photo to right). Sample collection tubing is connected directly to the top of the probe point (after rubber slip-on cap is removed) and the sample drawn through the base.

The purpose of soil vapor probes is to provide access to subsurface soil vapor so that an active sample can be collected. Soil vapor probes must be properly installed to collect representative soil vapor samples and to minimize the effects of changes in barometric pressure, temperature, or breakthrough of ambient air from the surface. Probes can be either temporary or permanent. The latter typically include a sandpack in the target depth interval of interest and a surface completion that includes a valve and/or a access port for periodic sampling.

7.9.1 Temporary Probes

Temporary probes typically consist of hollow steel rods driven into the subsurface using manual or direct push drilling methods. The temporary probes are driven to the bottom of the desired sampling interval using expendable or retrievable drive points. Then, the probe rods are withdrawn approximately 1 to 6 inches, leaving the expendable drive point in place and exposing the sampling interval. Narrow tubing with a threaded adaptor at the bottom end is inserted through the steel rods and threaded into the probe tip to form a gas tight seal. The use of tubing with a 1/4-inch to 3/8-inch outside diameter is most common (see Section 7.9.4). An example temporary probe sampling apparatus and typical installation are shown in Figure 7-14 and Figure 7-15.

Sample collection is performed through tubing that is run through the hollow drill rod and connected directly to the sampling probe tip. After collecting shallow samples using rods with retrievable tips, the rods can be advanced to collect deeper samples. The potential for cross contamination should be considered when using the same push rod for the collection of samples at multiple depths.

7.9.2 Permanent Probes

Permanent probes are constructed similar to groundwater monitoring wells installed using auger or direct push drilling techniques. However, permanent probes also can be installed manually within building interiors. Soils should be logged, field screened, and sampled for select contaminants during probe installation using auger or direct push drilling techniques. Documenting soil lithology can be important for development of conceptual site models, including an understanding subsurface vapor transport pathways and mechanisms, and for selecting vapor probe depths.

Permanent probes typically consist of small, inert tubing (e.g. 1/4-inch outside diameter; see Section 7.9.4) extending from the subsurface sampling interval to the ground surface and sealed in place with bentonite to prevent vertical air migration during sample collection. The subsurface end of the tubing is connected to a stainless steel screen or porous stone (airstone) probe tip to prevent particulates from entering the sample probe. Note that polyethylene probe tips are not recommended as VOCs might adsorb to the filter material. The probe tip is typically set halfway between the top and bottom of the sampling interval within a sand pack. Permanent screen implants are typically six inches in length. Placement of a few inches of sand below and above the implant is generally recommended for a total sample-interval sand pack length of approximately one-foot, although deviations can be considered with justification. A sampling interval of greater than one-foot increases the uncertainty in interpretation of measurements since the concentration is averaged over a larger vertical interval ((API 2005). This is especially important for subslab soil vapor samples, where the average concentration of VOCs within one-foot of the slab around preferential pathways into the building should be targeted (see Section 7.6.2.2). Figures 7-16 and Figure 7-17 present several examples of flush-mounted soil vapor points and a schematic diagram of vapor probe point designs.

Approximately 1 foot of dry granular bentonite should be placed on top of the sand pack to prevent infiltration of hydrated bentonite into the sand pack. The borehole is typically sealed to the surface, or to the bottom of the next highest sampling interval, with hydrated bentonite. When installing permanent probes at several depths in the same borehole, the deepest sample interval is always installed first. Figure 7-18 depicts the installation of a permanent vapor probe using a hand auger. Permanent probes should be finished to preclude infiltration of water or the exchange of ambient air in the sample tubing. Surface completions of permanent probes typically include a fitting that allows for soil vapor sample collection and a gas tight valve at the surface when the probe is not in use. Flush mounting or above ground vaults for surface completions are site specific and should be evaluated accordingly. Permanent probes should be purged of three system volumes immediately following installation (see Section 7.10.3) and allowed to equilibrate prior to sampling (see Section 7.10.2).

7.9.3 Additional Recommendations for Subslab Probes

Subslab probes are installed similarly to permanent probes. The probe consists of 1/4- to 3/8-inch (outside diameter) inert tubing (see Section 7.8.2) with a stainless steel or porous stone probe tip or “implant.” Probe implants should be placed within the first 6 to 12 inches of soil (see Section 7.6.2.2). If the probe is to be left in place, the surface termination should be a stainless steel or brass Swagelok compression fitting with a threaded plug to seal the probe. For temporary installations, the probe can be completed with 6 to 12 inches of tubing above the surface with a 2-way valve to seal it. Deeper vapor points can also be set beneath buildings slabs to investigate source-area vapor concentrations (see Section 7.6.2.3). Reviewing as-built plans and screening proposed vapor points using GPR or similar methods to check for rebar and other potential obstacles to drilling is recommended.

As discussed in Section 7.6.2.2, a targeted placement of subslab probes should include: 1) locations of known or suspected, localized subslab sources of contamination; 2) in the absence of the former, the center of the building slab, where concentrations of VOCs from deeper are anticipated to be the highest; 3) between the center of the building and outer, adjacent sources contamination; and 4) in the vicinity of cracks and gaps in the building slab where vapor intrusion is considered to be most likely (see also USEPA 2012d, CalEPA 2011). Examples of the latter include areas where utilities penetrate the building slab, or areas where cracks in the floor could serve as preferential vapor pathways.

Traditional subslab probes are installed by drilling a hole of appropriate diameter through the slab at the targeted location and installing a sample collection point directly into the underlying fill material. Using a rotary hammer drill, a 1¼-inch diameter hole is drilled approximately 1½ inches into the slab to make room for the Swagelok fitting. A 3/4-inch diameter hole is then drilled through the remaining slab thickness and 6 to 12 inches into the underlying sub-slab base material (typically engineered fill).

The inside of the hole should be cleaned out and wiped with a damp towel to remove the drilling dust and ensure an airtight seal. The probe assembly is then inserted into the hole so that the probe tip is just below the slab. The tubing should be cut to the appropriate length so that probe tip is just below the slab and the Swagelok termination is slightly recessed or flush with the slab surface. Clean sand is then poured into the hole until the probe tip is covered to form a filter pack. Granular bentonite is poured to the top of the 3/4-inch hole and hydrated. Care should be taken not to allow water to leak into the filter pack sand.

The Swagelok fitting is then sealed in place with a small amount of quick-drying cement (see Figure 7-19). Either plastic or stainless steel ferrules can be used for Swagelok fittings (plastic shown in photo; steel ferrules include an additional ring washer). To avoid cementing the probe closed, the cement should be poured no higher than flush with the top of the compression fitting. Cement should not be allowed to flow around the threaded plug. Figure 7-20 & Figure 7-21 depict completions for subslab vapor points.

Alternative approaches that can reduce the time, effort and cost of collecting subslab soil vapor samples are being developed. One example includes the “Vapor PinTM,” which is installed directly into the floor slab and does not require the installation of a separate, gas permeable probe tip and tubing into the underlying fill material (Figure 7-22; flush-mount shown). A core is removed from the slab in a similar manner as described above. The side of the boring should be brushed to remove loose material prior to installing a pin in order to obtain a strong seal. A shop vac or similar method should not be used to clean the hole due to the potential to disturb subsurface conditions. If done, then a minimum of 24 hours is recommended to reestablish equilibrium conditions.

The hollow, brass or stainless steel pin is then hammered into the boring (see Figure 7-22). A silicone sleeve around the pin provides a seal against the sides of the hole to prevent leakage of ambient air, eliminating the need for grout. The sampling train tubing is connected to the top of the pin and the sample is drawn directly through the base of the pin. Guidance for installation of the pins and leak tests should be followed if used at sites in Hawai’i (Cox-Colvin 2013, b). Similar devices are likely to be developed in the future and can be proposed for use on a site-specific basis. These types of pins have a good record for installation in concrete slabs but difficulty in obtaining an adequate seal has been reported for asphalt. If a slab crumbles during drilling, then silicon putty or similar, non-volatile material may be useful to help seal the annular space around a point.

A small amount of water can be added to holes drilled in slabs (“wet drilling”) if high levels of methane or other potentially explosive gases could be present beneath a slab or other capped area. This can help prevent sparks when the drill bit breaks through the bottom plane of the slab. If used, then an equilibrium time of at least two hours following installation of the vapor point (including vapor pins) is recommended.

7.9.4 Soil Vapor Probe Tubing

Inert, rigid-walled tubing, such as Teflon, nylon (e.g. Nylaflow), or stainless steel should be used as the primary tubing for soil vapor sampling probes (Ouellette 2004, SDC 2011, USEPA 2009). Tests using these materials show minimal (<10%) loss of VOCs during sample collection. Tubing within an outside diameter (OD) of 1/4-inch and 3/16-inch (0.1875) inside diameter (ID) and/or 3/8-inch OD and 1/4-inch (0.25) ID is most commonly used (see Figures 7-17 and 7-18).

Polyethylene and other flexible tubing (e.g., Tygon) as well as and copper tubing adsorb VOCs and should be avoided. Losses up to 80% of VOCS due to absorption has been documented with some types of flexible tubing. A few inches of flexible tubing that can be pinched closed may be needed in addition to the rigid-wall tubing (see Figure 7-15), however, or to connect rigid-wall tubing to sampling equipment (e.g., to syringes; see Figure 7-18). The use of larger diameter tubing to connect smaller rigid tubing introduces a potential for leakage during sampling (see Section 7.10.5),so swageloks should be used instead for connections where possible (e.g., see Figure 7-19). Sampling trains should be tested using a shut-in test prior to collection of samples regardless of the types of connections used.

Storage and handling of tubing is critical. Tubing should be stored in a sealed container to prevent contamination from ambient air or other sources. Avoid leaving tubing near open sources of vapors (e.g., fuel cans, cleaners, etc.) or near auto exhaust..

7.9.5 Soil Vapor Probe Abandonment

When soil vapor probes are no longer needed they should be properly abandoned. Abandonment procedures for temporary probes are the same as for any direct-push borehole (e.g., backfill with hydrated bentonite).

Well vaults should be over-drilled and removed. Several inches of the bentonite seal should be removed and the probe tubing cut as far down as practical. A thin layer (~one-inch) of bentonite should be placed back over the tubing and hydrated to seal it. The well-vault holes should then be filled with concrete and finished flush with the surrounding surface. The concrete used should be of suitable grade for the location. A layer of several inches of sand between the bentonite and the concrete will help prevent heaving of the concrete plug due to wetting/drying cycles of the bentonite. Probes installed in unpaved areas should be abandoned similarly except that clean surrounding soils should be used to fill the holes to grade.

Sub-slab probes should be drilled out, the inside of the hole cleaned with a damp cloth, and the hole filled flush with the slab with lime-based cement, or an epoxy cement or putty otherwise formulated for concrete repair. It is important to carefully clean the hole prior to pouring the cement to ensure a good seal so that the former probe hole does not become a conduit for VOCs to enter the building.