Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 7.10
ACTIVE SOIL VAPOR SAMPLING PROCEDURES

7.10 ACTIVE SOIL VAPOR SAMPLING PROCEDURES

Table 7-6 Sand Pack Porosity Volume (ml)

Filter Pack Length (inches)

Borehole Diameter (inches)

1.25

1.5

2.0

2.5

3.0

3.5

6

36

52

93

145

208

284

12

72

104

185

290

417

568

18

109

156

278

434

625

851

24

145

208

371

579

834

1,135

Note: Assumes sand pack porosity of 30%.


Table 7-7 Tubing Volume (ml)

Tubing Length
(feet)

Tubing Inner Diameter (inches)

1/16
(0.085)

1/8
(0.125)

3/16
(0.1875)

1/4
(0.25)

1

1

2

5

10

2

2

5

11

19

3

3

7

16

29

4

4

10

22

39

5

6

12

27

48

10

11

24

54

97

15

17

36

81

145

20

22

48

109

193

25

28

60

136

241


Figure-7-23a
Figure-7-23b
Figure-7-23c

Figure 7-23: Soil Vapor Probe Purging Devices Upper photo: Disposable syringe with 3-way Luer valve (see also Figure 7-18). Middle photo: Pump with flow meter. Lower photo: Large Summa used for purging. Smaller Summa to left used to collect sample after purging; purge Summa closed during sample collection


Figure-7-24

Figure 7-24: Example Vacuum Gauges for Purging and Sample Collection using a Summa Canister Sampling Train (see also Figure 7-26 and Figure 7-27).


Figure-7-25a

Figure-7-25b

Figure 7-25: Lung Boxes with Tedlar bag. Vacuum is drawn on sealed lung box, causing the Tedlar bag to pull vapor from the collection point and fill.


Figure-7-26a
Figure-7-26b

Figure 7-26: Summa canister sampling trains. Upper schematic: Diagram of Summa canister soil vapor sampling apparatus. Lower photo: Teflon tubing connected to flow controller with swage lock and to well point with short length of flexible tubing (allowed well point to be closed for followup sorbent tube sample). Note Tupperware shroud used for leak test. .


Figure-7-27a
Figure-7-27b
Figure-7-27c
Figure-7-27d

Figure 7-27: Example Soil Vapor Sample Collection Setups Upper photo: Battery powered sampling pump draws soil vapor through the Teflon tubing from the temporary soil vapor probe tip to the sample sorbent tube. A restrictor device reduces the pump flow to the required rate (usually set at laboratory). Note the black and silver air pump calibrator near the restrictor; before sampling, the flow rate is calibrated and recorded for each type of sorbent tube media. Middle schematic and photo: TO-17 sorbent tube soil vapor sampling apparatus. Vapor point tubing connected to sorbent tube inlet with a union and swage lock; pump or syringe connected to outlet of sorbent tube and used to draw sample (latter shown in photo). Lower left photo: Summa canister used to collect soil vapor from a permanent soil vapor point. The blue-bodied flow controller is laboratory-calibrated to restrict sample inflow to a predetermined flow rate. A gauge on the controller’s side indicates vacuum remaining in the canister. Lower right photo: Manifold setup to allow collection of duplicate samples (see also Figure 7-28). Small Summas used to collect samples; large Summa used to purge vapor point. Dual vacuum gauges in each setup used to monitor Summa vacuum and vacuum at well point (flow regulator installed between gauges). Shut-in leak test critical to ensure that sampling train is tight.


Table 7-8 Comparison of Tracer Leak Check Methods

Method

Advantages

Disadvantages

Tracer
Method 1

  • Smaller shroud requires less tracer gas;
  • Easier and faster to collect samples.
  • Integrity of sampling train not confirmed by laboratory analysis.

Tracer
Method 2

  • Single process to check entire apparatus;
  • Integrity of sampling train confirmed by laboratory analysis for tracer gas.
  • Unnecessarily replicates shut-in test;
  • Larger shroud requires more tracer gas;
  • Costlier and more time consuming to implement in the field;
  • Source of leak at sampling train vs vapor point or annular seal cannot be determined (although successful shut-in test would imply the latter).

Figure-7-28a
Figure-7-28b

Figure 7-28: Soil Vapor Sampling Trains Arranged for Shut-in Test (see also Figure 7-27)


Figure-7-29

Figure 7-29: Example PVC Coupling "Water Dam" Sealed to Floor with Inert Putty for Leak Testing Slab-mounted Vapor Point. After Cox-Colvin 2013, b. The water level is filled to a level above the tubing connection to the vapor point and monitored during a vacuum test prior to and during sample collection.


Figure-7-30a
Figure-7-30b
Figure-7-30c

Figure 7-30: Shroud Over Vapor Probe Surface Completion. Upper Photo: System consists of shroud (blue bucket), industrial-grade helium cylinder, field helium detector to measure helium within shroud, and syringe with vacuum gauge for purging. Purged sample tested in field for helium. Sampling train not shown in example. Middle Photo: System consists of Summa and flow controller sampling train, Tupperware shroud (bottom lined with foam door seal), non industrial-grade helium cylinder. Option for use of field helium detector with shroud and field testing of purged sample for helium (not shown in example). Bottom System: Similar to above but Tupperware shroud set into a ring of Play-Doh on concrete base to provide a tighter seal around vapor point. Large Summa used to purge well point; smaller Summa used to collect sample (see also Figure 7-23).


Figure-7-31a
Figure-7-31b

Figure 7-31: Method 2 Helium Shroud Leak Testing Systems Helium released into shroud and used as tracer to identify leaks in both sampling train and vapor probe annular seal. Upper left photo: Five-gallon bucket helium shroud placed over well point and Summa canister-flow controller sampling train (note two ports for injection and monitoring of helium inside shroud). Upper right photo: Use of garbage bag as shroud (helium injected under shroud to fill bag). Lower photos: Large Tupperware container converted to shroud, with fill ports for helium injection and monitoring, plus glove ports to open and close Summa canister for sample collection.


Table 7-9 Comparison of Leak Check Tracers

Compound

Advantages

Disadvantages

Isopropanol

  • Inexpensive and readily available
  • Detected using method TO-14/15 and SW8260
  • Can be used without a shroud
  • Denser than air
  • Cannot be selectively measured in the field
  • High concentrations can interfere with laboratory analysis
  • Potential use in gasoline

Helium

  • Can be selectively measured in the field
  • Will not interfere with TO-14/15 and SW8260 analysis
  • More expensive
  • Requires valves and fittings for cylinder
  • Sample must be analyzed using a separate method
  • Lighter than air

Difluoroethane

  • Inexpensive and readily available
  • Detected using method TO-14/15 and SW8260
  • Cannot be selectively measured in the field

The descriptions of active soil vapor sampling procedures in the following sections are general in nature and reflect commonly accepted designs and methods recommended by the USEPA (USEPA 1996c), industry standards (ASTM 2006f) and various other entities (e.g., MDNR 2005, SDC 20111, CalEPA 2012). Alternative designs may be more appropriate depending on sampling objectives

7.10.1 Soil Vapor Sample Timing and Frequency

Like sample location and depth, the timing and frequency of sample collection will necessarily be a site-specific decision and should be discussed with the overseeing HDOH project manager. General guidance is, however, presented below.

As discussed in Sections 7.2 and 7.5, the objective of the investigation is to develop a CSM that reflects the representative, average subsurface vapor concentrations and vapor intrusion conditions over time and during normal building operation over a period of many years (e.g., assumed exposure durations). Samples collected as part of an investigation should likewise be representative of assumed, long-term, average site conditions.

The collection of soil vapor samples from both the fill material immediately under the building slab and the suspected or known source area is recommended at sites where the distance to the source area is greater than 5 feet (but no closer than 2-3 ft to the water table) and a potentially significant source is present (see Section 7.6.2.3). This will help assess the need to seal cracks and utility gaps in the building floor as an added measure of precaution, in the event that nearby portions of the vapor plume exceed subslab, soil gas action levels even though VOCs meet action levels in subslab samples (see Section 7.14.1).

As discussed in Section 13.2 of the EHE overview, site-specific considerations regarding the timing and frequency of soil vapor sample collection include building HVAC system operation, seasonal weather variations and associated water table fluctuations and tidal effects on groundwater elevation. Multiple sampling events are recommended if the conceptual model identifies the VOC concentrations in source area vapors well above shallow soil gas action levels and the potential for significant, temporal fluctuations in soil vapor concentrations and or the potential for advective flow of vapors into overlying buildings. Recommendations include:

  • Collection and comparison of subslab soil vapor samples (or crawl space samples) during periods of the year when air conditioning is and is not routinely used (e.g., summer versus winter months);
  • Collection of deep and subslab soil vapor samples (or crawl space samples) during both the wet and dry season for sites where a significant smear zone is known or suspected to be present at the water table and exposure of contaminated soil in the smear zone could vary dramatically over the year.

The collection of seasonal soil vapor samples should be considered at sites where a substantial smear zone that could be exposed during falling water tables is known or suspected to be present. The collection of subslab soil vapor samples during periods of both falling and rising water table may be necessary on a site-specific basis to evaluate the effects of tidal pumping on subslab soil vapor concentrations at high-risk, coastal sites with significant free product on shallow groundwater.

At sites near the coast, the tides can affect groundwater levels, soil vapor samples should be collected at the same point in the tidal cycle in order to obtain data that are comparable from point-to-point and from sampling event to sampling event. The collection of subslab samples during both rising and falling tides (or more specifically water tables) may also be necessarily, especially if significant concentrations of vapor-phase contaminants have been identified in deeper, soil vapor samples.

A single round of soil vapor sample collection will generally be acceptable for sites that meet the following conditions (see HDOH 2007c): 1) Minimal volume of contaminated, vadose-zone soil is suspected to be present within 30 feet of the building slab (e.g., 10m3, not including capillary fringe zone soils), 2) Less than 30m2 area of floating product on water table present, 3) Larger area of floating product present but greater than 30-foot vertical separation distance, and/or 4) Water table fluctuations unlikely to expose a smear zone greater than three feet thick within 30 feet of the building slab. A minimum of one round of samples should be collected at chlorinated solvent sites where groundwater action levels are approached or exceeded or a significant source is present in the vadose zone.

At least two rounds of soil vapor sampling, one during the “dry” season and one during the “wet” season, are recommended prior to negation of potential vapor intrusion hazards for sites that meet the following conditions: 1) Sites with widespread, heavy contamination in vadose-zone soil and/or floating on groundwater within 30 vertical feet or 100 lateral feet of a building slab that do not meet the above-noted conditions or 2) Confirmation of remedial actions at sites where potential vapor intrusion hazards have been documented in the past (e.g., concentrations of VOCs in subslab soil vapor greater than action levels and/or impacts to indoor air above action levels or expected background identified and tied to vapor intrusion).

If more than one round of soil vapor samples are collected, the field procedures (e.g., purge volume), sample containers, and analytical methods should be consistent from one sampling event to the next to allow comparison of the site data over time. The CSM model should be refined to reflect the data collected over multiple sampling events and used to determine the need for additional actions.

Short-term (minutes or days), temporal variation of concentrations in soil vapors due to changes in temperature, barometric pressure, and wind speed due to passing storms are likely to be nominal for uncovered areas at depths of 2 feet bgs or more (USEPA 2007e). Infiltration from rainfall can potentially impact soil vapor concentrations by displacing soil vapor, dissolving volatile organic compounds, and by creating a “cap” above the soil vapor. In practice, infiltration from brief, large storms only penetrates into the soil on the order of inches. Soil vapor samples collected at depths greater than 3 feet bgs are therefore unlikely to be significantly affected. Soil vapor samples collected closer to the surface (less than 3 feet) without surface cover may be affected.

If the wetting front has penetrated to the sampling zone, it typically can be recognized by high vacuums during purging. If high vacuums are encountered when collecting a sample or drops of moisture are evident in the sampling train or sample, a soil vapor sample should not be collected (e.g., vacuum greater than seven inches Hg or 100 inches of water; see Section 7.10.3.2). In addition to potential short circuiting to the surface, imposition of a high vacuum on the soil could cause non vapor-phase VOCs to be stripped in free product, sorbed to soil or dissolved in soil moisture and bias the resulting vapor sample. Measurement of soil moisture can also be useful if shallow sampling is performed during or shortly after significant rainfall (e.g., greater than 1 inch; SDC 2011

7.10.2 Soil Vapor Probe Equilibration

Subsurface soil vapor conditions are disturbed during installation of soil vapor sampling probes. In general, temporary probes advanced with manual or direct push methods result in the least disturbance to soil vapor conditions and can be purged and sampled relatively soon after installation. Permanent probes result in greater subsurface disturbance and require a longer equilibration time.

Recent studies conducted by the USEPA have evaluated equilibration times for a variety of probe types (USEPA 2010c, USEPA 2010d). Data from these studies indicate that temporary probes (see Section 7.9.1) achieve approximately 80% equilibration within two hours of installation, while permanent probes installed in direct-push boreholes typically require eight to twenty-four hours to fully equilibrate. Probes installed in boreholes advanced with auger methods are expected to require up to 48 hours to equilibrate.

The time between probe installation and sampling will depend on the investigation objectives and the data quality requirements. For example, if a soil vapor survey is conducted using temporary points to map the extent of a vapor plume, and the sample data are not intended for use in risk assessment or site closure decisions, then sampling 30 minutes after installation would be acceptable. To obtain quality data for decision making, permanent soil vapor probes should be allowed to equilibrate for at least 24 (direct push) to 48 (augers) hours before sampling

As a default, the following equilibration times are recommended before proceeding with soil gas sampling (refer also to CalEPA 2012):

  1. For soil gas wells installed with the direct push method, do not conduct the purge volume test, leak test and soil gas sampling for at least two hours following completion of vapor probe installation;
  2. For soil gas wells installed with hollow stem or hand auger drilling methods, do not conduct the purge volume test, leak test and soil gas sampling for at least 48 hours following completion of vapor probe installation.
  3. For subslab soil gas probes installed in soil beneath the slab, do not conduct the purge volume test, leak test and soil gas sampling for at least two hours following completion of vapor probe installation
  4. For vapor collection pins installed directly into the slab, do not conduct the purge volume test, leak test and soil gas sampling for at least twenty minutes following completion of vapor probe installation (e.g., see Cox-Colvin 2013);

Cap the vapor pins immediately after they are installed in order to minimize the potential for cross slab air movement, including the potential migration of indoor air into the subslab area (e.g., air conditioned buildings under positive pressure).

7.10.3 Soil Vapor Probe Purging

Vapor probes should be purged of stagnant or ambient air in tubing and other equipment and filled with soil vapor prior to collection of a sample. The volume of air space in sand packs installed with the vapor point should be included in purging if less than 24 hours has lapsed since installation of the probes. The amount of time between purging and sample collection should be minimized.

The system volume is approximated as the sum of the volume of the open borehole interval (for temporary probes) or the sand pack porosity (for permanent probes) and the volume of tubing from the probe tip to the sample collection device.

Tables 7-6 and 7-7 show the sand pack volume (does not include tubing) and tubing volumes, respectively, for common borehole and tubing sizes.

Opinions vary on the optimum volume of vapor to be purged from a vapor point prior to the collection of a sample. Several published studies for relatively coarse soils indicated only minimal differences in VOC concentrations with different purge volumes (refer to SDC 2011). For the purposes of this guidance, it is recommended that temporary probes be purged of one to three system-volumes immediately prior to sampling. Permanent probes should be purged of three system-volumes after installation and then allowed to equilibrate.

Following equilibration, it is only necessary to purge three tubing-volumes prior to sample collection, as the sand pack is assumed to be in equilibrium with the surrounding native soils. The volume purged between different vapor points set at similar depths at a site should be approximately the same.

For in-slab, pin-type vapor points (e.g., Vapor PinsTM), minimal purging is required due to the small volume of air associated with the pin and tubing. Purging and monitoring prior to collection of a sample can be carried out until VOC and other target parameters (e.g., O2) appear to stabilize (e.g., using a PID). A minimum 30 second purge at a rate of 200ml/minute is recommended as a default.

Attempting to purge tight soils using a PID or similar instrument can induce an undesirably high vacuum on the soil, lead to inaccurate readings and even damage the instrument. Under these circumstances, an alternative is to collect an adequate volume of gas in a separate container to allow for field testing (e.g., use of a lung box and Tedlar bag; see Section 7.10.3.2). For example, a five-minute purge at 200mL/min can be used to fill a one-liter, Tedlar bag, which can then be tested using a multi-gas instrument as well as a landfill gas analyzer and/or helium detector, as needed. This approach also avoids the need to attach an instrument directly to the sample train tubing.

Probe purging can be accomplished using a syringe equipped with a three-way valve or a pump (Figure 7-23, see also Figures 7-16 and 7-18). Large Summa canisters can also be used for purging well points (ASTM 2006f; see Figure 7-23). Syringes are an inexpensive and simple approach for purging small volumes up to one liter. For larger purge volumes, a pump with variable flow rates and a flow meter should be used. The pump flow rate should not exceed 200 ml/minute unless it can be demonstrated that the vacuum imposed on the subsurface soil does not exceed seven inches of mercury. A PID can also be used to purge a well point provided that it does not cause an excess vacuum on the soil. Typical flow rates for PIDs range from 100 to 300 ml/minute.

Over purging a well point can have several drawbacks (SDC 2011). The larger the quantity of soil vapor drawn, the greater the uncertainty in the location of the collected sample. Large purge volumes also create a risk of short circuiting to atmospheric air along the outside of the probe body. Large purge volumes can also create vacuum conditions in the soil that could cause contaminant partitioning from soil or free product into the gas phase or cause a large volume of ambient air to be drawn into the formation after the purging equipment is removed.

7.10.3.1 Flow Rate

A flow rate of 200 milliliters per minute or less is recommended during purging and sampling (SDC 2011, CalEPA 2012). This is intended to keep the vacuum imposed on the soil to below seven inches Hg (100 inches of water) and avoid migration of otherwise sorbed VOCs into the air-filled pore space (see following section). Lower permeability soils may require lower flow rates in order to control the vacuum. A maximum flow rate of 200 ml/minute also helps to minimize the chance of breakthrough during the collection of sorbent-tube samples. Short circuiting to the atmosphere is of less concern for deep wells (e.g., >15ft bgs). A vacuum of seven inches Hg or less should be maintained during sampling, however, in order to minimize volatilization of sorbed VOCs (see below). Packers or other methods may also be required to isolate targeted depth zones.

Summa canisters should be used with flow controller devices supplied by the laboratory and calibrated to an appropriate flow rate. Flow rates are typically set for the flow controller by the laboratory and cannot be adjusted in the field. For example, a flow rate of 200 ml/minute equates to a five minute draw time for a one-liter canister. When purging a well point or collecting a vapor sample with a syringe, the flow rate is maintained by drawing the plunger back at a steady rate. When purging or collecting with a pump, a flow meter should be used to control and measure the flow rate. The flow rate should be read and recorded periodically (e.g., every five minutes or less).

7.10.3.2 Vacuum Conditions and Tight Soils

The purging and sample collection vacuum should be less than seven inches Hg (100 inches of water; SDC 2011, CalEPA 2012). Increasing the vacuum on the sampling system (e.g., resulting from low permeability soils, high purge or sample flow rates, or high soil moisture) can result in a biased sample. Imposition of a high vacuum on the soil could cause non vapor-phase VOCs in free product, sorbed to soil or dissolved in soil moisture to be stripped and bias the resulting vapor sample. As flow rates and vacuum levels increase, the risk of leakage in the sampling probes increases. High vacuums for sample points within a few feet of the water table can also cause water to be pulled into the sample container. This not only causes potential problems for the laboratory, but also compromises the integrity of the sample data since vapor-phase compounds could partition into the water during storage and shipment of the sample. If water is drawn into a sample container then the sample should be recollected (preferred) or the resulting sample data should be flagged and qualified in the site investigation report.

The vacuum should be measured and documented during purging and sample collection using a vacuum gauge placed between the probe and sample container (Figure 7-24). Note that the gauge on the flow controller for the Summa canister measures the vacuum in the canister, not the vacuum applied to the soil vapor probe.

A qualitative method to quickly determine if the permeability of the soil could lead to an excessive vacuum is to hook up a 20cc to 50cc gas-tight, plastic syringe to the probe and pull on the plunger (SDC 2011). If the plunger is difficult to pull (compare to pulling outside air) or if the plunger is pulled back towards the probe after released, then there is likely too little permeability to get an uncompromised sample. When sampling in relatively permeable soils (e.g., sands, or silty sands) using a syringe, a vacuum gauge is not typically needed as the sampler can feel the vacuum while drawing gas into the syringe.

If the purging or sampling vacuum exceeds seven inches Hg during sample collection, the sample collection flow rate should be reduced. This might be able to be accomplished by progressively closing the valve to the Summa canister. If this doesn’t work, then an alternative flow controller with a lower flow rate (e.g., <100ml/minute) might be necessary. The potential need for a low-flow controller should be assessed prior to the collection of samples based on the geology of the site and prior sampling experience, if available.

If a continued reduction in the flow rate does not reduce the sampling vacuum, then an alternative vapor point would be installed with a large sand pack emplaced in the zone of interest (SDC 2011). The sand pack should have an interstitial void volume of approximately three liters, which implies the use of approximately ten liters of sand for the pack. A vapor point is installed in the pack and completed at the surface. The top of the sand pack should be at least five feet below surface grade. The pack should be capped with bentonite to prevent break though to ambient air.

A vapor sample should only be collected after the sand pack has reached equilibrium with the surrounding, native soil. This can be expected to take approximately two weeks (SDC 2011). Only one purge volume equating to one tubing volume should be removed. Tubing size should be selected so that the purge volume does not exceed 200 milliliters. Purging 200 milliliters is unlikely to induce a significant vacuum in the probe given the substantially larger void volume in the sand pack. Sample size should be limited to one liter. The vacuum within the vapor probe should be measured during sampling to ensure that seven inches Hg is not exceeded during the purging and sample collection

7.10.4 Soil Vapor Sampling Trains

A variety of sampling systems can be used to collect representative soil vapor samples. Individual laboratories typically provide guidance on recommended designs of sampling trains (e.g., Air Toxics 2012, Air Toxics 2012b). The system should be selected based on the type of sample container, the sampling probe, and the overall sampling objectives. The primary requirements are that the system forms a gas-tight sample train from the probe to the sample container, with a means of controlling the sample flow rate and gauging the vacuum in the system.

Tedlar bags can also be filled using a lung box. A lung box is an air-tight container with ports for the soil vapor probe tubing and separate tubing to a pump. The Tedlar bag is connected to the vapor probe tubing and then placed inside the lung box. A pump is used to evacuate the lung box, which causes the Tedlar bag to expand, drawing in soil vapor (Figure 7-25).

A schematic diagram and photograph of a soil vapor sampling apparatus for collecting samples in Summa canisters is depicted in Figure 7-26 and includes the following major elements: the vapor probe tubing, a vacuum gauge, sample tubing (inert tubing such as Teflon, nylon, or stainless-steel), a flow controller, Summa canister, and gas-tight fittings.

The valves and gas-tight fittings must be closed at all times to prevent ambient air from entering the system except when actively purging or collecting a sample. This is especially critical if the sample collection vacuum is elevated (see Section 7.10.3.2).

Figure 7-27 shows several typical sampling apparatus in which soil vapor samples are being collected with a sorbent tube and Summa canisters.

In the bottom, right-hand photo of Figure 27, the dual vacuum gauges are used to read the vacuum in the Summa (gauge nearest Summa) and the vacuum imposed on the soil as the sample is being collected (gauge closest to well point). A small, pre-calibrated flow regulator is present between the gauges. The vacuum imposed on the soil will be near zero in highly permeable soils but could exceed 10 inches of mercury in tight, clayey soils. Both of these gauges should remain stable during the initial shut-in test of the sampling train (see Section 7.10.5.1).

7.10.5 Soil Vapor Probe Leak Testing

Leak tests are an important part of quality assurance and are strongly recommended for each vapor sample. The nature of leaks tests carried out as part of a site investigation should be clearly presented and discussed in the resulting report. Leaks in sampling train fittings or leaks at the vapor point annulus can result in dilution of the soil vapor samples with ambient air and under reporting of actual contaminant concentrations. Most leaks occur in the sampling train, rather than in the annulus around the vapor probe surface seal. Excessive vacuum conditions resulting from low porosity soils or high moisture content soils can exacerbate the potential for ambient air leakage. The use of Teflon tape in Swagelok fittings can also cause leaks and should be avoided.

Three types of leak tests are described: 1) A “shut-in” test to determine the tightness of the sampling train in the field, 2) A “water dam” test for field testing of the integrity of the vapor point when installed into a slab or other relatively impermeable surface and 3) A tracer test to determine the presence or absence of gas introduced around the vapor point and/or the sampling train in the sample that is collected. Performance of a shut-in test of the sampling train is recommended prior to the collection of all soil vapor samples. As described below, this allows the tightness of the sampling train to be quickly evaluated in the field. A water dam test, as described below, or equivalent test is recommended for vapor points installed in intact slabs prior to sample collection. This allows the tightness of the annular space around the vapor point to be quickly tested in the field.

Tracer tests are recommended to test the tightness of vapor points installed in soil, cracked slabs, pavement or other cases where a bentonite seal is used to prevent the infiltration of ambient air during sample collection. Tracer gas leak tests are also recommended for high-risk or high public profile sites where where lab data are desired to confirm sampling chain and vapor point tightness. As described below, two tracer gas methods are recommended: (1) Application of a leak check compound to the vapor probe surface seal or (2) Application a tracer gas to the entire sampling apparatus. Table 7-8 provides a comparison of the two leak check methods using a tracer gas.

Use of one of the two tracer methods described above is recommended for vapor points with connections that cannot be included in a shut-in test and for all samples collected from a depth of 5 feet or shallower, due to the increased risk of a leak in the annular seal. This will help to verify that short circuiting is not occurring at surface connection points and/or that there is an adequate annular seal for shallow samples. As discussed below, field measurement for the presence or absence of the tracer in the initial sample drawn from the well point can be used to help verify the integrity of the vapor point prior to submittal of samples to a laboratory (e.g., collection of an initial sample in a Tedlar bag and testing for helium using a hand-held field meter).

A simple shut-in test will be adequate for the routine collection of soil vapor samples when the depth of the probe is greater than 10 feet and all connections in the sampling train can be included in the test. Testing the tightness of the vapor point connection with the ground surface is less critical in these cases given the depth to the sampling point, although a seal should still be placed around the point. A combination of a field shut-in test with a water dam (for intact slabs) and/or the Method 1 tracer test described above will be adequate for the majority of remaining samples. The use of a full shroud over both the sampling train and vapor point could, however, be recommended or even required for sites with a high public profile, sites involved in legal actions, or sites where the integrity of previously collected samples is in question.

Note that simple release of tracer gas around sampling train connections and the vapor probe seal at the ground surface in the absence of a shroud is not recommended, since the concentration of tracer gas around the test points can be difficult to maintain and lengthy release times may be required to draw an adequate amount of tracer gas into the sample container and identify a significant leak.

Be aware that consultants have reported false positives for some types of helium field detectors due to very high concentrations of C5-C12 hydrocarbons in vapors in soil gas. This can be assessed prior to purging and sample collection by connecting the helium detector directly to the vapor point and evaluating the response. Ultra high-purity helium (e.g., Grade 5) is recommended for leak tests due to potential petroleum and other contaminants in cheaper, “party-grade” helium

High levels of light-end, petroleum vapors have also been reported to cause false, elevated readings of methane in vapor samples using a standard, landfill gas analyzer. The use of a carbon trap is recommended when evaluating methane using field instruments at sites where high levels of petroleum may be present in soil gas (see also Section 9.4). A carbon trap will retain VOCs, but not methane, and can allow for a more refined estimation of the level of methane present. If a carbon trap is reused, it is recommended to always have gas flow through a carbon trap in the same direction.

7.10.5.1 Sampling Train Shut-in Test

A sampling train shut-in test is recommended prior to the collection of all soil vapor samples. The shut-in test is performed by isolating the sampling train from the vapor sampling point and applying a vacuum to the sampling train. The applied vacuum should hold steady (not decrease) for at least 60 seconds. The start and end vacuum should be recorded and reported.

Figure 7-28 depicts example sampling trains arranged for a shut-in test. The system consists of a gas tight 2-way valve at the vapor probe termination, a 3-way valve to connect the vapor probe to the sample container, and a vacuum gauge and syringe with shut-off valve. The 2-way valve is closed and the sample container valve is closed to seal the sampling train. A vacuum is then applied by drawing back the syringe plunger. The vacuum in the sampling train is then monitored for one to five minutes, with a longer time used for more complex trains that have multiple connections or that otherwise might be especially susceptible to leaks. The sampling train can be considered to be adequate “tight” if the apparent loss in vacuum is less than 0.5 in mercury. The second example in the figure consists of a more simple arrangement, with a clamp and short length of flexible tubing included at the well point and used to seal the sampling train. A vacuum is applied by opening the valve on the Summa canister. The vacuum is monitored using the gauge on the flow controller. If the sampling train does not hold a vacuum, then all connections should be rechecked and the shut-in test repeated.

7.10.5.2 Water Dam Vapor Point Test

A “water dam” offers a simple and inexpensive method to test for leaks around vapor points installed into a slab or other relatively impermeable surface (e.g., see Cox-Colvin 2013b). For a flush-mount installation, water is poured directly into the depression cut into the floor around the vapor point (see Figure 7-22). For a stick-up installation, a coupling can be sealed to the floor around with non-volatile putty and then filled with water (Figure 7-29).

Pour enough distilled water into the water dam containment or the annular space of a flush-mount depression to immerse the tubing connection to the vapor point. Note that water soluble clays such as Play-Doh might absorb too much water to be suitable for tests that last more than one hour. Assemble the sample train and connect it to the vapor point. Perform a shut-in test to verify that the sample train can hold a vacuum for one to five minutes with no more than 0.5 in Hg loss of vacuum (see previous section). Purge the vapor point and monitor the water level in the dam. The water level might drop slightly due to absorption into the concrete. A sudden drop in water level, the appearance of water in sample tubing or other indication of water entering the sub-slab is most likely indicative of a significant leak, however. If this occurs then remove the distilled water and reposition or reseal the vapor point to address the leak.

7.10.5.3 Tracer Method 1 – Application of Tracer Gas to Surface Completion Point Only

Under Tracer Method 1, once the shut-in test has been successfully completed, a leak check compound is applied to the surface completion of the probe. In the first example, the leak check compound is applied by wetting a towel with liquid compound (e.g., Isopropanol) and placing it around the probe tubing at the ground surface (see Figure 7-28). Isopropanol is included as a targeted VOC in the lab analysis of the sample. In the second example, a small shroud is placed over the surface completion and the shroud filled with a tracer gas like helium (Figure 7-30). Leaks around the probe seal can be tested in the field by drawing a purge sample with a syringe and testing it for helium, as depicted in the first example. As an alternative or as a second check, helium can be included as a targeted VOC in the lab analysis of the sample (see second example). A concentration of 10-30% helium is typically targeted and maintained in the shroud. Advantages and disadvantages of different tracer compounds are discussed in the following sections.

On uneven surfaces it may be useful to place the shroud in a ring of bentonite clay to obtain a better seal around the base of the shroud and minimize the escape of helium. In the lower photo of Figure 7-30 the consultant used a ring of Play-Doh to seal the base of the shroud. Play-Doh is a non-toxic mixture of flour, water, salt, boric acid and mineral oil. Volatile chemicals associated with the mineral oil are negligible and not expected to interfere with leak detection or samples collected.

7.10.5.4 Tracer Method 2 – Application of Tracer Gas to Entire Sampling Apparatus

Figure 7-31 depicts several alternative, shroud configurations for enclosure of the entire sampling apparatus (Method 2), including sample container, all tubing and connections, and the vapor probe surface completion in a shroud, which is filled with tracer gas.

A concentration of 10-30% helium is typically targeted and maintained in the shroud. A hand-held helium detector can be used to monitor helium concentrations within the shroud. Purge tests can be conducted in the field to test for annular seal leaks. In addition, the sample submitted to the laboratory is analyzed for the tracer gas.

Five-gallon buckets modified to include ports for sampling train tubing and helium injection and monitoring are currently the most commonly used for full shrouds. Note that a Summa canister sampling train that includes an older, flow controller may not generally fit under a five-gallon bucket shroud. A smaller, more compact flow controller should be used instead. Garbage bags offer a last, easy-to-find resort, but can be difficult to keep sealed and inflated in the field, especially on a windy day and are not recommended for routine sampling.

An advantage of the full-system shroud is the ability to document the lack of a leak in both the sampling train and the vapor point seal by testing for helium in the sample analyzed by the laboratory. Combination with a field shut-in test of the sampling train as described in the previous section is still strongly recommended. This will allow significant leaks in the sampling train to be identified and addressed at the time of sample collection. If helium is identified in the sample that is subsequently collected and analyzed then it can also be concluded that the leak occurred around the vapor point annular seal. A disadvantage of full-system shrouds is the volume of helium required, as well as the increased time and cost of sample collection in the field. Monitoring the vacuum gauge on the flow controller without opening the shroud and loosing the helium is only possible if a clear container or bag is used. This is one advantage of the large Tupperware shroud.

7.10.5.5 Tracer Gas Concentration Measurement

For both methods, the concentration of tracer gas in the shroud should ideally be measured and monitored in the field using a hand-held field meter. A concentration of 10-30% helium is typically targeted and maintained. If the field meter is selective for the tracer gas compound (e.g., a helium detector used with helium tracer), then a pre-sample can be collected and checked in the field for tracer gas prior to collecting the formal sample. The formal sample should be analyzed by the laboratory for the leak check compound.

The amount of leaking, if any, can be quantified by comparing the tracer gas concentration measured in the shroud to the concentration measured in the sample. If the leak is less than ten percent of the original concentration of the tracer gas in the shroud then the sample can be considered acceptable (i.e., sample concentration less than 1% to 3% for original concentration in shroud of 10-30%). This indicates that no more than ten percent of the sample submitted to the lab was ambient air and that reported concentrations of VOCs are within ten percent of the actual concentration of VOCs at the vapor point in the field at the time the sample was collected. If a field meter is not used to monitor the concentration of tracer gas in the field then the results of the leak test can only be qualitatively evaluated and any detection of helium will be flagged as a potentially significant leak. Fully document and discuss any detections of leak check compound in the soil vapor investigation report.

7.10.5.6 Selection of Leak Check Compound

The selection of leak check compounds is site and analysis specific. Considerations include whether it is a known or suspected contaminant at the site or included in the laboratory’s list of target analytes for the method being used, and whether it can be monitored with portable measurement devices. Common leak check compounds are isopropanol (2-propanol or “rubbing alcohol”), helium, and difluoroethane (found in “office duster” cans). Each of the compounds has a variety of advantages and disadvantages.

Isopropanol is readily available, inexpensive, and does not require the use of a shroud, as it can be applied to a towel placed around the vapor probe, although it can also be used with a shroud by placing the towel inside the shroud. Isopropanol is also more dense than air and may be particularly useful for testing leaks associated with the anular space of the vapor point (see Figure 7-35). A further advantage of isopropanol is that it can be detected using methods TO-14, TO-15 or SW8260.

A disadvantage, however, is that selective field meters for isopropanol are not readily available and leaks cannot be readily identified in the field, as can be done with helium. Because it is handled at high concentrations, a relatively small and otherwise minor leak (i.e., <10%) can also significantly interfere with the analysis and require reporting limits for VOCs to be raised above target action levels. Quantification of leaks is likewise difficult, since the original concentration of vapor-phase isopropanol under the shroud can at best be estimated based on its vapor pressure. Isopropanol is also sometimes used as an additive in gasoline. This could again lead to false leaks for sampling at gasoline-release sites.

When using liquid tracer compounds, extreme care also needs to be taken to not contaminate the sampling train parts with tracer compound. Gloves should always be worn when handling the tracer compound. A new pair of gloves should be worn when handling and assembling sampling train components. Ideally, one field technician will be assigned to handle the leak check compound and a second field technician will be assigned to handle and assemble the sampling train.

Helium is commonly used as a leak detection tracer, especially for sampling trains. Helium concentrations can be readily measured in the field using a selective hand-held meter. A further advantage of helium is that its presence in a sample, even at high concentrations, will not interfere with TO-14, TO-15 or SW8260 analysis for VOCs. This allows for a more reliable quantification of leakage. However, the laboratory must run a separate analytical method to analyze for helium. A potential disadvantage of helium is that it is lighter than air. This requires that care be taken to ensure adequate mixing under a shroud if it is used as a tracer for leak detection around vapor points.

Difluoroethane (in office duster cans) is readily available and simpler to handle than helium cylinders and can be analyzed using methods TO-14, TO-15 or SW8260. However, like isopropanol, selective field meters are not readily available.

Table 7-9 provides a comparison of these leak check compounds.

7.10.6 Soil Vapor Sample Collection Steps

The following general steps should be followed when collecting soil vapor samples:

  1. Allow the sampling probe to equilibrate with the subsurface (see Section 7.10.2).
  2. Check all valves and fittings for integrity by either performing a vacuum leak test or applying a leak check compound during purging and sample collection (see Section 7.10.5).
  3. Purge the sampling apparatus as discussed in 7.10.3.
  4. Collect the soil vapor sample into the appropriate sample container.
  5. Disassemble the sampling apparatus making sure to close the valves on the sample container and the soil vapor sampling probe.
  6. Transport the sample to the analytical laboratory under appropriate conditions (based on the analytical method[s] employed) following standard chain-of-custody procedures.

7.10.7 Soil Vapor Sample Notes and Logs

Good field notes and logs are important components of soil vapor investigations. Take clear notes in the field. State the goals of each planned activity at the beginning of each day or shift. Document the names of personnel responsible for carrying out different activities, as well as site visitors. Be certain to include units of measurement. Note any nearby activities that could release chemicals to the air (e.g., smoking, recent painting, cleaning with solvents, generators, operation of motor vehicles, passing of cars on roadways, etc.).

Example information that should be included in a sample log includes:

  • Sample identification;
  • Names of sampling personnel;
  • Date and time of sample collection;
  • Sampling depth;
  • Sampling methods and devices;
  • Purging and sampling rates;
  • PID readings;
  • Soil vapor probe system volumes;
  • Volume of soil vapor extracted prior to sampling;
  • Sample volume;
  • If canisters are used, vacuum of canisters before and after sample collection;
  • Apparent moisture content of the sampling zone (e.g., dry, moist, etc.); and
  • Chain of custody protocols and records used to track samples from the sampling points to the laboratory

Use field forms to help remind workers of the information that needs to be recorded. Complete every part of the form provided, even if the response is “N/A.” Record data on field logs/forms and sample container tags at the same time. Take photographs during the field work, including broad overviews of the work site and close ups of specific activities. Use a checklist to verify that all equipment has been staged and accounted for prior to initiation of sample collection. Calibrate the field meters and record the method and results in the field notes and on any applicable field forms.

The above represents a necessarily brief overview of requirements for proper documentation of field work. Training by experienced experts and preparation of a detailed work plan can help minimize unanticipated problems during field activities.