Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 7.13
SOIL VAPOR AND INDOOR AIR SAMPLE ANALYSIS

7.13 SOIL VAPOR AND INDOOR AIR SAMPLE ANALYSIS

The analytical methods selected for analyzing soil vapor or indoor air samples depend on a number of factors. These include the targeted VOCs, desired detection and reporting limits and the manner in which the sample is collected. In Hawai`i, soil vapor or indoor air samples are in most cases forwarded to a fixed analytical laboratory on the mainland for analysis. Analytical methods should be consistent within each sampling event as well as for different sampling events to assist in the interpretation of the data.

7.13.1 Available Analytical Methods

7.13.1.1 Volatile Organic Compounds (VOCs)

A variety of analytical methods are available to measure soil vapor samples all of which give accurate results. Table 7-10 gives a summary of the available methods. Table 7-11 summarizes the methods most commonly used on Hawai‘i sites. Method versions denoted in the table by Roman letters (e.g., “A”, “B”, “C,” etc.) are for example only and may not include recent updates to the method. Discussion of the most appropriate analytical methods to meet the objectives of an investigation with the laboratory is strongly recommended. Less expensive methods such as TO-3, 8015 and 8021 for VOCs are, for example, primarily used for screening or monitoring purposes. Method 8015 is a comprehensive method for TPH whereas Methods 8260, TO-15 and TO-17 measure individual VOCs by mass spectrometry. Both methods work well for TPH as comprehensive data but GC/MS methods are recommended for final, decision making.

As discussed in the following section, Total Petroleum Hydrocarbons (TPH) should be reported as the sum of C5 to C12 (Summa canister samples) and/or C5-C18 (sorbent tube samples) for vapors associated with all types of petroleum fuels, including diesel and other middle distillate fuels. Unlike soil or groundwater, reporting of TPH compounds as “gasoline-range” or “diesel-range” is not applicable to soil vapors. As discussed in Section 7.13.1.2, vapors associated with diesel and other middle distillate fuels could, in theory, include a large component of C12 and higher aliphatic and to a lesser degree aromatic compounds (see HDOH 2012). Inclusion of heavier, vapor-phase compounds in the measurement of TPH will require the use of a sorbent method (e.g., TO-17). Recovery of aromatic compounds above C10 and aliphatic compounds above C12 is not currently practical for samples collected in a Summa canister (see Section 7.8.1). If a minimum one-liter sample cannot be drawn using a sorbent tube, for example due to potential saturation of the adsorbent material, then a concurrent Summa sample should be collected and used to report light-end VOCs and TPH (see Section 7.8.2). This will almost always be the case for soil vapors associated with petroleum. If initial sorbent data (or prior knowledge at a similar site) indicates that vapors are dominated (e.g., >90%) by C5-C12 compounds then subsequent TPH data can be obtained using Summa samples.

Other analytical methods not listed in Table 7-10 can be utilized on a site-specific basis. A description of the alternate analytical method, rationale for its selection, and analytical results should be fully documented in the final soil vapor or indoor air investigation report.

Table 7-10 Summary of Soil Vapor & Indoor Air Analytical Methods1

Method No. Type of Compounds Collection Device Methodology Detection Limit 2 Reference
TO-1 3 VOC Tenax® solid sorbent GC/MS or
GC/FID
0.02 – 200 µg/m3
(0.01-100 ppbv)
USEPA 1999b
TO-2 3 VOC Molecular sieve sorbent GC/MS 0.2 – 400 µg/m3
(0.1-200 ppbv)
USEPA 1999b
TO-3 VOC Cryotrap GC/FID 0.2 – 400 µg/m3
(0.1-200 ppbv)
USEPA 1999b
TO-4A Pesticides/
PCBs
Polyurethane foam GC/MD 0.5 – 2 µg/sample USEPA 1999b
TO-10A Pesticides/
PCBs
Polyurethane foam GC/MD 0.5 – 2 µg/sample USEPA 1999b
TO-12 NMOC Canister or on-line FID 200 – 400,000 µg/m3
(100-200,000 ppbv)
USEPA 1999b
TO-13A 3 PAH, TPH Polyurethane foam GC/MS 0.5-500 µg/m3
(0.6 – 600 ppbv)
USEPA 1999b
TO-14A VOC (nonpolar) Specially-treated canister GC/MS 0.4 – 20 µg/m3
(0.2-2.5 ppbv)
USEPA 1999b
TO-15 4 VOC Specially-treated canister GC/MS 0.4 – 20 µg/m3
(0.2-2.5 ppbv)
USEPA 1999b
TO-15A VOC Specially-treated canister GC/MS 0.005 µg/m3-0.02 µg/m3
(0.002-0 .04 ppbv)
USEPA 1999b
TO-17 3,4 VOC Single/multi-bed adsorbent GC/MS, FID 0.4 – 20 µg/m3
(0.2-2.5 ppbv)
USEPA 1999b
Method 3C N2, O2, CO2, and CH4 Canister GC/TCD 20,000 – 150,000 µg/m3
(10,000 ppbv)
USEPA 1999b
Method 16 H2S Tedlar® Bag, Canister GC/FPD 100 - 700 µg/m3
(50 ppbv)
USEPA 1999b
8015B/
8015D
TPH/VOC Tedlar® Bag, Canister, Glass vials GC/FID 300 – 3000 µg/m3
(100 – 10,000 ppbv)
USEPA 1998b
8021B VOC Tedlar® Bag, Canister, Glass vials GC/PID 4.0 – 60.0 µg/m3
(0.3 – 30 ppbv)
USEPA 1998b
8260B VOC Tedlar® Bag, Canister, Glass vials GC/MS 10.0 – 50.0 µg/m3
(0.6 – 25 ppbv)
USEPA 1998b
8270C SVOC Tedlar® Bag, Canister, Glass vials GC/MS 1,000 µg/m3
(20,000 – 100,000 ppbv)
USEPA 1998b
D1945-03(2010) natural gases and mixtures Tedlar® Bag, Canister, Glass vials GC/TCD 800 – 29,000 µg/m3
(10,000 ppbv)
ASTM 2010b
D1946-90(2011) H2, O2, CO2, CO, CH4,
C2H6, and C2H4
Tedlar® Bag, Canister, Glass vials GC/TCD 800 – 18,000 µg/m3
(10,000 ppbv)
ASTM 2011

Notes (Table adapted from API 2005):

  1. This is not an exhaustive list. Some methods may be more applicable in certain instances. Other updated, proprietary or unpublished methods may also apply. Passive samplers can also be used for collection and qualitative assessment of some compounds.
  2. Detection limits are compound specific and can depend upon the sample collection and the nature of the sample. Detection limits shown are for the range of compounds reported by the analytical methods.
  3. Trapping-type sampling method used to achieve high sensitivity. TO-17 is a one-time thermal desorption method; TO-13 is an extraction method that can be reanalyzed as needed. TO-13 can be used to quantify heavier TPH in vapors but may not adequately capture light-end VOCs (consult the laboratory).
  4. TO-15 or TO-17 recommended for final, decision making purposes.
GC - gas chromatography FPD - flame photometric detector
MD - multi-detector FID - flame ionization detector
MS - mass spectrometry SVOC - semivolatile organic compounds
NMOC - non-methane organic compounds VOC - volatile organic compounds
PAH - polycyclic aromatic hydrocarbons TCD - thermal conductivity detector


Table 7-11 HDOH-Recommended Laboratory Analytical Methods for Soil Vapor or Indoor Air Contaminants and Leak Detection Compounds
.Analyte Analytical Method

Reference

TPH TO-3, TO-14, TO-15,
TO-17, 8015
USEPA 1999b
USEPA 1998b
BTEX, MTBE, naphthalene TO-15, TO-17, 8012, 8260 USEPA 1999b
USEPA 1998b
VOCs (including difluoroethane and isopropanol alcohol) TO-14, TO-15, TO-1,
TO-2, TO-17, 8260, 8021
USEPA 1999b
USEPA 1998b
SVOCs (including PAHs) TO-17, 8270 (sorbent methods) USEPA 1998b
Oxygen, CO2, Nitrogen, Methane, Helium ASTM D-1946, 3C USEPA 1996j
Polynuclear aromatic hydrocarbons (PAHs) TO-13 USEPA 1999b
Notes:
  1. According to discussions between HDOH and laboratory staff, The best laboratory method to test for TPH in soil vapors appears to be a combination of both TO-15 (Summa canister samples) and TO-17 (sorbent tube samples) (HDOH 2012c). A sum of the individual carbon ranges can be more accurately determined from both methods. TO-3 can be far less sensitive than TO-15 and TO-17.

7.13.1.2 Total Volatile Petroleum Hydrocarbons

As discussed in Section 9 and the HEER Office EHE guidance (HDOH 2016 and updates), testing of vapors associated with petroleum should include a short list of target indicator compounds (e.g., BTEX, naphthalene and methane) and Total Petroleum Hydrocarbons (TPH), also referred to as Total Volatile Petroleum Hydrocarbons (TVPH). The target indicator compounds recommended for analysis at petroleum contaminated sites are listed in Section 9, Table 9-5.

Total Petroleum Hydrocarbons represents the sum of the vapor-phase, aliphatic and non-targeted, individual aromatic compounds. This is sometimes subdivided into a “gasoline-range (“TPHg)” category characterized by a dominance of light-end, C5–C12 compounds and a “diesel-range (TPHd”) category characterized by heavier-end, C10–C26 compounds. This is appropriate for testing of soil and water samples, based on the known or assumed type of fuel released.

A distinction between TPHg and TPHd compounds is misleading for vapor-phase petroleum, however, since vapors from diesel and other middle distillate fuels or fuels that include middle distillates (e.g., JP-4, a mixture of gasoline and kerosene, and JP-8, similar to diesel fuel) can contain a significant proportion of lighter end compounds, especially C5-C8 aliphatics. Requesting the lab to report vapor-phase TPH as the equivalent of “TPHd” (i.e., sum of C10+ compounds) could significantly underestimate the actual concentration of TPH in soil vapors.

This issue was investigated and discussed in the HEER Office study Field Investigation of the Chemistry and Toxicity of TPH in Petroleum Vapors: Implications for Potential Vapor Intrusion Hazards (HDOH 2012, 2012c; Brewer et al 2013). The study suggested that the proportion of C5-C8 aliphatic compounds in vapors associated with middle distillate fuels is highly variable but can be up to 50% or more of the total TPH. Excluding these vapors from the TPH analysis can significantly under-report the total TPH present in a vapor sample. The study also indicated that individual, targeted aromatic compounds such as BTEX typically make up less than 1% of the total petroleum vapors present.

Vapor intrusion risk associated with the TPH fraction of petroleum vapors has only recently begun to be investigated in detail (e.g., Brewer et al 2013; see also Section 9). Although “less toxic” with respect to toxicity factors and action levels, the higher proportion of TPH aliphatics in the vapors causes these compounds to be the primary risk driver with respect to potential vapor intrusion concerns. Ongoing evaluations of soil gas field data will help address the lack of published information on the relative risk of vapor intrusion (quantitatively considered) posed by TPH versus benzene and other individual compounds.

The HDOH study indicated that the ratio of TPH to other individual aromatic compounds, such as benzene, can vary within a study site and between sampling events. These spatial and temporal differences could reflect differences in weathering and biodegradation, subsurface migration and small-scale heterogeneity with the plume. This highlights the potential problems associated with one-time sampling events and limited vapor points (see Section 7.10.1).

For vapors associated with gasoline-only releases, TPH (or equivalent) should be reported as the sum of all compounds falling within the carbon range from C5 to C12 (non-BTEX aromatics typically reported to C10). For vapors associated with middle distillate (and heavier) fuels, including diesel, TPH should be reported as the sum of all compounds falling within the carbon range from C5 to C12, if a summa canister is used, and C5 to C18 if a sorbent tube is used. It is important to clarify this with the laboratory and document this in the report. The lab should not be requested to report “diesel range” TPH in the sample, since doing so excludes reporting of C5-C9 aliphatics in soil vapors and could significantly underestimate the total concentration of TPH-related compounds. As discussed in the HDOH EHE guidance, a more detailed analysis and evaluation of the carbon range makeup of TPH can be carried out on a site-specific basis as needed (e.g., development of more site-specific, TPH soil gas action levels; HDOH 2016).

The concurrent collection of soil vapor samples using both a Summa canister and a sorbent tube is recommended for the investigation of subsurface vapors associated with diesel or other middle distillate fuels (see Section 7.8). The draw volume for sorbent tube samples is typically limited to 50ml due to potential saturation of the sorbent media (see Section 7.8.2). The Summa canister sample is likely to be more representative of subsurface vapors, given its larger volume. This sample should be collected first, following purging, and tested for TPH as the sum of C5-C12 compounds, BTEX and other targeted compounds. The well point should then be closed (e.g., via a valve or tightly pinching the tubing) prior to unhooking the canister. This will prevent ambient air from entering the tubing if a vacuum has been imposed on the subsurface soil.

The sorbent tube sampling train should then be attached to the vapor point and a shut-in leak test performed. Following successful completion of a shut-in test, the well point should be opened and a minimum, 50ml sample drawn. The sorbent tube sample should be tested for TPH as the sum of C5-C18 compounds (e.g., using TO-17 methods). The resulting data should be compared to the reported level of TPH in the Summa canister sample. If the difference in minimal (i.e., <10%), then a conclusion can be drawn that a significant proportion of C12+ compounds are not present in the soil vapors and Summa canister samples can be used for future sample collection. A review of the TO-17 gas chromatograph for the sample can also be helpful to determine if a significant proportion of the TPH vapors consists of compounds greater than C12.

Vapor-phase TPH data for middle distillate release sites that do not include both light- and heavy-end compounds may not be accepted unless it can adequately demonstrated that heavier end compounds do not make up a significant proportion (e.g., >10%) of the total vapors. If the lab cannot report lighter-end compounds with their current setup then both “TPHg” and “TPHd” should be reported, and the sum of the two methods compared to target action levels (see also Section 7.13.2.

Targeted, individual compounds such as BTEX and naphthalene that are evaluated separately can be subtracted from the reported TPH for comparison to TPH indoor air or soil gas action levels. This can either be done by the laboratory (preferred) or based on the reported data if the compounds were included with the reported concentration of TPH (see Section 9). The approach used should be noted in the report.

The HEER Office indoor air and soil gas action levels for TPH action levels reflect assumptions regarding the toxicity-weighted sum of the individual carbon ranges. The action levels conservatively assume a mixture of a high proportion of more toxic, C9-C12 aliphatic compounds in petroleum vapors. These compounds are more typically associated with diesel and other middle distillate fuel vapors than vapors from gasoline. As a result, the default action levels may be excessively conservative for vapor intrusion evaluations of gasoline-only release sites. As discussed in the HDOH EHE guidance document, alternative action levels can be developed and proposed based on site-specific, TPH carbon range data (HDOH 2016). Alternative toxicity factors for TPH carbon ranges can similarly be proposed in a site-specific risk assessment (HDOH 2016).

As discussed in Appendix 1 of the HEER EHE guidance (HDOH 2016), the default action levels are likely to be too conservative for gasoline-only sites by a factor of three or more. For more site-specific evaluations, TPH can be reported in terms of the specific carbon ranges used to develop the action levels, including C5-C8 aliphatics, C9-C12+ aliphatics, and C9-C10+ aromatics. The concentration of individual TPH carbon ranges can be compared to indoor air or soil gas action levels presented in Appendix 1 of the EHE guidance. Site-specific TPH soil gas action levels can also be developed based on the average carbon range makeup of petroleum vapors (refer to HDOH 2012). Laboratory gas chromatograms should be obtained and included with site-specific evaluations of TPH carbon range chemistry and toxicity.

Note that the cumulative, noncancer risk must be calculated if carbon range-specific concentrations and action levels are used. This is necessary to ensure that the total concentration of vapor-phase TPH does not pose an unacceptable health risk. This is done by dividing the reported concentration of an individual carbon range by its respective action level, referred to as the “Hazard Quotient,” and then summing the calculated Hazard Quotients for each carbon range, referred to as the “Hazard Index.” If the calculated Hazard Index is less than 1.0 then the TPH does not pose a cumulative risk. If it exceeds 1.0 then potential cumulative risk needs to be further evaluated. In practice, noncancer, Hazard Indices should also be calculated for individual, targeted compounds such as BTEX and naphthalene and added to the total Hazard Index.

A key issue influencing reported TPH concentrations is the calibration procedure used by the laboratory. Is calibration done using a liquid or a vapor standard? The latter will provide more accurate data. Were typical gasoline and diesel calibration standards used, or were separate aliphatic hydrocarbon component standards used? Results will vary between labs if different types of calibration standards are used. Therefore, the calibration procedure should be fully documented in the final soil vapor or indoor air investigation report.

7.13.2 Choosing the Analytical Method

The primary criteria for choosing the appropriate method are:

  • The compounds of concern;
  • Required detection level and other data quality objectives (DQOs);
  • Sampling logistics; and
  • Cost.

The following questions should be considered prior to the selection of analytical methods for soil vapor or indoor air samples (API 2005):

  • What are the targeted chemicals of concern or other parameters (e.g., natural attenuation parameters)? The specific analytes targeted for the site investigation should be identified and noted (e.g., TPH, benzene, naphthalene). Generally, these will be the volatile and semi-volatile chemicals of concern identified during the overall site investigation. If indoor air samples are to be tested, targeted chemicals should be limited to chemicals identified in subslab or subsurface vapor samples. The vapor intrusion risk calculated for indoor air data should be specific to the targeted, subsurface VOCs of concern and exclusive of other contaminants in the sample from indoor or outdoor sources. The lab method(s) selected should optimize the number of targeted COPCs that can be reported in a single analysis and limit overlap between different methods.
  • What analytical method reporting limits are required to adequately assess the potential exposures? It is important to determine the lowest concentrations of chemicals of concern in soil vapor or other analytes that are expected to be required for evaluation of the subsurface vapor intrusion pathway and general site investigation needs. Refer to the EALs for indoor air and soil gas published in the HEER Office EHE guidance (HDOH 2016. Typical laboratory detection limits fall below action levels for soil gas but, in some cases, may be above purely risk-based action levels for indoor air. In this case the laboratory detection limit can be used as an alternative screening level (see also Volume 1 of the EHE guidance).
  • Do soil or groundwater analytical results, or other field data, indicate that concentrations of chemicals of concern in soil vapor will be high? If concentrations of chemicals of concern or other analytes in soil vapor are anticipated to be high, then the analytical method selected should address high concentrations. It is important to notify the laboratory of anticipated, high concentrations of VOCs in samples so that sample processing can be optimized. Including a summary table of PID data for sample points can assist the lab in selection of the most appropriate lab methods and help them optimize detection limits.

    In cases where very high concentrations of VOCs are anticipated, solid waste program methods for analysis of soil vapor samples typically reserved for landfill gas samples may be appropriate (USEPA 1998b). There is some concern that the solid waste program methods might be biased low for some chemicals of concern. Studies have indicated, however, that the solid waste program methods and air toxics methods produce similar results for TPH, BTEX and chlorinated hydrocarbons (e.g., Hartman 2004).

  • How are the samples to be collected? The analytical method selected, in many cases, will define the collection method (e.g., Summa canister) that should be used and typically the sample preparation that is required to analyze a sample (refer to Section 7.8).
  • Do the regulatory agencies require certification of the laboratory or that specific analytical methods be used? Some state or federal regulatory agencies require that samples be analyzed by specific methods. They can also require the laboratory that is conducting the analysis to be certified under a state or national program. In some cases, this can limit the use of field analytical methods. HDOH does not currently require analysis labs in Hawai`i to be certified for soil vapor analyses; however, the HEER Office recommends that lab certifications and/or other lab quality control measures be carefully considered when selecting an analysis lab. Be aware that work carried out at DoD facilities generally require use of certified laboratories.
  • Are there short turnaround times required for analytical results? Turnaround times will be influenced by shipping requirements, holding times, laboratory backlog, and analytical methods. Depending on the objectives and priorities of the site investigation, field analysis using a mobile laboratory (if available) may be preferable to shipment to a laboratory. Field analysis can provide nearly real time results.
  • Are the analytical methods appropriate for the soil vapor samples? Analytical methods are periodically updated with newer techniques. It is suggested that the user consult with the regulatory agency and a qualified analytical laboratory to identify analytical methods appropriate for the specific site.

As discussed above and in Section 9, it is important to also measure the total petroleum hydrocarbon concentration in soil vapor at petroleum hydrocarbon impacted sites. The total petroleum hydrocarbon measurement should be the full range of detectable hydrocarbons (i.e., C5 to C18), not of a specific product range of carbon numbers. Reporting of TPH as “gasoline range organics” or “diesel range organics” does not apply to indoor air or soil gas. This is because petroleum vapors from diesel can include a significant and even dominant proportion of lighter, aliphatic compounds even those these compounds make up only a small fraction of the fuel itself (refer to HDOH 2012). The higher, relative volatility of these compounds causes these compounds to dominate vapors associated with diesel and other middle distillate fuels.

The currently preferred laboratory method to test for TPH in soil vapors for final decision making purposes is a combination of both TO-15 (Summa canister samples) and TO-17 (sorbent tube samples) (see HDOH 2012, 2012c). Note that Methods TO-14 and TO-15 are similar. Method TO-15 offers additional target analytes over TO-14, however, and has largely replaced the latter. Based on discussions with laboratories, a sum of the individual carbon ranges can be more accurately determined using these methods. In theory, less expensive TO-3 methods can be far less sensitive than TO-15 and TO-17 to TPH. Data from the HDOH study do not indicate an obvious bias of TO-3 data for under reporting of TPH in soil gas samples, however. Alternative methods can be proposed on a site-specific basis.

A variety of issues, including low volatility and poor recovery from Summa canisters, make it problematic to quantify aromatic hydrocarbons greater than C10 and aliphatic hydrocarbons longer than C12 using methods TO-15 or SW8260. Sorbent tubes used in combination with Method TO-17 (or acceptable alternative) are capable of reporting the full range of vapor-phase, hydrocarbon compounds present in a sample, including aliphatics, aromatics and oxygenates. This is important because longer-chain hydrocarbons (C9+) are more toxic than shorter-chain hydrocarbons and their presence can significantly increase the vapor intrusion risk (HDOH 2012, see also HDOH 2016). Documenting the presence or absence of a significant proportion of these compounds in TPH vapors is necessary at the beginning of a site investigation.

The need to continue the collection of sorbent tube data at a site can be reviewed based on the results of the initial samples. It is reasonable to assume that this fraction of TPH, if present, is dominated by C12+ aliphatic compounds (refer to HDOH 2012). If C12 or higher compounds make up less than 10% of the total TPH present in the samples (i.e., sum of C5-C18 compounds) then the concurrent collection of sorbent tube samples can be discontinued. Labeled, laboratory chromatograms should be included in the investigation report to support this conclusion.

Consult with the laboratory to determine the calibration standard used for the TO-17 method. Document that calibration procedure in the final soil vapor or indoor air investigation report.

Detailed TPH carbon range data will be necessary for more site-specific risk evaluation (see Section 7.13.1.2 and HEER Office EHE guidance, HDOH 2016). The laboratory should be consulted to determine the most appropriate sample collection method (e.g., Summa vs sorbent tube) and lab method (e.g., TO-15 vs TO-17).

For vapors associated with diesel and other middle distillate fuels, sorbent tube methods that are able to report aromatic and aliphatic carbon ranges above C10 and C12 are preferred. Some labs may not be set up to report carbon range data using sorbent tube methods, however. In this case a combination of carbon range data (e.g., C5-C8 aliphatics, C9-C12 aliphatics and C9-C10 aromatics) and TPH data (e.g., TPH reported as sum of all compounds greater than C12, assumed to represent C12-C16 aliphatics and aromatics) may be necessary until it can be demonstrated that Summa data are adequate to evaluate TPH in general.

Many laboratories can quantify naphthalene using TO-15. Detection levels are normally adequate for soil vapor samples in comparison with correlative soil gas action levels (72 to 240 µg/m3), but may be too high for indoor air samples (action levels 0.072 to 0.12 µg/m3). Reporting naphthalene under TO-15 in combination with other targeted VOCs can avoid the need to for multiple samples and laboratory methods, especially for soil vapor samples. Check with the laboratory if indoor air sampling is to be carried out and naphthalene is a target compound.

7.13.3 Field Analytical Methods

On-site analysis can be very beneficial for vapor intrusion assessments as real-time data enable detection of preferential vapor migration sources or pathways, allow additional sampling locations to be added (spatially or vertically), allow the identification of spurious or otherwise non-representative data and enable measurement of the leak-test tracer compound to ensure valid soil vapor samples are collected. Simple, portable instruments can provide both qualitative and quantitative data depending upon the compound and the required detection levels. Field screening with hand-held PIDs or FIDs enable rapid identification of vapor migration routes around and into structures; although most field screening instruments are limited to the ppmv range for VOCs, which often do not provide sufficient sensitivity for vapor intrusion investigations. [Note that PIDs are not very sensitive to aliphatic compounds, which dominate petroleum vapors (ASTM 2006f; see also HDOH 2012).

Quantitative oxygen, carbon dioxide, and methane measurements also are possible using hand-held portable meters for concentrations in the percent range. Measurements of these compounds can help determine equilibration in newly installed wells, detect leaks in the sampling system, and also can be used to assess biodegradation of VOCs.

Mobile laboratories equipped with laboratory-grade instruments, including gas chromatographs and mass spectrometers, are capable of fully quantitative results meeting required QA/QC and detection limits as low as 1 ppbv. A field portable GC/MS (e.g. Hapsite by Inficon) is also available and gives quantitative soil vapor and indoor air analysis to levels as low as 1 ppbv.

7.13.4 Quality Control Samples

7.13.4.1 Field Quality Control

Figure-7-35

Figure 7-35: Typical Duplicate Sampling Apparatus (see also Figure 7-27). Left photo: Stainless steel "T" manifold to simultaneously collect primary and duplicate soil vapor samples in 500-ml Summa canisters. Right photo: Laboratory-supplied duplicate sampling apparatus to simultaneously collect primary and duplicate soil vapor samples in sorbent tubes.

Replicates

Field replicates are not routinely required for soil vapor investigations. Replicate samples are collected to provide information on the reproducibility of a sample intended to represent a pre-specified volume of soil or more specifically the vapors held within that soil. Reproducibility is a function of both field and laboratory error. This is relatively straight forward for soil investigations, where a designated Decision Unit (DUs) is subsampled by collection of a single, multi-increment sample (refer to Section 7.6.2.2 and Sections 3 and 5). Replicates are collected to verify that the number of increments collected in the Decision Unit, typically thirty to fifty, adequately capture the contaminant heterogeneity and provide a representative mean of targeted chemicals.

As discussed in Section 7.6.2.2, approaches for the designation of DUs in terms of soil vapors and vapor intrusion are still being studied. At this time the primary purpose of replicate soil vapor samples, if collected, is to evaluate the reproducibility of data for individual sample point locations, rather than for a DU as a whole. More specifically, the replicates can provide some information on the spatial variability of VOC concentrations in soil vapor at the scale of the sample volume collected. Collecting larger samples also helps to ensure that the data are more representative of the targeted area (e.g., six-liter versus one-liter Summa sample). This can be challenging at sites with tight soils, however.

If desired or recommended, field duplicate samples can be collected at a minimum of 10% of the active soil vapor or indoor air samples collected per sampling day per laboratory (if more than one laboratory is used). If less than ten samples are collected during each day or sampling event, a minimum of one duplicate sample is recommended per sampling day or event.

A field duplicate is a second sample collected in the field simultaneously with the primary sample at a single location. The duplicate sample is collected in a separate sample container from the same location and depth as the primary sample (Figure 7-35). The results of the duplicate field sample can be used to calculate a relative percent difference to provide information on consistency and reproducibility of field sampling and lab analysis procedures.

Trip Blanks

A trip blank should be included at a minimum of one trip blank per sampling day or shipment cooler for vapor or indoor air samples collected using sorbent tubes or passive samples. Ensure that the laboratory includes at least one trip blank for each batch of sorbent tubes to be shipped back for analysis.

The trip blanks and media should be the same as the collection devices to be used in the field and prepared at the same time and in the same manner by the laboratory. The trip blank is included with sample collection devices to be used in the field and stored, shipped, processed and analyzed in the same manner as the actual samples. The results of the trip blank sample can be used to evaluate if the storage, shipping and handling procedures are introducing contaminants into the samples, or if the original packing material or the laboratory equipment was potentially contaminated.

Trip blanks are not necessary for Summa canister samples (e.g., an unused canister), since the blank would only indicate if that particular canister had leaked. A minimum, residual vacuum of 3-5 inches of mercury is instead recommended in order to determine if the canister leaks or is otherwise tampered with prior to analysis by the laboratory. Labs also have a rigorous certification process for Summa canisters and flow regulators prior to shipment for sample collection.

Equipment Blanks

An equipment blank should be collected as part of an indoor (or less commonly outdoor) air study when very low VOC action levels are being applied. An equipment blank is collected by passing clean air or nitrogen through the soil vapor probe parts (tubing, tips, sample train) into the sample container at the beginning of the sampling event. The blank is then analyzed with the actual indoor samples to determine if any contaminants are in the equipment. Equipment blanks are not generally necessary for soil vapor samples, since it is less likely that contamination in tubing or other equipment will in itself cause reported levels of VOCs to exceed the comparatively higher soil gas action levels (e.g., 1,000 times higher than indoor air action level for residential soil gas action levels; HDOH 2016).

7.13.4.2 Laboratory Quality Control

The accuracy of an analytical method depends on sample handling and preparation and maintenance of the analytical equipment. Most analytical methods recommended by the USEPA include minimum quality control measures designed to assess the performance of the analytical procedures. Minimum quality control measures should include the calibration of instruments and an assessment of the analytical accuracy and precision (USEPA 2000d, API 2005). Analytical accuracy and precision are typically assessed through the use of method blanks and laboratory control samples (see Section 10). Additional details on quality control measures for analytical methods are included in the method documentation (USEPA 1998b; USEPA 1999b; USEPA 2004e).