The five-step process for investigating PVI applies in the event that a building does not satisfy the screening process and allow elimination of the exposure pathway. The site investigation phase starts with Step 4 - Implementing a concentration-based evaluation using existing data (
Site investigation approach flow chart.
The level of effort and the type of data required depend on site conditions in order to answer key questions such as:
One or more lines of evidence are needed when evaluating complex relationships between groundwater, soil, and air. When more than one line of evidence is collected, the process is referred to as a "multiple-lines-of-evidence" approach. It is up to the investigator to gather, evaluate, and weigh different types of data and information. Different, but not necessarily all, media can be sampled to evaluate multiple lines of evidence.
Soil gas sampling is a common approach used for PVI evaluations because of the influence of biodegradation on PHCs in the subsurface and the prevalence of indoor PHC sources (see Appendix L) that make indoor air data difficult to interpret. If the concentration of COCs in soil gas decreases to below action levels within the vadose zone, then the transport pathway is likely incomplete and additional sampling (for instance, indoor air) unnecessary.
The criteria for determining if the PVI pathway is complete can vary by state, region, and stakeholder. Existing concentration data can be compared to applicable vapor intrusion screening criteria (look-up values) to evaluate whether the pathway can be eliminated. This determination can be made independent of the vertical screening distance method outlined in Chapter 3. Check with the local regulatory agency for applicable concentration-based criteria (also see the ITRC PVI state survey, summarized in Appendix A).
Most PHC sites fall under one of the scenarios depicted in Figure 4-1. These scenarios are described in more detail in this section and can be used in selecting investigation strategies and approaches for the site.
An overview of the relevant investigative approaches is provided in
Scenario 1: Contamination Not in Contact with the Building
At PHC sites with contamination not directly in contact with the building, the initial investigation approach will most likely be soil gas sampling, since soil gas data reflect the processes that occur in the vadose zone (partitioning, sorption, biodegradation), from the contamination source to the overlying building. Alternative approaches may include collection of groundwater, soil, subslab soil gas, or indoor air and outdoor air data.
Scenario 2: Contamination in Contact with the Building
At PHC sites with contamination directly in contact with the building, collection of subslab soil gas samples may not be possible because of soil pore-space saturation. The initial investigation approach will most likely be indoor air or crawl space sampling and outdoor air sampling. In buildings with basements, near-slab soil gas samples may be collected around the perimeter or subslab soil gas samples collected below slab-on-grade garage floors. Alternative approaches may include collection of samples within the slab and flux chamber samples. If sumps are present, alternatives include the collection of sump water samples, sump headspace samples, or flux chamber samples.
Other Scenarios
Focus the Investigation
It may not be necessary to investigate all media shown in Figure 4-1. Focus only on the lines of evidence needed.
The following sections describe investigative approaches and sampling methods for the evaluation of PVI. The order of the investigative approaches described in the sections below does not reflect their priority. Details of the sampling methods presented are included in Appendix G.2 through Appendix G.5.
Groundwater data exist at most sites, typically from monitoring wells, but may only provide limited spatial coverage (both on and off site) and may be from wells screened over large depth intervals (10 feet to 15 feet). For evaluating the PVI pathway, it is best to use the groundwater data that have been collected from the most recent sampling event and that are representative of monitoring wells near the area of concern.
Additional groundwater samples may be collected to further characterize the potential for PVI. For evaluating the VI pathway, it is best if the groundwater samples are collected in a shallow interval across the top of the groundwater and as close to buildings as possible. Discrete sampling methods or small diameter wells are more suitable for VI investigations than conventional monitoring wells with long screens (see Appendix G.2).
If groundwater data indicate no potential for PVI risk, and if there are no sources in the vadose zone, then the pathway can be considered not of concern and further PVI assessment is not needed. If groundwater concentrations indicate LNAPL and the source is not in contact with the building, then further soil or soil gas sampling is recommended.
At many PHC sites, the initial investigation approach will most likely be soil gas sampling, since soil gas data reflect the processes that occur in the vadose zone (partitioning, sorption, biodegradation), from the contamination source to the overlying receptor. Three primary options are available for characterizing soil gas. These three options differ mainly by the sampling location relative to the building under investigation:
Methods for collecting soil gas samples, and additional factors in sample placement are provided in Appendix G.8. General advantages and disadvantages for each type of soil gas sampling are provided in Table G-6.
Vertical soil gas profiles can be acquired by installing a series of nested or clustered exterior or near-slab soil gas points at a range of depths. Such soil gas data may be useful for defining the zone of active biodegradation and demonstrating that the decrease in PHC concentrations with distance from the source is due to biodegradation.
When concentrations of PHCs in soil gas (5 feet bgs or greater) exceed allowable screening values, shallower soil gas samples (< 5 feet bgs) may potentially demonstrate that biodegradation is active and concentrations do not exceed screening levels at these locations. If field instruments or other on-site methods are used, step-out sampling locations and depths may be selected during a single mobilization. Alternatively, additional shallower samples may be collected during the same mobilization and analyzed in an off-site laboratory if results for the deeper samples exceed screening levels.
Indoor air data provide measurements at the point of exposure and represent the sum of influences of sources that contribute contaminants to indoor air. These sources may include ambient outdoor air and indoor sources (such as consumer products, petroleum vapors from cars in an attached garage, or petroleum vapors from home repair and remodeling), as well as the contribution from subsurface sources through VI. Indoor air measurements often complicate data interpretation when the data are collected without careful planning and well-documented execution.
Interpretation of indoor air sampling results for PHCs may be challenging when assessing the VI pathway for two primary reasons:
For these two reasons, indoor air sampling is unlikely to be the initial method used for a PVI investigation.
Conduct a building survey in advance of indoor air sampling to identify potential background sources. Removing the identified background sources (to the extent practical) before the sampling begins may be prudent, but be aware that additional, unidentified background sources may remain. A survey also provides an opportunity to educate occupants on what to expect during the sampling event and inform them of the activities that should be avoided immediately before and during the sampling period. Examples of building surveys can be found in ITRC’s 2007 guidance.
An 8-hour indoor air sampling period is often selected for commercial buildings. A 24-hour sampling interval is usually selected for residential structures. Stainless steel canisters are generally used for sampling intervals from 5 minutes to 24 hours. Alternative sampling devices (such as passive samplers) can be deployed for longer periods to reduce the effects of short term variability. However, PHC results for samples collected over longer periods are susceptible to false positives, potential interferences from occupant activities, and background sources, because hydrocarbons are ubiquitous in consumer products and ambient air.
Samples of indoor air represent the air quality at the time of sampling. Although indoor air is generally well mixed, temporal and seasonal variations occur in indoor air quality. Document conditions at the time of sampling, including heating, ventilation, and air conditioning (HVAC) system operation. Additionally, review the resulting laboratory data for representativeness and usability, according to the data quality parameters specified in the sampling and analysis plan.
Sample Duration
The sample duration for indoor air samples should be selected in an effort to provide the best estimate of the time-integrated average concentration to which an occupant may be exposed.
Indoor air samples may be collected with the HVAC system on or off, depending on the sampling objectives. To evaluate whether vapor intrusion is possible, sample with HVAC turned off and after the building has equilibrated for a few hours. This method represents a worst-case building scenario for VI. If assessing human risk exposure, indoor air samples should be collected under normal conditions.
Concurrent sampling of indoor air, ambient air, and subslab soil gas may provide data that allow a more detailed understanding of site conditions. Collecting multiple lines of evidence is particularly helpful at PHC sites because of the complex nature of the transport and exposure pathway, and because PHCs are ubiquitous in indoor air from background sources.
When indoor air is sampled, concurrent ambient air samples should also be collected. Collect ambient air samples at locations upwind of the building being investigated. Additionally, document information on significant point or nonpoint sources on the day of sampling (such as gasoline stations, automobiles, gasoline-powered engines, fuel and oil storage tanks, and locations that may generate significant petroleum vapors) when selecting ambient sample locations and interpreting the data. The ambient air data can be used as a tool to provide information regarding outdoor influences on indoor air quality (California DTSC 2011).
Air within a crawl space can be collected using indoor air sampling methods. These data may provide an additional line of evidence to evaluate whether vapor intrusion is occurring. A number of states and regions compare results for crawl space air samples to indoor air screening levels, which assume no attenuation between the crawl space and the indoor air. Detection of higher concentrations of PHCs in a crawl space than in indoor air samples collected in basement or upper floor areas may indicate a subsurface source.
Air exchange between the crawl space and ambient air may also vary significantly depending on construction and should be considered before sample collection. Methods for sampling in crawl spaces are described in Appendix G.
ITRC's Vapor Intrusion Pathway: A Practical Guideline (2007) notes that analytical data for soil samples are not ideal for evaluating VI risk because of the uncertainty associated with partitioning from soil to soil gas and the potential loss of VOCs during and after sample collection. To evaluate VI, contaminant concentrations measured in the soil sample must be converted to soil gas concentrations using assumptions about the partitioning of the contaminant into the gas phase. Soil- to-soil gas partitioning equations are readily available, but empirical data show a poor correlation between predicted soil gas values from soil data and actual measured values for PHCs (Golder Associates 2007).
Golder Associates has found that, in the case of PHCs, calculating soil gas values from contaminant concentrations measured in soil samples typically overestimates the actual concentrations in soil gas by orders of magnitude. This calculation overpredicts the risk for PVI (Golder Associates 2007). Several state agencies, however, have soil criteria for the VI pathway (Eklund et al. 2012).
If vapor transport modeling is conducted, then consider analyzing samples for physical properties in the scope of work (see Chapter 5).
The chemicals selected for analysis at a potential PVI site depend on the source and type of PHC contamination (see Appendix E, Types of Petroleum Sites), as well as the objectives of the investigation and the requirements of the agency providing regulatory oversight. The sampling and analysis plan should describe objectives of the investigation, analytical methods to be used, and quality requirements for the data.
A site-specific analyte list typically includes PHCs, but also might include TPH fractions and indicator compounds to assist in identifying and differentiating subsurface sources of volatile chemical contamination (Table 4-1).
|
Source |
Compounds |
|---|---|
|
Gasoline |
Benzene, toluene, ethylbenzene, xylenes, trimethylbenzenes, individual C–4 to C–8 aliphatics (such as hexane, cyclohexane, dimethylpentane, or 2,2,4-trimethylpentane) and appropriate oxygenate additives (such as MTBE and ethanol) |
|
Middle distillate fuels (No. 2 fuel oil, diesel, and kerosene) |
N-nonane, n-decane, n-undecane, n-dodecane, ethylbenzene, xylenes, trimethylbenzene isomers, tetramethylbenzene isomers, and naphthalene |
|
Manufactured gas plant sites |
Benzene, toluene, ethylbenzene, xylenes, indane, indene, naphthalene, and trimethylbenzene |
An assessment of biodegradation in soil gas usually includes the analysis of O2, CO2, and CH4. After oxygen is depleted, methanogenic bacteria convert petroleum hydrocarbons to methane and carbon dioxide. If methane is above 1%, then conditions are anaerobic, and sampling is likely near an LNAPL source. CO2, on the other hand, is typically the complement of oxygen, meaning that the combined sum should be around 21%. If there is an excess of CO2, then anaerobic biodegradation is likely occurring, and methane is being oxidized to CO2 under anaerobic conditions. Additionally, nitrogen may be considered an indicator as to whether there is replenishment of air or an advective flow of soil gas that flushes out the air. If nitrogen is displaced (much less than 79%) then either the bulk soil gas is migrating or the sample was collected under a vacuum.
Appendix G provides a detailed discussion of analytical methods for PVI investigations, and Table G-3 is a summary of analytical methods used for evaluation of petroleum hydrocarbons in the vapor phase.
Many regulatory agencies have requirements or guidance related to UST sites and remediation programs. Some agencies may also require specific analytical methods and the use of certified laboratories. Understanding these applicable regulatory requirements is part of designing a successful investigation.
Some PHC sites may require a detailed analysis of hydrocarbon sources. Forensic chemistry uses hydrocarbon profiling or fingerprinting for this purpose and is available in some commercial laboratories. Methods have been developed to analyze for vapor-phase PHC compounds (paraffins, isoparaffins, aromatics, naphthalenes, and olefins—also known as the PIANO analysis), including for air-phase matrices in VI investigations (Plantz et al. 2008).
The following section describes data quality considerations and factors to consider when evaluating the data.
If the project was planned using the Data Quality Objective (DQO) process (USEPA 2006b) or other standard project planning process, then the quantity and quality of data, including the measurement quality objectives, will have been specified in the sampling and analysis plan. Some common data quality issues are listed in Table 4-2. All of the data should be examined for these types of issues to ensure that data are of adequate quality prior to using the data to evaluate the VI pathway.
Evaluating data to assess the completeness and significance of the PVI pathway typically uses one or all of the following comparisons, as described in detail in ITRC's VI guidance (2007):
Issues to consider when evaluating PVI data include:
Vapor-transport modeling can be used during data evaluation to simulate the fate and transport of contaminant vapors from a subsurface source, through the vadose zone, and potentially into indoor air. Modeling at a potential PVI site can help guide vapor intrusion investigations, identify critical factors affecting transport, and help evaluate whether the aerobic biodegradation interface is likely to exist between the source and building foundation. The use of modeling, as well as a tiered analysis of increasing complexity, is described in greater detail in Chapter 5.
This step reflects the iterative nature of the PVI investigation in determining whether the site has been adequately characterized (ITRC 2007). Other questions to consider include the following:
If the conclusion is that data gaps still exist that prevent a decision on the potential for PVI, refer to Appendix G for additional tools to investigate the PVI pathway (such as building construction and HVAC operating conditions or vapor flux).
Once it has been determined that sufficient data have been collected, the final step in site investigation is the determination on the completeness of the PVI pathway. If the pathway is incomplete, no further evaluation of the PVI pathway is necessary. If the pathway is complete, however, the investigator must assess vapor control approaches as discussed in Chapter 6.