The site investigation phase starts with Step 4 – Implementing a concentration-based evaluation using existing data. If necessary, Step 5 involves conducting a PVI investigation. Step 6 is Data evaluation. Step 7 determines whether additional investigation is
warranted. Finally (Step 8), a conclusion on the completeness of the PVI pathway
must be made.
Figure 1. Site investigation approach flow chart.
Indoor air data is difficult to interpret. Additional details of investigative approaches are presented, including field procedures for sampling soil gas, groundwater, soil, near-slab and subslab soil gas, outdoor (ambient) air, and indoor air. Supplemental tools and other data that can be useful for VI investigations, including the use of tracers, differential pressure measurements, real-time and continuous analyzers,and forensic (“fingerprinting”) analysis, among others, are also covered in the PVI hot topic.
Step 4 – Conduct a Concentration-Based Evaluation Using Existing Data
Existing concentration data can be compared to applicable vapor intrusion screening criteria
(look-up values) to evaluate whether the pathway can be eliminated.
Step 5 – Select and Implement an Applicable Scenario and Investigative Approach
Investigative Scenario: Most PHC sites fall under one of the scenarios depicted in Figure 1.
Scenario 1: Contamination Not in Contact with the Building
The initial investigation approach will most likely be soil gas sampling. 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
The initial investigation approach will most likely be indoor air, outdoor air sampling and near-slab soil gas samples. 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.
Intermittent petroleum odors
The following sections describe investigative methods and sampling methods for evaluating PVI. Details of the sampling methods presented here included in this hot topic.
To evaluate the VI pathway from groundwater, 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. If the groundwater concentrations indicate the presence of NAPL but the source is not in contact with the building, then soil or soil gas sampling is recommended.
Soil Gas Sampling
Soil gas data reflect the processes that occur in the vadose zone (partitioning, sorption, biodegradation).
Three primary options are available for characterizing soil gas:
- Exterior soil gas
- Near-slab soil gas
- Subslab soil gas
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.
When concentrations of PHCs in soil gas (5 feet bgs or greater) exceed allowable screening values, shallower soil gas samples (<5 ft bgs) may potentially demonstrate that biodegradation is active and concentrations do not exceed screening levels at these locations.
Indoor Air Sampling
Indoor air data provide measurements at the point of exposure and represent the sum of influences of sources that contribute contaminants to indoor air. Interpretation of indoor air sampling results for PHCs may be challenging because of (1) frequent exceedance of benzene and other PHCs in ambient air in many urban areas and (2) ubiquitous indoor air sources for benzene and other PHCs. Therefore, indoor air sampling is unlikely to be the initial investigative method.
An 8-hour air sampling period is typically selected for commercial buildings, whereas a 24-hour sampling interval is usually used for residential structures. Stainless steel canisters are used for sampling interval between 5 minutes to 24 hours. Passive samplers can be deployed for longer periods to reduce the effects of short term variability. It is worth noting that PHC results for samples collected over longer periods are susceptible to false positives, due to ubiquitous presence of hydrocarbons in consumer products and ambient air.
To evaluate whether vapor intrusion is possible, sample with HVAC turned off and after the building has equilibrated for a few hours.
Ambient (Outdoor) Air Sampling
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.
Crawl Space Sampling
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. 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.
To evaluate vapor intrusion, 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.
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.
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 1).
Table 1. Indicator 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. If methane is above 1%, then conditions are anaerobic, and sampling is likely near an LNAPL source. CO2 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. 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.
Step 6 – Evaluate Data
The following section describes data quality considerations and factors to consider when evaluating the data.
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.
Table 2. Data quality issues to consider
|Data quality issue||Factors to keep in mind|
|False negatives||Consider that false negatives can be due to losses in sampling equipment, leaks, and other factors. Ask yourself:
o Did containers fill to the target pressure?
o Was the leak detection compound applied and measured correctly?
o Were canister pressures recorded, for both start time and end times?
Issues to consider when evaluating PVI data include:
- Screening levels for PHCs in groundwater generally do not account for biodegradation. As a result, use of groundwater models or attenuation factors generally overpredicts the potential for PVI.
- In general, soil data overpredict the potential for PVI for vadose zone PHC sources where there is adequate separation between the source and the receptor, and biodegradation is occurring.
- PHC concentrations in soil gas also decrease with distance from the source because of biodegradation if sufficient O2 is present. Vertical profiles of the soil gas PHCs between the source and receptor can be an effective approach to document the effect of biodegradation on soil gas concentrations and whether the VI pathway is complete.
- Since slabs are known to “breathe” in both directions because of fluctuations in barometric pressure and building factors, measured PHCs in samples might also come from the indoor air of the overlying structure and from leaking drains and other utilities.
- Indoor air data are typically compared to indoor air screening levels and to ambient (outdoor) air levels. The evaluation may become more complex if there is contribution of PHCs from indoor air background sources, outdoor air sources, occupant activities, building materials, or other nonsubsurface sources. Appendix L includes information and references for background concentrations of PHCs.
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.
Step 7 – Determine Whether Additional Investigation is Warranted
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:
- Have the site contaminants been properly delineated?
- Has the potential for PVI at all possibly affected buildings been assessed?
- Are there sufficient data to reach a vapor control decision at the site?
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).
Step 8 – Decide Whether PVI Pathway is Complete
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.