Investigation Methods and Analysis Toolbox
Abs: This article describes various sampling and analysis methods available for vapor intrusion investigations. This information will help with select the techniques that will best meet the data objectives.
Important Considerations in VI Investigation
Vapors and VI are an unfamiliar territory for many practitioners in this field (regulators, stakeholders, consultants, subcontractors). Practitioners commonly make errors with soil vapor programs and soil vapor data in three general areas: units, screening or target levels, and project goals/objectives.
Planning tools, such as the USEPA’s DQO process, can be used to help ensure that data of the right quality, type, and amount are collected.
The USEPA’s DQO process includes the following seven steps:
- State the problem that necessitated the study and define the overall objectives of the study.
- List specific questions that need to be addressed in order to meet the study objectives.
- Identify what types of samples, data, and other information are needed.
- Define study boundaries (spatial and temporal), including the lateral and vertical extent of the contamination in all media, as well as multiple exposure areas that may be at the site.
- Develop “if, then” decisions that will be made based on results of the investigation.
- Specify tolerable errors in the decisions to be made, as well as the measurement quality objectives for analytical data.
- Optimize the sampling design. Consider your options for before heading to the field.
Specifying analytical data quality is covered under measurement quality objectives outlined in the DQO process and may include the following:
- Identify COCs and screening levels.
- Choose sampling and analytical methods with appropriate reporting limits.
- Complete presampling building survey (interior sampling).
- Establish appropriate sampling conditions, number of samples, and duration of sampling.
- Identify and collect quality control samples (field blanks, duplicates).
In general, groundwater data overpredicts PVI risk because groundwater screening values are usually developed without considering biodegradation in the vadose zone. PVI pathway could be evaluated with preexisting groundwater data, interpolation of nearby data and obtained new groundwater data.
Preexisting groundwater data should be obtained from wells screened across the water table at the time of sampling. If groundwater data immediately upgradient (and closest to the contamination source) from the structure are not available, surrounding data points can be used to construct contaminant isoconcentration maps. For collecting groundwater data suitable for VI assessment, screen intervals, screen length, well installation, well development, well purging, well sampling should be properly designed and conducted.
Groundwater samples should be collected as close, horizontally and vertically, to the structures as possible because concentrations are not always uniform within a plume because of heterogeneities in source areas and in the subsurface media. After an initial VI investigation has been completed, long-term groundwater monitoring to reevaluate the VI pathway may be appropriate where groundwater concentrations exceeding screening levels are close to, but not currently within, the applicable distance criterion to a potential receptor.
Figure 1.Low-flow sampling using a peristaltic pump.
Alternative groundwater sampling methods that may have application to VI investigations include passive samplers and low-flow purging and sampling. Passive diffusive bag samplers (PDBS) currently may be the most common tool for sampling VOCs. PDBS should be deployed just below the water level in a well for a minimum of two weeks to equilibrate with the well water. Significant water table fluctuations during that period will affect the appropriate depth intervals for the samplers. In any event, the depth to water in the well should be measured when the PDBS are installed and removed, and the position of the samplers relative to the water level should be clearly described in the report presenting the PDBS data.
If evaluating the VI pathway is the only sampling objective, use two modifications to the typical low-flow purging and sampling:
- Set the pump intake level as close to the water table as possible without significant risk that the water level will drop and expose the pump intake. For wells in formations with average or high permeability, about 1.5 to 2 feet below the static water level should be an adequate intake location.
- The purging objective is to flush two volumes of groundwater through the sampling array (such as tubing and pump). While measuring water quality indicator parameters is preferred (but not necessary), drawdown should be measured and should not be excessive.
Soil data are not typically used for evaluating the VI pathway because of the uncertainty associated with using partitioning equations and the potential loss of VOCs during sample collection In order to perform VI risk calculations using soil data, contaminant concentrations in soil must be converted to soil gas concentrations using assumptions about the partitioning of the contaminant into the gas phase. In the case of PHCs, calculated soil gas values from soil data often overestimate actual soil gas concentrations.
When sampling soil for VOCs, the soil samples should be collected using procedures specifically designed to minimize volatilization losses. Existing soil data should be used as part of the lines-of-evidence approach. In general, soil matrix data are not recommended as a stand-alone screening tool for a VI investigation.
Site-specific soil properties such as bulk density, grain density, total porosity, moisture content, and fraction organic carbon can be measured from soil samples and the results used to replace default input parameters when models are used. Air permeability of the vadose zone can be determined from either in situ measurements or laboratory measurements. In situ measurements test a larger portion of the subsurface than a laboratory measurement of a small core sample and are the preferred method.
Soil headspace concentrations could be used as criteria for defining sufficient thickness of non source (clean) soil for screening out sites for VI investigations. Headspace concentrations are affected by the size of the container, amount of sample, the size of the available headspace, temperature, development time, hold time, and analysis time. In addition, the permeability of the container and any contamination from the container should also be considered.
Figure 2.Screening core samples with Photoionization Detector.
Measurement of Indoor Air (Interior)
Indoor air samples are normally collected after other environmental samples (for example, groundwater or soil gas) indicate the need to conduct an internal building-specific assessment. The analyte list should minimally focus on compounds identified in subsurface samples at concentrations above screening levels, their possible breakdown products, and potential compounds that may be useful as marker compounds.
The temporal variability depends on the duration of the sample (residential settings: typically over a 24-hour sampling period; commercial and industrial settings: normally over a 8-hour sampling period; commercial receptors with work days longer than 8 hours: multiple samples).
Time-integrated sampling is typically used when conducting indoor air exposure assessments associated with VI investigations. A time-integrated sample represents a sample taken at a known sampling rate over a fixed period of time. Two methods are commonly used: collection of samples in an evacuated canister and collection of samples on adsorbent media.
Collection of Samples in an Evacuated Canister
The sampling canister is a passivated or specially-lined inert container (such as a Summa or Silco canister) that is sent to the field under vacuum and is certified clean and leak free. The canister fills with air at a fixed flow rate over a preset period of time with use of a flow controller that is calibrated and set in the laboratory. The newest hardware allows for collection periods of up to seven days.
The main advantages of canister sample collection are the capability to analyze multiple samples from the same canister and the ease of deployment and retrieval. Canister methods are most commonly used in North America.
Figure 3.Stainless steel canisters.
Collection of Samples on Adsorbents
Sample collection on an absorbent is an option for VOCs and a requirement for SVOCs and can be done actively or passively. Active sampling requires drawing air at a calibrated flow rate through a tube containing adsorbent media over a specified time period. The flow rate and sampling volume used are determined based on the adsorbent used, the COCs, and the amount (mass) of adsorbent contained in the tube. The samples are taken to the laboratory for thermal or chemical desorption and subsequent analysis. The sample pump flow rate should be verified and documented both at the start and finish of sample collection using a calibrated flowmeter.
Passive sampling of indoor air is similar to active sampling methods in which vapor constituents are collected onto adsorbents, but the collection of constituents is based on the diffusion of the compound onto the adsorbent and does not rely on pumps.
Any sampling approach should take into account the different exposure scenarios (such as day care or medical facilities) that exist within the building and any sensitive populations that may be exposed to the contaminated vapors.
Ambient (Outdoor) Air Sampling
When conducting indoor air sampling as part of a VI study, outdoor ambient air samples should be collected concurrently. Ambient air samples are collected to characterize site-specific outdoor air background conditions. Outdoor air samples should be collected from a representative location, preferably upwind and away from wind obstructions such as trees and buildings. The intake should be at about 3 to 5 feet off the ground (at the approximate midpoint of the ground story level of the building) and about 5 to 15 feet away from the building. Ambient sampling should begin at least one hour, and preferably two hours, before indoor air monitoring begins and continue until at least thirty minutes before indoor monitoring is complete.
Supplemental Tools and Data Useful for VI Investigations
Emission Flux Chamber Method
Flux chambers are enclosures that are placed directly on the surface (ground or floor) for a period of time, and the resulting contaminant concentration in the enclosure is measured. Flux chambers are best-suited for situations where measurement from bare soils is desired, and can also be used as a qualitative tool to locate surface fluxes of VOC contamination and entry points into structures.
Determination of Slab-Specific Attenuation Factor Using Tracers
Measurement of a conservative tracer inside the structure and in the subslab soil gas can allow a site-specific attenuation factor to be calculated. The calculated attenuation factor can then be used to estimate the indoor air concentration of other COCs by multiplying the measured subslab soil gas concentration by the attenuation factor for the tracer (or “marker compound”).
Determination of Room Ventilation Rate Using Tracers
The indoor air concentration is inversely proportional to the room ventilation rate: a two-fold increase in ventilation rate decreases the indoor air concentration by two-fold. The default ventilation rates used by the USEPA and many other agencies are conservative: room exchange rates of once every 1 to 4 hours for residences and once every hour for commercial buildings. ASTM Method E 741 describes techniques for measuring ventilation rates using gaseous tracers such as helium or sulfur hexafluoride.
Differential Pressure Measurements
Measurement of the pressure gradient between the structure and outdoors can assist in interpreting measured indoor concentrations of contaminants. A correlation between indoor air concentration and relative pressure can provide information on the contaminant source.
Real-Time and Continuous Analyzers
Real-time analyzers can be used to collect multiple, less expensive data that can be used to locate problem structures, vapor migration routes into structures, and VOC sources inside the structures, as well as provide the functionality to collect samples at varying depths below ground surface.
Forensic Data Collection and Analysis
Forensic approaches attempt to determine the source of any detected VOCs through a detailed study of the nature of contamination, with a focus on lines of evidence to potential sources. Forensic approaches have been used to determine whether the source of subslab contaminants were from the overlying structure or from the vadose zone.
Figure 4.Real-time soil gas analysis on site using a portable GC
A variety of weather conditions can influence soil gas or indoor air concentrations. For soil gas, the importance of these variables is greater the closer the samples are to the surface and is unlikely to be important at depths greater than 3 to 5 feet below the surface or the structure foundation. Indoor air may be more susceptible to weather conditions, therefore collection of meteorological data can be helpful to assessing the VI risk.
In some cases, geologic layers can form partial or complete barriers to upward vapor transport toward overlying buildings. It may be possible to identify the presence of such geologic barriers using pneumatic testing, analogous to a groundwater pumping test, in which one well is used for extraction and other wells are used for monitoring the vacuum response.
Manipulating Pressure Differentials
One possible method for distinguishing subsurface VI from background sources is to collect indoor air samples with and without manipulating the pressure differential from the subsurface to indoor air. This method can be accomplished by pressurizing the building or depressuring the region beneath the floor slab.