Development of More Cost-Effective Methods for Long-Term Monitoring of Soil Vapor Intrusion to Indoor Air Using Quantitative Passive Diffusive-Adsorptive Sampling Techniques

Dr. Todd McAlary | Geosyntec Consultants, Inc.


Objectives of the Demonstration

The objectives of this work were to provide data to demonstrate and validate passive diffusive samplers for assessing soil vapor, indoor air, and outdoor air concentrations of volatile organic compounds (VOCs) at sites with potential human health risks attributable to subsurface vapor intrusion to indoor air. The project compared the accuracy and precision of passive diffusive samplers to conventional methods, identified capabilities and limitations of various sampler types, and provided scientific data to support regulatory review and acceptance where appropriate.

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Technology Description

Quantitative passive sampling can be defined as the collection of vapors by diffusion or permeation in response to concentration gradients (rather than pressure gradients, as are used by Summa canisters and pumped Automated Thermal Desorption [ATD] tubes) at a known and controlled uptake rate, such that the time-weighted average (TWA) concentration can be calculated from the mass of each analyte collected over a given period of time. The passive sampler acts as a sink for the analytes, which establishes the concentration gradients so no external power source is required; hence, the sampling is termed “passive.” There are several different commercially available quantitative passive samplers with different sizes, shapes, materials of construction, sorbents, and protocols. The project team was selected to include individuals highly experienced with passive samplers (in general) and each of the five samplers tested (in particular). The passive samplers tested included: (1) SKC Ultra and Ultra II; (2) Radiello®; (3) Waterloo Membrane Sampler; (4) ATD tubes; and (5) 3M Organic Vapor Monitor 3500.

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Demonstration Results

The project included laboratory testing under controlled conditions for 10 VOCs, including chlorinated compounds (ethenes, ethanes, and methanes) and petroleum hydrocarbons (aromatics and aliphatics), spanning a range of properties and including some compounds expected to pose challenges associated with retention and recovery by the sorbents (naphthalene, methyl ethyl ketone). Laboratory tests were performed under conditions of different temperature (17 to 30°C), relative humidity (30 to 90 percent relative humidity [%RH]), sampler face velocity (0.014 to 0.41 meters per second), concentration (1 to 100 parts per billion by volume), and exposure duration (1 to 7 days). These conditions were selected to challenge the samplers across a range of conditions likely to be encountered in indoor and outdoor air field sampling programs. High concentration laboratory tests were also conducted at 1, 10, and 100 parts per million by volume to evaluate concentrations of interest for soil vapor monitoring using the same 10 VOCs and constant test conditions (90%RH, 30 minutes [min] exposure, 22°C). Inter-laboratory testing was also performed to assess the variance in the analytical results attributable to the differences between several laboratories used in this study.

The project also included field testing of indoor air, outdoor air, sub-slab vapor, and deeper soil vapor at several Department of Defense (DoD) facilities. Indoor and outdoor air samples were collected over durations of 3 to 7 days and Summa canisters were collected over the same durations as an active sample for comparison. Subslab and soil vapor samples were collected with durations ranging from 10 min to 12 days, at depths of about 0.5 (immediately below floor slabs), 4, and 12 feet. Passive samplers were employed with uptake rates ranging from about 0.05 to almost 100 milliliters per minute and analysis by both thermal desorption and solvent extraction. Mathematical modeling was performed to provide theoretical insight into the potential behavior of passive samplers in the subsurface, and to help select those with uptake rates that would minimize the “starvation effect.” The starvation effect refers to a negative bias that occurs when a passive sampler with a high uptake rate removes VOC vapors from the surroundings faster than they are replenished, essentially scrubbing the local atmosphere of VOCs. A flow-through cell apparatus was also tested as an option for sampling existing sub-surface probes that are too small to accommodate a passive sampler.

The results of this demonstration show that all of the passive samplers provided data that met the accuracy, precision, ease of use, and cost success criteria under some or most conditions. Compared to conventional active sampling methods, the passive sampler’s precision was generally comparable; ease of use was generally comparable or better; and cost was comparable or better (improving with larger numbers of samples). Accuracy met the success criterion in most cases, and exceptions were attributable to one or more of five possible causes: (a) poor retention of the analytes on the sampler; (b) poor recovery of the analytes from the sorbent; (c) starvation effects; (d) uncertainty in uptake rate for the specific combination of sampler/compound/conditions; or (e) blank contamination. These biases can be prevented in most cases through careful selection of the sampler, as well as sorbent and exposure duration for specific target analytes. Positive biases were less common than negative biases, and attributed either to blank contamination or to uncertainty in the uptake rates. Most of the passive samplers provided highly reproducible results, which is encouraging because the accuracy can be verified using inter-method duplicate samples (e.g., a limited number of conventional samples collected beside selected passive samples for the same duration as a quality assurance/quality control check), and the field-calibrated uptake rates will be appropriate for other passive samples of the same type collected under similar conditions. This project also demonstrated for the first time the reliable use of passive samplers for quantifying soil vapor concentrations.

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Implementation Issues

Passive samplers offer some potential benefits compared to conventional sampling methods and may reduce costs and the implications of temporal variability, which would reduce liabilities for DoD. The overall cost of monitoring with passive samplers is comparable to or lower than monitoring with conventional methods because of the simplicity of the sampling protocols (less time required for sample deployment and collection) and reduced shipping charges. Passive samplers are generally easy to use and minimal training is required for most applications. A modest increase in effort is needed to select the appropriate sampler, sorbent, and exposure duration for the site-specific chemicals of concern and desired reporting limits compared to Summa canisters and U.S. Environmental Protection Agency (USEPA) Method TO-15; however, the level of effort is not much different than the design process for active ATD tube sampling for analysis by USEPA Method TO-17. As the number of samples in a given program increases, the initial cost of sampling design becomes a smaller fraction of the overall total cost, and the passive samplers gain a significant cost advantage. For best results, the selection of the appropriate sampler, sorbent, and exposure duration for a particular set of target chemicals and reporting limits should be reviewed carefully by an experienced professional and the sampling program should include trip-blank samples. Inter-method duplicate samples can also be included to provide field-calibrated uptake rates as an additional accuracy check where needed.

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Points of Contact

Principal Investigator

Dr. Todd McAlary

Geosyntec Consultants, Inc.

Phone: 416-637-8747

Fax: 647-775-1501

Program Manager

Environmental Restoration