A New High Purge Volume Sampling Method for Sub-slab Soil Gas Monitoring for the Evaluation of the Vapor Intrusion Pathway
Paul Nicholson, Geosyntec Consultants, Inc., Guelph, ON, Canada          
Todd McAlary, Geosyntec Consultants, Inc., Guelph, ON, Canada
David Bertrand, Geosyntec Consultants, Inc., Guelph, ON, Canada
Hester Groenevelt, Geosyntec Consultants, Inc., Guelph, ON, Canada

Active Sub-Slab Depressurization System for Mitigation of Chlorinated Solvent Vapor Intrusion at a Retail Complex
Michael J. Murphy, MACTEC Engineering & Consulting, Inc., Wakefield, MA         
Phillip J. Muller, MACTEC Engineering & Consulting, Inc., Wakefield, MA
Charles Collet, MACTEC Engineering & Consulting, Inc., Wakefield, MA
Thomas Hanlon, MACTEC Engineering & Consulting, Inc., Wakefield, MA
Ryan Belcher, MACTEC Engineering & Consulting, Inc., Portland, ME

A New Quantitative Passive Sampler for Vapor Intrusion Pathway Evaluation: The PDMS Membrane Sampler
Todd N. Creamer, Geosyntec Consultants, Inc., Acton, MA          
Hester Groenevelt, Geosyntec Consultants, Inc.
Tadeusz Górecki, University of Waterloo, Ontario, Canada
Suresh Seethapathy, University of Waterloo, Ontario, Canada

Indoor Vapor Intrusion at Manufactured Gas Plant (MGP) Sites Lessons learned:  Four Case Studies
Atul M. Salhotra, RAM Group of Gannett Fleming, Inc., Houston, TX

A New High Purge Volume Sampling Method for Sub-slab Soil Gas Monitoring for the Evaluation of the Vapor Intrusion Pathway

Paul Nicholson, Geosyntec Consultants, Inc. 130 Research Lane, Guelph, ON, N1G 5G3, Tel: 519-822-2230, Email: pnicholson@geosyntec.com
Todd McAlary, Geosyntec Consultants, Inc. 130 Research Lane, Guelph, ON, N1G 5G3, Tel: 519-822-2230, Email: tmcalary@geosyntec.com
David Bertrand, Geosyntec Consultants, Inc. 130 Research Lane, Guelph, ON, N1G 5G3, Tel: 519-822-2230, Email: dbertrand@geosyntec.com
Hester Groenevelt, Geosyntec Consultants, Inc. 130 Research Lane, Guelph, ON, N1G 5G3, Tel: 519-822-2230, Email: hgroenevelt@geosyntec.com

Many regulatory guidance documents call for the collection of sub-slab soil gas samples as a line of evidence for assessing soil vapor intrusion to indoor air. Empirical data collected to date shows considerable spatial and temporal variations in the soil gas concentrations.  This raises questions about the number, frequency and spacing of samples and indeed whether these data are valuable.  A new High Purge Volume (HPV) sampling method will be discussed in this presentation that can be used to assess volume-integrated sub-slab organic vapor concentrations, reduce uncertainty regarding concentrations between and beyond probe locations, and provide data that can be used for design of mitigation systems, as warranted.

Conventional sub-slab sample methods recommend purging a small volume of soil gas (< 1 Liter) prior to filling a Summa canister (1 to 6 Liters) for submitting to the laboratory for analysis. The collection of such small sample volumes characterizes short-term and small-scale variations in target analyte concentration.  Chronic health risks are typically assessed assuming 30 years of exposure, during which hundreds of thousands of liters of soil gas may enter a typical building.   Longer-duration and/or larger volume samples can provide a more representative integrated average concentration that is less sensitive to small-scale and localized spatial variability, which would be more appropriate for evaluating human health risk. 

HPV sampling consists of extracting soil gas at a flow rate of 10 to 100 cubic feet per minute (cfm) over several hours.  The effluent vapor concentrations, flow rate and applied vacuum are monitored over the duration of the test.  This data can be used to assess the spatial distribution of sub-slab concentrations over large areas.  This is a rapid screening technique that is particularly useful in evaluating the sub-slab soil gas concentrations in large buildings where the collection of closely-spaced sub-slab sample would be prohibitively expensive. This technique also reduces the risk of failing to identify the presence of vapors which may exist in locations between sample collection probes.  Case studies including helium tracer tests, sub-surface permeability tests, flow and vacuum measurements, sample collection for laboratory analysis and comparative data will be discussed.

Active Sub-Slab Depressurization System for Mitigation of Chlorinated Solvent Vapor Intrusion at a Retail Complex

Michael J. Murphy, MACTEC Engineering & Consulting, Inc., 107 Audubon Road, Wakefield, MA 01880, Tel: 781-245-6606, Fax: 781-246-5060, Email:mjmurphy@mactec.com
Phillip J. Muller, MACTEC Engineering & Consulting, Inc., 107 Audubon Road, Wakefield, MA 01880, Tel: 781-245-6606, Fax: 781-246-5060, Email:pjmuller@mactec.com
Charles Collet, MACTEC Engineering & Consulting, Inc., 107 Audubon Road, Wakefield, MA 01880, Tel: 781-245-6606, Fax: 781-246-5060, Email:cacollet@mactec.com
Thomas Hanlon,  MACTEC Engineering & Consulting, Inc., 107 Audubon Road, Wakefield, MA 01880, Tel: 781-245-6606, Fax: 781-246-5060, Email:trhanlon@mactec.com
Ryan Belcher, MACTEC Engineering & Consulting, Inc., 511 Congress Street, Portland, ME, 04112-7050, Tel: 207-775-5401, Fax: 207-772-4762, Email:rtbelcher@mactec.com

Vapor intrusion impacts on indoor air quality in a large New England retail complex had been identified based on measured indoor air concentrations of trichloroethene and tetrachloroethene above regulatory requirements.  Groundwater and soil vapor investigations had previously identified the subsurface impact zone within the footprint of the retail complex.  An active sub-slab depressurization system was planned as a component of the overall site remedial strategy.  Pre-design activities included a soil boring program and vacuum testing.  The system design included four separate depressurization units, corresponding to the four separate retail spaces.  The main depressurization system includes a regenerative blower, knockout tank, system controls and alarms, and vapor treatment and discharge.  Three smaller systems utilize radon type fans attached to vapor extraction wells.  Installation of the systems was accomplished with vapor extraction wells and associated piping networks installed with minimized intrusive activity within the building and minimal impact to functionality of the building interior.  The system is electrically powered and system controls include automated vacuum monitoring and automated telephone notification of lost vacuum.   Initial sampling and analysis was conducted to demonstrate indoor air quality improvement and achievement of regulatory concentration targets.  A regular air monitoring program was implemented to meet regulatory requirements and document the continued effectiveness of the mitigation system. 

A New Quantitative Passive Sampler for Vapor Intrusion Pathway Evaluation: The PDMS Membrane Sampler

Todd N. Creamer, Geosyntec Consultants, Inc. 289 Great Rd, Ste. 105, Acton, MA, 01720, Tel: 978-263-9588, Email: tcreamer@geosyntec.com
Todd McAlary, Geosyntec Consultants, Inc., Email: tmcalary@geosyntec.com
Hester Groenevelt, Geosyntec Consultants, Inc., Email: hgroenevelt@geosyntec.com
Tadeusz Górecki, University of Waterloo, Ontario, Canada, Email: tgorecki@uwaterloo.ca
Suresh Seethapathy, University of Waterloo, Ontario, Canada, Email: sseethap@scimail.uwaterloo.ca

Effective evaluation of the soil vapor intrusion to indoor air pathway faces complications from topics such as 1) very low indoor air target reporting limits; 2) perceptions of high spatial and temporal data variability; and 3) the high cost of investigations.  A new technology, polydimethylsiloxane (PDMS) membrane samplers, will be discussed in this paper that can be used for quantitative passive monitoring of organic vapor concentrations, and is capable of achieving reliably sub-part per billion reporting limits for many commonly-targeted volatile analytes. 

Calculations of indoor air screening levels or excess lifetime risk caused by chronic exposure to volatile organic compounds are often based on concentrations measured in indoor air or sub-slab soil gas.  Because conventional whole-gas sampling typically is limited to 24 hours (or much less for soil gas) and one to six liters sample volume, short-term and small-scale variations in target analyte concentration can introduce data bias and the perception of variability when the long-term or large-scale effects of these variations may not be significant.  Such short term variations in time and in space can be minimized by integrating sample collection over larger scales.  Longer-duration and/or larger spatially-integrated samples can reduce cost while improving the measurements used to calculate excess risk. 

PDMS sampler deployment and retrieval can produce significant advantages in the field because they are simple to install and are much smaller than Summa canisters.  Their preparation, handling and analysis by TO-15 are all also significantly less expensive.  In some instances, conventional whole-gas sampling may be supplemented or replaced by passive sampling.  When used strategically for monitoring of vapor intrusion, passive sampling can significantly reduce both cost and data variability.  Case studies and comparative data will be discussed.

Indoor Vapor Intrusion at Manufactured Gas Plant (MGP) Sites Lessons learned:  Four Case Studies

Atul M. Salhotra, RAM Group of Gannett Fleming, Inc., 5433 Westheimer Rd, Houston, TX 77056, Tel: 713-784-5151, Fax: 713-784-6105, Email: asalhotra@ramgp.com 

The vapor intrusion pathway was evaluated at four former MGP sites in different stages of characterization and remediation. The evaluation included (i) collection and analysis of soil gas, soil, and groundwater samples; (ii) building survey, and (iii) comparison of analytical data to risk based standards. None of the measured concentrations at any of the sites exceeded conservative risk based standards.

Site 1: This site is surrounded by commercial properties. A one storey brick and cinder block building with a concrete slab, is located onsite and leased by the Post Office. Investigation was conducted as a proactive measure to determine whether the postal workers might be exposed to vapors from former MGP impacts.

Site 2: The currently vacant site is located in a mixed residential commercial area. The site has been characterized, and source removal activities are imminent. The investigation was conducted to evaluate potential impacts to three adjacent residential receptors.    

Site 3: This site has undergone considerable remediation; however, residual soil and groundwater concentrations exist off-site, below a bank building beyond which are residences. The investigation evaluated potential impacts to the receptors in the bank and the residences. Permanent soil vapor probes were installed and data collected quarterly for one year to evaluate seasonal variability.

Site 4:  A single story warehouse building without a basement at the former above-grade gasholder site, is currently occupied by the Salvation Army. This investigation focused on the collection of soil gas and soil geotechnical samples to evaluate soil gas inhalation risk to the Salvation Army workers.  

The paper will discuss the site stratigraphy, site hydrogeology, data collected and the lessons learned at each site. A comparative evaluation of the vapor inhalation risk at each site as well as the cost of investigation will be presented.

Acknowledgements are due to project managers at Ameren for providing the opportunity to proactively perform these studies and several members of RAM Group and Gannet Fleming Inc. who helped in data collection and data evaluation.  

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