A Practical Approach to Distinguishing Vapor Intrusion Indoor Air Impacts from Background
Adam J. Last, Corporate Environmental Advisors, West Boyston, MA

Indoor Air as a Source of VOC Contamination in Shallow Soils Below Buildings
Philip C. de Blanc, Groundwater Services, Inc., Houston, TX

Overview of Two Large-Scale Residential Sub-Slab Depressurization Systems Installation Programs
Lucas A. Hellerich, Metcalf & Eddy, Inc., Wallingford, CT
Erzsébet M. Pόcsi, Metcalf & Eddy, Inc., Wallingford, CT

A Vadose Zone Soil Gas Sampler – Its Design, Implementation and Performance

Tai T. Wong, O’Connor Associates Environmental Inc., Suite 200, 318 – 11th Avenue SE, Calgary, Alberta, Canada T2G 0Y2, Tel: 403-294-4200, Fax: 403-294-4240, Email: tai-wong@oconnor-associates.com
John G. Agar, O’Connor Associates Environmental Inc., Suite 200, 318 – 11th Avenue SE, Calgary, Alberta, Canada T2G 0Y2, Tel: 403-294-4200, Fax: 403-294-4240, Email: john-agar@oconnor-associates.com

Human health risks resulting from the inhalation of vapors from volatile organic chemicals (VOCs) in contaminated soils and groundwater are sometimes the most critical risks that must be addressed during the environmental risk management and remediation of contaminated sites. Potential indoor air quality impacts can be assessed by collecting air samples indoors and submitting them for laboratory chemical analyses. However, indoor air quality is often affected by VOC vapors released indoors by cigarette smoke, building materials and household solvents and thus may not be a reliable indicator of the impact caused by subsurface contamination. Many U.S. and Canadian environmental regulatory agencies have accepted an indirect method of assessment. The method is comprised of soil gas sampling near the basement or ground floor slab of a building and transport modeling to estimate soil gas flow rates and VOC flux into a building. The VOC flux concentration is then used to evaluate the potential human exposure to soil or groundwater derived VOCs and estimate the associated human health risks. The soil gas transport model most commonly used is the Johnson and Ettinger model which is an axisymmetric analytical model. Since 2001 O’Connor Associates has been using a soil gas sampler (SGS) and sampling procedures specifically designed for collecting representative soil gas samples in the vadose zone adjacent to a building basement or a ground floor slab. Using the SGS and a vacuum canister, a constant volume of representative soil gas can be collected at a fixed location for every sampling event. In order to more accurately simulate soil gas transport, a 3-D finite element soil gas transport code has been adapted to study the influence of such factors as soil type, degree of saturation, sampler volume, sampling rate, proximity to the building structure and atmospheric pressure variations. These results are then used to evaluate the design requirements and performance of the SGS. This paper presents comparative analytical results for samples collected from indoor air, sub-slab backfill and SGSs along with predicted sub-slab and indoor VOC concentrations.

A Practical Approach to Distinguishing Vapor Intrusion Indoor Air Impacts from Background

Adam J. Last, M.S., P.E., LSP, Senior Project Manager, Corporate Environmental Advisors, 127 Hartwell Street, West Boylston, MA 01583, Tel: 508-835-8822 ext. 260, Fax: 508-835-8812, Email: alast@cea-inc.com

Evaluating the origins of indoor air impacts at a contaminated site requires a well thought out qualitative and quantitative evaluation.  When the potential for vapor intrusion (i.e., migration of volatile organic compounds from contaminated soil, groundwater or soil gas into indoor air) has been identified the following three consecutive steps are typically taken: (1) field screening of soil vapor from probes, (2) laboratory analysis of soil vapor from probes, and (3) laboratory analysis of indoor air samples.  Based on site-specific information steps may be combined, skipped or added.  The purpose of this session is to introduce vapor intrusion assessment methods and to present assessment methods that may be used to distinguish between contaminants in indoor air that are attributable to vapor intrusion and those that are attributable to background sources.

The presence of numerous background sources of volatile organic compounds (VOCs) in residential, commercial and industrial buildings and human health risks posed by low VOC concentrations makes assessing and remediating the source of contaminants in indoor air a complex problem.  Through a combination of assessment and data evaluation techniques one can help determine if the contaminants detected in indoor air are attributable to the release being assessed.

The presentation shall include:

• Introduction
• Assessing Vapor Intrusion – Why, When and How?
• Sampling Indoor Air – Why, When and How?
• Where are the contaminants coming from?
• Common background sources of VOCs
• Site Specific, Federal and State Background
• Using tracer gases
• Seasonal Fluctuations
• Comparing indoor air data to soil vapor data
• Establishing ratios: indoor air data to soil vapor data
• Interpreting ratios
• Case Study
• Distinguishing vapor intrusion indoor air impacts from background

Indoor Air As A Source of VOC Contamination in Shallow Soils Below Buildings

Thomas E. McHugh, Ph.D., Groundwater Services, Inc., 2211 Norfolk, Suite 1000, Houston, Texas, 77098-4044, Tel: (713) 522- 6300, Fax: (713) 522-8010, Email: temchugh@gsi-net.com
Phillip C. de Blanc, Ph.D., Groundwater Services, Inc., 2211 Norfolk, Suite 1000, Houston, Texas, 77098-4044, Tel: (713) 522- 6300, Fax: (713) 522-8010, Email: pcdeblanc@gsi-net.com
Roger J. Pokluda, Groundwater Services, Inc., 2211 Norfolk, Suite 1000, Houston, Texas, 77098-4044, Tel: (713) 522- 6300, Fax: (713) 522-8010, Email: rjpokluda@gsi-net.com

Both USEPA and many state guidance documents recommend sub-slab sampling as a key component of site investigations to determine if vapor migration from underlying soil is a completed exposure pathway (USEPA, 2002; WIDHFS, 2003; San Diego County, 2004; PADEP, 2004). If VOCs are detected in the sub-slab, then migration from the subsurface is assumed to be occurring and further evaluation is required to determine the extent of the impact. This guidance is predicated on the assumption that VOCs detected in sub-slab samples have originated from deeper within the subsurface. However, detection of VOCs in sub-slab samples is not sufficient to conclude that VOCs are migrating from the subsurface towards a building. VOCs detected in sub-slab samples can originate from indoor sources, migrating through the slab by diffusion or advection. Commonly used conceptual models of vapor intrusion include VOC migration from the subsurface into buildings but do not consider the potential for VOC migration from buildings into the subsurface (USEPA, 2002, Johnson and Ettinger, 1991, Parker 2003).

However, the advective and diffusive forces that result in the migration of VOCs from the subsurface into buildings are equally likely to result in the migration of VOCs from buildings into the subsurface under conditions under suitable pressure or concentration gradients. In this paper we present: i) simple analytical modeling indicating that indoor sources of VOCs may cause sub-slab impacts through advection across the building foundation, ii) field analyses from a site where indoor sources rather than subsurface contamination was the source of VOCs detected in sub-slab samples, and iii) recommendations for field investigation methods to allow the discrimination of subsurface versus indoor sources of sub-slab VOCs.

Overview of Two Large-Scale Residential Sub-Slab Depressurization System Installation Programs

Lucas A. Hellerich, Ph.D., P.E., Metcalf & Eddy, Inc., 860 North Main Street Extension, Wallingford, CT 06492, Tel: 203-269-7310, Fax: 203-269-8788, Email: lucas.hellerich@m-e.com
Erzsébet M. Pόcsi, Metcalf & Eddy, Inc., 860 North Main Street Extension, Wallingford, CT 06492, Tel: 203-269-7310, Fax: 203-269-8788, Email: erzsebet.pocsi@m-e.com
William A. Baker, IV, P.E., Metcalf & Eddy, Inc., 860 North Main Street Extension, Wallingford, CT 06492, Tel: 203-269-7310, Fax: 203-269-8788, Email: will.baker@m-e.com
Ronald Curran, Bureau of Waste Management, State of Connecticut Department of Environmental Protection, 79 Elm Street, Hartford, CT 06106-5127, Tel: 860-424-3705, Fax: 860-424-4057, Email: ronald.curran@po.state.ct.us
Graham Stevens, Bureau of Waste Management, State of Connecticut Department of Environmental Protection, 79 Elm Street, Hartford, CT 06106-5127, Tel: 860-424-3705, Fax: 860-424-4057, Email: graham.stevens@po.state.ct.us

Sub-slab depressurization (SSD) systems, commonly used to mitigate radon, create a vacuum beneath a building to prevent soil gas from entering the building as a result of pressure gradients that naturally exist between the building and the sub-slab region; the extracted soil gas is then vented directly to the atmosphere.  This paper describes two large-scale residential SSD system installation case studies.  The SSD systems were designed and installed to mitigate intrusion of soil gas, which contained low levels of volatile organic compounds, into (1) 100+ individual houses and (2) several buildings in a multi-structure condominium complex.     

The SSD installation methodology consisted of the following components: stakeholder involvement, site assessment, feasibility study, pilot testing/design, installation, performance testing, and operations & maintenance.  Public meetings were held and homeowner feedback was elicited to achieve an end product that not only mitigated vapor intrusion, but also was acceptable to the homeowner.  The system design process incorporated the results of site-specific assessments and field pilot testing.  These systems were installed in a design-build fashion using a variety of construction techniques.  Following installation, the SSD systems were performance tested to ensure that the resulting suction field encompassed the entire sub-slab area. 

Numerous examples of the SSD system installations are presented.  SSD system designs/components and construction techniques, issues, and challenges specific to the two case studies are discussed.  System performance data and lessons learned from the SSD installations also are presented.  In addition, a comparison of the operation of the engineered SSD systems to several radon mitigation systems previously installed using typical radon industry techniques is conducted to reveal some interesting results.

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