May 27, 2021

« All News

Geosyntec Personnel Authored Six Articles for Groundwater Monitoring & Remediation Focus Issue

Robert Ettinger, Elham Shirazi, Ph.D.  (California); Paul Nicholson, P.Eng., Darius Mali, P.Eng., Todd McAlary, Ph.D., P.Eng., P.G., (Ontario); and Chase Holton, Ph.D., P.E. (Colorado) made significant contributions to the Groundwater Monitoring & Remediation Focus Issue on Vapor Intrusion released in May 2021.

Robert Ettinger is a Senior Principal Environmental Scientist based in California with more than 20 years of professional experience focused on soil vapor extraction system design, vapor emission estimation, and subsurface contaminant vapor migration to indoor air.

Elham Shirazi is a Senior Staff Professional based in California with a focus on modeling approaches to address the vapor intrusion pathways. She has extensive experience in environmental engineering and wastewater treatment.

Paul Nicholson is a Senior Engineer based in Ontario with more than 17 years of experience in environmental consulting. His practice focuses on assessing and mitigating human health risks associated with sub-surface vapor intrusion, and he helped develop the High-Volume Sampling method and associated modeling to enhance the characterization and mitigation of the vapor intrusion pathway.

Darius Mali is a Project Engineer with more than 10 years of experience in environmental consulting, including numerous groundwater sampling styles, soil vapor sampling, high-volume sampling, and remedial design.

Todd McAlary is a Senior Principal Engineer with more than 30 years of international consulting experience focused on the evaluation of contaminant fate and transport in soil and groundwater. He specializes in assessing and mitigating the migration of volatile organic compound vapor from the sub-surface environment into buildings and in the assessment of human health risks associated with inhalation exposure.

Chase Holton is a Senior Engineer based in Colorado with experience in contaminant fate and transport, including evaluation and mitigation of the vapor intrusion pathway. He has additional experience in site characterization, environmental research and consulting, and soil and groundwater remediation.

Groundwater Monitoring & Remediation (GWMR) is a resource for researchers and practitioners in the field. It is a quarterly journal that offers the best in application oriented, peer-reviewed papers together with insightful articles from the practitioner's perspective. Each issue features papers containing cutting-edge information on treatment technology, columns by industry experts, news briefs, and equipment news. GWMR plays a unique role in advancing the practice of the groundwater monitoring and remediation field by providing forward-thinking research with practical solutions.


Improving Risk-Based Screening at Vapor Intrusion Sites in California
Authors: Matthew Lahvis, Robert Ettinger
The attenuation factor (AF) of 0.03 recommended by the U.S. Environmental Protection Agency (USEPA) is increasingly being used by regulatory agencies for the development of subsurface vapor screening levels for vapor intrusion (VI). There are concerns, however, over the database used to derive the AF and the AF's applicability to building types and geographies not included in USEPA database. To derive a more technically defensible AF for subsurface vapor screening in California, a database consisting of 8415 paired indoor and subsurface vapor samples collected from 485 buildings at 36 sites in California was compiled. Filtering was applied to remove data of suspect quality that were potentially affected by background (non‐VI) sources. Filtering reduced the size of the database to 788 indoor air and subsurface vapor pairs, 80% of which were trichloroethylene (TCE) measurements. An AF of 0.0008 was derived from only TCE vapor data, based on the ability of the AF to reliably identify buildings with indoor air concentrations above screening levels in 95% of cases where subsurface vapor screening levels were exceeded. The AF derived from this study demonstrated limited sensitivity to the variables typically considered important in VI characterization, which was partially attributed to relatively weak correlation of indoor air and subsurface vapor concentration data. The results of this study can be used to improve VI screening in California and other states and help focus limited resources on sites posing the greatest potential risk.

Simplified Approach for Calculating Building-Specific Attenuation Factors and Vapor Intrusion Mitigation System Flux-Based Radius of Influence
Authors: Paul Nicholson, Darius Mali, Todd McAlary
This article describes a simplified method to calculate a building‐specific subslab to indoor air attenuation factor using data collected during pressure‐field extension testing similar to industry standards for radon mitigation. It also describes a simplified method to calculate the radius of influence for a conventional suction point using a mass flux‐balance model. The analysis is based on three simple measurements: (1) the extraction flow rate, (2) cross‐slab applied vacuum at a radial distance of 3 feet, and (3) cross‐slab applied vacuum at a radial distance of 10 feet. The intent is to provide a practitioner with a rapid and useful screening‐level assessment of whether the benefits of reduced mitigation system costs warrant an investment in a more detailed mathematical analysis of the flow and vacuum data. In addition, this may also help a practitioner to make real‐time decisions regarding placement of communication test points during pressure‐field extension testing.

Modeling Fate and Transport of Volatile Organic Compounds (VOCs) Inside Sewer Systems
Authors:  Mohammadyousef Roghani, Ying Li, Nader Rezaei, Ariel Robinson, Elham Shirazi, Kelly G. Pennell
Hazardous waste site investigations have shown that volatile organic compounds (VOCs) can be transported via sewer pipes and migrate into indoor spaces. Despite field data confirming the presence of this exposure pathway, there is lack of context-based numerical models that provide guidance to characterize and predict VOCs concentration in sewer gas at vapor intrusion sites. Particularly, this poses a challenge when assessing and mitigating risks associated with these exposure pathways. Therefore, a numerical model has been developed to simulate the concentration of VOCs in sewer gas in different stages throughout the sewer lines. The developed model considers various input parameters, including temperature, sewer liquid depth, groundwater depth, and sewer construction characteristics to incorporate local and operational conditions. The model's output is verified using field data from a sewer system constructed near a Superfund site. Moreover, a sensitivity analysis was conducted to evaluate the model's response to variation of the external input parameters. To the best of our knowledge, this study is the first attempt to model VOCs concentration in sewer gas, particularly to address vapor intrusion. The developed model can be used as a numerical tool to support the development of sewer assessment guidelines, risk assessment studies, and mitigation strategies.

Subslab Depressurization Versus Subslab Ventilation: Insights from Recent Research
Author: Todd McAlary
Many vapor intrusion (VI) mitigation systems involve some form of gas extraction from below the concrete floor slab of a building to create a static vacuum below the slab that meets or exceeds a value specified in a guidance document or standard. This also results in some degree of ventilation below the floor slab, which reduces vapor concentrations and achieves some level of mass removal of target chemicals. The relative contribution of vacuum and ventilation to protecting building occupants is generally not quantified. Recent research completed under ESTCP Project ER‐201322 yields some new insights using new lines of evidence. The results warrant changes in the guidance documents and standards of practice to promote and enable VI mitigation system designs that vary according to the transmissivity of the material below the floor slab and the rate of vertical leakance of indoor air across the floor slab and incorporate additional lines of evidence including the mass removal rate.

Observation of Conditions Preceding Peak Indoor Air Volatile Organic Compound Concentrations in Vapor Intrusion Studies
Authors: Chris Lutes, Chase Holton, Brian Schumacher, John Zimmerman, Andrew Kondash, Robert Truesdale
Temporal and spatial variability of indoor air volatile organic compound (VOC) concentrations can complicate vapor intrusion (VI) assessment and decision‐making. Indicators and tracers (I&T) of VI, such as differential temperature, differential pressure, and indoor radon concentration, are low‐cost lines of evidence to support sampling scheduling and interpretation of indoor air VOC sampling results. This study compares peak indoor air chlorinated VOC concentrations and I&T conditions before and during those peak events at five VI sites. The sites differ geographically and in their VI conceptual site models (CSM). Relative to site‐specific baseline values, the results show that cold or falling outdoor temperatures, rising cross slab differential pressures, and increasing indoor radon concentrations can predict peak VOC concentrations. However, cold outdoor air temperature was not useful at one site where elevated shallow soil temperature was a better predictor. Correlations of peak VOC concentrations to elevated or rising barometric pressure and low wind speed were also observed with some exceptions. This study shows how the independent variables that control or predict peak indoor air VOC concentrations are specific to building types, climates, and VI CSMs. More I&T measurements at VI sites are needed to identify scenario‐specific baseline and peak related I&T conditions to improve decision‐making.

Groundwater Monitoring & Remediation Focus Issue: Advancements in Vapor Intrusion Risk Assessment
Authors: Matthew Lahvis, Chase Holton, Henry Schuver
This brief editorial introduces and summarizes the Focus Issue.

Watch the Videos

More Information

About the Groundwater Monitoring & Remediation Focus Issue:
About Groundwater Monitoring & Remediation:
For consultation regarding vapor intrusion, contact Todd McAlary at This email address is being protected from spambots. You need JavaScript enabled to view it. or Chase Holton at This email address is being protected from spambots. You need JavaScript enabled to view it..