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Subsurface Vapor Intrusion Modeling and Assessments

Geosyntec Consultants maintains an international reputation as one of the preeminent firms for the assessment and management of subsurface vapor intrusion to indoor air. We earned our reputation by providing decades of innovation in applied research and practical project experience to the challenges presented by vapor intrusion at sites all over the world.

Clients seek us out for customized solutions founded in an understanding of physical, chemical, and biological processes. We design our solutions to add value and address risk by targeting corrective actions where they are warranted. Additionally, our professionals published the most widely cited and used vapor intrusion model (Johnson and Ettinger, 1991), and we have performed pioneering modeling of aerobic degradation of hydrocarbon vapors with the 3-D model of Abreau and Johnson (2005).

Geosyntec engineers and scientists are co-authors or contributing editors of guidance and regulatory documents that outline and advocate the latest advancements and research related to the remediation of vapor intrusion around the world. These include standards, information supplements, and handbooks issued by the U.S. Environmental Protection Agency, the Electric Power Research Institute, the Interstate Technology and Regulatory Council, and ASTM International, formerly known as the American Society for Testing and Materials. The efforts associated with this public involvement represent significant steps forward in the evolution of vapor intrusion assessment and control technology.

Our speciality areas include soil gas sampling, indoor air quality monitoring, mathematical modeling, and ventilation assessment.

Soil Gas Sampling

Geosyntec's soil gas surveys include the use of pioneering approaches in mobile gas chromatography, pneumatic testing, high volume purge sub-slab soil gas sampling (McAlary et al., 2010), tracer testing, sampling in low permeability materials (McAlary et al., 2009), and multi-phase partitioning assessments. Our practitioners developed sampling protocols for the Electric Power Research Institute (EPRI, 2005) and the United Kingdom Energy Institute (in press) and have conducted extensive applied research and development of passive quantitative samplers for use in soil gas, indoor, and outdoor air sampling.

Indoor Air Quality (IAQ) Monitoring

Our professionals have assessed indoor air quality as part of vapor intrusion investigations since 1992. We provide technical direction for one of the largest vapor intrusion investigations in the world, which required us to develop a new method of sampling and laboratory analysis to achieve reporting limits as low as three parts-per-trillion. We have established long-term relationships with the world's leading laboratories and research institutes and have extensive experience with both traditional and emerging methods for sampling volatile organic compounds (VOCs), semi-volatile compounds, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and other, less common compounds.

Mathematical Modeling

Geosyntec offers clients world-class experience in the application of vapor transport models. Our professionals co-developed the most widely used mathematical model for vapor intrusion to indoor air and use modeling to assist with targeted investigation design (Johnson and Ettinger, 1991) . We study large-scale, long-term vapor transport modeling and develop models with non-equilibrium phase partitioning for transient analysis of mass removal during soil vapor extraction. Our professionals established several forensic methods for discriminating indoor and outdoor sources of common VOCs from subsurface sources of VOCs to indoor air.

Ventilation Assessment

Geosyntec practitioners apply heating, ventilating, and air-conditioning (HVAC) system manipulation to assess points of soil gas entry. This allows us to establish target locations for sealants or other barriers, assess the sources of VOCs to indoor air, and implement means to mitigate the VI pathway. In some cases, minor modifications to an HVAC design or operations are sufficient to eliminate the vapor intrusion pathway while reducing or eliminating otherwise costly disruptions of manufacturing or retail operations.

Advancing the State of the Practice

Geosyntec continues to advance the state of the practice for subsurface vapor intrusion modeling and assessments through applied research and development. In recent years, the firm was retained by the U.S. Department of Defense to compare four quantitative passive samplers to the conventional Summa canister method of sampling and analysis.

Our practitioners also have performed applied research with the University of Waterloo to develop another quantitative passive sampler, the Waterloo Membrane Sampler™, and commercialize it for use in vapor intrusion assessments. Quantitative passive samplers provide long-term average concentrations, which are more relevant for assessing human health risks.