Decommis­sioning & De­contamination (# 84)

A Conversation with Ian Hers

 The possibility of harmful vapours from volatile contaminants in groundwater or soil being released and entering through cracks or other openings in the below-grade portions of buildings is an important consideration when new development is planned at contaminated or brownfield sites.

A growing body of science indicates that even relatively low-level exposure to some contaminants may pose chronic health risks. We have seen that vapour intrusion (VI) is a highly site-specific process, but the potential for VI exists for a range of sites. Agencies in many countries have developed regulations and guidance for assessing VI pathways.


Much attention is focused on trichloroethylene, a commonly used degreaser in industrial operations, as well as other common contaminants that are found in petrol, other fuels, and solvents.

Dr. Ian Hers, based in our Vancouver office, is Golder’s worldwide practice leader for VI services. Drawing from international best practices and new modelling developments, Golder can help site owners analyse and manage risks from VI. 


What do you frequently observe in your work?  

Relatively small releases of trichloroethylene and other chlorinated hydrocarbons such as percholoroethylene – still used at many dry cleaners – can contaminate large volumes of groundwater. Dissolved plumes often migrate off-site, resulting in the potential for VI in residential and commercial buildings above these plumes. For compounds that do not readily naturally degrade, VI can be a relatively pervasive problem, and elevated indoor air concentrations have been measured in some cases even when groundwater concentrations are relatively low. There have been occurrences of VI for both residential and commercial buildings, and for a range of foundations, such as basements, crawl spaces, or at-grade slabs.  

How does soil vapour characterisation enter the picture?  

An assessment process is essential to get a good understanding of site characteristics, the source of the vapours that may enter a building, and migration patterns. Better predictive models give us the ability to assess whether and how a contaminant vapour plume can be expected to migrate, and the potential indoor air concentrations.

Recent work has also gone into quantifying the occurrence and rate of natural aerobic biodegradation of petroleum hydrocarbon vapours that, for many sites, will render vapours to negligible levels before they reach the building.   

What role does building design play?  

Building foundation design can be influential, as can the heating ventilation and air conditioning (HVAC) system and how buildings "breathe." By that I mean a combination of HVAC operation and weather conditions may result in cyclic changes in pressure gradients that complicate assessments. We are finding that near- and within-building VI characterisation can be complex when there is shallow contamination.

How can mitigation, if required, be done efficiently? 

At existing buildings with VI issues, sub-slab venting and HVAC modifications are possible solutions. For new construction at brownfield sites, there are a range of options, including installation of low permeability barriers such as geomembrane liners and venting systems to prevent vapour infiltration. Using aerated floors as part of a more efficient below-building venting system is another development. There are also sustainable solutions with solar or wind-power energy that, under certain conditions, provide for effective mitigation.

VI services are designed to help property owners understand the potential issues and liabilities, and can help demonstrate to businesses and land developers involved in due diligence and property transfers whether or not there is a problem and mitigation is warranted.

Ian Hers, P.Eng,, holds a PhD in Civil Engineering and is a Principal at Golder and a senior environmental engineer.