How Does Vapour Intrusion Happen?
Vapour intrusion is a well known phenomenon affecting the indoor air quality in residential and commercial environments. In the past, industrial and commercial facilities disposed of, or spilled chemicals into soils within their property boundaries without recognizing the potential issues that could be caused. Often these chemicals were unregulated at the time. Although some of the substances evaporated and contributed to local air pollution, many substances, due to their intrinsic properties, migrated into the soil and encountered ground water leading to aqueous plumes of contamination. Over time the contaminants traveled with the groundwater to nearby inhabited areas or commercial/industrial facilities beyond the original property boundaries. These chemicals silently affect the water and through various mechanisms, the chemicals can enter indoor environments leading to indoor air quality issues in homes and businesses. In this way, depending on the type and quantity of chemical used, the nature of the soil, geological formations and water table, chemical concerns can travel far from the source of the problem and persist long after the offending parties cease ownership of the original business/location where the contamination originated.
Dry Cleaning and PERC
The table below highlights the indoor air quality criteria for sites impacted by sub-surface contamination for a set of chlorinated volatile organic compounds that we refer to when assessing vapour intrusion issues. Note that many more possible contaminants exist.
|Health Based Indoor Air Criteriaa|
|Residential µg/m3||Commercial µg/m3|
The last entry in the table is tetrachloroethylene, also known as perchloroethylene, PCE or PERC. PERC is still used in dry cleaning today. Spills can seep into soils and subsequently form a plume. PERC itself and many of the breakdown compounds are also a cause for concern. The IARC and NIOSH, respectively have designated PERC as a “probable carcinogen” and a “potential occupational carcinogen”.
Dealing With the Problem
Today, we are often in a position to be able to detect and mitigate the effects of vapour intrusion. Investigative techniques can follow a prescriptive formula to determine chemical contaminants based on the historical activities in the building and the surrounding areas (i.e., Phase I environmental assessment). Further investigation in the form of a Phase II environmental assessment is completed if preliminary investigations warrant. Lastly, if contamination is determined some remedies include:
1.) Remediation of the site, in an attempt to remove the bulk of the source material by excavating the contaminated material on site and pumping the ground water.
2.) Installation of a soil vapor extraction system. This requires a shed with an extraction pump and multiple pipes near the building to reduce pressures near the foundation. This provides a preferential pathway for the contaminants which are captured by a vapour recovery system that is regenerated regularly. In addition, regular monitoring and maintenance of the soil vapour extraction system is required.
3.) Installation of a heat recovery ventilator (HRV) indoors. This unit will exchange outside fresh air with indoor air continuously thus diluting the levels of contamination observed, as well as all other contaminants that may be present in the building. HRVs keep fresh air circulating into the building while minimizing the increased cost of energy usage by recovering some of the energy in the exhaust air and transferring it to the fresh air coming into the building.
4.) Installation of sub-slab extraction devices. In this case, one or more pipes may be installed to extract vapours from the zone below the basement slab. The pipe can be vented at roof level or directly outside at the basement level. The system can be operated either passively or actively using a fan. By creating a low partial vacuum in the zone below the slab and establishing a preferential pathway for migration of the contaminants, intrusion of contaminants into the building envelope is significantly reduced. This can ensure adequate indoor air quality respecting the target contaminants and will maintain airborne concentrations below a level where health problems are likely to occur from chronic exposure.
One or a combination of the above described solutions can be used. Periodic air testing is recommended to verify that the mitigation system is operating effectively.
As of 2018 Industry Canada listed 3547 dry cleaning and laundry service businesses across the country. The abundance of dry cleaning facilities increases the likelihood that spills can or have occurred in areas where a local population can be adversely affected. Businesses that purchase former dry cleaners or buildings nearby can inherit a problem that becomes both a health concern and a financial liability. Airzone has assisted clients in assessing vapour intrusion issues for over 15 years. We have provided indoor air and soil vapour measurement services to some of the largest vapour intrusion projects in North America. We can perform both air sampling on-site and analysis at our CALA accredited lab.