A Close Call for Residents as Blasting Operation Generates Poisonous Gas

The alarms from the residential carbon monoxide detectors in three residences in a Kingston, Ontario suburb went off on the evening of March 24, 2006. They persisted long enough to prompt the residents to vacate their homes and contact the Kingston Fire Department (KFD). Upon investigating, the KFD determined that there was no fire to consider as a source of carbon monoxide gas, and contacted the Ontario Ministry of the Environment (MOE) for assistance. The carbon monoxide level, measured in the evacuated residences, was in excess of 100 parts per million (NIOSH exposure limit is 50ppm).

Carbon monoxide is a tasteless, odourless and toxic gas. It is a product of incomplete combustion most commonly produced by conventional combustion devices such as furnaces, wood stoves, water heaters, barbeques and internal combustion engines. Nevertheless, the KFD ruled out any faulty combustion equipment in the homes. The City of Kingston Public Works Department conducted tests on the air in the nearby sewers. The MOE Air Quality specialists also performed air sampling/gas chromatographic-mass spectrometric analysis. Neither result revealed the presence of carbon monoxide.

The investigation then focused on a subdivision that was being developed immediately to the east of the affected residences. Blasting activities to facilitate the installation of underground services were being conducted at the development property. It was determined that approximately 5,000 kilograms of commercial Ammonium Nitrate Fuel Oil (ANFO) explosives had been detonated over a four day period leading up the incident. The carbon monoxide alarms started within approximately 18 hours of the cessation of blasting.

Investigations completed by me, in collaboration with MOE Air Quality scientific staff, attributed the carbon monoxide gas in the homes to the use of commercial explosives for excavation in fractured limestone bedrock. It was presumed that low water table conditions at the time of blasting allowed for subsurface migration of the gas via rock fractures, resulting in contamination of the air in the homes.

Consultants for the subdivision development described the stratigraphy of the property as comprising silts and clays overlying fractured limestone bedrock. Test pit logs indicated that bedrock was encountered at depths ranging from 0.5 to 2.7 m below grade. The ground water table was below the surface of the bedrock layer at the time of this incident, and the immediate topmost layer of the bedrock was dry. The overburden was comprised of silt and clay, which effectively "capped" the bedrock layer, providing for an effective seal of the dry limestone layer.

MOE Air Quality scientific staff collaborated with professional geoscientists from the MOE Regional Hydrogeology section to develop a working model of the process. The detonation of ANFO generates carbon monoxide gas. It was theorized that the dry bedrock layer became charged with carbon monoxide. The gas then began to migrate horizontally through the fractures in the bedrock in the dry zone, and then entered the nearby homes via sump holes and any cracks or openings in the basements. ANFO was considered to be the source by the process of elimination and the coincidental timing of the alarms. All other combustion processes were ruled out. The homes were repeatedly closed up, with attendant rise in carbon monoxide level, despite the shutdown of all combustion sources in the homes. The homes were then ventilated. The elevated carbon monoxide level persisted for six days before residents were allowed back into their homes.

Several journal articles describing carbon monoxide movement through the ground arising from the use of explosives have been published. In fact, ANFO detonation has been implicated in carbon monoxide production. Specific documents reviewed as part of my investigation included a paper entitled "Carbon Monoxide Poisoning and Death After the Use of Explosives in a Sewer Construction Project" dated July 14, 1999. It is the result of a case study from the National Institute of Occupational Safety and Health (NIOSH). This paper describes an incident that occurred in 1997 in which a detonation of approximately 120 kg (265 lbs) of explosives, specifically ANFO, approximately 18 m from of a trench excavation for a sewer line, produced elevated levels of carbon monoxide gas within the trench. The carbon monoxide level found in the trench was sufficient enough to injure three workers, one fatally. As a result of this case study, a "Hazard ID" from NIOSH and the Center for Disease Control was issued in March 1998.

Another document reviewed was a paper from the National Research Council Research Press web site. This document is entitled "Carbon Monoxide Poisoning Associated With Blasting Operations Close To Underground Enclosed Spaces. Part 1,carbon monoxide Production and Migration Mechanisms," written by Richard Martel et al, and published on June 16, 2004. This document determined that ANFO-type explosives used for blasting operations can generate significant amounts of carbon monoxide gas that can migrate through fractured bedrock and possibly enter enclosed spaces (sewage systems, manholes, basements of houses, etc.).

My review of the available literature on this subject indicated that approximately 10 to 24 L of carbon monoxide could be generated for each kilogram of ANFO explosive detonated. The research also showed that carbon monoxide gas could remain in the subsurface anywhere from seven days to up to one month, depending on site-specific conditions.

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Copyright 2006, Association of Professional Geoscientists of Ontario (APGO)