Explosives detonated in a surface construction blast produce large quantities of gases that expand rapidly to fracture the rock. The gases contain toxic fumes such as carbon monoxide. If explosives are detonated in the rock below groundwater level, gas bubbles containing toxic fumes can form in the groundwater and stay trapped in the groundwater for many days. We discuss the mechanism that forms and traps gas bubbles. Groundwater flow can transport gas bubbles laterally and allow toxic fumes to vent into the atmosphere a significant distlmce from the blast. This may cause a health risk to workers or inhabitants of buildings near a blast. The gas trapping and gas migration mechanisms are supported by measurements from a constmction case where toxic fumes migrated laterally below ground. We discuss the construction conditions and subsurface conditions that are likely to allow the formation of gas bubbles and the subsurface migration of the gas bubbles that contain toxic fumes. We recommend some simple changes to blasting and construction procedures that can reduce the risk of subsurface migration of toxic fumes.
Explosives detonated in a surface construction blast produce smoke, which is usually seen immediately after the blast. Smoke is generally composed of water vapor, solid products of combustion, and other gases including toxic gases, such as carbon monoxide and nitrogen oxides. If the conditions are favorable, toxic fumes can dissipate rapidly after a surface construction blast. However, disappearance of the visible components of blast smoke is not sufficient to indicate that the toxic fumes have dissipated, because some toxic fumes such as carbon monoxide are colorless and odorless. During detonation, gas bubbles containing toxic fumes can also form below ground. Migrating gas bubbles in the groundwater can transport toxic fumes. We believe this was a previously unrecognized hazard. Toxic levels can develop in the atmosphere at the blast site or some distance from the blast site when the gas bubbles burst and the gas vents from below ground. Deep and narrow excavations that have poor air circulation will be most susceptible to build up of toxic fumes.