Deoxygenated gas poses a serious and life-threatening hazard (confined space hypoxia) for engineeringprojects in London, particularly those in the Upnor Formation of the Lambeth Group. We review its causes andpostulate that it is induced by dewatering or positive pressure tunnelling bringing air into contact with thesesediments, causing oxidation. Subsequent resaturation results in pockets of deoxygenated gas. Drawdown of theLower Aquifer during the industrial growth of London brought air into contact with Lambeth Group sediments on aregional scale. Its recharge has trapped and compressed deoxygenated gas beneath overlying impermeable clays.Historically, the mineral glauconite has been blamed for these hazards, but it lacks potency as a reducing agent andremains unaltered in many oxidised Lambeth Group sediments. Pyrite and organic carbon are both plausible but amore potent agent is green rust, which readily and rapidly oxidises on contact with air, and which is only briefly
observed in fresh core samples. This research identifies two key ground engineering hazards: the misidentification ofrisk by reliance on observations of glauconite; and the likelihood of encountering pressurised deoxygenated gaswithin Lambeth Group sediments in underground projects following short- or long-term changes in ground waterlevel.