Abstract Surface expressions occur when hot oil, water and vapor associated with steam injection in heavy oil reservoirs flows uncontrollably to surface, creating safety and environmental hazards. A new method for early-time identification of these events and monitoring of remediation efforts is proposed. Transient high gamma ray (THG) is a high-resolution, high-amplitude measurement that samples a large volume of rock when vapor containing radon condenses at a cooled well and gamma ray increases by a factor of 10 to 200. The effect is transient because gamma ray returns to normal when this transport process is reversed, as wellbore temperature equilibrates. Absence of this high-amplitude, high-resolution signal through shallow air sands above a heavy oil reservoir is confirmation that vapor breakthrough has not occurred. For wells located in an area where surface expressions have occurred, the method identified contaminated sands and detected flow-behind-casing in an uncemented well. Gamma ray transects in an area of surface expressions identified hot spots, caused by plumes of vapor that condense just below the surface. In at-risk areas, observation wells can provide assurance that injected steam is contained. When surface expressions occur, the method can be used to optimize and confirm the effectiveness of mitigation efforts.