Aqueous and surface expression of subsurface GHGs: Subsurface mass transfer effects

Releases of greenhouse gases (GHGs) from the subsurface can result in atmospheric emissions and the degradation of water quality. These effects require attention in today's changing climate to properly quantify emissions, reduce risk and inform sound policy decisions. Flowing subsurface GHGs, including methane and carbon dioxide, present a risk in the form of two environmental expressions: i) to the atmosphere (surface expression) and ii) to shallow groundwater (aqueous expression). Results based on high-resolution observations in an analog experimental system and analytical modelling show that these expressions depend on the rate of gas flow and the velocity of the flowing groundwater. In deeper systems, the emission of flowing subsurface GHGs could be significantly limited by dissolution into groundwater, adversely impacting water resources without surficial evidence of an underlying issue. This work shows that mass transfer in the subsurface must be considered to quantify, monitor and mitigate risks of leaking subsurface GHGs. (C) 2019 Elsevier Ltd. All rights reserved.