Quantification of Gas Fluxes through Frozen Sand and Snow Cover using a Multi-Tracer Approach

English Abstract:

Seasonal freezing of the vadose zone affects approximately 50% of the exposed land in the Northern Hemisphere, while 30 % has a seasonal snow cover. The occurrence and thickness of frozen soil and snow cover are directly responsible for water, energy and gas fluxes from the surface to aquifers and vice-versa. Fluxes of CO₂ and other trace gases such as CH₄ and N₂O have been shown to occur during winter and to contribute to the annual budget, but they are often not measured or considered. Microbial processes, temperature and soil permeability are involved in controlling these gas fluxes, which need to be better understood and quantified. A multi-tracer approach is used to identify gas transport mechanisms and quantify gas fluxes. The experimental site is located on a sand deposit (esker), southwest of Montreal (Canada). Radon and CO₂ dynamics are currently monitored in the first meters of soil and in the snowpack throughout the 2015 winter and spring seasons. Soil water content, frost depth, snow cover and air pressure in soil and snow are also measured. Air permeability in these porous media is determined. Radon, a non-reactive gas that is produced homogeneously in the soil, is used to identify transport processes. Biological processes controlling CO₂ production can then be discriminated. In addition to these naturally produced gases, SF₆ is injected at the soil/snow interface as a tracer to evaluate the possibility for gases to seep to the atmosphere or to the unsaturated zone during winter. Transport mechanisms of gases through frozen and thawing soil as well as through heterogeneous snow cover will be discussed, in particular the role of barometric pumping as opposed to diffusion.