Preferential flow of gas in soils

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Gas move in soils because soils are porous media with connected pores. This gas movement allows billions of organisms such as plants, bacteria, and fungus to breathe and decompose products (e.g. hydrocarbons). These activities generate gases that must be eliminated from the soil toward the atmosphere to avoid toxicity. Gases, as for water, do not homogeneously move in soils. Gases use pathways through only a portion of the soil, called 'preferential flow pathways', which favour faster movement of gases than it would be predicted with models if the gases would be moving through the bulk soil volume. Preferential flow is caused by soil heterogeneities of all sizes. PF affects the growth of plants and micro-organisms, chemical-reactions, efficiency of soil decontamination systems and gas capturing systems in landfills, etc. Very little is known on PF of gases in soils. The main objective of this research is to understand and quantify PF of gas in soils under several conditions (e.g. frozen, wet, with macropores). Laboratory experiments will be conducted on repacked soil columns and on large monoliths in which soil heterogeneity is as observed in the field. The monoliths will be scanned (CT scanner as in hospitals) to describe soil porosity in three dimensions and correlate the porosity structure with PF of gas. This project is an important step in improving knowledge on gas flux in soil. It will help in developing more appropriate models, soil management and decontamination, improving data interpretation relative to plant, microbes, oxydo-reduction reactions, and efficiency of engineering processes in soils.