The Role of Fractional Dynamic Wettability as a Primary Determinant for Flow and Transport in Soils and the Vadose Zone.

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In some soils when water is applied to the surface by rain or other means the water beads on the surface and runs off. Then, over time the soil changes to become able to soak up the water. This process is called dynamic fractional wettability or soil water repellency. Dynamic fractional wettability drastically changes the behaviour of water and its availability in soils for plant growth, nutrient cycling, and it role in contaminant transport. The process of changing wettability of soils is very complex and unpredictable under many important environmental conditions. For example, after a forest wildfire the soil is commonly rendered highly repellent leading to excessive runoff of rain with resultant erosion. Also, the amount of water that is retained in repellent soil is greatly reduced which directly affects plant growth, seed germination, soil evapotranspiration, and soil temperature. Water that does pond on the surface due to the repellency can drain down fractures and large pores in the soil (like old root channels) and bypass the shallow soil and most plant roots to subsequently arrive in groundwater much more quickly. Similar concerns of dynamic fractional wettability below ground surface occur in soils after petroleum spills or at old industrial sites (Brown Fields). Unfortunately, our current scientific knowledge and ability to predict how water moves and is stored or released from soils that have the tendency to repel water to varying degrees over time (dynamic fractional wettability) is critically lacking. The current state of our scientific knowledge is such that there are currently no mechanistic computer models that can accurately describe water behaviour in these complex dynamic soils and/or resultant recharge and transport of contaminants to ground waters under these conditions. Research over the last few decades has revealed that soil water fractional wettability/repellency is far more common than previously known/recognised and is potentially a primary driver for water movement under a number of important environmental conditions. The objectives of this NSERC Discovery Grant research program are to advance our fundamental conceptual understanding and ability to mathematically model and predict water flow and contaminant transport in soils expressing dynamic fractional wettability/water repellency. This will be done through field experiments, highly instrumented laboratory testing/experiments, pore-scale imaging in representative soil materials, and model development. The outcome will be new/better models of soil water flow, soil water storage, and related contaminant transport in soil and groundwater that can be used to help solve many related and pressing environmental problems subject to the primary effects of dynamic fractional wettability.