The amount of water contained in soil pores may be small in volume, but quantifying it is vitally important for understanding many geophysical processes. Despite its significance, however, soil water density is still a poorly understood physical property. A recent review article in Reviews of Geophysics described different experimental and theoretical approaches to measuring soil water density and proposed a unified method for quantifying soil water density variation. Here, the authors of the paper explain the challenges of measuring and theorizing soil water density.
How is the density of soil water different from the density of free water?
Soil water density is calculated as the mass of water retained in soils divided by soil water volume. How and why soil water density differs from free water density (0.997 grams per cubic centimeter) has been a long-standing puzzle for scientists. Different researchers have observed or predicted contradictory results with soil water density as high as 1.680 g/cc or as low as 0.752 g/cc.
Our research shows that soil water density can be either higher or lower than free water density because inter-water molecular pressure can vary greatly within an individual soil pore, depending on the prevailing soil-water interaction mechanisms.
What physicochemical mechanisms cause soil water density to be different?
The physicochemical interactions between soil and water are what alters the density of soil water and its physical properties. These interactions include capillarity, osmosis, interlamellar cation hydration, surface hydroxyl hydration, and multilayer adsorption. The capillary mechanism mainly stems from forces among water molecules at the air-water interface (i.e., surface tension), whereas the other mechanisms originate from the molecular forces between water molecules and various species of solid matrix (such as mineral crystal, surface hydroxyl, and cation), and can be regarded as adsorption.
Our work has revealed that, even though both capillarity and adsorption decrease the free energy of soil water, the former will decrease soil water density and pore water pressure whereas the latter will increase soil water density and pore water pressure.
Why is the difference difficult to measure or theorize?
Experimental challenges come from the lack of direct tools to probe nanoscale pore structures of clayey soils and water molecules. Clayey soils often exhibit myriad nanoscale pores, which can greatly expand their volume upon wetting. Therefore, it is hard to find appropriate fluid or gas to displace the volume of soil water without disturbing soil pore structures. In addition, the whole elementary soil-water system (i.e., solid, water, and air) should maintain a given water potential during experiments, which is challenging for existing experimental techniques.
Theoretical challenges come from the frequent conflation between water pressure and water potential in soil-related scientific and engineering communities. Water pressure is frequently treated as identical to matric potential, and thus uniformly lower than the ambient air pressure in soil under unsaturated conditions. Such conception leads to the dilemma that the uniform negative water pressure suggests the soil water is in tension, and thereby exhibits an expanded volume and a lower density than free water, whereas the soil water density is frequently measured to be much higher than the free water density.
How could variations in soil water density be better conceptualized?
The key to conceptualizing soil water density is to identify the distinct physical features between adsorption and capillarity. Both adsorption and capillarity lower the free energy of soil water and provide the capacity of a soil to attract or retain water molecules. Generally, adsorption dominates the low water content or low water potential regime, whereas capillarity dominates the high water content or high water potential regime (e.g. Lu, 2016).
In addition to this difference in energetic magnitude, capillarity tends to generate tensile pressure and thus lower water density, whereas adsorption leads to compressive pressure and molecular structural changes, thereby increasing water density. As such, soil water density variation can be conceptualized in terms of a characteristic function of soil water content called the soil water density curve.
As a result of the interplay between adsorption and capillarity, soil water density will first drastically decrease and then slightly increase with increasing water content.
What are some of the unresolved questions where additional research, data, or modeling is needed?
The long-standing puzzle of soil water density drives us to identify a missing piece in the current theories on soil-water interaction: what is the physical link between pore water pressure and water potential? To answer this question, we recently proposed a new concept called “soil sorptive potential” and used it to unify all physicochemical potentials and to bridge the gap between pore water pressure and water potential (Lu & Zhang, 2019; Zhang & Lu, 2018).
Theoretically, under an equilibrium water potential, the pore water pressure within an individual soil pore is always not uniform but varies spatially across the soil-water system. The inter-molecular or local pressure, together with the temperature, is the governing physical variable defining the thermodynamic state of pore water.
This newly-synthesized spatially-varying pore water pressure will help to resolve many dilemmas and puzzles in a range of physical science disciplines with interests in pore fluid’s physical properties and processes such as phase change, heat transfer, electrical conductivity, chemical adsorption and transport, soil freezing and thawing, effective stress, and film flow. Therefore, it will be scientifically fruitful to reassess current theories that have been established on the basis of capillary pressure for variably saturated earthen materials.
—Chao Zhang and Ning Lu (Email: [email protected]), Department of Civil and Environmental Engineering, Colorado School of Mines