Identifying Soil Adsorptive Water by Soil Water Density

Soil and water interact with each other in two physically distinct mechanisms: capillarity and adsorption. Capillarity occurs in soil pores when curved air–water interfaces exist under unsaturated conditions, whereas adsorption always occurs on or within soil particles under both saturated and unsaturated conditions. Capillarity is governed by pore size distribution, air–liquid interfacial tension, and affinity of soil to water (contact angle). Adsorption, on the other hand, is governed by soil composition and specific surface area. As such, soil water content can be physically divided into adsorptive and capillary water, each with distinct basic physical properties, e.g., pore pressure regime, viscosity, and density. Consequently, each of these will play very different roles in physical processes occurring in soil such as fluid flow, heat transfer, electron conduction, stress, and deformation.

To date, nearly all theories or models for describing soil basic properties do not distinguish capillary and adsorptive water, because only recently theories (e.g., Lu and Zhang 2019), models (e.g., Lu 2016), and experimental techniques (e.g., Zhang and Lu 2018) to quantify capillary and adsorptive water have emerged.

The soil water density greater than unity at varying water contents has been observed and different physical interaction mechanisms have been identified (Zhang and Lu 2018). The internal interaction among water molecules due to capillarity produces tensile stress and decreases the water density, whereas the external interaction by adsorption results in compressive water pressure and thus increases the soil water density. Therefore, abnormal soil water density offers a unique physical criterion to define and separate capillary water, $w_c$, and adsorptive water, $w_a$.

Here, helium gas pycnometer (e.g., Zhang and Lu 2018) is used to directly measure the soil water density with various water contents. The incremental soil water density, or the local density of water at the drying or wetting front, can be deduced from the calculated increment of soil water density. Fig. 1 illustrates the results of typical incremental soil water density curve [Fig. 1(a)] and independent soil water retention curve (SWRC) data [Fig. 1(b)] for an expansive clay (Ningming clay). The pycnometer measurements show that the water density in Ningming clay can be as high as $1.20(\mathrm{g}/{\mathrm{cm}}^3)$ at very dry condition and gradually decreases to 1.00 as the water content increases to the gravimetric water content of 0.130. This indicates that the water dwells within soil in an adsorptive state with a density higher than the unity of free water, and capillary water does not start to condensate until the water content reaches the maximum adsorptive water content, $w_a^{\max }$. The value $w_a^{\max }$ can also be quantified by using the SWRC model of Lu (2016) as shown in Fig. 1(b). The identified maximum adsorptive water content by the soil water density function [$0.130(\mathrm{g}/\mathrm{g})$] matches well with that [$0.129(\mathrm{g}/\mathrm{g})$] from the SWRC data.