The supplies of O2 and water and the mechanical impedance of soil are determined by soil structural form and soil water content. The range in soil water content in which limitations for plant growth associated with matric pressure, aeration, and mechanical resistance are minimal was defined as the least limiting water range (LLWR). This study was carried out to evaluate the LLWR as an index of the structural quality of soils. Undisturbed soil cores were taken from the 5‐ to 10‐cm depth of a silt loam and a loamy sand, cropped to corn (Zea mays L.) and red clover (Trifolium pratense L.). Soil water retention, soil resistance, air‐filled porosity, and bulk density (Db) were measured. Water contents at critical limits associated with field capacity (−0.01 MPa), wilting point (−1.5 MPa), air‐filled porosity (10%), and soil resistance (2.0 MPa) were predicted and the LLWR calculated for each measured Db. The natural variation in Db on both soils gave rise to a wide variation in LLWR. Values of LLWR varied from 0 to 0.14 cm3 cm−3 for the silt loam soil and from 0.05 to 0.13 cm3 cm−3 for the loamy sand soil. At Db above 1.36 g cm−3 for the silt loam and 1.43 g cm−3 for the loamy sand, the LLWR declined sharply with increasing bulk density. Further research relating LLWR to crop response is required before LLWR can be recommended as a soil structural quality index for crop production.

Spontaneous imbibition of wetting liquids in porous media is a ubiquitous natural phenomenon which has received much attention in a wide variety of fields over several decades. Many traditional and recently presented capillary-driven flow models are derived based on Hagen-Poiseuille (H-P) flow in cylindrical capillaries. However, some limitations of these models have motivated modifications by taking into account different geometrical factors. In this work, a more generalized spontaneous imbibition model is developed by considering the different sizes and shapes of pores, the tortuosity of imbibition streamlines in random porous media, and the initial wetting-phase saturation. The interrelationships of accumulated imbibition weight, imbibition rate and gas recovery and the properties of the porous media, wetting liquids, and their interactions are derived analytically. A theoretical analysis and comparison denote that the presented equations can generalize several traditional and newly developed models from the literature. The proposed model was evaluated using previously published data for spontaneous imbibition measured in various natural and engineered materials including different rock types, fibrous materials, and silica glass. The test results show that the generalized model can be used to characterize the spontaneous imbibition behavior of many different porous media and that pore shape cannot always be assumed to be cylindrical.

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Colloid transport may facilitate off-site transport of radioactive wastes at the Hanford site, Washington State. In this study, column experiments were conducted to examine the effect of irrigation schedule on releases of in situ colloids from two Hanford sediments during saturated and unsaturated transientflow and its dependence on solution ionic strength, irrigation rate, and sediment texture. Results show that transient flow mobilized more colloids than steady-state flow. The number of short-term hydrological pulses was more important than total irrigation volume for increasing the amount of mobilized colloids. This effect increased with decreasing ionic strength. At an irrigation rate equal to 5% of the saturated hydraulic conductivity, a transient multipulse flow in 100 mM NaNO3 was equivalent to a 50-fold reduction of ionic strength (from 100 mM to 2 mM) with a single-pulse flow in terms of their positive effects on colloid mobilization. Irrigation rate was more important for the initial release of colloids. In addition to water velocity, mechanical straining of colloids was partly responsible for the smaller colloid mobilization in the fine than in the coarse sands, although the fine sand contained much larger concentrations of colloids than the coarse sand.