Water flow and solute transport processes in the unsaturated zone

Abstract

This paper gives a review of our current conceptual understanding of the basic processes of water flow and chemical transport in the untsaturated (vadose) zone and of various deterministic mathematical models that are being used to describe these processes. During the past few decades, tremendous effort has been directed toward unravelling the complexities of various interactive physical, chemical, and microbiological mechanisms affecting unsaturated flow and transport, with contributions being made by soil scientists, geochemists, hydrologists, soil microbiologists, and others. Unfortunately, segmented, disciplinary research has contributed to a lack of experimental and theoretical understanding of the vadose zone, which, in turn, has precluded the accurate prediction and management of flow and contaminant transport through it. Thus a more unified and interdisciplinary approach is needed that considers the most pertinent physical, chemical, and biological processes operative in the unsaturated zone. Challenges for both fundamental and applied researchers to reveal the intricacies of the zone and to integrate these with currently known concepts are numerous, as is the urgency for progress inasmuch as our soil and ground water resources are increasingly subjected to the dangers of long‐term pollution. Specific research areas in need of future investigation are outlined.

Advances in modeling of water in the unsaturated zone

“…Nielsen et al (1986) has summarized in detail our current understanding of physical processes of importance for water flow in the unsaturated zone. Smith (1983a) reviewed research in the U.S.A. on unsaturated soil water flow published in the 1979-1982 quadrennium, and a new progress report on unsaturated flow modeling for 1983-1986 was given by van Genuchten and Jury (1987).…”

Saturated and unsaturated salt transport in peat from a constructed fen

“…where s is the kinetically sorbed concentration to the solid (M), and α OSA is the first-order rate coefficient between dissolved and adsorbed concentration (T −1 ) which has been found to be a function of pore-water velocity and cannot be derived by batch experiments alone (Nielsen et al, 1986).…”

“…1 and 2; van Genuchten and Wagenet, 1989) and the classical CDE (Eq. 3;van Genuchten and Alves, 1982;Nielsen et al, 1986), and the third is the one-site adsorption equation (OSA; Eqs. 5 and 6; van Genuchten et al, 1974;Nielsen et al, 1986) which was only used for Na + .…”

“…Solute transport in the subsurface may be subject to physical and chemical non-equilibrium (Nielsen et al, 1986) invalidating the use of the conventional convection-dispersion equation (CDE) to simulate it. Physical non-equilibrium is thought to be a process of a heterogeneous flow field with spatial differences in hydraulic conductivity due to dead-end pores (Coats and Smith, 1964;Zurmühl and Durner, 1996), non-moving intra-aggregate water (Philip, 1968;Passioura, 1971) or stagnant water in thin liquid films around soil particles (Nielsen et al, 1986).…”

“…Physical non-equilibrium is thought to be a process of a heterogeneous flow field with spatial differences in hydraulic conductivity due to dead-end pores (Coats and Smith, 1964;Zurmühl and Durner, 1996), non-moving intra-aggregate water (Philip, 1968;Passioura, 1971) or stagnant water in thin liquid films around soil particles (Nielsen et al, 1986). In this mobile-immobile model (MIM; Coats and Smith, 1964;van Genuchten and Wierenga, 1976), the liquid phase is partitioned into a mobile and an immobile region.…”