Customizing Soil-Water Expertise for Different Users

Our lives depend upon and determine the fluxes of water and chemicals in the environment. Atmospheric, aquatic, and terrestrial systems are all characterized by transfer processes that make our lives possible. Some of these processes deliver the air, water, and nutrients that we need to produce food and fiber. Other transfer processes relocate our wastes as environmental contaminants that must be properly managed. As society grows in absolute numbers, so, too, must our concern for maintaining the balance between the wise use of our natural resources in a sustainable manner on the one hand, and the misuse of these resources through short-sightedness and mismanagement on the other hand. Utilization of our resources must be accompanied by protection of them, and knowledge of the role that transfer processes play in this balancing act is important. Management for the long term means that wise decisions in the short term are based on two key issues. First, there is a crucial need to further understand how natural processes, particularly transfer processes, operate. Without this knowledge base, we are unable to formulate logical and lasting solutions to environmental problems. While soil scientists have always focused on tabulating land characteristics in the form of soil surveys, there now is the need to translate these static characterizations into dynamic land qualities, such as soil transfer processes. As important, but less appreciated, is the fact that scientists are becoming increasingly accountable to our clients, the public, for approaches to solve problems that are important to society. This is particularly true for those scientists knowledgeable in transfer processes, for the obvious reasons of public focus on environmental management and pollution prevention. The decisions regarding the impact of our science will be debated, enacted, and enforced outside the scientific community. As we now realize, this means we must consider solutions to environmental problems that are endorsed not only by the scientific community, but also by the public citizenry and regulatory bodies. Many soil and water scientists are experts on transfer processes in the unsaturated zone of the soil.

Diffusion-Linked Microbial Metabolism in the Vadose Zone

Figure 7.1 is a schematic of nutrient and contaminant transformations and cycling in the vadose zone. As detailed in Harris and Arnold (1995), higher plants take up C, N, P, and S in their most oxidized forms and use, via photosynthesis, the Sun’s energy and low-energy electrons from the oxygen in water to convert the oxidized forms of these essential elements into the relatively high energy reduced forms comprising plant biomass. Following plant death, the biomass residues enter the soil and are attacked by soil organisms as a source of food. The plant residues are depolymerized and the reduced, high-energy monomers are assimilated in part into soil organism biomass, and in part are used as electron donors to combine with the most thermodynamically efficient electron acceptors for dissimilatory energy generation to drive growth and maintenance reactions. In aerobic zones, oxygen is the preferred electron acceptor as long as it is nonlimiting. Death of soil organisms produces dead biomass which re-enters the biological reactor. Ultimately, via respiration in aerobic soils, all the reduced C, N, P, and S materials are released as their oxidized forms, and oxygen is reduced to water to complete the cycle. Ideally, the cycle is conservative, particularly from the standpoint of nonleakage of nutrients, such as nitrate, into the groundwater. Similarly, contaminants entering the vadose zone, either as a function of agronomic use or by accident, should ideally be integrated into the natural nutrient cycles and converted to harmless by-products for assimilation and dissimilation by soil organisms and higher plants (Liu, 1994). Management of nutrient and contaminant transformations by the soil organisms requires a thorough understanding of the ecophysiological and solute transport ground rules that control the nature and rates of transformation options available to the soil organisms. In models of chemical transport and transformation through the vadose zone, colonies of microorganisms are frequently treated as a homogeneous biofilm reactor (Grant and Rochette, 1994). Often, modeling efforts are focused on environmental conditions external to the microbial colony. This consideration of the colony as a biofilm with relatively constant nutrient uptake rates ignores the growth differentiation that occurs as the colony develops

Incorporation of Interfacial Areas in Models of Two-Phase Flow

The mathematical study of flow in porous media is typically based on the 1856 empirical result of Henri Darcy. This result, known as Darcy’s law, states that the velocity of a single-phase flow through a porous medium is proportional to the hydraulic gradient. The publication of Darcy’s work has been referred to as “the birth of groundwater hydrology as a quantitative science” (Freeze and Cherry, 1979). Although Darcy’s original equation was found to be valid for slow, steady, one-dimensional, single-phase flow through a homogeneous and isotropic sand, it has been applied in the succeeding 140 years to complex transient flows that involve multiple phases in heterogeneous media. To attain this generality, a modification has been made to the original formula, such that the constant of proportionality between flow and hydraulic gradient is allowed to be a spatially varying function of the system properties. The extended version of Darcy’s law is expressed in the following form: qα=-Kα . Jα (2.1) where qα is the volumetric flow rate per unit area vector of the α-phase fluid, Kα is the hydraulic conductivity tensor of the α-phase and is a function of the viscosity and saturation of the α-phase and of the solid matrix, and Jα is the vector hydraulic gradient that drives the flow. The quantities Jα and Kα account for pressure and gravitational effects as well as the interactions that occur between adjacent phases. Although this generalization is occasionally criticized for its shortcomings, equation (2.1) is considered today to be a fundamental principle in analysis of porous media flows (e.g., McWhorter and Sunada, 1977). If, indeed, Darcy’s experimental result is the birth of quantitative hydrology, a need still remains to build quantitative analysis of porous media flow on a strong theoretical foundation. The problem of unsaturated flow of water has been attacked using experimental and theoretical tools since the early part of this century. Sposito (1986) attributes the beginnings of the study of soil water flow as a subdiscipline of physics to the fundamental work of Buckingham (1907), which uses a saturation-dependent hydraulic conductivity and a capillary potential for the hydraulic gradient.

Fundamentals of Transport Equation Formulation for Two-Phase Flow in Homogeneous and Heterogeneous Porous Media

Most porous media of practical importance are hierarchical in nature; that is, they are characterized by more than one length-scale. When these length-scales are disparate, the hierarchical structure can be analyzed by the method of volume averaging (Anderson and Jackson, 1967; Marie, 1967; Slattery, 1967; Whitaker, 1967). In this approach, macroscopic quantities at a given length-scale are defined in terms of a boundary value problem that describes the phenomena at a smaller length-scale, and information is filtered from one scale to another by a series of volume and area integrals. Other methods, such as ensemble averaging (Matheron, 1965; Dagan, 1989) or homogenization theory (Bensoussan et al, 1978; Sanchez-Palencia, 1980), have been used to study hierarchical systems, and developments specific to the problems under consideration in this chapter can be found in Bourgeat (1984), Auriault (1987), Amaziane and Bourgeat (1988), and Sáez et al. (1989). The transformation from the Darcy scale to the large scale is a recurrent problem in reservoir and aquifer engineering. A detailed description of reservoir properties is available through geostatistical analysis (Journel, 1996) on a fine grid with a length-scale much smaller than the scale of the blocks in the reservoir simulator. “Effective” or “pseudo” properties are assigned to the coarse grid blocks, while the forms of the large-scale equations are required to be the same as those used at the Darcy scale (Coats et al., 1967; Hearn, 1971; Jacks et al., 1972; Kyte and Berry, 1975; Dake, 1978; Killough and Foster, 1979; Yokoyama and Lake, 1981; Kortekaas, 1983; Thomas, 1983; Kossack et al., 1990). A detailed discussion of the comparison between the several approaches is beyond the scope of this chapter; however, one can read Bourgeat et al. (1988) for an introductory comparison between the method of volume averaging and the homogenization theory, and Ahmadi et al. (1993) for a discussion of the various classes of pseudofunction theories.

Microwave Observations of Soil Hydrology

The upper few centimeters of the soil are extremely important because they are the interface between soil science and land-atmosphere research and are also the region of the greatest amount of organic material and biological activity (Wei, 1995). Passive microwave remote sensing can provide a measurement of the surface soil moisture for a range of cover conditions within reasonable error bounds (Jackson and Schmugge, 1989). Since spatially distributed and multitemporal observations of surface soil moisture are rare, the use of these data in hydrology and other disciplines has not been fully explored or developed. The ability to observe soil moisture frequently over large regions could significantly improve our ability to predict runoff and to partition incoming radiant energy into latent and sensible heat fluxes at a variety of scales up to those used in global circulation models. Temporal observation of surface soil moisture may also provide the information needed to determine key soil parameters, such as saturated conductivity (Ahuja et al., 1993). These sensors provide a spatially integrated measurement that may aid in understanding the upscaling of essential soil parameters from point observations. Some specific issues in soil hydrology that could be addressed with remotely sensed observations as described above include (Wei, 1995): (1) criteria for soil mapping based on spatial and temporal variance structures of state variables, (2) identifying scales of observation, (3) determining soil physical properties within profiles based on surface observations, (4) quantifying correlation lengths of soil moisture in time and space relative to precipitation and evaporation, (5) examining the covariance structure between soil water properties and those associated with water and heat fluxes at the land-atmosphere boundary at various scales, and (6) determining if vertical and horizontal fluxes of energy and matter below the surface can be ascertained from surface soil moisture distributions. In this chapter, the basis of microwave remote sensing of soil moisture will be presented along with the advantages and disadvantages of different techniques. Currently available sensor systems will be described.

Emerging Measurement Techniques for Vadose Zone Characterization

Variables and parameters required to characterize soil water flow and solute transport are often measured at different spatial scales from those for which they are needed. This poses a problem since results from field and laboratory measurements at one spatial scale are not necessarily valid for application at another. Herein lies a challenge that vadose zone hydrologists are faced with. For example, vadose zone studies can include flow at the groundwater-unsaturated zone as well as at the soil surface-atmosphere interface at either one specific location or representing an entire field or landscape unit. Therefore, vadose zone measurements should include techniques that can monitor at large depths and that characterize landsurface processes. On the other end of the space spectrum, microscopic laboratory measurement techniques are needed to better understand fundamental flow and transport mechanisms through observations of pore-scale geometry and fluid flow. The Vadose Zone Hydrology (VZH) Conference made very clear that there is an immediate need for such microscopic information at fluid-fluid and solid-fluid interfaces, as well as for methodologies that yield information at the field/landscape scale. The need for improved instrumentation was discussed at the ASA-sponsored symposium on “Future Directions in Soil Physics” by Hendrickx (1994) and Hopmans (1994). Soil physicists participating in the 1994-1999 Western Regional Research Project W-188 (1994) focused on “improved characterization and quantification of flow and transport processes in soils,” and prioritized the need for development and evaluation of new instrumentation and methods of data anlysis to further improve characterization of water and solute transport. The regional project documents the critical need for quantification of water flow and solute transport in heterogeneous, spatially variable field soils, specifically to address preferential and accelerated contaminant transport. Cassel and Nielsen (1994) describe the contributions in computed tomography (CT) using x-rays or magnetic resonance imaging (MRI) as “an awakening,” and they envision these methodologies to become an integral part of vadose zone research programs. The difference in size between measurement and application scales poses a dilemma for the vadose zone hydrologist.

Present Directions and Future Research in Vadose Zone Hydrology

The last decade has been an active one for research in vadose zone hydrology (VZH). There are a host of new experimental devices, lots of new theories, and a bright new generation of scientists eager to unlock the mysteries of the discipline. It would be tempting to say that we are well on our way to conquering the most difficult problems that face experimentalists and modelers in the VZH field. However, as is so often the case in science, new problems are discovered in the process of solving other ones. I have been given the task of providing an overview of the current directions in our field, and of pointing out unsolved problems and future research directions for the discipline. Execution of such a task is well beyond my abilities or vision, so what you will get is a compendium of my personal preferences and bias. I chose several methods carry out my charge. First, I examined the poster abstracts to get an idea of the content and breadth of the offerings for the symposium “Vadose Zone Hydrology—Cutting Across Disciplines.” Next, I examined 1 year’s worth of articles in the S-l section of the Soil Science Society of America Journal and in Water Resources Research at a 10-year interval to get an idea of the changes in people’s interests in research over that time span. Finally, I polled my own research group and asked some colleagues what the really tough problems were in the discipline of VZH. One way to find out what is going on in the world of VZH is to examine the poster abstracts from the above-named conference. Table 17.1 presents an organizational summary of the 78 posters by subject matter. It is clear that the most active areas are property measurement, monitoring, and characterizing large-scale systems, which reflects both the influx of new monitoring devices and also increased attention paid to details of scale-dependence, interpolation, disturbance during monitoring, and other issues that have become research areas in their own right.

Water Flow in Desert Soils Near Buried Waste Repositories

Desert soils are frequently considered the safest choice for storing radioactive and chemical wastes (Winograd, 1974; National Research Council, 1976, 1995; Gee et al., 1992; IT Corporation, 1994). The reason is that nearly all rainwater that percolates into a desert soil is assumed to be taken up by plant roots and transpired back into the atmosphere. However, there is still recharge occurring in desert areas, although the magnitude of this recharge can vary greatly from one area to another (Rockhold et al., 1995). Therefore, proper siting of waste-disposal facilities is extremely important. This requires a good understanding of the factors that affect recharge and that minimize water flow through the waste layer. For a brief discussion of recharge in arid and semiarid regions, as well as an introduction to eight comprehensive papers on the topic, see Gee and Tyler (1994). This chapter has two major thrusts. The first part examines field measurements that have been made of the amounts of water that pass below the root zone to deeper depths in arid (and semiarid) environments. Also, some historical precipitation records are examined. The second part deals with modeling water flow in sloping, layered soils as might occur naturally or in cover designs that use capillary barriers. A simple analytical expression is presented for pressure-head distribution and diversion. This is followed by results from numerical modeling in order to test the effects of more complex boundary conditions, including steady versus nonsteady rainfall. Most rainwater that percolates into a desert soil is taken up by plant roots and transpired back into the atmosphere (Phillips, 1994). In addition, there is considerable surface evaporation. The combined processes of transpiration and evaporation, plus the lack of rainfall, cause desert soils to be dry most of the time. Furthermore, many of these soils are dry to great depths. However, not all desert soils are dry. Soils with minimal or no vegetation may contain considerably more water than soils with vegetation. Shifting sand dunes with deep, uniform sandy soils and minimal vegetation are often quite moist (Berndtsson and Chen, 1994).

Non ideal Transport of Reactive Solutes in Porous Media : Cutting Across History and Disciplines

The potential for human activities to adversely affect the environment has become of increasing concern during the past three decades. As a result, the transport and fate of contaminants in subsurface systems has become one of the major research areas in the environmental/hydrological/earth sciences. An understanding of how contaminants move in the subsurface is required to address problems of characterizing and remediating soil and groundwater contaminated by chemicals associated with industrial and commercial operations, waste-disposal facilities, and agricultural production. Furthermore, such knowledge is needed for accurate risk assessments; for example, to evaluate the probability that contaminants associated with a chemical spill will reach an aquifer. Just as importantly, knowledge of contaminant transport and fate is necessary to design “pollution-prevention” strategies. A tremendous amount of research on the transport of solutes in porous media has been generated by several disciplines, including analytical chemistry (chromatography), chemical engineering, civil/environmental engineering, geology, hydrology, petroleum engineering, and soil science. This research includes the results of theoretical studies designed to pose and evaluate hypotheses, the results of experiments designed to test hypotheses and investigate processes, and the development and application of mathematical models useful for integrating theoretical and experimental results and for evaluating complex systems. While much of the previous research has focused on transport of nonreactive solutes, it is the transport of “reactive” solutes that is currently receiving increased attention. Reactive solutes are those subject to phase-transfer processes (e.g., sorption, precipitation/dissolution) and transformation reactions (e.g., biodegradalion). Of special interest in the field of contaminant transport is so-called nonideal transport. In the most general sense, nonideal transport can be described as transport behavior that deviates from the behavior that is predicted using a given set of assumptions. A homogeneous porous medium and linear, instantaneous phase transfers and transformation reactions are the most basic set of assumptions for ideal solute transport in porous media. As discussed in a recent review, transport of reactive contaminants is often nonideal (Brusseau, 1994). The potential causes of nonideal transport include rate-limited and nonlinear mass transfer and transformation reactions, as well as spatial (and temporal) variability of material properties.

Site-Specific Management of Flow and Transport in Homogeneous and Structured Soils

Among research publications in soil science, few have had a greater impact than those by Nielsen et al. (1973) or Biggar and Nielsen (1976). According to Science Citation Index, the former paper, entitled “Spatial variability of field-measured soilwater properties,” has been cited by scientific peers over 390 times. The 1976 paper, entitled “Spatial variability of the leaching characteristics of a field soil,” has been cited over 232 times. Experimental work presented in both papers represents the first-ever attempt at a large field-scale study of steady-state water and solute transport (Wagenet, 1986). Among the seminal findings of these two papers were as follows: (1) extensive spatial variability existed in soil hydraulic and solute transport properties within a relatively homogeneous field (important in the work of Pilgrim et al., 1982; Addiscott and Wagenet, 1985; Feddes et al., 1988; van dcr Molen and van Ommen, 1988); (2) soil water content, bulk density, and soil particle size exhibited normal frequency distributions, while distributions for hydraulic conductivity, hydraulic diffusivity, pore water velocity, and hydrodynamic dispersion were lognormal (work extended by van der Pol et al.. 1977; Rao et al., 1979); (3) frequency distributions were far superior to field-average parameter values (especially for lognormally distributed properties) in describing field transport behavior (demonstrated by Rao et al., 1979; Trangmar et al., 1985); (4) a simple unit hydraulic gradient method was shown to estimate saturated hydraulic conductivity accurately (results extended by Libardi et al., 1980; van Genuchten and Leij, 1992); (5) good correspondence was found between solute velocity and pore water velocity (key assumption in Jury and Fluhler, 1992); (6) and theoretical predictions of a linear relation between hydrodynamic dispersion and pore water velocity were shown to be obeyed at the field scale (result used widely by solute transport modelers, as discussed in Nielsen et al., 1986). The seminal works by Nielsen et al. (1973) and Biggar and Nielsen (1976) produced several new directions in soil science and vadose zone hydrology research. The most interesting was a series of papers that rejected the theoretical basis and practicality of using deterministic equations, and instead introduced stochastic approaches to describe field-scale water and solute fluxes.