Published online 13 September 2005
Published in Vadose Zone J 4:891-898 (2005)
DOI: 10.2136/vzj2004.0182
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
REVIEWS AND ANALYSES
Pedology
A Hydrogeological Perspective
T. N. Narasimhan*
Dep. of Materials Science and Engineering, Dep. of Environmental Science, Policy and Management, 210 Hearst Memorial Mining Bldg., Univ. of California, Berkeley, CA 94526-1760
* Corresponding author (tnnarasimhan{at}lbl.gov)
Received 25 December 2004.
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ABSTRACT
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In a finite earth with concerns about sustainable use of natural resources, earth science disciplines are moving toward appreciating their respective roles in understanding the interlinked functioning of hydrological, erosional, and nutritional cycles. This move provides motivation for exploring the connections between hydrogeology and pedology as components of a larger whole. As pointed out by Jenny (1941) in his concluding chapter, soils can be studied through mapping their spatial distribution (Marbut's geographical method), and through comprehending soil attributes as functions of soil-forming factors (Jenny's functional method). As Jenny further recognized, the conversion of the functional "knowledge to specific field conditions is impossible unless the areal distribution of soil formers is known." Ultimately, therefore, the vision of pedology is "the union of the geographical and the functional method," to comprehend "the soil body in its natural position," and to understand spatial distribution of soil assemblages. Hydrogeology is concerned with the physical and chemical modification of the materials of the earth's crust by water, and the transport of energy, suspended and dissolved constituents and nutrients from the time water enters the subsurface to the time it goes back to the atmosphere. Hydrogeology thus provides a rationale, through the unifying action of flowing water, to achieve an integration of the geographical and the functional methods envisioned by Hans Jenny. This paper examines the conceptual-philosophical threads that connect pedology and hydrogeology. In the process, the role of soil geomorphology is also addressed.
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INTRODUCTION
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THE ONSET OF THE 21ST CENTURY is witnessing a coming together of different earth science disciplines, necessitated by persuasive evidence that human activities the world over are seriously impacting the Earth's biological habitat. These impacts are attributable to human perturbation of the subtle interactions that exist among the hydrological, erosional, and nutrient cycles in unacceptable ways. As earth scientists attempt to deal with these concerns, a motivation emerges to step back and look at the roles of various earth science disciplines in understanding the three cycles that are vital for the existence of all life. An upshot is that the vital cycles are helping to unite various earth science disciplines that have hitherto cherished their separate identities.
Pedology is devoted to the study of soil attributes, the factors that influence them, and their geographical distribution. Hydrogeology is devoted to the study of water in the lithosphere, from the time it enters the earth's subsurface to the time it leaves to enter the atmosphere. The focus of pedology is on the nature of the solid phase of the near-surface portions of the earth's crust, while the focus of hydrogeology is on the liquid phase. However, water and the solid phase of the earth's crust are intimately interrelated. It is impossible to study one without considering the other. The intrinsic connection of soil to the landscape is the focus of soil geomorphology, which devotes attention to erosional processes that generate soil material, their transport, and deposition. Soil geomorphology, unlike hydrogeology, focuses attention on the transport of the solid phase, and the attendant consequences.
The purpose of this paper is to examine the connections between pedology and hydrogeology, recognizing the role of geomorphology. Its spirit is conceptual and philosophical. The motivation is that the comparative ideas presented will help highlight the foundational ideas that unite pedology and hydrogeology.
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PEDOLOGY
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Pedology can be looked at in two ways, as articulated by Hans Jenny. First, it is identified "with the section of the domain of soil science that studies the soil body in its natural position." (Jenny, 1941, p. vii). Second, "As pedologists, we are interested only in those strata on the solid surface of the earth the properties of which are influenced by climate, organisms, etc." (Jenny, 1941, p. 17). In keeping with this, a soil is assumed to be, "... those portions of the crust of the earth the properties of which vary with soil-forming factors" (Jenny, 1941, p. 17). Although a non-expert may have a mental image that a soil is something that lies close to the land surface, Jenny's perception permits of a broader interpretation of what a soil is. For example, weathering, which can reach down to considerable depths in the earth's crust, is considered to be one of the many factors of soil formation (Jenny, 1941, p.12). Jenny's broad view of pedology provides ample scope for appreciating the linkages between pedology on the one hand, and on the other, earth science disciplines such as hydrogeology.
In his celebrated work Hans Jenny strove to transform pedology from merely being descriptive, and render it quantitative in a manner analogous to physical sciences. He drew on the method of functional analysis to formulate quantitative laws that permit mathematical treatment of soil properties as functions of independently variable soil-formation factors. As a working hypothesis, he assumed that the five factors, climate (cl), organisms (o), topography (r), parent rock (p), and time (t), plus some accessory factors such as seasonal variation of climate and groundwater table are sufficient to define any soil (Jenny, 1941, p. 262). Preceding Jenny, Shaw (1928) treated time as the independent variable, holding others constant. Analogously, Milne (1936) used topography as the only independent variable in his catena concept whose central theme was that certain groups of soils, with notably different attributes, were predictably linked in their occurrence under similar conditions of topography.
Jenny's state factor model recalls to mind thermodynamic systems whose states are treated as functions of several variables. Such variables are known as state variables, and the equation relating them is referred to as an equation of state. Any one of the state variables can be expressed as a function of the rest. The chosen variable is a dependent variable while the rest are independent variables. Given this, the total change of the chosen variable depends on the sum of the changes caused by the other variables. Mathematically, this may be stated as follows. Suppose the system is characterized by five state variables, x1, x2, x3, x4, and x5. Then, if we chose x1 to be the dependent variable, then, the total change of x1 is written mathematically as,
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This equation is very similar in form to Jenny's Eq. [5] (Jenny, 1941, p. 19),
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Although similar in mathematical form, Eq. [1] and [2] have an important difference. Physical systems represented by Eq. [1] are closed systems in the sense that any one of the five state variables can be treated as dependent on the other four. However, in Jenny's conceptualization represented by Eq. [2], the soil property, s, is not part of the set of factors, cl, o, r, p, t. This system is open, and therefore is of much greater complexity than a thermodynamic system. Additional complexity is introduced due to the fact that climate, organisms, topography, and parent rock are themselves characterized by many variables that are difficult to measure and quantify.
Even though Jenny's model is phenomenological, Jenny himself and others have advanced pedology through judicious application of the model to carefully compiled field observations. In practice, observational data are presented as graphs or equations, depicting the variation of a chosen soil attribute as a function of one of the five independent variables, the remaining four held constant. This methodology has led to well-recognized notions such as climosequence, toposequence, chronosequence, and so on.
One final observation regarding the State Factor Model is in order. Jenny restricted his attention solely to the relation between a soil property and a soil factor as observed in the soil body in place. He did not concern himself with the processes that led to a given state or the transitions between states. He simply left such treatments for a later occasion (Amundson, 2005).
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SOIL GEOMORPHOLOGY
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During the 1970s, emphasis on the topographic factor led to the emergence of soil geomorphology. As pointed out by Gerrard (1992), this field builds on the observed fact that soil patterns and landscape elements often coincide, and that the knowledge of one allows predictions to be made on the other. This coincidence can occur on any spatial scale. In addition to the study of these pattern correlations, soil geomorphology is also concerned with the erosional processes that generate the substrate from which soils form, as well as the mass transport and mixing processes that govern in situ soil evolution (Brimhall et al., 1992). For example, Dietrich et al. (1995) have attempted to understand the spatial distribution of soil depth on hill slopes as a function of the curvature of hill slopes. In this case, slope curvature is the topographic factor that governs soil depth.
The emphasis based on physical erosion and deposition outlined above necessitates a distinction between hill-slope soils and soils of areas of very low relief. Soils of valley floors and ridge tops, characterized by low relief, are influenced relatively more by chemical and biological processes, than by physical processes. In this connection, Tricart and Cailleux (1972) note that chemical erosion is approximately proportional to the intensity of soil-forming processes, and that the normal evolution of soils is all the more advanced as mechanical erosion is restricted. Here, "chemical erosion" signifies the dissolution of chemical elements from the host material, and their subsequent removal by water. The existence of a typical, complete soil with defined horizons shows that mechanical erosion is slower than soil-formation processes. Clearly, soil genesis is governed by complex interaction of physical and chemical factors.
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HYDROGEOLOGY
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Hydrogeology is concerned with geological processes of the earth's crust that are influenced by water. Within this broad definition, the term "groundwater" is used in this work to denote all water below the land surface, that which coexists with air above the watertable in the vadose zone, and that which fully saturates the pores below. Although departing from convention, this inclusive definition of groundwater is in keeping with the fact that unsaturated and saturated flows together influence soil formation. In the USA, modern hydrogeology commenced during the late 19th century, when groundwater systems across the nation began to be systematically studied. The underlying principles governing groundwater occurrence, movement and storage were enunciated by Meinzer (1923), whose contributions to hydrogeology are comparable in stature to those of Jenny to pedology. Between 1940 and 1960, many physical and chemical observations about the nature of groundwater systems came together to result in the concept of gravity-driven groundwater flow systems (Meyboom, 1962; Toth, 1962). Intimately intertwined with groundwater flow systems is the concept of hydrochemical facies. All geological manifestations influenced by water are sought to be understood within the framework flow systems and hydrochemical facies. Groundwater flow systems are deciphered with the help of water level or moisture tension measurements in two or three dimensions. Such data are usually gathered with nested piezometers or tensiometers.
Groundwater Flow Systems and Their Manifestations
Precipitation, infiltrating through the soil and the vadose zone, reaches the water table to recharge the groundwater reservoir. Within the vadose zone and the saturated zone below, groundwater flow pattern is governed by undulations in topography (geometry), by spatial variations of permeability (heterogeneity), and by climate (boundary condition). The result is that flow is vertically organized into local (or, shallow), intermediate, and deep systems (Toth, 1962; Meyboom, 1962). Stagnation zones typically separate these systems (Fig. 1)
. Within each system, groundwater moves vertically down in recharge areas, gradually becoming horizontal as it moves away, and finally moving up in discharge areas. As one would expect, discharge areas are distinguished by the presence of sloughs, marshes, wetlands, alkaline soils, saline encrustations, and other features. Both at entry in the recharge area, and at exit in the discharge area, groundwater interacts intimately with the soil. Because of its ability to transport clay and colloidal particles in suspension, and its ability to dissolve and precipitate, the direction and magnitude of groundwater flow has a major influence on the evolution of a soil profile. Together, the flow systems represent the lower part of the hydrological cycle.
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Fig. 1. Schematic illustration of topographically controlled shallow, intermediate, and regional groundwater flow systems. Flow is from right to left. Vertical scale exaggerated (after Toth, 1962).
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The flow systems illustrated in Fig. 1 represent average steady-state conditions over long periods of time. However, hydrogeology is also concerned with transient flow systems in which flow patterns vary with time. Such conditions are specially relevant in shallow, local systems within which water movement may be subjected to competing influences of gravity and evapotranspiration. As an example, Fig. 2
(Meyboom, 1966) shows changes in vertical groundwater flow patterns in a hummocky terrain from normal fall/winter conditions (upper), spring recharge conditions (middle), to summer discharge conditions (lower). This prairie pothole terrain lies in an area of regional recharge, subject to pronounced discharge conditions due to evapotranspiration during the summer. In many small upland water sheds, there could be transient variations in recharge and discharge patterns on much shorter time scales, as when the watershed responds to individual storm events. These periodic changes in the direction of water movement should be expected to have much influence on soil formation in these areas.
Time-dependent changes in regional groundwater flow patterns may occur on a variety of time scales. Changes on time scales of centuries to millenniums or larger may be caused by climate change, topographic changes caused by erosion and deposition, or tectonic uplift. These changes may cause modification of recharge-discharge area distributions, alteration of groundwater-surface water relationships, adjustments in evapotranspiration patterns, and consequent effects on aqueous-solid chemical interactions. Connection between paleopedology and paleohydrology is now a topic of active inquiry. For example, Ashley and Driese (2000) describe the complex history of pedogenesis influenced by episodic volcanic input, episodic expansion and contraction of a nearby alkaline lake, and fluctuation in groundwater level over a period of some 25000 yr.
Apart from mere flow, the role of groundwater as an agent of deformation is of relevance to soil genesis on hill slopes. In upland watersheds, mass wasting of colluvial wedges is an important soil-forming process (Dietrich et al., 1995). Colluvial wedges comprise unconsolidated material lying over bedrock, often fractured. When the colluvial wedges attain a critical thickness, they become susceptible to failure. Failure, however, occurs following severe rainstorms when the build-up water pressure at the toe of the wedge exceeds the weight of the thin overlying material. Toe failure then propagates upslope, leading to a landslide (Wilson et al., 1989).
The ability of infiltrating water to transport suspended particles through the vadose zone can play a role in soil formation. Wood and Petraitis (1984) reported an interesting field observation on carbon dioxide (CO2) generation and distribution from the southern high plains of Texas. During routine monitoring of partial pressures of CO2, oxygen (O2), nitrogen (N), and argon (Ar) down to 36 m below the land surface, they discovered that partial pressures of CO2 at depth exceeded those measured in the active soil zone, which is normally considered to be the principal source of soil CO2. The most probable hypothesis advanced by the authors is that dissolved and particulate organic C introduced by infiltrating water is oxidized to CO2 by aerobic microbial communities that use O2 diffusing in from the atmosphere. This conclusion was corroborated by stable isotope measurements.
Organisms constitute one of Jenny's five soil-forming factors. In this context, the relationships between plants and groundwater enumerated by Meinzer (1927) become relevant. Based on well-documented field observations, Meinzer distinguished between groundwater species (phreatophytes) and nongroundwater species of plants. Especially characteristic of arid environments, plants distinguished as groundwater species possess the ability to extract water for sustenance from the water table or extract moisture from the deeper part of the vadose zone where the water has risen from the water table through capillary rise. Some plants belonging to groundwater species such as greasewood [Sarcobatus vermiculatus (Hook) Torrey] and desert willow (Chilopsis linearis Sweet) may send tap roots down to more than 15-m below land surface (Robinson, 1958). In desert environments, the distribution of groundwater species may exhibit a concentric zoning around an essentially barren playa at the center. At some distance beyond the center, the groundwater species may entirely disappear. In between, the distribution and density of particular species is primarily a function of depth to the watertable and water chemistry (Fig. 3)
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Fig. 3. Profiles across Badwater Basin (A) and Cottonball Basin (B) of Death Valley showing the distribution of different kinds of plants. Phreatophytes dependent on groundwater not excessively saline are restricted to the sides of the saltpan where gravel fans abound. Salt-tolerant phreatophytes are restricted to the edges of the saltpan. (Hunt et al., 1966, Fig. 16).
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The observations described above suggest that physical and biological processes relevant to the genesis of soils extend down to considerable depths, and that the domain of pedology may extend to considerable depths below the water table.
Groundwater Chemistry
The chemical evolution of the Earth's lithosphere, including its epidermis, the soil, is profoundly influenced by groundwater as an agent of chemical denudation and transport. The 1950s brought important developments that coupled aqueous geochemistry with the dynamics of groundwater flow systems. Based on a study of more than 10 000 groundwater samples, Chebotarev (1955) noted that groundwater tends to evolve chemically in predictable ways from areas of recharge to areas of discharge. Water entering the soil in recharge areas tends to be highly oxidizing due to the presence of free O2, and slightly acidic due to CO2 dissolved from the atmosphere as well as from decomposing organic matter in the soil. As groundwater travels through the crust over long distances and continuously interacts with the solid phases, it has a tendency to move toward the composition of sea water, following the anion-sequence, bicarbonate to bicarbonate + sulfate to sulfate + chloride to chloride, with increasing age of the water.
The notion of hydrochemical facies was introduced by Back (1960), to denote the diagnostic chemical aspect of aqueous solutions occurring in groundwater systems, reflecting the progress of chemical processes within the framework of groundwater motion. Figure 4
shows the flow pattern deciphered from mapping of the hydrochemical facies in the Atlantic coastal plain by Back (1960). Contrary to what one may intuitively expect from the eastward dipping cretaceous sediments, portions of the Potomac River where cretaceous sediments crop out are actually areas of local discharge, rather than being areas of recharge. To systematically interpret groundwater chemical analyses, it is common to plot analytical data on trilinear diagrams to identify characteristic cation and anion facies of a given area, or to present fence diagrams that show overall spatial distribution of facies (Back, 1961).
The concept of electrochemical evolution of groundwater (Freeze and Cherry, 1979) was borrowed into geochemistry from corrosion science by Hem and Cropper (1959), and Garrels (1960). This concept highlighted the importance of redox processes in earth systems, and initiated the use of Eh-pH diagrams to represent the stability of mineral phases in aqueous geochemical systems. The electrochemical perspective soon revealed that as one follows the groundwater path from recharge to discharge one can discern the redox sequence in the anionic water composition, as had earlier been suggested by Chebotarev (1955). In general, consumption of free O2 along the flow path is followed successively by reduction of nitrate 
, ferric iron 
, and sulfate (SO4).
Thermodynamically, the stability of mineral phases are intimately related to water chemistry, especially pH and the redox state. As a consequence, the distribution of hydrochemical facies go hand in hand with thermodynamically compatible mineral assemblages. That such associations exist between aqueous chemistry and mineral assemblages constitutes a basic theme in the geochemical study of sedimentary rocks and economic mineral deposits. Analogously, soil mineral assemblages may be considered as pedo-facies, complementing hydrochemical facies. Incompatibility between water chemistry and mineral assemblages may provide clues about thermodynamic disequilibrium, and ongoing transition of a soil from one state to another.
These important developments in hydrochemistry were also complemented by increased understanding of ion-exchange and adsorption in clays and oxide minerals. Since the 1980s, considerable attention has been devoted to understand the role of microorganisms in catalyzing chemical reactions in diverse aqueous subsurface environments (Ehrlich, 1998). Microorganisms, adapted to survive under narrow Eh-pH windows, appear to be pervasive in almost all shallow hydrogeological systems, extracting energy for their sustenance from specific chemical reactions. The practical utility of these developments has been greatly accentuated by exponential growth in computational devices starting from the 1970s. At present, coupled groundwater flow and hydrochemical models are being routinely used to quantitatively understand the genesis of rock and mineral systems on all scales, from a meter to basin-wide scales of hundreds of kilometers.
An Example
The role of groundwater in soil genesis can be illustrated with the example of secondary (supergene) enrichment of copper (Cu) ores through leaching of copper sulfide (Cu2) minerals from host rock, and their subsequent precipitation. This phenomenon is simultaneously of interest in economic geology, geochemistry, hydrogeology, and pedology. The Cu ore, chalcopyrite (Cu-iron sulfide, CuFeS2) usually occurs as disseminated grains or massive veins in some granitic and volcanic rocks, along with pyrite (FeS2). When these rocks are exposed at the land surface, and are subjected to weathering by oxidizing water, pyrite dissolution generates solutions rich in sulfuric acid that dissolve and transport Cu ions downward. The leached host rock, enriched in red iron oxides and characterized by an open box-work structure is referred to as gossan. As the infiltrating Cu-rich solutions reach the water table, they encounter a different chemical environment, marked by O2-deficient, reducing conditions. Under these conditions Cu precipitates as dense sulfides, forming a blanket of rich Cu ore. The top of the enrichment zone represents the position of the paleo water table. Should the water table decline after supergene enrichment, the ores may be oxidized once again to form a zone of oxidized minerals just above the original water table. The zoned structure of a supergene sulfide zone is schematically illustrated in Fig. 5
. The vertical scale of supergene environment may be a few meters in areas of shallow watertable, or exceed a few hundred meters in desert areas (for example, Chilean Cu deposits). The supergene enrichment process is somewhat analogous to the formation of spodosol, which involves, among other things, in the acid-leaching of aluminum (Al) and iron from the surficial layers, and their enrichment in lower soil layers.
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Fig. 5. Schematic description of supergene sulfide enrichment of copper. Depth to the enriched zone may exceed hundreds of meters in some desert environments (Bateman, 1950).
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DISCUSSION
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Despite its accessibility to observation close to the land surface, soil is a material of great complexity, whose genesis is not easy to decipher. Soil forms from its parent rock through a combination of mechanical (transport and mixing) and chemical (dissolution, precipitation, ion exchange, sorption) processes, mediated by water, microorganisms, plant roots, and animal activity. In introducing his soil-formation-factor model, Jenny was optimistic that science will have advanced sufficiently at a future date to quantify, with numbers, the independent variables that govern soil formation and "solve" his phenomenological equation. But, as more and more field observations become available, it is becoming apparent that the number of factors involved are so large, and the nature of their interactions so complex that it is impossible to solve the phenomenological equation in any precise sense. It is in the very nature of earth and biological systems that they defy precise physical description and mathematical solution.
The fact that the soil-formation-factor equation cannot be solved does not negate the practical utility of the model. In a modest, semi-quantitative way, current hydrogeological knowledge can be beneficially used to gain insights into the genesis of soils, their spatial relationships with the landscape, and their potential response to human activities.
Groundwater influences soil genesis physically and chemically. Physically, it transports suspended particles, microorganisms, heat, and dissolved chemical constituents. In addition, changes in water pressure cause soil porosity to change through deformation. Chemically, it plays a fundamental role in reactions arising from mixing of waters as well as from water-solid interactions. Because of these attributes, groundwater is relevant to pedology on all scales, from that of a single soil profile involving spatial variations over a few inches to the distribution of soil types in a large river basin spanning thousands of square kilometers.
As has been shown, motion of groundwater is at the core of hydrogeology. It is therefore most meaningful to look at the soil from the perspective of groundwater motion. Within a single soil profile, vertical groundwater movement (upward or downward) plays a role in the layering (horizon formation) of soils through mechanical transport of fine particles (eluvial and illuvial processes) as well as through chemical leaching and precipitation. Clearly, the direction of groundwater flow is important in determining the direction in which the layering proceeds. Thus the forces that drive regional groundwater motion become important in understanding soil formation. For example, at the bottom of major intermontane valleys (e.g., the San Joaquin Valley of California), shallow, intermediate, and deep flow systems are all driven upward by regional forces. One would expect soil layering, mineralogy, and texture in these areas to be strongly influenced by consequences of vertical upward movement of water.
Away from discharge areas, on valley flanks, there will be competition between upward evapotranspirative influence and downward gravity flow. The two regimes will be separated by a horizontal divide above the water table, whose position will fluctuate through the year. During times of rainfall infiltration, it will move up to the land surface, and it will move down during periods of evaporative losses. In these areas, the transient changes in the direction and magnitude of water movement will influence soil formation in significant ways. One would suspect that the layered structure of various soil groups would, in some measure, reflect the temporal variations of vertical water movement in the vadose zone.
Whereas the physics of water movement will have relatively more pronounced influence on the local profile, groundwater as a chemical agent will likely have much influence on the basin-wide scale, from the water divide to the valleys. It is now well recognized that rocks and mineral assemblages associated with aqueous geochemical systems are best understood jointly in terms of pH and redox state. Applying this reasoning to soils, one should expect that basin-wide variations in soil mineralogy will be amenable to systematic interpretation in term of pH and Eh. One practical difficulty in this regard is that oxidation potential is very difficult to measure in the field, both due to instrumentation and due to effects of scale (sample size). A consequence is that it is very common in the literature to use pH as a measurable soil parameter, but not oxidation potential. Perhaps there is a semiquantitative way to circumvent this problem, by taking the electrochemical approach. As pointed out by Freeze and Cherry (1979), as water ages along the groundwater path, it becomes progressively reducing. This tendency for reduction can be noticed in the successive disappearance of O2, nitrates, ferric iron, and sulfate. It seems likely that by combining this redox tendency with pH measurements, soil mineralogy may become amenable to more systematic interpretation.
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CONCLUDING REMARKS
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Over the past two centuries, earth sciences have evolved by dividing earth systems into separate units and generating detailed understanding of the functioning of each unit. Fields such as petrology, geomorphology, pedology, structural geology, economic geology, and so on attest to this phase of the evolution of the earth sciences. Having generated enough detailed understanding of well-defined narrow problems, earth scientists are now moving toward understanding the complex ways in which the various separate units are interacting with each other as is manifest in the functioning of the Earth as a whole. The need for this has emerged both from intellectual as well as practical needs. It has become apparent that the Earth's biological habitat, including that of humans, is vitally governed by the Earth's hydrological, erosional, and nutrient cycles, which are delicately linked to each other. Many branches of the earth sciences are now engaged in understanding these delicate linkages so that humans can learn to live with these cycles in a sustainable way. As earth scientists enter this phase of understanding the functioning of the Earth's vital life-sustaining cycles, the differences among the various earth sciences disciplines assume less importance, and the emphasis shifts toward a broader understanding of the Earth. Fortunately, the intellectual-philosophical approaches introduced by Jenny, Meinzer, and others provide us with a sound framework to take on the emerging challenges. Pedologists and soil chemists have compiled a voluminous body of data on mutual relationships between water chemistry and various soil attributes. The framework of hydrogeology contributes to pedology by focusing attention on moving water, which through its mechanical and chemical actions, simultaneously influences the temporal evolution of a soil profile at a given location, and the areal distribution of related soil types over watersheds.
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ACKNOWLEDGMENTS
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Thanks are due to Ronald Amundson, John Cherry, William Dietrich, Jonathan Stock, Isaac Winograd, and Thomas C. Winter for critical review of the manuscript. Thoughtful comments by David Stonestrom and two anonymous reviewers helped greatly in the revision process. Thanks to Edward Landa, whose invitation to present something historical and philosophical at the 2004 SSSA meeting in Seattle motivated the preparation of this paper. This work was supported partly by the Agricultural Extension Service, through the Division of Natural Resources, University of California. Partial support was also provided by the Director, Office of Energy Research, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 through the Earth Sciences Division of Ernest Orlando Lawrence Berkeley National Laboratory.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
