This book is about groundwater. It is about the geological environments that control the occurrence of groundwater. It is about the physical laws that describe the flow of groundwater. It is about the chemical evolution that accompanies flow. It is also about the influence of man on the natural groundwater regime; and the influence of the natural groundwater regime on man.
The term groundwater is usually reserved for the subsurface water that occurs beneath the water table in soils and geologic formations that are fully saturated. We shall retain this classical definition, but we do so in full recognition that the study of groundwater must rest on an understanding of the subsurface water regime in a broader sense. Our approach will be compatible with the traditional emphasis on shallow, saturated, groundwater flow; but it will also encompass the near-surface, unsaturated, soil-moisture regime that plays such an important role in the hydrologic cycle, and it will include the much deeper, saturated regimes that have an important influence on many geologic processes.
We view the study of groundwater as interdisciplinary in nature. There is a conscious attempt in this text to integrate chemistry and physics, geology and hydrology, and science and engineering to a greater degree than has been done in the past. The study of groundwater is germane to geologists, hydrologists, soil scientists, agricultural engineers, foresters, geographers, ecologists, geotechnical engineers, mining engineers, sanitary engineers, petroleum reservoir analysts, and probably others. We hope that our introductory treatment is in tune with these broad interdisciplinary needs.
If this book had been written a decade ago, it would have dealt almost entirely with groundwater as a resource. The needs of the time would have dictated that approach, and books written in that era reflected those needs. They emphasize the development of water supplies through wells and the calculation of aquifer yields. The groundwater problems viewed as such are those that threaten that yield. The water supply aspects of groundwater are still important and they will be treated in this text with the deference they deserve. But groundwater is more than a resource. It is an important feature of the natural environment; it leads to environmental problems, and may in some cases offer a medium for environmental solutions. It is part of the hydrologic cycle, and an understanding of its role in this cycle is mandatory if integrated analyses are to be promoted in the consideration of watershed resources, and in the regional assessment of environmental contamination. In an engineering context, groundwater contributes to such geotechnical problems as slope stability and land subsidence. Groundwater is also a key to understanding a wide variety of geological processes, among them the generation of earthquakes, the migration and accumulation of petroleum, and the genesis of certain types of ore deposits, soil types, and landforms.
The first five chapters of this book lay the physical, chemical, and geologic foundations for the study of groundwater. The final six chapters apply these principles in the several spheres of interaction between groundwater, the earth, and man. The following paragraphs can be viewed as an introduction to each of the later chapters.
The endless circulation of water between ocean, atmosphere, and land is called the hydrologic cycle. Our interest centers on the land-based portion of the cycle as it might be operative on an individual watershed. Figures 1.1 and 1.2 provide two schematic diagrams of the hydrologic cycle on a watershed. They are introduced here primarily to provide the reader with a diagrammatic introduction to hydrologic terminology. Figure 1.1 is conceptually the better in that it emphasizes processes and illustrates the flow-system concept of the hydrologic cycle. The pot-and-pipeline representation of Figure 1.2 is often utilized in the systems approach to hydrologic modeling. It fails to reflect the dynamics of the situation, but it does differentiate clearly between those terms that involve rates of movement (in the hexagonal boxes) and those that involve storage (in the rectangular boxes).
Inflow to the hydrologic system arrives as precipitation, in the form of rainfall or snowmelt. Outflow takes place as streamflow (or runoff) and as evapotranspiration, a combination of evaporation from open bodies of water, evaporation from soil surfaces, and transpiration from the soil by plants. Precipitation is delivered to streams both on the land surface, as overland flow to tributary channels; and by subsurface flow routes, as interflow and baseflow following infiltration into the soil. Figure 1.1 makes it clear that a watershed must be envisaged as a combination of both the surface drainage area and the parcel of subsurface soils and geologic formations that underlie it. The subsurface hydrologic processes are just as important as the surface processes. In fact, one could argue that they are more important, for it is the nature of the subsurface materials that controls infiltration rates, and the infiltration rates influence the timing and spatial distribution of surface runoff. In Chapter 6, we will examine the nature of regional groundwater flow patterns in some detail, and we will investigate the relations among infiltration, groundwater recharge, groundwater discharge, baseflow, and streamflow generation. In Chapter 7, we will look at the chemical evolution of groundwater that accompanies its passage through the subsurface portion of the hydrologic cycle.
Before closing this section, it is worth looking at some data that reflect the quantitative importance of groundwater relative to the other components of the hydrologic cycle. In recent years there has been considerable attention paid to the concept of the world water balance (Nace, 1971; Lvovitch, 1970; Sutcliffe, 1970), and the most recent estimates of these data emphasize the ubiquitous nature of groundwater in the hydrosphere. With reference to Table 1.1, if we remove from consideration the 94% of the earth’s water that rests in the oceans and seas at high levels of salinity, then groundwater accounts for about two-thirds of the freshwater resources of the world. If we limit consideration to the utilizable freshwater resources (minus the icecaps and glaciers), groundwater accounts for almost the total volume. Even if we consider only the most “active” groundwater regimes, which Lvovitch (1970) estimates at 4 × 106 km3 (rather than the 60 × 106 km3 of Table 1.1), the freshwater breakdown comes to: groundwater, 95%; lakes, swamps, reservoirs, and river channels, 3.5%; and soil moisture, 1.5%.
|Parameter||Surface area (km2) x 106||Volume (km3) x 106||Volume (%)||Equivalent depth (m)*||Residence time|
|Oceans and seas||361||1,370||94||2,500||~4,000 years|
|Lakes and reservoirs||1.55||0.13||<0.01||0.25||~10 years|
|River channels||<0.1||<0.01||<0.01||0.003||~2 weeks|
|Soil moisture||130||0.07||<0.01||0.13||2 weeks–1 year|
|Groundwater||130||60||4||120||2 weeks–10,000 years|
|Icecaps and glaciers||17.8||30||2||60||10–1,000 years|
|Atmospheric water||504||0.01||<0.01||0.025||~10 days|
|Biospheric water||<0.1||<0.01||<0.01||0.001||~1 week|
This volumetric superiority, however, is tempered by the average residence times. River water has a turnover time on the order of 2 weeks. Groundwater, on the other hand, moves slowly, and residence times in the 10’s, 100’s, and even 1000’s of years are not uncommon. The principles laid out in Chapter 2 and the regional flow considerations of Chapter 6 will clarify the hydrogeologic controls on the large-scale movement of groundwater.
Most hydrology texts contain detailed discussions of the hydrologic cycle and of the global water balance. Wisler and Brater (1959) and Linsley, Kohler, and Paulhus (1975) are widely used introductory hydrology texts. A recent text by Eagleson (1970) updates the science at a more advanced level. The massive Handbook of Applied Hydrology, edited by Chow (1964a), is a valuable reference. The history of development of hydrological thought is an interesting study. Chow (1964b) provides a concise discussion; Biswas’ (1970) book length study provides a wealth of detail, from the contributions of the early Egyptians and the Greek and Roman philosophers, right up to and through the birth of scientific hydrology in western Europe in the eighteenth and nineteenth centuries.
The primary motivation for the study of groundwater has traditionally been its importance as a resource. For the United States, the significance of the role of groundwater as a component of national water use can be gleaned from the statistical studies of the U.S. Geological Survey as reported most recently for the year 1970 by Murray and Reeves (1972) and summarized by Murray (1973).
Table 1.2 documents the growth in water utilization in the United States during the period 1950–1970. In 1970 the nation used 1400 × 106 m3/day. Of this, 57% went for industrial use and 35% for irrigation. Surface water provided 81% of the total, groundwater 19%. Figure 1.3 graphically illustrates the role of ground-water relative to surface water in the four major areas of use for the 1950–1970 period. Groundwater is less important in industrial usage, but it provides a significant percentage of the supply for domestic use, both rural and urban, and for irrigation.
The data of Table 1.2 and Figure 1.3 obscure some striking regional variations. About 80% of the total irrigation use occurs in the 17 western states, whereas 84% of the industrial use is in the 31 eastern states. Groundwater is more widely used in the west, where it accounts for 46% of the public supply and 44% of the industrial use (as opposed to 29% and 16%, respectively, in the east).
|Cubic meters/day x 106*||Percent of 1970
|Total water withdrawals||758||910||1,023||1,175||1,400||100|
In Canada, rural and municipal groundwater use was estimated by Meyboom (1968) at 94 × 106 m3/day, or 20% of the total rural and municipal water consumption. This level of groundwater use is considerably lower than that of the United States, even when one considers the population ratio between the two countries. A more detailed look at the figures shows that rural groundwater development in Canada is relatively on a par with rural development in the United States, but municipal groundwater use is significantly smaller. The most striking differences lie in irrigation and industrial use, where the relative total water consumption in Canada is much less than in the United States and the groundwater component of this use is extremely small.
McGuinness (1963), quoting a U.S. Senate committee study, has provided predictions of future U.S. national water requirements. It is suggested that water needs will reach 1700 × 106 m3/day by 1980 and 3360 × 106 m3/day by the year 2000. The attainment of these levels of production would represent a significant acceleration in the rate of increase in water use outlined in Table 1.2. The figure for the year 2000 begins to approach the total water resource potential of the nation, which is estimated to be about 4550 × 106 m3/day. If the requirements are to be met, it is widely accepted that groundwater resources will have to provide a greater proportion of the total supply. McGuinness notes that for the predictions above, if the percent groundwater contribution is to increase from 19% to 33%, ground-water usage would have to increase from its current 262 × 106 m3/day to 705 × 106 m3/day in 1980 and 1120 × 106 m3/day in the year 2000. He notes that the desirable properties of groundwater, such as its clarity, bacterial purity, consistent temperature, and chemical quality, may encourage the needed large-scale development, but he warns that groundwater, especially when large quantities are sought, is inherently more difficult and expensive to locate, to evaluate, to develop, and to manage than surface water. He notes, as we have, that groundwater is an integral phase of the hydrologic cycle. The days when groundwater and surface water could be regarded as two separate resources are past. Resource planning must be carried out with the realization that groundwater and surface water have the same origin.
In Chapter 8, we will discuss the techniques of groundwater resource evaluation: from the geologic problems of aquifer exploration, through the field and laboratory methods of parameter measurement and estimation, to the simulation of well performance, aquifer yield, and basin-wide groundwater exploitation.
If groundwater is to continue to play an important role in the development of the world’s water-resource potential, then it will have to be protected from the increasing threat of subsurface contamination. The growth of population and of industrial and agricultural production since the second world war, coupled with the resulting increased requirements for energy development, has for the first time in man’s history begun to produce quantities of waste that are greater than that which the environment can easily absorb. The choice of a waste-disposal method has become a case of choosing the least objectionable course from a set of objectionable alternatives. As shown schematically on Figure 1.4, there are no currently-feasible, large-scale waste disposal methods that do not have the potential for serious pollution of some part of our natural environment. While there has been a growing concern over air- and surface-water pollution, this activism has not yet encompassed the subsurface environment. In fact, the pressures to reduce surface pollution are in part responsible for the fact that those in the waste management field are beginning to covet the subsurface environment for waste disposal. Two of the disposal techniques that are now being used and that are viewed most optimistically for the future are deep-well injection of liquid wastes and sanitary landfill for solid wastes. Both these techniques can lead to subsurface pollution. In addition, subsurface pollution can be caused by leakage from ponds and lagoons which are widely used as components of larger waste-disposal systems, and by leaching of animal wastes, fertilizers, and pesticides from agricultural soils.
In Chapter 9 we will consider the analysis of groundwater contamination. We will treat the principles and processes that allow us to analyze the general problems of municipal and industrial waste disposal, as well as some more specialized problems associated with agricultural activities, petroleum spills, mining activities, and radioactive waste. We will also discuss contamination of coastal groundwater supplies by salt-water intrusion. In all of these problems, physical considerations of groundwater flow must be coupled with the chemical properties and principles introduced in Chapter 3; and the coupling must be carried out in light of the concepts of natural geochemical evolution discussed in Chapter 7.
Groundwater is not always a blessing. During the construction of the San Jacinto tunnel in California, tunnel driving on this multi-million-dollar water aqueduct was held up for many months as a result of massive unexpected inflows of groundwater from a system of highly fractured fault zones.
In Mexico City during the period 1938–1970, parts of the city subsided as much as 8.5 m. Differential settlements still provide severe problems for engineering design. The primary cause of the subsidence is now recognized to be excessive groundwater withdrawals from subsurface aquifers.
At the Jerome dam in Idaho, the dam “failed,” not through any structural weakness in the dam itself, but for the simple reason that the dam would not hold water. The groundwater flow systems set up in the rock formations adjacent to the reservoir provided leakage routes of such efficiency that the dam had to be abandoned.
At the proposed Revelstoke dam in British Columbia, several years of exploratory geological investigation were carried out on an ancient landslide that was identified on the reservoir bank several miles above the damsite. The fear lay in the possibility that increased groundwater pressures in the slide caused by the impoundment of the reservoir could retrigger slope instability. An event of this type took almost 2500 lives in 1963 in the infamous Vaiont reservoir disaster in Italy. At the Revelstoke site, a massive drainage program was carried out to ensure that the Vaiont experience would not be repeated.
In Chapter 10 we will explore the application of the principles of groundwater flow to these types of geotechnical problems and to others. Some of the problems, such as leakage at dams and inflows to tunnels and open pit mines, arise as a consequence of excessive rates and quantities of groundwater flow. For others, such as land subsidence and slope instability, the influence arises from the presence of excessive fluid pressures in the groundwater rather than from the rate of flow itself. In both cases, flow-net construction, which is introduced in Chapter 5, is a powerful analytic tool.
There are very few geologic processes that do not take place in the presence of groundwater. For example, there is a close interrelationship between groundwater flow systems and the geomorphological development of landforms, whether by fluvial processes and glacial processes, or by natural slope development. Groundwater is the most important control on the development of karst environments.
Groundwater plays a role in the concentration of certain economic mineral deposits, and in the migration and accumulation of petroleum.
Perhaps the most spectacular geologic role played by groundwater lies in the control that fluid pressures exert on the mechanisms of faulting and thrusting. One exciting recent outgrowth of this interaction is the suggestion that it may be possible to control earthquakes on active faults by manipulating the natural fluid pressures in the fault zones.
In Chapter 11, we will delve more deeply into the role of groundwater as an agent in various geologic processes.
The study of groundwater requires knowledge of many of the basic principles of geology, physics, chemistry and mathematics. For example, the flow of groundwater in the natural environment is strongly dependent on the three-dimensional configuration of geologic deposits through which flow takes place. The groundwater hydrologist or geologist must therefore have some background in the interpretation of geologic evidence, and some flair for the visualization of geologic environments. He should have training in sedimentation and stratigraphy, and an understanding of the processes that lead to the emplacement of volcanic and intrusive igneous rocks. He should be familiar with the basic concepts of structural geology and be able to recognize and predict the influence of faulting and folding on geologic systems. Of particular importance to the student of groundwater is an understanding of the nature of surficial deposits and landforms. A large proportion of ground-water flow and a significant percentage of groundwater resource development takes place in the unconsolidated surficial deposits created by fluvial, lacustrine, glacial, deltaic, and aeolian geologic processes. In the northern two-thirds of North America an understanding of the occurrence and flow of groundwater rests almost entirely on an understanding of the glacial geology of the Pleistocene deposits.
Geology provides us with a qualitative knowledge of the framework of flow, but it is physics and chemistry that provide the tools for quantitative analysis. Groundwater flow exists as a field just as heat and electricity do, and previous exposure to these more classic fields provides good experience for the analysis of groundwater flow. The body of laws that controls the flow of groundwater is a special case of that branch of physics known as fluid mechanics. Some understanding of the basic mechanical properties of fluids and solids, and a dexterity with their dimensions and units, will aid the student in grasping the laws of groundwater flow. Appendix I provides a review of the elements of fluid mechanics. Any reader who does not feel facile with such concepts as density, pressure, energy, work, stress, and head would be well advised to peruse the appendix before attacking Chapter 2. If a more detailed treatment of fluid mechanics is desired, Streeter (1962) and Vennard (1961) are standard texts; Albertson and Simons (1964) provide a useful short review. For the specific topic of flow through porous media, a more advanced treatment of the physics than is attempted in this text can be found in Scheidegger (1960) and Collins (1961), and especially in Bear (1972).
The analysis of the natural chemical evolution of groundwater and of the behavior of contaminants in groundwater requires use of some of the principles of inorganic and physical chemistry. These principles have long been part of the methodology of geochemists and have in recent decades come into common use in groundwater studies. Principles and techniques from the field of nuclear chemistry are now contributing to our increased understanding of the groundwater environment. Naturally-occurring stable and radioactive isotopes, for example, are being used to determine the age of water in subsurface systems.
Groundwater hydrology is a quantitative science, so it should come as no surprise to find that mathematics is its language, or at the very least one of its principal dialects. It would be almost impossible, and quite foolish, to ignore the powerful tools of the groundwater trade that rest on an understanding of mathematics. The mathematical methods upon which classical studies of groundwater flow are based were borrowed by the early researchers in the field from areas of applied mathematics originally developed for the treatment of problems of heat flow, electricity, and magnetism. With the advent of the digital computer and its widespread availability, many of the important recent advances in the analysis of groundwater systems have been based on much different mathematical approaches generally known as numerical methods. Although in this text neither the classical analytical methods nor numerical methods are pursued in any detail, our intention has been to include sufficient introductory material to illustrate some of the more important concepts.
Our text is certainly not the first to be written on groundwater. There is much material of interest in several earlier texts. Todd (1959) has for many years been the standard introductory engineering text in groundwater hydrology. Davis and De Wiest (1966) place a much heavier emphasis on the geology. For a text totally committed to the resource evaluation aspects of groundwater, there are none better than Walton (1970), and Kruseman and De Ridder (1970). A more recent text by Domenico (1972) differs from its predecessors in that it presents the basic theory in the context of hydrologic systems modeling. Among the best texts from abroad are those of Schoeller (1962). Bear, Zaslaysky, and Irmay (1968), Custodio and Llamas (1974), and the advanced Russian treatise of Polubarinova-Kochina (1962).
There are several other applied earth sciences that involve the flow of fluids through porous media. There is a close kinship between groundwater hydrology, soil physics, soil mechanics, rock mechanics, and petroleum reservoir engineering. Students of groundwater will find much of interest in textbooks from these fields such as Bayer, Gardner, and Gardner (1972), Kirkham and Powers (1972), Scott (1963), Jaeger and Cook (1969), and Pirson (1958).
The first two sections of this chapter provide an introduction to the topics we plan to cover in this text. It is equally important that we set down what we do not intend to cover. Like most applied sciences, the study of groundwater can be broken into three broad aspects: science, engineering, and technology. This textbook places heavy emphasis on the scientific principles; it includes much in the way of engineering analysis; it is not in any sense a handbook on the technology.
Among the technical subjects that are not discussed in any detail are: methods of drilling; the design, construction, and maintenance of wells; and geophysical logging and sampling. All are required knowledge for the complete groundwater specialist, but all are treated well elsewhere, and all are learned best by experience rather than rote.
There are several books (Briggs and Fiedler, 1966; Gibson and Singer, 1971; Campbell and Lehr, 1973; U.S. Environmental Protection Agency, 1973a, 1976) that provide technical descriptions of the various types of water well drilling equipment. They also contain information on the design and setting of well screens, the selection and installation of pumps, and the construction and maintenance of wells.
On the subject of geophysical logging of boreholes, the standard reference in the petroleum industry, where most of the techniques arose, is Pirson (1963). Patten and Bennett (1963) discuss the various techniques with specific reference to groundwater exploration. We will give brief mention to subsurface drilling and borehole logging in Section 8.2, but the reader who wants to see examples in greater number in the context of case histories of groundwater resource evaluation is directed to Walton (1970).
There is one other aspect of groundwater that is technical, but in a different sense, that is not considered in this text. We refer to the subject of groundwater law. The development and management of groundwater resources must take place within the framework of water rights set down by existing legislation. Such legislation is generally established at the state or provincial level, and the result in North America is a patchwork quilt of varying traditions, rights, and statutes. Piper (1960) and Dewsnut et al. (1973) have assessed the situation in the United States. Thomas (1958) has drawn attention to some of the paradoxes that arise out of conflicts between hydrology and the law.
CHOW, V. T. 1964. Hydrology and its development. Handbook of Applied Hydrology, ed. V. T. Chow. McGraw-Hill, New York, pp. 1.1–1.22.
MCGUINNESS, C. L. 1963. The role of groundwater in the national water situation. U.S. Geol. Surv. Water-Supply Paper 1800.
MURRAY, C. R. 1973. Water use, consumption, and outlook in the U.S. in 1970. J. Amer. Water Works Assoc., 65, pp. 302–308.
NACE, R. L., ed. 1971. Scientific framework of world water balance. UNESCO Tech. Papers Hydrol., 7, 27 pp.