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Estimating Groundwater Recharge
- Richard W. Healy
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- 30 September 2010
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Understanding groundwater recharge is essential for successful management of water resources and modeling fluid and contaminant transport within the subsurface. This book provides a critical evaluation of the theory and assumptions that underlie methods for estimating rates of groundwater recharge. Detailed explanations of the methods are provided - allowing readers to apply many of the techniques themselves without needing to consult additional references. Numerous practical examples highlight benefits and limitations of each method. Approximately 900 references allow advanced practitioners to pursue additional information on any method. For the first time, theoretical and practical considerations for selecting and applying methods for estimating groundwater recharge are covered in a single volume with uniform presentation. Hydrogeologists, water-resource specialists, civil and agricultural engineers, Earth and environmental scientists and agronomists will benefit from this informative and practical book. It can serve as the primary text for a graduate-level course on groundwater recharge or as an adjunct text for courses on groundwater hydrology or hydrogeology. For the benefit of students and instructors, problem sets of varying difficulty are available at http://wwwbrr.cr.usgs.gov/projects/GW_Unsat/Recharge_Book/
References
- Richard W. Healy
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7 - Chemical tracer methods
- Richard W. Healy
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- 30 September 2010, pp 136-165
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Summary
Introduction
Tracers have a wide variety of uses in hydrologic studies: providing quantitative or qualitative estimates of recharge, identifying sources of recharge, providing information on velocities and travel times of water movement, assessing the importance of preferential flow paths, providing information on hydrodynamic dispersion, and providing data for calibration of water flow and solute-transport models (Walker, 1998; Cook and Herczeg, 2000; Scanlon et al., 2002b). Tracers generally are ions, isotopes, or gases that move with water and that can be detected in the atmosphere, in surface waters, and in the subsurface. Heat also is transported by water; therefore, temperatures can be used to trace water movement. This chapter focuses on the use of chemical and isotopic tracers in the subsurface to estimate recharge. Tracer use in surface-water studies to determine groundwater discharge to streams is addressed in Chapter 4; the use of temperature as a tracer is described in Chapter 8.
Following the nomenclature of Scanlon et al. (2002b), tracers are grouped into three categories: natural environmental tracers, historical tracers, and applied tracers. Natural environmental tracers are those that are transported to or created within the atmosphere under natural processes; these tracers are carried to the Earth’s surface as wet or dry atmospheric deposition. The most commonly used natural environmental tracer is chloride (Cl) (Allison and Hughes, 1978). Ocean water, through the process of evaporation, is the primary source of atmospheric Cl. Other tracers in this category include chlorine-36 (36Cl) and tritium (3H); these two isotopes are produced naturally in the Earth’s atmosphere; however, there are additional anthropogenic sources of them.
Acknowledgments
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8 - Heat tracer methods
- Richard W. Healy
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- Estimating Groundwater Recharge
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- 30 September 2010, pp 166-179
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Summary
Introduction
The flow of heat in the subsurface is closely linked to the movement of water (Ingebritsen et al., 2006). As such, heat has been used as a tracer in groundwater studies for more than 100 years (Anderson, 2005). As with chemical and isotopic tracers (Chapter 7), spatial or temporal trends in surface and subsurface temperatures can be used to infer rates of water movement. Temperature can be measured accurately, economically, at high frequencies, and without the need to obtain water samples, facts that make heat an attractive tracer. Temperature measurements made over space and time can be used to infer rates of recharge from a stream or other surface water body (Lapham, 1989; Stonestrom and Constantz, 2003); measurements can also be used to estimate rates of steady drainage through depth intervals within thick unsaturated zones (Constantz et al., 2003; Shan and Bodvarsson, 2004). Several thorough reviews of heat as a tracer in hydrologic studies have recently been published (Constantz et al., 2003; Stonestrom and Constantz, 2003; Anderson, 2005; Blasch et al., 2007; Constantz et al., 2008). This chapter summarizes heat-tracer approaches that have been used to estimate recharge.
Some clarification in terminology is presented here to avoid confusion in descriptions of the various approaches that follow. Diffuse recharge is that which occurs more or less uniformly across large areas in response to precipitation, infiltration, and drainage through the unsaturated zone. Estimates of diffuse recharge determined using measured temperatures in the unsaturated zone are referred to as potential recharge because it is possible that not all of the water moving through the unsaturated zone will recharge the aquifer; some may be lost to the atmosphere by evaporation or plant transpiration. Estimated fluxes across confining units in the saturated zone are referred to as interaquifer flow (Chapter 1). Focused recharge is that which occurs directly from a point or line source, such as a stream, on land surface. Focused recharge may vary widely in space and time. If the water table intersects a stream channel, estimates of stream loss are called actual recharge, or just recharge. If the water table lies below the stream channel, estimates are referred to as potential recharge. For simplicity, all vertical water fluxes are referred to as drainage throughout this chapter. Whether the estimated quantity represents actual or potential recharge or drainage depends on the circumstances of each individual study.
2 - Water-budget methods
- Richard W. Healy
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Summary
Introduction
A water budget is an accounting of water movement into and out of, and storage change within, some control volume. Universal and adaptable are adjectives that reflect key features of water-budget methods for estimating recharge. The universal concept of mass conservation of water implies that water-budget methods are applicable over any space and time scales (Healy et al., 2007). The water budget of a soil column in a laboratory can be studied at scales of millimeters and seconds. A water-budget equation is also an integral component of atmospheric general circulation models used to predict global climates over periods of decades or more. Water-budget equations can be easily customized by adding or removing terms to accurately portray the peculiarities of any hydrologic system. The equations are generally not bound by assumptions on mechanisms by which water moves into, through, and out of the control volume of interest. So water-budget methods can be used to estimate both diffuse and focused recharge, and recharge estimates are unaffected by phenomena such as preferential flow paths within the unsaturated zone.
Water-budget methods represent the largest class of techniques for estimating recharge. Most hydrologic models are derived from a water-budget equation and can therefore be classified as water-budget models. It is not feasible to address all water-budget methods in a single chapter. This chapter is limited to discussion of the “residual” water-budget approach, whereby all variables in a water-budget equation, except for recharge, are independently measured or estimated and recharge is set equal to the residual. This chapter is closely linked with Chapter 3, on modeling methods, because the equations presented here form the basis of many models and because models are often used to estimate individual components in water-budget studies. Water budgets for streams and other surface-water bodies are addressed in Chapter 4. The use of soil-water budgets and lysimeters for determining potential recharge and evapotranspiration from changes in water storage is discussed in Chapter 5. Aquifer water-budget methods based on the measurement of groundwater levels are described in Chapter 6.
Frontmatter
- Richard W. Healy
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3 - Modeling methods
- Richard W. Healy
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Summary
Introduction
Simulation models are widely used in all types of hydrologic studies, and many of these models can be used to estimate recharge. Models can provide important insight into the functioning of hydrologic systems by identifying factors that influence recharge. The predictive capability of models can be used to evaluate how changes in climate, water use, land use, and other factors may affect recharge rates. Most hydrological simulation models, including watershed models and groundwater-flow models, are based on some form of water-budget equation, so the material in this chapter is closely linked to that in Chapter 2. Empirical models that are not based on a water-budget equation have also been used for estimating recharge; these models generally take the form of simple estimation equations that define annual recharge as a function of precipitation and possibly other climatic data or watershed characteristics.
Model complexity varies greatly. Some models are simple accounting models; others attempt to accurately represent the physics of water movement through each compartment of the hydrologic system. Some models provide estimates of recharge explicitly; for example, a model based on the Richards equation can simulate water movement from the soil surface through the unsaturated zone to the water table. Recharge estimates can be obtained indirectly from other models. For example, recharge is a parameter in groundwater-flow models that solve for hydraulic head (i.e. groundwater level). Recharge estimates can be obtained through a model calibration process in which recharge and other model parameter values are adjusted so that simulated water levels agree with measured water levels. The simulation that provides the closest agreement is called the best fit, and the recharge value used in that simulation is the model-generated estimate of recharge.
9 - Linking estimation methods to conceptual models of groundwater recharge
- Richard W. Healy
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- 30 September 2010, pp 180-204
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Summary
Introduction
The selection of appropriate methods for estimating groundwater recharge should be tied to a conceptual model of recharge processes; assumptions inherent in any method must be consistent with that conceptual model. The emphasis of Chapters 2 through 8 was on estimation methods. Various categories of methods were described and systematically analyzed with particular attention to underlying assumptions. The objectives of this final chapter are to illustrate how methods for estimating recharge are tied to conceptual models and to provide some broad guidelines for selecting methods.
Section 9.2 provides a reexamination of the conceptual model development discussed in Chapter 1 in light of the information provided in the intervening chapters. A comparison of the various families of methods is provided in Section 9.3; tables summarize recharge processes, space and time scales of applicability, and the relative expense and complexity of methods. Section 9.4 contains discussions of conceptual models of recharge processes that have been developed and used within different groundwater regions of the United States. Also included in Section 9.4 are discussions of methods that have been applied in support of those conceptual models and a necessarily brief sampling of recharge studies that have been conducted within each region. The chapter concludes with final thoughts related to future developments in estimating groundwater recharge.
Preface
- Richard W. Healy
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Summary
Groundwater is an integral part of natural hydrologic systems. Humans have used groundwater for thousands of years. Its use has increased greatly over time, but only in the last few decades has our appreciation of the limitations of its supply and its vulnerability to contamination grown to the point where steps are being taken to protect this valuable resource. One of the most important components in any assessment of groundwater supply or aquifer vulnerability is the rate at which water in the system is replenished – the rate of recharge.
A number of textbooks are devoted to hydrogeology, groundwater flow, and contaminant transport (e.g. Freeze and Cherry, 1979; Domenico and Schwartz, 1998; Todd and Mays, 2005). The importance of recharge is cited in all of these textbooks, but only limited information is provided on the description and analysis of techniques for estimating recharge. Similarly, undergraduate and graduate courses on hydrogeology, groundwater flow, and contaminant transport are offered at many universities, but we know of no university level courses specifically devoted to groundwater recharge. This book attempts to fill these gaps by providing a systematic and comprehensive analysis of methods for estimating recharge.
6 - Physical methods: saturated zone
- Richard W. Healy
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Summary
Introduction
Among the most widely used techniques for estimating recharge are those based on measurement of groundwater levels over time and space. The abundance of available groundwater-level data and the simplicity of these methods facilitate straightforward application. The water-table fluctuation method uses fluctuations in groundwater levels over time to estimate recharge for unconfined aquifers; it is the focus of most of this chapter. Included in the discussion of the method are an analysis of mechanisms that can cause water-table fluctuations and a review of methods for estimating specific yield. Other methods addressed in this chapter are based on the Darcy equation and include an approach developed by Theis (1937), the Hantush (1956) method for estimating interaquifer flow, and the application of flow nets. The chapter also includes a discussion of approaches based on time-series analyses of measured groundwater levels. The content of this chapter draws from and expands upon the material presented in Healy and Cook (2002).
Groundwater-level data
Many local, state, and federal agencies maintain databases of measured groundwater levels in individual countries. Within the United States, the US Geological Survey maintains the largest database on real-time and historic groundwater levels (Table 2.1). Groundwater levels can be measured manually by using a graduated measurement tape to determine the depth to water in a well from a reference point at the top of the well casing. Historically, groundwater levels in some observation wells were automatically recorded by using a float that was attached by a steel tape or wire to a wheel sensor; a strip-chart or paper-punch device was used to record movement of the wheel (Rasmussen and Andreasen, 1959). Submersible pressure transducers have come into widespread use for monitoring groundwater levels since the 1990s (Freeman et al., 2004). These electronic devices can automatically sense and record groundwater levels at user-selected frequencies. The depth (relative to the reference point on the well casing) at which a transducer is placed in a well must be carefully measured. If groundwater elevation is desired, the elevation of the reference point needs to be determined.
1 - Groundwater recharge
- Richard W. Healy
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- 30 September 2010, pp 1-14
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Summary
Introduction
Groundwater is a critical source of fresh water throughout the world. Comprehensive statistics on groundwater abstraction and use are not available, but it is estimated that more than 1.5 billion people worldwide rely on groundwater for potable water (Clarke et al., 1996). Other than water stored in icecaps and glaciers, groundwater accounts for approximately 97% of fresh water on Earth (Nace, 1967; Shiklomanov and Rodda, 2003). As the world population continues to grow, more people will come to rely on groundwater sources, particularly in arid and semiarid areas (Simmers, 1990). Long-term availability of groundwater supplies for burgeoning populations can be ensured only if effective management schemes are developed and put into practice. Quantification of natural rates of groundwater recharge (i.e. the rates at which aquifer waters are replenished) is imperative for efficient groundwater management (Simmers, 1990). Although it is one of the most important components in groundwater studies, recharge is also one of the least understood, largely because recharge rates vary widely in space and time, and rates are difficult to directly measure.
The rate, timing, and location of recharge are important issues in areas of groundwater contamination as well as groundwater supply. In general, the likelihood for contaminant movement to the water table increases as the rate of recharge increases. Areas of high recharge are often equated with areas of high aquifer vulnerability to contamination (ASTM, 2008; US National Research Council, 1993). Locations for subsurface waste-disposal facilities often are selected on the basis of relative rates of recharge, with ideal locations being those with low aquifer vulnerability so as to minimize the amount of moving water coming into contact with waste (e.g. US Nuclear Regulatory Commission, 1993). A high profile example of the importance of susceptibility to contamination is the study for the proposed high-level radioactive-waste repository at Yucca Mountain, Nevada. Tens of millions of dollars were invested over the course of two decades in efforts to determine recharge rates at the site (Flint et al., 2001a).
Contents
- Richard W. Healy
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- 30 September 2010, pp v-viii
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4 - Methods based on surface-water data
- Richard W. Healy
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- 30 September 2010, pp 74-96
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Summary
Introduction
Streamflow data are commonly used to estimate recharge rates in humid and subhumid regions, in part because of the abundance of streamflow data and the availability of computer programs for analyzing those data. Most of the methods described in this chapter are easy to use, but application of any of the methods should be accompanied by a careful analysis of the underlying assumptions. The methods estimate exchange rates between groundwater and surface-water bodies. That exchange can represent focused recharge from a losing stream, or, as in the case of groundwater discharge to a stream, the exchange can reflect diffuse recharge that occurs over widespread areas. Some of these methods may be unfamiliar to groundwater hydrologists because they were not developed specifically for the study of groundwater recharge; instead, they were developed for purposes such as sizing of culverts and bridge openings, predicting low-flow rates in streams, or developing an understanding of stream-water quality and the ability of a stream to assimilate solutes and contaminants. The fact that base-flow or recharge estimates are generated as byproducts of these methods does not diminish the usefulness or applicability of the methods in recharge studies.
Techniques presented herein include the stream water-budget method, seepage meters, Darcy methods, streamflow duration curves, traditional streamflow hydrograph analyses (including hydrograph separation and recession-curve displacement), and chemical and isotopic hydrograph separation techniques. Some of these methods are designed specifically for estimating focused recharge; others are for estimating diffuse recharge. Discussions are centered on groundwater movement to or from streams, but the principles discussed and the methods described are equally applicable for groundwater exchange with other surface-water bodies, such as lakes, reservoirs, and wetlands. Proper application of any method requires a good conceptual model of the hydrologic system and a solid understanding of underlying assumptions. Prior to presentation of individual methods, background discussions are given on the exchange of groundwater and surface water and on the relationship between base flow and recharge. These discussions illustrate assumptions inherent to the methods and provide some guidelines for assessing the validity of those assumptions.
Index
- Richard W. Healy
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5 - Physical methods: unsaturated zone
- Richard W. Healy
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Summary
Introduction
Estimates of recharge can be obtained from measurement of downward water flux or change in water storage within the unsaturated zone. Methods based on physical (as opposed to chemical) data collected within the unsaturated zone are not among the more commonly used techniques for estimating recharge, but they offer some distinct advantages. The methods actually produce estimates of drainage rates below the depth of measurement within the unsaturated zone. The usual assumption is that the draining water will eventually reach the water table, at which time it can properly be called recharge. But there can be a long lag time over which water traverses that depth interval. Estimates generated by these methods are referred to as drainage in this chapter. In general, these methods produce point estimates of drainage. The question of how representative a measurement at a single point is of flux through the unsaturated zone as a whole requires careful consideration. These methods can be costly to implement and require intensive instrumentation that is susceptible to measurement inaccuracies. Nonetheless, under certain circumstances, such as rapid movement of a wetting front from land surface to a shallow water table, these methods have a unique ability to provide detailed insight into recharge processes and factors that influence recharge rates.
The methods can be divided into two classes: water-budget methods and methods based on the Darcy equation. Unsaturated-zone water budgets relate changes in the amount of water stored in the unsaturated zone to infiltration, drainage, and evapotranspiration. These methods include the zero-flux plane method and lysimetry. Included in the section on the zero-flux plane method is a discussion on measurement of water storage and change in storage within the unsaturated zone. Darcy methods require measurement or estimation of the hydraulic gradient and hydraulic conductivity at the ambient water content. Natural variability in hydraulic conductivity complicates the application of the Darcy method. Lysimeters can provide precise measurements of drainage rates, but the instruments can be expensive to install and maintain. A brief overview of techniques for measuring water content, pressure head, water-retention characteristics, and hydraulic conductivity of unsaturated-zone sediments, provided in Section 5.2, lays the groundwork for discussions of the zero-flux plane method (Section 5.3), the unsaturated-zone Darcy method (Section 5.4), and the use of lysimeters (Section 5.5).