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A Primer on Groundwater Management Michael E. Campana, University of New Mexico Introduction Groundwater management today involves many facets. Ground-water managers must be cognizant of many things – not just the science of ground-water hydrology, but a literal laundry list of other disciplines: economics; surface-water hydrology; law; engineering; ecology; management; chemistry; and communications. To that list some (including the author) would add a dash of sociology, public relations, psychology, and political science. Suffice it to say that management of ground-water basins in the 21st century will demand more of our managers than ever before. Consider one of the most fundamental questions in ground-water management: how much water can a ground-water basin yield? Gone are the days when we spoke of the “safe yield” of ground-water basins, when safe yield could be defined almost any way we wished. Indeed, Todd (1959) defined a basin’s safe yield as “the amount of water which can be withdrawn from it annually without producing an undesired result.” As Alley and Leake (2004) stated, we have now journeyed from safe yield to sustainability. But as they also stated, sustainability is no more concise than safe yield, and is a value-laden concept that means different things to different people. The definition of ground-water sustainability is quite similar to that of safe yield, and just as broad and ambiguous: “the development and use of ground-water resources in a manner that can be maintained for an indefinite time without causing unacceptable environmental, economic or social consequences” (Alley and Leake, 2004). Depending upon what the word “social” subsumes, we could add “cultural”, “political” and “legal” as modifiers of “consequences”. Arguably one of the most important aspects of sustainability that has recently come to the fore is the effect of ground-water development on riparian and aquatic ecosystems. Hydrologists have been aware of surface-water – ground-water interactions for years, but recently the environmental implications of those interactions have gained new impetus (Grantham, 1996; Glennon, 2002). Ground-water managers must be prepared to consider such implications in defining the sustainability of ground-water basins. In a recent paper, Devlin and Sophocleous (2005) describe the difference between the terms “sustainability” and “sustainable pumping”. As mentioned above, sustainability is a value-laden concept that encompasses far more than just the extraction of groundwater, whereas sustainable pumping (or “sustainable development”) refers to the pumping rate that can be maintained indefinitely without dewatering or mining an aquifer. This paper will not be a comprehensive treatise on groundwater management, but a primer or guide to some of the issues confronting 21st century groundwater managers. 26

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Characteristics of Groundwater Groundwater comprises the largest stock of liquid fresh water on the earth, almost 100 times the volume of the fresh liquid surface water (Maidment, 1993) – about 10,530,000 cubic kilometers. Worldwide ground-water withdrawals are approximately 750-800 cubic kilometers per year (Shah et al. 2000). What is groundwater? Groundwater is simply water beneath the land surface in the saturated zone, which is the region where the openings (pores or fractures) in the rock or soil are completely filled with water. The top of the saturated zone is the water table. Groundwater is distinguished from other forms of subsurface water by the fact that it occurs under completely saturated conditions (note that there has been a disturbing trend by some in recent years to call all subsurface water “groundwater”, which is, strictly speaking, incorrect). Aquifers are earth materials (sediments, “soft” and “hard” rocks) that store and transmit groundwater in quantities sufficient to supply wells. Note several things about this definition. What do we mean by “quantities sufficient to supply wells”? What kind of wells? A domestic well that needs only to produce a few tenths of a liter per second (Lps)? Or an irrigation or municipal well that produces 100 Lps? As we saw with safe yield and sustainability, ambiguity rules in the definition of aquifer. However, among ground-water hydrologists, there is general agreement about what types of materials have the potential to be “good” aquifers: “clean” (i.e., little silt or clay) sands and gravels, cavernous (karst) carbonate rocks, and young basalts. Recharge and discharge are two important characteristics of aquifers and ground-water basins. Recharge refers to water that moves across the water table to replenish an aquifer. Recharge can come from the infiltration of precipitation, which moves through the soil and thence across the water table, or via seepage from a stream or other surface water body. Discharge is groundwater leaving the saturated zone and exiting at the land surface or to a surface-water body. Groundwater can discharge naturally via: springs and seeps; seepage to surface water; and evaporation and transpiration (evapotranspiration). Artificial ground-water discharge occurs by wells and galleries. Groundwater possesses several characteristics that distinguish it from its counterpart in the hydrological cycle, surface water. Aside from the fact that it is hidden, groundwater’s major differences from surface water are that: • It moves more slowly than surface water. In most cases, ground-water velocities on the order of a few meters per day are the rule, not the exception. • Ground-water systems generally have much greater storage capacities than surface water systems. • Ground-water response times vary greatly, but in general are much greater than those for surface water systems. In response to a stimulus (pumping, change in 27

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recharge), it may take minutes, hours, days, months, years, centuries or millennia for changes to manifest themselves at certain points in the system. • Contaminants in ground-water systems move more slowly than they do in surface waters. • Contaminants in groundwater can potentially affect much larger volumes in ground-water systems than in surface water ones and take much longer to be flushed from the system. It should be noted that under certain conditions, such as in cavernous carbonate aquifers like the Edwards aquifer in Texas, USA, or those underlying the Yucatán Peninsula of México, ground-water flow may be quite similar to surface-water flow in terms of velocities (high) and response times (short). Ground-Water Management 1011: The Water Budget It is rather easy to write a water budget for a ground-water basin prior to development. Following Bredehoeft et al. (1982) we can write, for a ground-water basin under steady state (undisturbed or equilibrium) conditions: Ro = Do (1) Where Ro and Do are the mean ground-water recharge and discharge rates, respectively, under virgin (predevelopment/pre-pumping) conditions. After imposing a pumping rate of Q, the water budget becomes (Bredehoeft et al., 1982): (Ro + ΔRo) - (Do + Δ Do) - Q + dV/dt = 0 (2) where: Δ Ro = change in the mean recharge rate; Δ Do = change in the mean discharge rate; Q = pumping rate; and dV/dt = rate of change of groundwater storage in the system. Each term in equations (1) and (2) has dimensions of L3T-1 (units of Lps, cubic feet per second, cubic meters per second, acre-feet per year, etc.). If we substitute equation (1) into (2), the result is (Bredehoeft et al. 1982): Δ Ro - Δ Do - Q + dV/dt = 0 (3) 1 Note for readers who are not familiar with the term “101”: The term “101” is in reference to an introductory-level class at the university that in many cases introduces students to the fundamentals of a given discipline. Likewise, “102,” which is used in the section on “The Water Budget Myth,” would refer to a similar course that would be taken after the introductory course. 28

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Equation (3) is the new budget for the ground-water basin under transient (time- dependent) conditions due to ground-water development via pumping. What is clear from equation (3) is that the original steady state values of discharge and recharge, Ro and Do, are unimportant. They do not enter into the calculation. The changes in recharge and discharge are important. This will be illustrated below. Changes in ground-water recharge and discharge rates in response to pumping can occur in a number of ways, For example, changes in natural discharge rates can occur through reduction in: 1) ground-water flow to surface-water bodies (lakes, streams, wetlands, oceans, etc.); 2) spring flow; and 3) evapotranspiration (via the direct evaporation of groundwater and by phreatophytes, plants that withdraw water directly from the water table) in response to a lowered water table. Increases in recharge can occur because of: 1) lowered water tables, which would permit water that otherwise could not infiltrate, to infiltrate into the soil and recharge the aquifer; and 2) induced recharge from surface- water bodies. Let’s now return to why changes in recharge and discharge are important. If we now want to specify a budget for sustainable development, then dV/dt = 0 and (Bredehoeft, 2002): Δ Ro - Δ Do = Q (4) The quantity (Δ Ro - Δ Do) is called the capture attributable to the pumping (Bredehoeft, 2002). To reach a new equilibrium, the capture must equal the pumping rate. Ground-Water Management 102: The Water-Budget Myth Bredehoeft et al. (1982) wrote their paper, aptly entitled Groundwater: The Water-Budget Myth, to elucidate the principles first promulgated by Theis (1940), who said that when a new stress (pumping) is superimposed upon a ground-water system in equilibrium, the new discharge must be balanced by: 1) a reduction in natural discharge; 2) an increase in natural recharge; 3) a loss in ground-water storage (decrease in the amount of groundwater in the subsurface); or 4) a combination of these. They felt the need to do this because they noted the prevalent belief among water managers and some hydrologists that the predevelopment water budget – essentially the natural recharge rate – determines the magnitude of ground-water development. This is the so-called “Water-Budget Myth” (hereafter referred to as the WBM), a term coined by Bredehoeft et al. (1982). They also noted that Brown (1963) attempted to dispel this misconception, but in a highly technical fashion that was not readily understandable by any but a few ground-water hydrologists. Later, Alley et al. (1999), in an excellent U.S. Geological Survey Circular, stated succinctly (page 16): “Some hydrologists believe that a predevelopment water budget for a ground-water system (that is, a water budget for the natural conditions before humans used the water) 29

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can be used to calculate the amount of water available for consumption (or the safe yield). This concept has been referred to as the ‘Water-Budget Myth’.” Bredehoeft (2002) again dispelled the WBM in a short, lucid paper in which he simulated development in a basin using a numerical ground-water flow model. His model was based on a hypothetical but realistic ground-water basin in the arid Basin and Range Province of the western USA (presumably in Nevada, which has codified the WBM in its water law by restricting a basin’s maximum pumping rate to its natural recharge rate). Bredehoeft’s (2002) results showed that capture is a dynamic process and the important factor in determining how a ground-water system reaches a new equilibrium. He concluded his paper with these words (Bredehoeft, 2002, p. 345): “These ideas are not new. Theis spelled them out in 1940. Somehow the groundwater community seems to lose sight of these fundamental principles.” Devlin and Sophocleous (2005) state that the persistence of the WBM may be related to confusion over the concepts of sustainability and sustainable pumping. They argue that the determination of natural recharge Ro can be important in sustainability studies, but it is not necessary to determine sustainable pumping rates, which is what we have been concerned with in our discussion of the WBM. If ground-water managers learn nothing else, they need to understand the Water-Budget Myth, why it is a myth, and what does determine the response of a ground-water basin to pumping. They also need to understand the concepts of sustainable pumping rates (or sustainable development) and sustainability, and the differences between them. Ground-Water and Surface-Water Interactions Groundwater and surface water (where it exists) are often in close hydrologic communication. Groundwater can discharge to surface-water bodies or it can receive water (recharge) from such bodies. Some aquifers may receive seepage from surface water in some areas and discharge to surface water in other areas. Similarly, contaminants can also migrate between surface water and groundwater. In fact, there is now an increasing realization that ground-water systems and surface-water systems are inextricably linked and should really be treated as a single hydrological unit (Winter et al., 1998). In terms of water management and regulation - both water quality and water quantity - this interconnection is not always recognized. Ground-water development can greatly impact streamflow, so much so that flow from the aquifer to the stream can be induced or, where it previously existed, increased. This can lead to several problems, which may include infringement upon surface water rights and the introduction of contaminated surface water into the aquifer. The groundwater – surface water interface, known as the hyporheic zone, is itself an environmental zone that not only harbors life, but also is an important region for the transfer of nutrients and chemicals between aquatic and terrestrial environments. 30

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Biogeochemical reactions occurring in the hyporheic zone also can have important implications for water quality (von Gunten and Lienert, 1993). Groundwater is also important to the functioning of wetlands and bogs (Winter et al., 1998). Groundwater and associated surface water should be managed conjunctively, which includes utilizing Aquifer Storage and Recovery (ASR) – the storing of surface water underground for use at a later date. Existing water codes may not recognize the intertwined nature of these two resources and attempt to treat them as two separate entities, which may complicate the manager’s task. Effects of Groundwater Overdrafting Wetlands and Riparian/Aquatic Ecosystems Ground-water overdrafting, or withdrawal of groundwater from storage, has recently been recognized as a potential threat to wetlands, riparian areas, and aquatic ecosystems. Surface ecosystems can be dependent upon groundwater; examples include wetlands, often located in ground-water discharge zones and riparian ecosystems, which may depend upon shallow groundwater (Grantham, 1996). Streamflow or other surface-water bodies may be depleted by ground-water pumping. Effects on ecosystems have not often been considered in the allocation of ground-water resources (Grantham, 1996). A number of areas in the USA have been impacted by ground-water withdrawals. One of the best-known areas is the Upper San Pedro River of southern Arizona, where rapid growth has depleted the flow in the river, an important water resource and a world-class birding area (Grantham, 1996, pp. 42-43; Glennon, 2002, Chapter 4). The Santa Cruz River flowing through Tucson, AZ, has been dramatically impacted by the rapid growth of Tucson, which, until recently, relied exclusively on groundwater (Glennon, 2002). Development of groundwater from the Edwards aquifer system in the San Antonio, TX, area, has threatened spring discharge and related ecosystems, and has ESA (Endangered Species Act) implications (Longley, 1992). The effects of ground-water overdrafting on ecosystems has only recently been recognized. In her landmark report, Grantham (1996) reviewed over 6,300 citations from the fields of hydrology, hydrogeology, and wetland ecology and found fewer than 30 papers dealing directly with the effects of overdrafting on ecosystems. Few people are aware of her excellent report, primarily because it was published in the “gray literature” and, to the author’s knowledge, was never published in the refereed literature nor distributed widely. Glennon’s (2002) popular and well-documented book has done much to bring this important issue before not only the public but also the scientific, regulatory, and engineering communities. Ground-water managers would be well-advised to read both these publications and incorporate their recommendations into their management plans. 31

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Other Effects Ground-water overdrafting can have other serious effects that the ground-water manager must be aware of. These include: • Land subsidence and sinkhole formation • Sea-water intrusion • Changes in ground-water quality Land subsidence can be a serious problem. In the San Joaquin Valley of California, USA, the maximum land surface subsidence has been over 9 meters (Galloway et al., 1999), and parts of México, D.F., have subsided this much as well. Sinkholes (or cenotes in Spanish), a “catastrophic” form of land subsidence, can be a problem when excessive pumping from carbonate (limestone and dolomite) aquifers occurs. Sea-water intrusion can destroy fresh ground-water resources by replacing the pumped fresh water with saline water. The aquifers beneath Brooklyn, NY were destroyed in this manner. Changes in ground-water quality, aside from the extreme case of sea-water intrusion, can be induced when pumping causes the introduction of poor-quality water from adjacent aquifers, surface-water bodies, or from greater depths in the same aquifer being pumped. Ground-Water Quality and Quantity Just as it is important to manage surface water and groundwater conjunctively, so is it important to jointly manage and regulate ground-water quality and quantity. Although this sounds so obvious as to be trite, not all political jurisdictions recognize this. Indeed, in some of the USA states, ground-water quantity and quality are regulated by different agencies within the same state. The ground-water manager must nonetheless integrate these two aspects of groundwater into her management plan, recognizing that quality determines water use and water use affects quality. Concluding Remarks The 21st century ground-water manager must be many-faceted. We have attempted to illustrate some of the more important fundamental aspects of ground-water management. A very rudimentary acronym can be used to summarize the principles of ground-water management: SIMPLE. • Sustainable long-term ground-water extraction. Be cognizant of the Water-Budget Myth and strive to manage the system sustainably. • Integrated water resources management. Manage surface water and groundwater conjunctively; realize that ground-water quality and quantity cannot be separated; and recognize the interrelationship among land use, water use, and water quality. IWRM also implies involvement of stakeholders in the decision-making process. 32

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• Maintenance of ecosystems. Understand the effects pumping can have on ecosystems, and consider these effects in management plans and water resource allocation. • Protection of ground-water quality. This involves avoiding the pollution of aquifers by properly locating, designing, and managing municipal landfills, agricultural operations (farms, feedlots, etc.), hazardous waste disposal sites, sewage disposal facilities (e.g., septic tanks), industrial facilities, and other contaminant sources so that effluent from these sources contaminates neither surface water or groundwater. • Large storage volume utilization. Ground-water reservoirs often have huge storage capacities. This capacity can be used as a: 1) buffer against drought; 2) place to store excess surface water, and 3) reserve to extract groundwater unsustainably for a short period if the situation warrants it. • Equity. Everyone and everything (i.e, flora and fauna) must be treated equitably. Easier said than done, but it nevertheless is an important aspect of 21st century ground-water management. Remembering the concepts embodied in the SIMPLE approach, will help ground-water managers cope with the demands of the 21st century. For additional information, interested readers should refer to the excellent, highly- readable (by the intelligent layperson) U.S. Geological Survey Circulars by Alley et al. (1999) and Winter et al. (1998). These and other USGS publications are downloadable at http://water.usgs.gov/pubs/. 33

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References Alley, W. M. and S.A. Leake. 2004. The journey from safe yield to sustainability. Ground Water 42(1): 12-26. Alley, W.M., T.E. Reilly and O.L. Franke. 1999. Sustainability of Ground-Water Resources. U.S. Geological Survey Circular 1186, 79p. Bredehoeft, J.D. 2002. The water budget myth revisited: why hydrogeologists model. Ground Water 40(4): 340-345. Bredehoeft, J.D., S.S. Papadopulos and H.H. Cooper. 1982. Groundwater: the Water- Budget Myth. In Scientific Basis of Water-Resource Management, Studies in Geophysics, Washington , DC: National Academy Press, pp. 51-57. Brown, R.H. 1963. The cone of depression and the area of diversion around a discharging well in an infinite strip aquifer subject to uniform recharge. U.S. Geological Survey Water-Supply Paper 1545C, pp. C69-C85. Devlin, John F. and Marios Sophocleous. 2005. The persistence of the water budget myth and its relationship to sustainability. Hydrogeology Journal 13(4): 549-554. Galloway, D., D.R. Jones and S.E. Ingebritsen. 1999. Land Subsidence in the United States. U.S. Geological Survey Circular 1182, 177p. Glennon, Robert. 2002. Water Follies: Groundwater Pumping and the Fate of America’s Fresh Waters. Washington, DC: Island Press, 314p. Grantham, C. 1996. An Assessment of the Ecological Impacts of Ground Water Overdraft on Wetlands and Riparian Areas in the United States. Moscow, ID: Research Technical Completion Report, Idaho Water Resources Research Institute, University of Idaho, 103p. Longley, G. 1992. The subterranean aquatic ecosystem of the Balcones Fault Zone Edwards Aquifer in Texas – threats from overpumping. In J. Stanford and J. Simons (eds.) Proceedings of the First International Conference on Ground Water Ecology. Bethesda, MD: American Water Resources Association, Technical Publication Series TPS92-2, pp. 291-300. Maidment, D.R. 1993. Hydrology. In Maidment, D.R. (ed.) Handbook of Hydrology. New York: McGraw-Hill, pp. 1.1-1.15. Shah, T., D. Molden, R. Sakthivadivel and D. Seckler. 2000. The global groundwater situation: overview of opportunities and challenges. Colombo, Sri Lanka: International Water Management Institute. 34

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Theis, C.V., 1940. The source of water derived from wells: essential factors controlling the response of aquifers to development. Civil Engineering 10: 277-280. Todd, D.K. 1959. Ground Water Hydrology. New York: John Wiley and Sons. Von Gunten, H. and C. Lienert. 1993. Decreased metal concentrations in ground water caused by controls on phosphate emissions. Nature 364: 220-222. Winter, T.C., J.W. Harvey, O. L. Franke and W.M. Alley. 1998, Ground Water and Surface Water: A Single Resource. U.S. Geological Survey Circular 1139, 79p. 35