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Introduction to Groundwater

CHAPTER MENU

1. 1.1 Why Study Groundwater?
2. 1.2 Brief History of Groundwater

This book is concerned with the theory and practice of groundwater hydrology, the science of water in subsurface environments. It is divided into two main parts; (1) basic concepts dealing with the origin, movement of groundwater, and its recovery from wells, and (2) important areas of practice that these days includes groundwater sustainability, geogenic, and anthropogenic problems of contamination. The book is developed around a process‐oriented theme that organizes hydrologic phenomena based on physical and mathematical principles. It emphasizes the application of knowledge to the solution of practical problems focused on aquifers and human impacts, as well as hydraulic testing and groundwater contamination.

One of the important ways to learn about groundwater problems is to experience them firsthand. Thus, the book relies on case studies and demonstrations of techniques through worked problems. Although most of the hydrogeological world is hidden from view, there are exciting things to see in the field. We brought some of these features to life through the colored photographs and illustrations.

1.1 Why Study Groundwater?

There are a variety of reasons why scientists and engineers study groundwater. First and foremost, groundwater is a key source of drinking water that is essential to life on earth, as we know it. The earth has something like 1375 million cubic kilometers with most occurring as non‐potable seawater. Groundwater, interestingly, is a tiny fraction, just 0.06% of the earth's available water. However, this relatively small volume is critically important because it represents 98% of the freshwater readily available to humans (Zaporozec and Miller, 2000). Abundant fresh water is tied up in glaciers and icecaps, but essentially unavailable. Of the other available reservoirs, large rivers have been particularly important for their role in sustaining societies for millennia. However, now, the explosive growth of human populations around the world over the last 150 years has required unsustainable development of groundwater supplies, which is among the most serious problems affecting humanity today.

Groundwater is found in aquifers, which have the capability of both storing and transmitting groundwater. An aquifer is defined formally as a geologic unit that is sufficiently permeable to supply water to a well. Commonly, the large volumes of water stored in aquifers could be counted on as reliable source during periods of drought lasting months or years. Moreover, hydrogeologists have always been taught that groundwater is a renewable resource, recharged by rain and snow‐melt runoff. Indeed, major aquifer systems store impressively large quantities of water. For example, the High Plains Aquifer located in America's Midwest covers an area of 450,000 km2. It stored an impressive 4000 km3, roughly equivalent to Lake Huron, the fifth‐largest lake in the world. Now, that important function is threatened by groundwater technologies capable of depleting groundwater in just a few generations.

Figure 1.1 The Nubian Sandstone Aquifer System underlies Egypt, Libya, Chad, and Sudan. The oases shown were localized discharge areas for waters leaking upward from the aquifer. East Oweinat is an area where groundwater is mined for irrigation of crops

(FWS).

The sad reality is that aquifers can be pumped to the extent that they easily flip to being a nonrenewable resource. Our book is chock full of examples of water supplies in aquifers that are no longer renewable, such as the High Plains aquifer system in the United States. Particularly illustrative in this respect has been the historical utilization of groundwater in the oases of the Western Deserts of Egypt (Figure 1.1) The main oases there occur are associated with erosional depressions within the largely desert landscape. They exist because underlying the Sahara Desert is the Nubian Sandstone Aquifer System. Fracture systems facilitated the natural upward flow of artesian groundwater that discharge into these oases. This water has nurtured human societies for nearly 25,000 years (Caton‐Thompson and Gardner, 1932). Moreover, these oases (Figure 1.1) have been centers of trade even for millennia with the first occupation before the Old Kingdom (~2500 BCE) time in Egypt.

The Kharga Oasis (Figure 1.1) was likely a seasonal home for paleolithic humans, who migrated there as early as 25,000 years B.P. Groundwater originally discharged as a series of mound springs in the valley floor. Nearby is the Dakhla Oasis which also provided unique agricultural products during Middle Kingdom of Egypt (Boozer, 2015). Crops in these oases required irrigation from groundwater given that the mean annual rainfall is less than 1 mm year. These oases came under control of the Romans about 30 BCE and with expanded irrigation (Figure 1.2) supplied the empire with olives, dates, and wine (Kaper and Wendrich, 1998).

The Nubian Sandstone aquifer is the largest aquifer in the world with an areal extent of 2.6 million km2. Rainfall is the original source of water, having infiltrated the subsurface in the south‐western corner of Egypt and flowed northeastward hundreds of kilometers to the northeast Studies with krypton isotopes and other isotopes (Sturchio et al., 2004) indicated groundwater ages from 200,000 years (sampled at Dahkla) to 1 million years (at Bahariya).

Since the 1930s, increasing numbers of deep, high‐capacity groundwater wells eventually resulted in water‐level declines. For example, through the 1950s, deep wells in the Kharga Oasis flowed at the surface. By 1975, all the deep wells had ceased flowing (LaMoreaux et al., 1985). The future now appears to be groundwater mining using deep (800–1000 m) high‐capacity wells until storage is depleted.

Groundwater in most places is at risk of depletion because of its essential role in irrigation and food production. Potential solutions to aquifer restoration are technical complicated, expensive, and impractical in developing countries (Schwartz et al., 2020). Thus, although aquifers often facilitate food production in arid lands for a while, the water will eventually be used up when rates of production exceed rates of replenishment. Shown in Figure 1.3 is a satellite view the East Oweinat Project, located in southern Egypt (map Figure 1.1). This is one of several ambitious projects to diversify agriculture geographically within Egypt. In this case, irrigation of desert lands is accomplished by mining groundwater water from the Nubian Sandstone Aquifer System. The circles on the figure are indicative of central pivot irrigation with crops like wheat and potatoes.

Figure 1.2 The photograph shows the remains of a Roman‐age water well at the Dakhla, Oasis. Groundwater discharged originally as artesian and mound springs, which were associated with fault and fracture zones

(With permission from Phil LaMoreaux).

Figure 1.3 This image covers a 33.3 km × 47.2 km area of south‐western Egypt and shows irrigated lands of the East Oweinat Project. Crops are watered with pivot irrigation systems using groundwater mined from the Nubian Sandstone Aquifer System

(National Aeronautics and Space Administration/https://photojournal.jpl.nasa.gov/targetFamily/Earth/last accessed under 5 May 2023).

One feature of groundwater that makes it valuable as a resource is its physical and chemical quality. Unlike many surface‐water supplies, natural groundwater has few suspended solids, small concentrations of bacteria and viruses, and often only minimal concentrations of dissolved mineral salts. These characteristics make groundwater an idea source of water to support human life. The connection of the groundwater pathway in hydrologic cycle to the land surface, unfortunately, provides the opportunity for humans to pollute natural groundwaters and devalue the resource.

Not surprisingly, then, issues of groundwater pollution and the protection of groundwater resources provide another important reason to study groundwater. In the latter part of the 20th century, hydrogeologists became aware of the health threat posed by contamination and the daunting technical challenges in cleaning up contaminated sites.

The human attack on groundwater comes from many different directions. Significant contamination in groundwater has come from the careless disposal of human and animal wastes; haphazard disposal of industrial wastes, and contamination associated with mining and oil operations; leaks from storage tanks, pipelines, or disposal ponds; and everyday activities such as farming and solid waste disposal. Developed countries have moved aggressively to clean up or mitigate historical problems of contamination and to prevent new problems from developing. Other countries will require more time and money to confront the problems of contamination.

Since publication of the first edition of this book in 2002, new problems of contamination have emerged. From a health perspective, the most serious are geogenic contaminants (due to geological processes), such as arsenic and...