9.3 Problems Associated with Groundwater

Describe problems with groundwater that result from human activities.

Groundwater is essential for human needs, particularly in arid regions, where most or all water used by people comes from the ground. Heavy reliance on groundwater can create conflict among neighboring communities drawing from the same aquifer. There are two major problems associated with human use of groundwater resources: (1) withdrawal of groundwater faster than it is replenished and (2) pollution of groundwater.

Too Much Too Fast: Groundwater Overdraft and Mining

The height of the water table in an unconfined aquifer is the result of a balance between groundwater recharge (inputs) and groundwater discharge (outputs). Human activities can disrupt this hydrologic balance by groundwater overdraft and groundwater mining.

Groundwater Overdraft

Groundwater overdraft is the removal of water from an aquifer faster than the aquifer is recharged at the site of a well. Groundwater overdraft often forms a cone of depression: a cone-shaped lowering of the water table around the well from which water is being removed (Figure 9.27). Overdraft and the resulting cone of depression change the topography of the water table near the well. As a result, the pattern of hydraulic pressure and the direction of water movement in the aquifer can also change. Water can migrate toward the lowered water table at the cone of depression.

Figure 9.27

GEO-GRAPHIC: Groundwater overdraft may lower the water table. When water is withdrawn from an aquifer faster than the aquifer is recharged, the water table drops. In the example on the right side of the figure, water withdrawals from a well used by a large factory have lowered the water table out of the reach of a smaller well.

Animation

Lowered water table

http://qrs.ly/1a457cf

groundwater overdraft

Withdrawal of water from an aquifer faster than the aquifer is recharged at the site of a well.

cone of depression

The cone-shaped lowering of the water table resulting from groundwater overdraft.

Figure 9.28

GEO-GRAPHIC: Saltwater intrusion. Hydraulic pressure exerted by the weight of ocean water forces salt water inland. A freshwater aquifer exerts pressure against the salt water, creating an equilibrium line between the two. Groundwater overdraft can reduce the hydraulic pressure of the freshwater and cause the equilibrium line to retreat inland, allowing salt water to contaminate the well.

Groundwater overdraft can result in several additional problems, including a lowered water table at other wells, contamination of wells by salt water, and land subsidence.

  1. Lowered water table: If neighboring wells are not deep, the cone of depression can lower the water table and cause them to go dry (see Figure 9.27).

  2. Saltwater intrusion: In coastal regions, a freshwater aquifer may lie on top of salty groundwater. Groundwater overdraft may cause the salty water to migrate higher in the aquifer and contaminate wells. This process is called saltwater intrusion (Figure 9.28). Saltwater contamination of a well is permanent and irreversible.

    Animation

    Saltwater intrusion

    http://qrs.ly/9w457cg

    saltwater intrusion

    The contamination of a well by salt water as a result of groundwater overdraft.

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  3. Land subsidence: The pressure of the water in the pores of sediments keeps sediment particles apart while the ground is saturated. When water is removed, these pores can collapse under the weight of the sediments. When this happens, the elevation of the land surface drops as the sediments are compacted (Figure 9.29A) In most cases, once the pores in the aquifer collapse, they can no longer hold water, and the aquifer is lost permanently. Near Mendota, in the San Joaquin Valley, California, the land elevation has dropped more than 8.5 m (28 ft) due to groundwater overdraft. In some cases of land subsidence, large surface fissures (or cracks) can open up (Figure 9.29B).

Figure 9.29

GEO-GRAPHIC: Land subsidence. (A) The collapse of pores caused by groundwater overdraft results in land subsidence. (B) These large subsidence fissures have opened up in the town of Queen Creek, in southern Arizona.
(B. Arizona Department of Water Resources)

Groundwater Mining

Groundwater mining is the process of extracting groundwater from areas where there is little to no groundwater recharge. In areas where there is no recharge, once groundwater has been mined, it is gone permanently.

groundwater mining

The process of extracting groundwater where there is little to no groundwater recharge.

North America’s largest aquifer is the Ogallala Aquifer (also called the High Plains Aquifer), which lies beneath the Great Plains (Figure 9.30A). About 200,000 water wells tap into the Ogallala Aquifer. Most of them are connected to center-pivot irrigation systems used for agricultural fields (Figure 9.30B). Agriculture in the Great Plains generates some $20 billion in revenue each year, or about one-ninth of the total U.S. agricultural production.

Figure 9.30

The Ogallala Aquifer. (A) The Ogallala Aquifer (as mapped in 2009) stretches from South Dakota to northern Texas. Its zone of saturation is thickest in Nebraska, where it is about 300 m (1,000 ft) thick. (B) This aerial photo shows the circular irrigation patterns produced by farms using the Ogallala Aquifer near the Texas Panhandle.
(B. © Robert S. Ogilvie)

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The water in the Ogallala is fossil groundwater: water that entered the aquifer long ago and is no longer being replenished. Most of it came from the now-melted Laurentide ice sheet that covered much of North America during the most recent glacial period (see Section 6.2). In the southern portions of the Ogallala, the climate is too arid to recharge the aquifer. There, farmers and ranchers are mining the Ogallala’s groundwater. As a result, the water table is dropping across the southern stretches of the aquifer, by more than 1.6 m (5 ft) each year in some places (Figure 9.31). The deeper the water table becomes, the more energy and expense is required to pump water because it has a longer way to go to reach the surface.

Figure 9.31

Water table changes in the Ogallala Aquifer. The water table has dropped more than 46 m (150 ft) in historic times in some areas, mostly in northern Texas.

fossil groundwater

Water that entered an aquifer long ago and is no longer being replenished.

Aquifers in arid regions, where there are no permanent surface water bodies, are always composed of fossil groundwater. Many countries face looming problems with the loss of groundwater, including China, India, Pakistan, and most countries in the Middle East. About one-fourth of India’s food is grown using groundwater that is not being replaced. Saudi Arabia, in the hyperarid Arabian Desert, has developed an agricultural economy centered on mining fossil groundwater, as highlighted in Picture This.

Groundwater Pollution

Common sources of groundwater pollution include leaks from landfills (or dumps), septic systems, and gas station tanks; agricultural chemicals; animal sewage from factory farms; and mining activities. The process of mining natural gas from shale rock is also linked to groundwater pollution (see Section 13.5).

In some areas, toxic chemicals are (or were) intentionally dumped on the ground or injected into aquifers from wells to dispose of them cheaply. In the 1950s and 1960s, the large utility company Pacific Gas and Electric (PG&E) dumped hexavalent chromium into a collecting pool. From there, the chemical seeped into the ground and migrated several miles through the aquifer. The wells in nearby Hinkley, California, gradually became contaminated, and rates of various types of cancers skyrocketed in the small town. Many of the residents became ill and died. Although PG&E denied wrongdoing, the company was held accountable in court because of the efforts of the environmental activist Erin Brockovich. Environmental regulations are now in place to stop (or at least reduce) such practices.

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Picture This

(Map: NASA Earth Observatory image created by Robert Simmon and Jesse Allen, using Landsat data provided by the United States Geological Survey; photo, © Matt Green)

Groundwater Mining

This Landsat satellite image acquired in January 2012 shows center-pivot irrigation plots for grain, fruit, and vegetable farming in Saudi Arabia. This method of farming is increasingly used in arid regions where fossil groundwater is available. The fossil groundwater in Saudi Arabia is at least 10,000 years old. It entered the ground when the climate was wetter during the most recent glacial period (see Section 6.2).

The green agricultural fields visible in the desert indicate that groundwater is being mined. At the center of each green circle is a well pipe and a diesel pump that pulls water up from the aquifer below. Here, water is pumped from as deep as 1 km (0.6 mi) beneath the surface. The water moves through a tubular arm called a gantry that stretches across the circle’s radius (inset). The gantry slowly sweeps around the field. The diameter of each circle is about 1 km (0. 6 mi).

Consider This

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Once pollutants have entered an aquifer, they disperse as a contaminant plume: a cloud of pollution that migrates through the aquifer away from its source (Figure 9.32). Any pollutants, natural or anthropogenic, that enter the aquifer stay there for centuries or longer. Therefore, the loss of an aquifer through groundwater pollution is, in most cases, permanent.

Figure 9.32

Aquifer contaminant sources. Sources of groundwater contaminant plumes (in light orange) come from a variety of human activities. Contaminant plumes migrate in the direction of water flow within an aquifer.

Video

Groundwater contamination

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contaminant plume

The cloud of pollution that migrates through an aquifer away from its source.

Various techniques for cleaning up polluted groundwater have been tried. These techniques include the use of bacteria to digest the contaminants and the use of chemicals to react with the contaminants, changing them into other, less harmful chemicals. The process of cleaning a contaminated aquifer is called groundwater remediation (Figure 9.33). In most instances, however, the best option becomes closing contaminated wells and stopping the “upstream” sources of contaminants.

Figure 9.33

SCIENTIFIC INQUIRY: How is contaminated groundwater cleaned? In 1980, the U.S. Environmental Protection Agency Superfund Program was established to clean up the nation’s worst toxic contamination. Over 80% of EPA Superfund sites are areas of contaminated groundwater. The method of cleanup used depends on the type of contaminant, the physical circumstances at the site, and the cost. Generally, contaminated water is either pumped from the ground and treated, or it is treated while in the ground, a method called in situ remediation.

groundwater remediation

The process of cleaning a contaminated aquifer.

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