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.

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.

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.

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.

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.

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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.

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.

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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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.

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.

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