Like many of our other valuable natural resources, groundwater is being exploited. In some areas, overuse threatens the groundwater supply. In other places, groundwater withdrawal has caused the ground and everything resting on it to sink. Still other localities are concerned with contamination of the groundwater supply.

Treating Groundwater as a Nonrenewable Resource

For many, groundwater appears to be an endlessly renewable resource, for it is continually replenished by rainfall and melting snow. In contrast, groundwater in some regions has been and continues to be treated as a nonrenewable resource. Where this occurs, the amount of water available to recharge the aquifer is significantly less than the amount being withdrawn.

The High Plains, a relatively dry region that extends from South Dakota to western Texas, is one example of an extensive agricultural economy that is largely dependent on irrigation using groundwater (Figure 1). Underlying about 112 million acres (450,000 square kilometers, 175,000 square miles) in parts of eight states, the sandstones and gravels of the High Plains accounts for about 30 percent of all groundwater withdrawn for irrigation in the United States. In the southern part of this region, which includes the Texas Panhandle, the natural recharge of the aquifer is very slow, and the problem of declining groundwater levels is acute.

Where intense irrigation has been practiced for an extended period, depletion of groundwater can be severe. Declines in the water table at rates as great as 1 meter (3 feet) per year have led to an overall drop of between 15 and 60 meters (50 and 200 feet) in some areas. Under these circumstances, it can be said that the groundwater is literally being “mined.” Even if pumping were to cease immediately, it would take thousands of years for the groundwater to be fully replenished.

Groundwater depletion has been a concern in the High Plains and other areas of the West for many years, but it is worth pointing out that the problem is not confined to that part of the country. Increased demands on groundwater resources worldwide have overstressed aquifers in many areas, not just in arid and semiarid regions. Recent research indicates that more than 70 percent of aquifers globally are being depleted, and rates of decline are accelerating.

Land Subsidence Caused by Groundwater Withdrawal

As you will see later in this chapter, surface subsidence can result from natural processes related to groundwater. However, the ground may also sink when water is pumped from wells faster than natural recharge processes can replace it. This effect is particularly pronounced in areas underlain by thick layers of loose sediments. As water is withdrawn, the water pressure drops, and the weight of the overburden is transferred to the sediment. The greater pressure packs the sediment grains more tightly together, and the ground subsides.

Many areas can be used to illustrate such land subsidence. A classic example in the United States occurred in the San Joaquin Valley of California (Figure 2). Other well-known cases of land subsidence resulting from groundwater pumping in the United States include Las Vegas, Nevada; New Orleans and Baton Rouge, Louisiana; portions of southern Arizona; and the Houston–Galveston area of Texas. In the low-lying coastal area between Houston and Galveston, land subsidence ranges from 1.5 to 3 meters (5 to 10 feet). The result is that about 78 square kilometers (30 square miles) are permanently flooded.

Outside the United States, one of the most spectacular examples of subsidence occurred in Mexico City, a portion of which is built on a former lake bed. Groundwater extraction combined with compacting sediments have resulted in portions of the city subsiding 7.5 meters (25 feet) or more, and rates of subsidence indicate some areas could drop another 19 meters (62 feet) in the next 150 years. In Shanghai, China, groundwater depletion through 2005 resulted in an average surface drop of 1.9 meters (~6 feet), and although rates are decreasing in many Shanghai areas, some districts of the vast city are still experiencing subsidence rates of 2.5 centimeters (1 inch) per year.

Groundwater Contamination

The pollution of groundwater is a serious matter, particularly in areas where aquifers provide a large part of the water supply. One common source of groundwater pollution is sewage. Its sources include an ever-increasing number of septic tanks, as well as farm wastes and inadequate or broken sewer systems.

If sewage water that is contaminated with bacteria enters the groundwater system, it may become purified through natural processes. The harmful bacteria can be mechanically filtered by the sediment through which the water percolates, destroyed by chemical oxidation, and/or assimilated by other organisms. For purification to occur, however, the aquifer must be of the correct composition. For example, extremely permeable aquifers (such as highly fractured crystalline rock, coarse gravel, or cavernous limestone) have such large openings that contaminated groundwater may travel long distances without being cleansed. In this case, the water flows too rapidly and is not in contact with the surrounding material long enough for purification to occur. This is the problem at Well 1 in Figure 3A.

Conversely, when the aquifer is composed of sand or permeable sandstone, the water can sometimes be purified after traveling only a few dozen meters through it. The openings between sand grains are large enough to permit water movement, yet the movement of the water is slow enough to allow ample time for its purification (Well 2, Figure 3B).

Other sources and types of contamination also threaten groundwater supplies. These include widely used substances such as highway salt, fertilizers that are spread across the land surface, and pesticides. In addition, a wide array of chemicals and industrial materials may leak from pipelines, storage tanks, landfills, and holding ponds. Some of these pollutants are classified as hazardous, meaning that they are either flammable, corrosive, explosive, or toxic. As rainwater oozes through the refuse, it may dissolve a variety of potential contaminants. If the leached material reaches the water table, it mixes with the groundwater and contaminates the supply.

Because groundwater movement is usually slow, polluted water might go undetected for a long time. In fact, contamination is sometimes discovered only after drinking water has been affected and people become ill. By this time, the volume of polluted water might be very large, and even if the source of contamination is removed immediately, the problem is not solved. Although the sources of groundwater contamination are numerous, there are relatively few solutions.

Once the source of the water contamination has been identified and eliminated, the most common practice is simply to abandon the water supply and allow the pollutants to be flushed away gradually. This is the least costly and easiest solution, but the aquifer must remain unused for many years. To accelerate this process, polluted water is sometimes pumped out and treated. Following removal of the tainted water, the aquifer is allowed to recharge naturally or, in some cases, the treated water or other freshwater is pumped back in. This process is costly and time-consuming, and it may be risky because there is no way to be certain that all the contamination has been removed. Clearly, the most effective solution to groundwater contamination is prevention.

Saltwater Intrusion

A growing threat to coastal aquifers is that of saltwater intrusion, where saline water from the ocean contaminates freshwater and groundwater sources. Along coastlines, fresh groundwater from inland regions meets up with saline groundwater; the dense saltwater forms a wedge beneath the freshwater at a transition zone. In most areas, the pressure of a significant amount of freshwater prevents much inland movement of dense saltwater. However, where sea level rise combines with decreased levels of groundwater or over pumping of groundwater, saltwater may encroach further into freshwater aquifers. In Hilton Head Island, North Carolina, saltwater is intruding into aquifers at a rate of 122 meters (400 feet) per year. Hundreds of millions of dollars have been spent to move treated water from the mainland onto the island and to build water treatment and storage facilities.

• Groundwater can be “mined” by being extracted at a rate that is greater than the rate of replenishment. When groundwater is treated as a nonrenewable resource, as it is in parts of the High Plains aquifer, the water table drops, in some cases by 60 meters (200 feet).

• The extraction of groundwater can cause pore space to decrease in volume and the grains of loose Earth materials to pack more closely together. This overall compaction of sediment volume results in the subsidence of the land surface.

• Contamination of groundwater with sewage, highway salt, fertilizer, industrial chemicals, or saltwater is another issue of critical concern. Once groundwater is contaminated, the problem is very difficult to solve, requiring expensive remediation or even abandonment of the aquifer.

• saltwater intrusion: The movement of saline water into freshwater aquifers.