Groundwater

Introduction

Groundwater is water that has accumulated deep in the ground in permeable rocks or soils known as an aquifer. These water-holding formations are located beneath most areas of Earth. Rain filters down through the surface layers of rock and soil and is stored in the aquifer. This process acts as a natural filtration system, removing harmful substances from the water.

Groundwater has long been an important natural resource for humankind. By using groundwater, many civilizations have been able to avoid disease spread by surface water that is contaminated with sewage. Groundwater may also be present in areas that have little surface water, allowing civilization to flourish in areas it would otherwise not. Today, groundwater is still a valuable resource that people rely on. However, dangerous pollutants have contaminated some groundwater, making it unsafe to drink. In places where water treatment is available, this is less of a problem, but in undeveloped areas, some people are exposed to dangerous substances through groundwater.

Historical Background and Scientific Foundations

Groundwater begins to form when rain falls to the ground or snow melts. The water sinks into the ground and some of it evaporates or is absorbed by the roots of plants. As it is sinking down, the water is known as “recharge water,” because it will soon refill the aquifer. Drawn by gravity, the water continues downward until it reaches an impermeable layer. It rests there, usually in cracks in rocks or voids in sand and gravel. These saturated layers are known as the aquifer, and the top level they reach is called the water table. On the shores of some lakes, the water table may reach to the surface and water can be seen filling holes dug on the shoreline.

The water table may rise or fall, depending on weather conditions and the amount of water drawn from the aquifer. During summer droughts, more water might be pumped out of an aquifer than is replaced by rainfall and the water table will fall. If there is an adequate amount of snow or rain in the winter, the aquifer will be recharged. Problems can arise when more water is drawn from an aquifer than is quickly replaced. In parts of Texas, Oklahoma, Kansas, and New Mexico, water tables have dropped more than 100 ft (30 m) from historical measurements. This raises the possibility that the aquifer could be pumped dry, causing water supplies to run out. The same thing can happen when water is recharged into the aquifer very slowly. Enough water may be replenished by rainfall and snowmelt each year, but it takes a very long time to reach the aquifer. As people use the water from the aquifer, it can become depleted, even though enough recharge water will eventually reach it. This situation could occur if Las Vegas, Nevada, which gets most of its water from the Colorado River, starts pumping additional supplies from its aquifer.

Many ancient civilizations relied heavily on groundwater and used it as a resource to help them expand. The ancient Romans are famous for their extensive and complex waterworks. Unlike many cities that came before and after Rome, each resident could rely on a plentiful supply of clean water, easily accessible at neighborhood fountains. The greater supply of water simply allowed more people to live in the city than otherwise could. The purity of the water permitted Rome to grow and flourish relatively free from water-borne diseases that spread very easily in crowded cities.

The most obvious large water source in Rome is the river Tiber. It flows through the city and would have been easily accessible in ancient times. However, the river was a poor source of drinking water because of the large amount of silt in the water. As the city grew, most of the sewers emptied into the river, making it unsuitable for drinking. There are some small springs that erupt within the city, but they did not provide enough water to slake a city of a million people. Today, engineers try to sink wells and pump water from the local aquifer, but the ancient Romans did not have the technology to do this on a large scale. Instead, Romans built aqueducts to access the pure groundwater of the surrounding hills, which flowed to the surface in artesian springs. Most aqueducts were subterranean tunnels, carved out of the local rock. When aqueducts had to cross a valley, they were raised onto the stone arches we commonly associate with Roman aqueducts.

Rome, of course, was not the only ancient city to successfully utilize groundwater, nor was it even among the first. The Minoan settlement of Knossos, flourishing on the island of Crete around 2000 BC, supplied its water from nearby springs using a system of pipes. In Egypt, people accessed groundwater with wells as early as 3000 BC. By the fifth century BC, the Greek physician Alcmaeon of Croton had observed that water quality affected a person’s health. The Romans realized that mine waste could contaminate groundwater, making it poisonous. After the fall of the Roman Empire, their water system fell into disrepair. During the Middle Ages, the size of the city of Rome decreased dramatically, partially due to the lack of clean water.

The use of groundwater experienced a resurgence in the nineteenth century with the invention of better pumping technology. At this time, the city of London relied heavily on both deep and shallow wells to draw drinking water. In 1854, an outbreak of cholera ravaged a few neighborhoods in the city. It was eventually linked to a shallow well that had been contaminated by sewage. Dr. John Snow (1813–1858) determined that those who utilized pumped groundwater were not exposed to the disease. Not only was this an important episode in the development of germ theory, but it illustrates one reason why groundwater supplies became so desirable and heavily used.

Issues and Impacts

When groundwater is polluted, serious public health problems can occur. Because many cities and towns distribute groundwater as drinking water, contamination in the groundwater is efficiently spread to many people. There are many kinds of contamination that can affect groundwater. Some wells are especially vulnerable to fecal contamination, usually from farm animals. Aquifers can be contaminated with saltwater, either from the ocean or from underground sources. Contamination with industrial chemicals is a serious problem in some areas. Additionally, there is growing concern that trace

WORDS TO KNOW

AQUIFER: Rock, soil, or sand that is able to hold and transmit water.

ARTESIAN SPRING OR WELL: Groundwater that flows from the aquifer to the surface without the need for a pump.

WATER TABLE: The level of the aquifer under the ground.

amounts of pharmaceuticals, illicit drugs, and personal care products are making their way into the water supply.

There are many ways in which groundwater can become polluted with dangerous substances. Tainted soil, improperly disposed chemicals, and leaky septic systems can all leach dangerous substances into the aquifer. Poorly maintained sewer and storm-drain systems can leak sewage or tainted runoff, which makes its way into groundwater. Runoff from livestock or poultry farms can sometimes infiltrate groundwater, particularly if it runs down wells. Contaminated surface water can also taint groundwater. The water from some streams, rivers, and lakes seeps into the ground to the aquifer. This effect can be great during floods or times of heavy precipitation when the ground is saturated, the water table rises, and streams rise in their banks, capturing soil that it normally doesn’t overflow. If the bodies of water are contaminated or pick up tainted soil, the pollution can be spread to the aquifer. Many kinds of contaminants can be spread in these ways, from industrial chemicals and salt to dangerous bacteria.

Groundwater tainted with human or animal feces can cause a public health emergency. Many animals, especially cattle, shed dangerous E. coli bacteria in their feces. When humans ingest the bacteria, the first symptoms of illness are usually gastrointestinal: vomiting, intestinal cramps, fever, and bloody diarrhea. The disease can progress to kidney failure, circulation collapse, loss of limbs, and death. The small town of Walkerton in Ontario, Canada, was the site of a serious outbreak of E. coli in the spring of 2000, leading to the deaths of seven people. On May 12, heavy rain swept cattle feces into one of the central wells that served the town, contaminating the aquifer with E. coli. Unbeknownst to the town, the water manager had not been treating the well water properly with chlorine, allowing the bacteria to grow. In all, 500 people complained of E. coli symptoms, several children died, and the town, residents, and government spent millions of dollars decontaminating the water system.

Any water that contains raw or treated sewage has the potential to contain pharmaceuticals and personal care products (PPCPs). These substances enter the water supply through human and veterinary use and waste from manufacturing. PPCPs include grooming products like shampoo, toothpaste, and shaving cream; pharmaceuticals such as hormones, steroids, and antibiotics; and illegal drugs and the substances the body metabolizes them into. Since many of the pharmaceutical products and illegal drugs are used specifically for their ability to affect cell receptors, they can have powerful effects even in small amounts. Though PPCPs are a larger problem in surface water bodies, the interactions between groundwater and surface water mean that PPCPs have been found in almost every source that has been examined. PPCP contamination in groundwater sources is of special concern because the inaccessibility of most aquifers makes any pollution difficult to remove.

Chemicals from industrial sources, farms, homes, and businesses can pollute groundwater with leaks or due to improper disposal. Underground tanks are a large source of groundwater pollution because these tanks can leak for a long time before the spill is discovered. Sometimes, underground tanks are abandoned or forgotten, with no one taking responsibility for their maintenance. Underground tanks are responsible for a great number of gasoline, fuel oil, and diesel leaks. In the past, landfills were not as heavily regulated as they are today. Hazardous chemicals like paint, oil, solvents, pesticides, and wastes containing heavy metals were thrown out with regular household trash. As a result, many old or abandoned landfills have the potential to pollute groundwater when rain falls on them and drains into the underlying aquifer.

Today, known hazardous wastes are disposed of in special landfills that are fitted with impermeable liners and systems to collect leaching chemicals. In the past, disposal sites have been less secure or the containment was breached sometime after the chemicals were deposited. This can lead to groundwater contamination by allowing the stored chemicals to escape. Many industrial companies use stabilization ponds or lagoons to store and treat chemical waste before disposal. Farms also utilize them to break down animal waste. These ponds use light, bacteria, chemical reactions, or a combination of these factors to break down the wastes. If the ponds are not properly lined, or if they break, leak, or overflow, organic chemicals or disease-causing bacteria can escape the ponds and taint groundwater.

Polluted aquifers have a limited ability to restore themselves to purity, depending on the type and extent of pollution. The U.S. Geological Survey (USGS) has been studying an aquifer under a former military base on Cape Cod, in Massachusetts. Treated wastewater that still contained dissolved contaminants was allowed to seep into the aquifer for more than 60 years. The scientists have concluded that while the aquifer will be able to recover, it will take more than 30 years from the time the dumping ceased. With greater demand on limited water resources, this recovery period restricts access to necessary water.

New processes designed to treat the whole aquifer and accelerate the restoration of clean groundwater are being used in many places. The State of California has studied the use of the chemical potassium permanganate

(KMn0₄) to break down volatile organic compounds (VOCs) that are polluting aquifers. By injecting the chemical into the aquifer in strategic places, the contaminants are decomposed, leaving clean water to be pumped to the surface. Special bacteria are also being used to remove chemicals like benzene (C₆H₆) and toluene (C₇H₈) from groundwater. Under the right conditions, bacteria can be introduced into the polluted aquifer and decompose the dangerous chemicals in place. Oxygen (O) from regular air has the ability to break down hazardous chemicals before they reach sensitive groundwater. Injecting air under leaking tanks encourages the growth of natural microorganisms that degrade dangerous chemicals. A new, more extreme method of protecting aquifers is known as in situ vitrification. Large electrodes are used to heat contaminated soil to extreme temperatures. The soil melts and cools into a sold block of glass, trapping the dangerous chemicals so they cannot escape into the groundwater. This method is particularly useful because it deals with all kinds of contaminants, rather than just a specific one. This process also minimizes chemical exposure to workers and the outside environment.

With growing population, pollution, and natural scarcity in some areas, clean water is an increasingly valuable resource. Since 50% of drinking water in the United States is groundwater, it is necessary to protect aquifers from pollution and carefully use their water. Water is also necessary to operate power plants, irrigate crops, water livestock, and dispose of waste. In the United States, a complicated system of laws, rules, and precedents governs the rights to use groundwater. One of the earliest rules governing the use of groundwater allowed landowners the right to use the water pumped from their land. Another similar rule gave rights to whoever pumped the groundwater to the surface. These systems ignored the limited nature of groundwater as a resource and the fact that an aquifer generally extends beneath the property of many people.

In the eastern part of the United States, where groundwater is relatively plentiful and there are fewer large-scale agricultural demands, the legal precept known as “reasonable use” dominates, meaning everyone is entitled to enough groundwater within reasonable limits. In the drier, more agricultural western areas, water rights are allocated according to who has the oldest claim on the resource. This has created some problems, with some sources having more claims than available resources.

The long-term sustainability of groundwater as a vital resource depends on many factors. Pollution of groundwater sources must be controlled if humans are to make use of the water. Similarly, contaminated sources will need to be restored to purity to meet the increasing water demands of a growing population. There are also some groundwater sources that are naturally undrinkable because of large amounts of salt or other minerals. The ability to treat the water from these aquifers may become necessary in the future. In some cases, alternate sources of water will need to be found for areas that rely on aquifers that cannot be replenished, or are replenished so slowly that replacements cannot meet demand. In all cases, conservation of groundwater resources will become increasingly important as human demand for water grows.

See Also Groundwater Quality

BIBLIOGRAPHY

Periodicals

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Virginia Tech Civil Engineering Department. “Groundwater Primer.” June 7, 1998. http://www.cee.vt.edu/ewr/environmental/teach/gwprimer/gwprimer.html (accessed April 19,2008).

Kenneth T. LaPensee