Groundwater Contamination

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Natural groundwater is typically rich in dissolved elements and compounds derived from the soil, regolith, and bedrock through which the water has migrated. Some of these dissolved elements and compounds are poisonous, whereas others are tolerable in small concentrations but harmful in high concentrations. Human and industrial waste contamination of the groundwater is increasing, and the overuse of groundwater resources has caused groundwater levels to drop and has led to other problems, especially along coastlines. Seawater may move in to replace depleted freshwater, and the ground surface may subside when the water is removed from the pore spaces in aquifers.

The U.S. Public Health Service has established limits on the concentrations of dissolved substances (called total dissolved solids, or t.d.s.) in natural waters that are used for domestic and other purposes. The table of "Drinking Water Standards for the United States" lists these limits for the United States. Many other countries, particularly those with chronic water shortages such as many in the Middle East, have much more lenient standards. Sweet water is preferred for domestic use and has fewer than 500 milligrams (mg) of total dissolved solids per liter (L) of water. Fresh and slightly saline water, with t.d.s. of 1,000-3,000 mg/L, is suitable for use by livestock and irrigation. Water with higher concentrations of t.d.s. is unfit for humans or livestock. Irrigation of fields using waters with high concentrations of t.d.s.




Total Dissolved Solids (T.D.S.)


< 500 mg/L


500-1,000 mg/L

Slightly saline

1,000-3,000 mg/L

Moderately saline

3,000-10,000 mg/L

Very Saline

10,000-35,000 mg/L


> 35,000 mg/L

is also not recommended, as the water will evaporate but leave the dissolved salts and minerals behind, degrading and eventually destroying the productivity of the land.

Either a high amount of total dissolved solids or the introduction of a specific toxic element can reduce the quality of groundwater or contaminate it. Most of the total dissolved solids in groundwater are salts derived from dissolution of the local bedrock or soils derived from the bedrock. Salts can also seep into groundwater supplies from the sea along coastlines, particularly if the water is being pumped out for use. In these cases seawater often moves in to replace the depleted freshwater. This process is known as seawa-ter intrusion, or seawater incursion.

Dissolved salts in groundwater commonly include bicarbonate (HC03-) and sulfate (S042-) ions, often associated with other ions. Dissolved calcium (Ca2+) and magnesium (Mg+) ions can cause the water to become "hard." Hard water is defined as containing more than 120 parts per million dissolved calcium and magnesium. The dissolved ions in hard water make it difficult to lather soap, and they form a crusty mineralization buildup on faucets and pipes. Adding sodium (Na+) in a water softener can soften hard water, but people with heart problems or those who are on a low-salt diet should not do this. Hard water is common in areas where the groundwater has moved through limestone or dolostone rocks, which contain high concentrations of Ca2+ and Mg2+-rich rocks that groundwater easily dissolves.

Groundwater may have many other contaminants, some natural and others the result of human activity. Human pollutants including animal and human waste, pesticides, industrial solvents, road salts, petroleum products, and other chemicals are a serious problem in many areas. Some of the biggest and most dangerous sources of groundwater contamination include chemical and gasoline storage tanks, septic systems, landfills, hazardous waste sites, military bases, and the general widespread use of road salt and chemicals such as fertilizers or pesticides.

The Environmental Protection Agency has led the cleanup from spills from leaking chemical storage tanks in the United States. There are estimated to be more than 10 million buried chemical storage tanks in the United States, containing chemicals such as gasoline, oil, and hazardous chemicals. These tanks can leak over time, and many of the older ones have needed to be replaced in the past two decades, bringing in a new generation of tanks that should last longer and corrode less.

Home and commercial septic systems pose serious threats to some groundwater systems. Most are designed to work effectively and harmlessly, but some were not installed properly or were poorly designed.

In many cases groundwater supplies have been contaminated by chemicals and other contaminants that were poured down drains, entering the septic system and then the groundwater system.

There are more than 20,000 known and abandoned hazardous waste sites in the United States. Some of these contain many barrels of chemicals and hazardous materials that can and do leak, contaminating the water supply. Landfills may also contain many hazardous chemicals—when landfills are designed they are supposed to incorporate a protective impermeable bottom layer to prevent chemicals from entering the groundwater system. But some chemicals that are erroneously placed in the landfill sometimes burn holes in the basal layer, making their way (with a myriad of other chemicals) into the groundwater system.

In parts of the country that freeze, road salts are commonly used to reduce the amount of ice on the roads. These salts dissolve in rainwater and can eventually make their way down into the aquifers as well, turning an aquifer salty. Together with chemicals from lawn and farm field fertilization and application of pesticides, the amount of these chemicals starts to become significant for the safety of the water quality below ground.

Groundwater contamination, whether natural or human-induced, is a serious problem because of the importance of the limited water supply. Pollutants in the groundwater system do not simply wash away with the next rain, as many dissolved toxins in the surface water system do. Groundwater pollutants typically have a residence time, or average length of time that it remains in the system, of hundreds or thousands of years. Many groundwater systems are capable of cleaning themselves from natural biological contaminants using bacteria, but other chemical contaminants have longer residence times.

Arsenic in Groundwater

In parts of the world many people have become sick from arsenic dissolved in the groundwater. Arsenic poisoning leads to a variety of horrific diseases, including hyperpigmentation (abundance of red freckles), hyperkeratosis (scaly lesions on the skin), cancerous lesions on the skin, and squamous cell carcinoma. Arsenic may be introduced into the food chain and body in several ways. In Guizhou Province, China, villagers dry their chili peppers indoors over coal fires. unfortunately, the coal is rich in arsenic (containing up to 35,000 parts per million arsenic), and much of this arsenic is transferred to the chili peppers during the drying process. Thousands of the local villagers now suffer arsenic poisoning, with cancers and other forms of the disease ruining families and entire villages.

Most naturally occurring arsenic is introduced into the food chain through drinking contaminated groundwater. Arsenic in groundwater is commonly formed by the dissolution of minerals from weathered rocks and soils. In Bangladesh and West Bengal, India, 25-75 million people are at risk for arsenosis, because of high concentrations of natural arsenic on groundwater.

Since 1975 the maximum allowable level of arsenic in drinking water in the United States has been 50 parts per billion. The EPA has been considering adopting new standards on the allowable levels of arsenic in drinking water. Scientists from the National Academy recommend that the allowable levels of arsenic be lowered to 10 parts per billion, but this level was overruled by the Bush administration. The issue is cost: the EPA estimates that it would cost businesses and taxpayers $181 million per year to bring arsenic levels to the proposed 10 parts per billion level, although some private foundations suggest that this estimate is too low by a factor of three. They estimate that the cost would be passed on to the consumer, and residential water bills would quadruple. The EPA estimates that the health benefits from such a lowering of arsenic levels would prevent between 7 and 33 deaths from arsenic-related bladder and lung cancer per year. These issues reflect a delicate and difficult choice for the government. The EPA tries to "maximize health reduction benefits at a cost that is justified by the benefits." How much should be spent to save 7-33 lives per year? Would the money be better spent elsewhere?

Arsenic is not concentrated evenly in the ground-water system of the United States, or anywhere else in the world. The U.S. Geological Survey issued a series of maps in 2000 showing the concentration of arsenic in tens of thousands of groundwater wells in the United States. Arsenic is concentrated mostly in the Southwest, with a few peaks elsewhere such as southern Texas, parts of Montana (due to mining operations), and parts of the upper plains states. Perhaps a remediation plan that attacks the highest concentrations of arsenic would be the most cost-effective and have the highest health benefit.

Contamination by Sewage

A major problem in groundwater contamination is sewage. If chloroform bacteria get into the ground-water, the aquifer is ruined, and care must be taken and samples analyzed before water is used for drinking. In many cases sand filtering can remove bacteria, and aquifers contaminated by chloroform bacteria and other human waste can be cleaned more easily than aquifers contaminated by many other elemental and mineral toxins.

Although serious, detailed discussion of groundwater contamination by human waste is beyond the scope of this encyclopedia, the reader is referred to the sources listed at the end of the chapter for more detailed accounts.

Seawater Intrusion in Coastal Aquifers

Encroachment of seawater into drinking and irrigation wells is an increasing problem for many coastal communities around the world. Porous soils and rocks beneath the groundwater table in terrestrial environments are generally saturated with fresh water, whereas porous sediment and rock beneath the oceans is saturated with salt water. In coastal environments there must be a boundary between the fresh ground-water and the salty groundwater. In some cases this is a vertical boundary, whereas in other cases the boundary is inclined with the denser salt water lying beneath the lighter fresh water. In areas where there is complex or layered stratigraphy, the boundary may be complex, consisting of many lenses.

In normal equilibrium situations the boundary between the fresh and salty water remains rather stationary. In times of drought the boundary may move landward or upward, and in times of excessive precipitation the boundary may move seaward and downward. As sea levels rise the boundary moves inland and wells that formerly tapped fresh water begin to tap salt water. This is called sea water intrusion or encroachment.

Many coastal communities have been highly developed, with many residential neighborhoods, cities, and agricultural users obtaining their water from groundwater wells. When these wells pump more water out of coastal aquifers than is replenished by new rainfall and other inputs to the aquifer, the fresh water lens resting over the salt water lens is depleted. This also causes the salt water to move in to the empty pore spaces to take the place of the fresh water. Eventually as pumping continues the fresh water lens becomes so depleted that the wells begin to draw salt water out of the aquifer, and the well becomes effectively useless. This is another way that salt water intrusion or encroachment can poison groundwater wells. In cases of severe drought the process may be natural, but in most cases seawater intrusion in caused by over-pumping of coastal aquifers, aided by drought conditions.

Many places in the United States have suffered from seawater intrusion. For instance, many East Coast communities have lost use of their wells and had to convert to water piped in from distant reservoirs for domestic use. In a more complicated scenario western Long Island of New York experienced severe seawater intrusion into its coastal aquifers because of intense overpumping of its aquifers in the late 1800s and early 1900s. Used water that was once returned to the aquifer by septic systems began to be dumped directly into the sea when sewers were installed in the 1950s, with the result that the water table dropped more than 20 feet over a period of 20 years. This drop was accompanied by additional seawater intrusion. The water table began to recover in the 1970s when much of the area converted to using water pumped in from reservoirs in the Catskill Mountains to the north of New York City.


Most of the world's freshwater is locked in glaciers or ice caps, and about 25 percent of the freshwater is stored in the groundwater system. Water in the groundwater system is constantly but slowly moving, being recharged by rain and snow that infiltrates the system, and discharging in streams, lakes, springs, and extracted from wells. Water that moves through a porous network forms aquifers, and underground layers that restrict flow are known as aquicludes. Fracture zone aquifers comprise generally nonpermeable, nonporous crystalline rock units, but faults and fractures that cut the rock create new or secondary porosity along the fractures. If exposed to the surface, these fractures may become filled with water and serve as excellent sources of water in dry regions.

The groundwater system is threatened by pollutants that range from naturally dissolved but deadly elements such as arsenic, to sewerage, to industrial wastes and petroleum products that have leaked from underground storage containers, or were carelessly dumped. some chemical elements have a short residence time in the groundwater system and are effectively cleaned before long, but other elements may last years or thousands of years before the ground-water is drinkable again.

See also hydrosphere; meteoric; soils.


Alley, William M., Thomas E. Reilly, and O. L. Franke. Sustainability of Ground-Water Resources. Reston, Va.: United States Geological Survey Circular 1186, 1999.

Ford, D., and P. Williams. Karst Geomorphology and

Hydrology. London: Unwin-Hyman, 1989. Keller, Edward A. Environmental Geology. 8th ed. Engelwood Cliffs, N.J.: Prentice Hall, 2000. Kusky, T. M. Floods: Hazards of Surface and Groundwater Systems. New York: Facts On File, 2008. Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth, an Introduction to Physical Geology. 5th ed. New York: John Wiley & Sons, 2004. United States Geological Survey. "Water Resources." Available online. URL: Accessed December 10, 2007.

Halley, Edmond (1656-1742) British Astronomer, Geophysicist, Mathematician, Meteorologist, Physicist Edmond Halley was born on November 8, 1656, in Shoreditch, England, and is best known for the comet bearing his name, Halley's Comet. Halley married Mary Tooke in 1682, and the couple had three children. He died on January 14, 1742.

Edmond Halley studied mathematics at an early age while at St. Paul's School in London, before moving to the Queen's College at Oxford in 1673. During his undergraduate years at Oxford, Halley published several scientific papers on sunspots and the solar system.

After graduating from Oxford Halley visited the South Atlantic Ocean island of St. Helena to examine the southern stars, then returned to England in 1678, publishing his observations of the southern sky as his Catalogus Stellarum Australium in 1679. This work led to his being awarded a master of arts degree from Oxford and his election as a fellow of the Royal Society.

Some eight years after his voyage to the South Atlantic Halley published the second volume from his field observations, this one on the southern trade winds and monsoons, and his deduction that atmospheric motions were ultimately driven by solar heating of the atmosphere. Halley became interested in gravity and studied 16th century Austrian mathematician Kepler's laws of planetary motion and met with Sir Isaac Newton on the matter in 1684. He found that Newton had derived proof of Kepler's laws. Halley convinced Newton to publish his works and even paid the cost of printing.

In 1691 Halley applied for the position of Savilian Professor of Astronomy at Oxford, but since his views on religion were atheistic, the archbishop of Canter bury opposed his appointment and gave it instead to David Gregory, who was supported by Newton.

After working to develop actuarial models for the British government, Halley returned to science and was given command of a vessel to sail to the South Atlantic to study variations in the magnetic compass. After an initial voyage was terminated because of insubordination by the crew in 1698, he sailed from September 1699 to September 1700, then in 1701 published his observations of the magnetic field as his General Chart of the Variation of the Compass, the first chart ever to show magnetic isogonic lines, contour lines that show places of constant magnetic declination.

After his detractors at Oxford died, Halley was appointed Savilian Professor of Geometry in 1703 and was given an honorary doctor of law in 1710. While he was in the Savilian Professorship, Halley pursued historical astronomy and published his analysis of past accounts of comet sightings as Synopsis Astronomia Cometicae in 1705. He noted comet sightings from the years 1456, 1531, 1607; 1682, and noting the 75-76 year repeat cycle of the comets, he suggested that these sightings were of the same comet, and that it would return in 1758. When the comet did return, it became known as Halley's comet. See also astronomy; comet; Sun.


Cook, Alan H. Edmond Halley: Charting the Heavens and the Seas. Oxford: Clarendon Press, 1998.

Hess, Harry (1906-1969) American Geologist

Harry Hammond Hess is best known for formulating a theory on the origin and evolution of ocean basins. Drawing on observations from which Alfred Wegener proposed his theory of continental drift in 1912, Hess visualized a process occurring deep below the oceanic crust that caused seafloor spreading. In this model the seafloor is created at ridges and sinks at trenches back into the Earth's mantle. This concept provided a model that catapulted the plate tectonics theory into the earth sciences mainstream.

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