Name three sources of groundwater pollution and how do pollutants get into the water?
About half the population in the United States relies to some extent on groundwater as a source of d...
Published on: Mar 3, 2016
Transcripts - Name three sources of groundwater pollution and how do pollutants get into the water?
Name three sources of groundwater pollution and how do
pollutants get into the water?
About half the population in the United States relies to some extent on groundwater as a source of
drinking water, and still more use it to supply their factories with process water or their farms with
irrigation water. However, if all water uses such as irrigation and power production are included,
only about 25 percent of the water used nationally is derived from groundwater. Still, for those who
rely on it, it is critical that their groundwater be unpolluted and relatively free of undesirable
A groundwater pollutant is any substance that, when it reaches an aquifer, makes the water unclean
or otherwise unsuitable for a particular purpose. Sometimes the substance is a manufactured
chemical, but just as often it might be microbial contamination. Contamination also can occur from
naturally occurring mineral and metallic deposits in rock and soil.
For many years, people believed that the soil and sediment layers deposited above an aquifer acted
as a natural filter that kept many unnatural pollutants from the surface from infiltrating down to
groundwater. By the 1970s, however, it became widely understood that those soil layers often did
not adequately protect aquifers. Despite this realization, a significant amount of contamination
already had been released to the nation's soil and groundwater. Scientists have since realized that
once an aquifer becomes polluted, it may become unusable for decades, and is often impossible to
clean up quickly and inexpensively.
Types of Groundwater Contamination
Groundwater pollution caused by human activities usually falls into one of two categories: point-source
pollution and nonpoint-source pollution.
Fertilizers and pesticides applied to crops eventually may reach underlying aquifers, particularly if
the aquifer is shallow and not "protected" by an overlying layer of lowpermeability material, such as
clay. Drinking-water wells located close to cropland sometimes are contaminated by these
agricultural chemicals. Point-source pollution refers to contamination originating from a single tank,
disposal site, or facility. Industrial waste disposal sites, accidental spills, leaking gasoline storage
tanks, and dumps or landfills are examples of point sources. Chemicals used in agriculture, such as
fertilizers, pesticides, and herbicides are examples of nonpoint-source pollution because they are
spread out across wide areas. Similarly, runoff from urban areas is a nonpoint source of pollution.
Because nonpoint-source substances are used over large areas, they collectively can have a larger
impact on the general quality of water in an aquifer than do point sources, particularly when these
chemicals are used in areas that overlie aquifers that are vulnerable to pollution. If impacts from
individual pollution sources such as septic system drain fields occur over large enough areas, they
are often collectively treated as a nonpoint source of pollution.
Some groundwater pollution occurs naturally. The toxic metal arsenic, for instance, is commonly
found in the sediments or rock of the western United States, and can be present in groundwater at
concentrations that exceed safe levels for drinking water.
Radon gas is a radioactive product of the decay of naturally occurring uranium in the Earth's crust.
Groundwater entering a house through a home water-supply system might release radon indoors
where it could be breathed.
One of the best known classes of groundwater contaminants includes petroleum-based fuels such as
gasoline and diesel. Nationally, the U.S. Environmental Protection Agency (EPA) has recorded that
there have been over 400,000 confirmed releases of petroleum-based fuels from leaking
underground storage tanks.
Gasoline consists of a mixture of various hydrocarbons (chemicals made up of carbon and hydrogen
atoms) that evaporate easily, dissolve to some extent in water, and often are toxic. Benzene, a
common component of gasoline, is considered to cause cancer in humans, whereas other gasoline
components, such as toluene, ethylbenzene, and xylene, are not believed to cause
Aquifers in industrialized areas are at significant risk of being contaminated by chemicals and
petroleum products. In most developed countries, various laws attempt to prevent land and water
pollution, and to clean up contaminated areas when they occur. Developing countries and countries
in economic distress are less likely than developed nations to assess the risk of groundwater
contamination by land-use activities. cancer in humans but may be toxic in other ways. One
interesting property of gasoline is that it is less dense than water, and so it tends to float on top of
the water table.
Another common class of groundwater contaminants includes chemicals known as chlorinated
solvents. One example of a chlorinated solvent is dry-cleaning fluid, also known as
perchloroethylene. These chemicals are similar to petroleum hydrocarbons in that they are made up
of carbon and hydrogen atoms, but the molecules also have chlorine atoms in their structure.
As a general rule, the chlorine present in chlorinated solvents makes this class of compounds more
toxic than fuels. Unlike petroleum-based fuels, solvents are usually heavier than water, and thus
tend to sink to the bottoms of aquifers. This makes solvent-contaminated aquifers much more
difficult to clean up than those contaminated by fuels.
Cleaning Up Contaminated Groundwater
Groundwater typically becomes polluted when rainfall soaks into the ground, comes in contact with
buried waste or other sources of contamination, picks up chemicals, and carries them into
groundwater. Sometimes the volume of a spill or leak is large enough that the chemical itself can
reach groundwater without the help of infiltrating water.
Groundwater tends to move very slowly and with little turbulence, dilution, or mixing. Therefore,
once contaminants reach groundwater, they tend to form a concentrated plume that flows along with
groundwater. Despite the slow movement of contamination through an aquifer, groundwater
pollution often goes undetected for years, and as a result can spread over a large area. One
chlorinated solvent plume in Arizona, for instance, is 0.8 kilometers (0.5 miles) wide and several
Several federal laws focus on either preventing or remediating groundwater contamination, often
caused by industrial, commercial, or petroleum pollutants. While these federal laws have provided
an overall framework for these activities, the regulatory implementation of these laws is usually
carried out by states in cooperation with local governments. Often, federal laws are adopted by the
states largely unchanged.
The two major federal laws that focus on remediating groundwater contamination include the
Resource Conservation and Recovery Act (RCRA) and the Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA), also known as Superfund. RCRA regulates storage,
transportation, treatment, and disposal of solid and hazardous wastes, and emphasizes prevention of
releases through management standards in addition to other waste management activities. CERCLA
regulates the cleanup of abandoned waste sites or operating facilities that have contaminated soil or
groundwater. CERCLA was amended in 1986 to include provisions authorizing citizens to sue
violators of the law.
The Cleanup Process
Several steps normally are taken to clean up a site once contamination has been discovered. Initially
a remedial investigation is conducted to determine the nature and extent of the contamination. In
the risk assessment phase, scientists evaluate if site contaminants might harm human health or the
environment. If the risks are high, then all the various ways the site might be cleaned up are
evaluated during the feasibility study. The record of decision is a public document that explains
which of the alternatives presented in the feasibility study will be used to clean up a site.
Usually, the most protective, lowest cost, and most feasible cleanup alternative is chosen as the
preferred cleanup method. The selected cleanup method is designed and constructed during the
remedial design/remedial action phase. The operations and maintenance phase then follows.
Periodically the remedial action is evaluated to see if it is meeting expectations outlined in the
record of decision.
Methods of Cleanup
The various ways to respond to site contamination can be grouped into the following categories:
Containing the contaminants to prevent them from migrating from their source;
Removing the contaminants from the aquifer;
Remediating the aquifer by either immobilizing or detoxifying the contaminants while they are still
in the aquifer;
Treating the groundwater at its point of use;
Abandoning the use of the aquifer and
finding an alternative source of water.
Several ways are available to contain
groundwater contamination: physically, by
using an underground barrier of clay,
cement, or steel; hydraulically, by pumping
wells to keep contaminants from moving past
the wells; or chemically, by using a reactive substance to either immobilize or detoxify the
contaminant. When buried in an aquifer, zero-valent iron (iron metal filings) can be used to turn
chlorinated solvents into harmless carbon dioxide and water.
The most common way of removing a full range of contaminants (including metals, volatile organic
chemicals, and pesticides) from an aquifer is by capturing the pollution with groundwater extraction
wells. After it has been removed from the aquifer, the contaminated water is treated above ground,
and the resulting clean water is discharged back into the ground or to a river. Pump-and-treat, as
this cleanup technology is known, can take a long time, but can be successful at removing the
majority of contamination from an aquifer.
Another way of removing volatile chemicals from groundwater is by using a process known as air
sparging. Small-diameter wells are used to pump air into the aquifer. As the air moves through the
aquifer, it evaporates the volatile chemicals. The contaminated air that rises to the top of the aquifer
is then collected using vapor extraction wells.
Bioremediation is a treatment process that uses naturally occurring microorganisms to break down
some forms of contamination into less toxic or non-toxic substances. By adding nutrients or oxygen,
this process can be enhanced and used to effectively clean up a contaminated aquifer. Because
bioremediation relies mostly on nature, involves minimal construction or disturbance, and is
comparatively inexpensive, it is becoming an increasingly popular cleanup option.
Some of the newest cleanup technologies use surfactants (similar to dishwashing detergent),
oxidizing solutions, steam, or hot water to remove contaminants from aquifers. These technologies
have been researched for a number of years, and are just now coming into widespread use. These
and other innovative technologies are most often used to increase the effectiveness of a pump-an-
Depending on the complexity of the aquifer and the types of contamination, some groundwater
cannot be restored to a safe drinking quality. Under these circumstances, the only way to regain use
of the aquifer is to treat the water at its point of use. For large water providers, this may mean
installing costly treatment units consisting of special filters or evaporative towers called air
strippers. Domestic well owners may need to install an expensive whole-house carbon filter or a
reverse osmosis filter, depending on the type of contaminant.
SEE ALSO ATTENUATION OF POLLUTANTS; CHEMICALS FROM AGRICULTURE;
GROUNDWATER; LANDFILLS: IMPACT ON GROUNDWATER; LEGISLATION: FEDERAL WATER;
MODELING GROUNDWATER FLOW AND TRANSPORT; POLLUTION OF GROUNDWATER:
VULNERABILITY; SEPTIC SYSTEM IMPACTS.
William R. Mason
Boulding, J. Russell. Practical Handbook of Soil, Vadose Zone, and Ground-water Contamination:
Assessment, Prevention, and Remediation. Boca Raton, FL: Lewis Publishers, 1995.
Wiedemeier, Todd H. et al. Natural Attenuation of Fuels and Chlorinated Solvents in the Subsurface.
New York: John Wiley & Sons, 1999.
Johnson, Robert et al. "MTBE: To What Extent Will Past Releases Contaminate Community Supply
Wells?" Environmental Science & Technology 34 no.9 (2000): 210A.
"Methyl Tertiary Butyl Ether (MTBE)." U.S. Environmental Protection Agency.
Swain, Walter. "Methyl Tertiary-Butyl Ether (MTBE)." <>.http://ca.water.usgs.gov/mtbe/>.
"Water Pollutants." Recommended EPA Web pages. U.S. Environmental Protection Agency.
MTBE: GASOLINE ADDITIVE
Methyl tert-butyl ether (MTBE) is used almost exclusively as a gasoline additive to help reduce
harmful tailpipe emissions from motor vehicles. MTBE has been credited with improving air quality
by significantly reducing carbon monoxide and ozone levels in areas where the additive has been
used. Unfortunately, this is a case where the United States may have "robbed Peter to pay Paul": a
growing number of studies have found that MTBE has contaminated groundwater and surface water
in those same additive-use areas.
As a part of their National Water Quality Assessment, the U.S. Geological Survey (USGS) found
MTBE in 21 percent of 480 wells located in specific areas of the United States that use MTBE in
gasoline to abate air pollution. In the rest of the United States, MTBE detection frequency in
groundwater was only about 2 percent. Furthermore, after controlling for factors such as population
density, commercial and industrial land use, and the presence of gasoline stations, the USGS found
that the use of MTBE in gasoline increases the probability of detecting MTBE in groundwater by a
factor of about 4 to 6.
MTBE readily dissolves in water and can move rapidly through soils and aquifers. Because it is
resistant to microbial degradation, it migrates faster and farther in the ground than other gasoline
components, thus making it is more likely to contaminate public water-supply systems. According to
the USGS, the vulnerability of aquifers to MTBE contamination appears to be most dependent on the
chemical's use, the population density, and the presence of industry, commerce, and gasoline
stations in the vicinity of sampled wells. Hydrogeologic factors such as well depth, groundwater
level, and presence of roads seem to be less important.
There is widespread concern about MTBE in drinking-water sources because of potential human-health
effects and its offensive taste and odor. The U.S. Environmental Protection Agency has
tentatively classified MTBE as a possible human carcinogen, but has not yet established a drinking-water
regulation. The agency, however, has issued a drinking-water advisory of 20 to 40 micrograms
per liter (20 to 40 parts per billion) on the basis of taste and odor thresholds.
Although water can be treated using existing technologies such as air stripping or granular activated
carbon (GAC), such treatment is difficult and time consuming because of MTBE's physical and
chemical properties. Air stripping is a process in which contaminated water is passed through a
large column filled with loose packing material while upward-flowing air evaporates volatile
chemicals from the water. MTBE does not readily separate from water into the vapor phase, often
requiring high air-to-water ratios.
The GAC treatment technique pumps contaminated water through a bed of activated carbon to
remove organic compounds. Since MTBE does not adsorb well to organics such as carbon, high
volumes of the contaminated water must pass repeatedly through a GAC system before MTBE is
Based on what is now known about MTBE, scientists and regulators have recommended significantly
reducing or eliminating the use of MTBE in gasoline to protect drinking water. They are also
recommending that safer alternatives to MTBE such as ethanol be used in gasoline to guarantee that
clean air benefits are preserved.