Ground-water Contamination - An Explanation of Its Causes and Effects

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Ground-water Contamination - An Explanation of Its Causes and Effects
A Special Report by Geraghty & Miller, Inc., Consulting Ground Water Geologists of Port Washington, NY


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Groundwater ( jstor )
Contaminants ( jstor )
Groundwater contamination ( jstor )
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An Explanation of Its Causes and Effects

Of all the kinds of pollution that are steadily degrading man's physical environment in the United
States, probably the least recognized is the hidden deterioration in the quality of ground water-the
vast subsurface water reservoir that is of such vital importance to cities, industries, agricultural
enterprises, and private homeowners all across the nation. Although the dumping of liquid wastes into
the ground has been going on insidiously for a long time, the evidence of damage to ground-water
quality has only begun to show up in comparatively recent times.

The causes of the pollution are many. Practically all buried sewers and pipelines leak, for
example. Waste-storage lagoons or ponds are rarely lined, and because all earth materials are permeable
to at least some degree, the downward seepage of the liquid wastes must go on. Once in the ground,
the wastes are hidden from view, and slowly begin to move through the geologic formations until they
inevitably enter the ground-water systems that may be in use as sources of water supply. While natural
processes such as filtration and dilution can in some cases help to reduce the seriousness of
ground-water pollution, many wastes remain essentially unchanged in composition after they enter a
ground-water body, and relentlessly travel through the earth until they either enter someone's water
well or are discharged into a stream or river.

What exactly is meant by the term "ground-water contamination"? Basically, it
denotes impairment of the quality of ground water by chemical or bacterial pollution to a
degree that inhibits the use of the water or that creates an actual hazard to public health
through poisoning or the spread of disease. Such contamination, as discussed in detail later
in this report, may be caused in many ways. A partial list of common sources of ground-
water contamination is given in the following table.


Cesspools and septic tanks
Leaky sanitary and storm sewers
Animal feed lots
Leaks in oil and other pipelines
Leaks in petroleum storage tanks
Waste retention ponds and lagoons
Infiltration to wells from contaminated
rivers or lakes
Internal circulation in wells
Irrigation return water

10. Insecticides, herbicides and fertilizers
11. Mine drainage
12. Seepage from sanitary land fills
13. Sea-water encroachment
14. Salt used for highway deicing
15. Upward coning of salty ground
16. Accidental or deliberate spills
17. Waste-disposal wells
18. Oil-field brines


It is important to recognize that the pollution of ground water is often not detected until a
considerable part of an aquifer system has been adversely affected. Once an aquifer is polluted, a
very long time and a very large financial expenditure may be required to clean it up, even after the
original source of the contamination is removed. This is in contrast to surface-water pollution,
which can often be corrected in a very short time by physically skimming, pumping, or flushing the

Because this entire process is hidden from view beneath the land surface and seldom reveals
itself until actual damage occurs to a water supply, most people have little or no knowledge of the
magnitude of the problem from a national viewpoint. Even governmental agencies responsible for
water investigations remain largely in the dark in most parts of the country about the extent and
seriousness of ground-water contamination, and have only begun to give serious attention to this
problem in recent years. The present report is intended to provide an overview of the subject in
order to acquaint decision-makers, especially in industry and government, with the principal factors
involved in ground-water contamination and with the general state of knowledge concerning how
and where it is taking place.

Background Facts About Ground Water

The current use of fresh water in the United States for all purposes except the
generation of hydroelectric power is about 300 bgd (billion gallons per day). Most of this
water is supplied by rivers and streams, but in recent decades, ground water pumped
from wells has begun to play a very significant role in water supply; so that ground-water
resources now furnish 22 percent of the nation's fresh-water needs and undoubtedly will
provide an even larger percentage by 1980, when the national water requirement will be
about 600 bgd. The widespread availability of ground water and its desirable properties,
such as clarity, bacterial purity, and generally consistent temperature and chemical
quality, are a few of the characteristics that have led to the present day expansion of
water-well systems in the nation. Another important reason for this trend is that more
and more public water-supply systems, industries, and commercial establishments, es-
pecially in densely populated regions, can no longer find the large tracts of land that are
required for construction of new surface-water reservoirs.

The actual ground-water reserves of the United States are difficult to determine, but
are certainly immense in comparison with other water resources. It has been estimated,
for example, that the total volume of fresh ground water stored in the various geologic
formations of Florida greatly exceeds the storage capacity of all five of the Great Lakes,
and that the usable portion of ground water in the entire country is 150 times greater
than the amount of water used in the country in 1965.

However, in spite of the large reserves, highly productive water-bearing beds or
aquifers are irregularly distributed, so that no overall statement can be made about the
yields of wells in all parts of the country. This is because the aquifers differ considerably
in areal extent, thickness, and permeability; in addition, the ways in which wells are
drilled and developed also govern how much water can be extracted from an aquifer. A
further complication is that some aquifers contain water having a naturally poor quality.


Causes of Ground Water Contamination

Every day, a wide variety of harmful substances is regularly introduced into sub-surface
formations and the aquifers of the nation. These substances enter ground-water supplies by direct
injection through wells, by percolation of liquids spilled at the land surface or leached from soluble
solids at the surface, by leaking or broken sewers and pipelines, by downward seepage from waste
lagoons, by infiltration of polluted surface water into the ground, and in many other ways. Even
windblown wastes have caused contamination of ground-water supplies. To the list should be added
contamination of ground water caused by infiltration of irrigation tail water, sea-water encroach-
ment, and upward coning of salty ground water into fresh-water aquifers.

One newly recognized threat to ground-water quality comes from improperly designed sanitary
landfills and dumps. At some such sites, garbage and rubbish are dumped directly into ground
water, especially in low-lying swampy and coastal areas. However, even if the wastes are not in
direct contact with the ground water, rainfall entering the landfill through its exposed surface can
react with the contaminants to form a concentrated leachate that percolates downward to the

Cesspools and septic tanks, which are in use in every part of the country that has not been
sewered, are another source that in many localities has already caused measurable pollution. In
Long Island, N.Y., for instance, sewage effluent from hundreds of thousands of cesspools and septic
tanks enters directly into a permeable glacial aquifer, and the result has been a fairly widespread
deterioration in water quality that has caused abandonment of some public water-supply wells
tapping this particular water-bearing formation. In another case, a sudden outbreak of infectious
hepatitis in Michigan was traced to contaminated septic tank effluent that traveled through
fractures in limestone to the water table and then spread to domestic wells.

Disposal of industrial wastes in pits or waste lagoons is another widespread practice, and is of
particular concern where metal and plating wastes are being dumped. When improperly constructed,
such lagoons can leak toxic wastes such as hexavalent chromium, cadmium, cyanide, and caustic
soda into the sub-surface.

Percolation of liquids spilled at the land surface can be another serious threat if the surficial
materials are permeable and allow downward percolation. For example, many hydrocarbon spills
take place at airports during cleaning or fueling of aircraft, and frequently, such spills penetrate
into the ground, travel downward, and come to rest on top of the water table. Pipeline spills may
also pose a hazard to ground-water supplies. In 1969, for example, a total of 410 hydrocarbon
spills were reported in 30 states in the nation, most of them due to corrosion, defective pipelines,
and damage to pipelines by construction equipment. Storage of chemicals, chemical wastes, or
petroleum products in steel or concrete tanks also presents a potential hazard because corrosion of
the metal or cracking of the concrete may ultimately permit seepage of contaminants into an
aquifer. Leaching of soluble solids stored on the land surface is still another practice that can be

responsible for contamination of ground wafer. Such incidents occur, for example, where rainwater
dissolves soluble materials and enters the ground from coal-mine waste piles and deicing salt piles.

Deep-well disposal (disposal of liquid wastes through wells into porous and permeable rock
formations) is a comparatively new practice in the nation that also may threaten some ground
waters. Chemical, petro-chemical, and pharmaceutical companies operate many such injection wells
in the country at present. Generally, the wells are designed to inject or discharge waste fluids into
salt-water-bearing rocks, but improperly designed wells and mechanical failure of the well installa-
tions themselves have caused serious contamination of fresh ground-water sources in some instances.
Oil and gas wells can also be a threat, as in one case in Pennsylvania where acid drainage water
from abandoned coal mines gained entrance through such wells into fresh-water aquifers.

Another kind of ground-water contamination takes place when salt water encroaches and
enters into a fresh water-bearing aquifer. Such salt-water intrusion occurs mainly in coastal areas
where pumping has decreased the natural seaward flow of fresh ground water. However, it can also
take place in inland localities when pumping induces deep salty waters to cone upward into
fresh-water beds.

Agricultural practices also may result in ground-water contamination. Of most serious concern
is irrigation return water that becomes more and more concentrated and saline as a result of
evaporation and recirculation. A related problem stems from the wide use of pesticides and


Biological contamination of ground water may occur when human or animal wastes enter an
aquifer. Microorganisms present in the wastes may be carried in the ground water into nearby water
wells, where they may cause disease. Medical history is full of examples of this kind. One of the
most famous cases occurred during the cholera epidemic of 1854 in London when, for the first
time, a medical doctor traced the source of the illness to one particular well that turned out to be
contaminated with sewage.

Numerous studies have been made to determine the role of septic tanks and cesspools in
ground-water contamination. The migration of the bacterial pollutants through the earth has been
studied by collecting samples from test wells, and indications are that the bacteria seldom travel
more than 100 feet from a source point (the exception is in cases where the aquifer is fractured or
cavernous, and where bacteria are able to travel rapidly for great distances). In general, the studies
have shown that bacteria are largely removed by filtration. Although most microorganisms die out
rapidly in ground water, bacterial pollution may occur locally in heavily populated suburban areas
where numerous septic tanks discharge large quantities of waste into an aquifer.


Inorganic chemical contamination differs from biological contamination in several important
ways. Most important are the indestructibility of inorganic chemicals, the persistence of the
pollution created by their presence, and the difficulty in their removal. In the United States, the
U.S. Public Health Service has specified certain "minimum" standards -for concentrations of
substances in drinking water (see table below). The standards show, for example, that concentra-
tions of such toxic elements as arsenic and hexavalent chromium greater than 0.05 ppm (parts per
million) would cause rejection of the water supply if used for drinking purposes.

Several of the elements listed in the following table are particularly toxic, for instance
cadmium and lead. Cadmium in ground water in excessive concentrations is often found where
electroplating wastes have been discharged into the ground. Lead in excessive concentrations is
often found in ground water where leaded high-octane gasoline has entered the aquifer through
leaks in buried pipelines or by surface spills that have percolated down to the aquifer.

(Source: U.S. Public Health Service, 1962)

Recommended limits Mandatory limits
Substance of concentrations, of concentrations,
in mg/1 in mg/1

Alkyl benzene sulfonate (ABS) 0.5
Arsenic (As) 0.01 0.05
Barium (Ba) 1.0
Cadmium (Cd) -0.01
Carbon chloroform extract (CCE) 0.2
Chloride (Cl) 250.0
Chromium hexavalentt) (Cr+6) 0.05
Copper (Cu) 1.0
Cyanide (CN) 0.01 0.2
Iron (Fe) 0.3
Lead (Pb) 0.05
Manganese (Mn) 0.05
Nitrate (NO3) 45.0
Phenols 0.001
Selenium (Se) 0.01
Silver (Ag) 0.05
Sulfate (SO4) 250.0
Total dissolved solids (TDS) 500.0
Zinc (Zn) 5.0

In the arid western parts of the nation, inorganic chemical contamination is of great concern
to agricultural water users. Generally, the quality criteria most often applied relate to total salt
concentration (total dissolved solids), chloride, sodium, boron, and bicarbonate. Water low in salts
is usually the most desirable for irrigation, but sometimes only water containing several thousand
ppm of salts is available. Lack of proper flushing and high evaporation rates may cause salt
accumulation in the root zone with a resulting decrease in crop yields. Reuse of water for irrigation
along a stream is a frequent source of build-up of salts in the water and in the ground-water
reservoir in general. In this connection, it is of interest to note that many of the rivers in the
southwestern United States have a naturally high content of dissolved solids, being fed by saline
ground water that emerges in the form of springs along the upstream portion of the drainage basin.

Organic chemical contamination is most often caused by such substances as detergents,
gasoline, oil, and phenolic compounds. Detergents in ground water are a common contaminant in
many parts of the nation. Most often these problems are caused by cesspools or by direct
discharges into "dry wells" of common household detergents. The detergent ABS alkyll benzene
sulfonate), widely used before 1965, is itself not considered to be toxic or a menace to health, but
its foaming appearance and taste are regarded as problems, and do affect the ultimate usefulness of
an aquifer. In addition, ABS in ground water points to the possible presence of other harmful
contaminants from liquid wastes.

Phosphates contained in detergents and chemical fertilizers may constitute a hazard to
ground-water quality if present in excessive concentrations. Although research has shown that
phosphate concentrations in sewage-contaminated ground water are greatly reduced immediately
down-gradient from cesspools and septic tanks, excessive concentrations in built-up urban areas may
cause the abandonment of public water-supply wells or may require the installation of expensive
treatment facilities to eliminate the contaminant. Nitrates derived from fertilizers and septic tanks
similarly may cause contamination of ground water if present in excessive concentrations.

Gasoline and other hydrocarbons often end up as ground-water contaminants through pipeline
breaks or spills at the land surface. The presence of minute concentrations of hydrocarbons may
result in abandonment of wells because of strong odors and tastes. Frequently, toxic metals such as
lead or chemical additives complicate the contamination pattern. Recovery or flushing out of all
hydrocarbon contamination in a ground-water system is virtually impossible because capillary
adhesion of hydrocarbon particles takes place. Phenolic wastes derived from oil refineries or
chemical plants are another substance that often ends up in ground-water systems. The presence of
this contaminant is generally discovered through taste and odor, which can often be recognized at
concentrations as low as 0.001 ppm, the recommended U.S. Public Health Service limit for phenol
in drinking water.


The nature, size, and degree of hydraulic interconnection of openings or pore spaces determine
how easily fluids can move through geologic formations. Openings in rock may differ greatly, from
large openings feet in diameter in cavernous limestone and volcanic rocks to minute pore spaces in
silts and clays. The permeability of the rock, or its water-yielding characteristic, is generally
calculated in the field by means of controlled pumping tests. Permeability differs from place to
place depending on rock type and the nature of bedding and fracturing. Of the dense hard rock
varieties, cavernous limestones and jointed lava beds generally have the highest permeability,
followed by regular limestone, sandstone, shale, and granitic and metamorphic rocks. Of the
unconsolidated rocks, gravels and sands have the highest permeability. Silty materials have a much
lower permeability, followed by clays, which have extremely low permeabilities and allow little
water to pass through. Often, fine-grained sediments such as clays act to absorb contaminants,
thereby filtering out wastes.

Geologic units are often stratified, so that a particular site may be underlain by a series of
layers of rocks, each with its own hydraulic characteristics. In the glaciated northeastern part of the
United States, for example, it is not uncommon to find, below a layer of top soil, beds of sand and
gravel having a high permeability, inter-bedded with beds of silt and clay of low permeability, in
turn overlying fractured crystalline bedrock formations. Since fluids tend to move most rapidly in
the more permeable zones, stratification plays an important role in the dispersal of contaminants.


The configuration and slope of the water table are important considerations in estimating the
directions and rates of travel of wastes in the subsurface environment. Contaminants dumped in a
region where the water table is practically flat and where little movement of ground water is
occurring will tend to stay in place. On the other hand, where there is a steep slope on the water
table, the contaminant will tend to spread rapidly down-gradient from the site.

The thickness and composition of the unsaturated zone overlying the saturated zone are also
important factors. Especially in cases of biological contamination, a thick unsaturated zone will act
to absorb and filter biological pollutants before they can be introduced into the ground-water body
itself. In the opposite case, where the unsaturated zone is thin, there may be little retarding or
absorptive action and the pollutant arrives at the saturated zone almost immediately.




or will float (hydrocarbons for instance) on top of the saturated zone (see Figure 1, which shows a



Once at the top of the water table, fluid wastes will either penetrate into the ground-water body
or will float (hydrocarbons for instance) on top of the saturated zone (see Figure 1, which shows a
typical case involving the percolation of contaminants through the unsaturated zone into a water-table
aquifer). The contaminant will then move with the ground water toward its ultimate discharge point,
which commonly is a spring or a river. Frequently, however, ground-water flow patterns are modified
because of pumping from nearby wells. In such cases, the hydraulic gradient or slope of the water
table is toward such a well and the contaminants converge upon the center of pumping and emerge in
the well discharge (see Figure 2, which shows contamination of a water-table aquifer by leaching of
surface solids). In most cases, this is exactly how ground-water pollution is discovered.

The normal rate of flow of ground water in typical saturated formations is generally low, on the
order of inches or a few feet per day. Cavernous limestones, on the other hand, may transmit water at
rates of up to hundreds of feet per day depending on hydraulic gradients.

Under natural conditions and in the absence of pumping, almost all water-table aquifers discharge
ground water continuously into a nearby surface-water body such as a lake or river. Thus, the ground
water is always entering the lake or stream and the aquifer itself cannot be contaminated by wastes
carried by the stream. When a well is put in operation in such an aquifer near a stream, however,
ground-water levels are lowered and the hydraulic gradient between the well and river may be reversed,
causing surface water to flow toward the well. If the stream is polluted, contaminated river water may
thereby be induced to flow to the well. This situation is shown on Figure 3.


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According to the laws of fluid movement in saturated material, the direction of ground-water
flow will always be toward points where the hydraulic head is lowest. In many parts of the country,
saline ground water in deep aquifers is under high artesian heads, so that it can be induced to move
upward into fresh water-aquifers where heads are lower, if the two zones are interconnected through
abandoned or improperly sealed wells. Figure 4 shows how deep saline water may flow upward and
spread into a fresh-water aquifer in this way. Leakage of contaminants may also take place when
contaminated water enters broken or corroded well casings and discharges into another aquifer (see
Figure 5, which depicts the resulting contamination of a water-supply well).



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Numerous aquifer zones have already been contaminated to various degrees by a wide variety of
natural and man-made pollutants, and the growing realization of the seriousness of this problem has
led to a demand for legislation and protective measures. Unfortunately, as yet, there is no Federal law
that specifically prohibits dumping of dangerous wastes on the land. Most states do have laws that
prohibit dumping that can be injurious to health, but enforcement is often lax.

Federal agencies such as the Atomic Energy Commission and Federally licensed utilities and
manufacturers involved in dumping noxious wastes are now required to file environmental impact
statements before such dumping can be approved under the Environmental Policy Act of 1969. But
there are no such provisions controlling the private sector. There is also no specific Federal legislation
regarding underground disposal of wastes by means of deep wells. No state is known to have legislation
that denies such installations, and with a few exceptions most states permit deep-well disposal. Only
four states (Michigan, Ohio, West Virginia, and Texas) have specific legislation that regulates industrial

waste-water injection. Thus, in the absence of Federal action on ground-water contamination, the
various State and regional environmental and pollution agencies may be expected to place greater
emphasis in the future on prevention, control, and removal of polluted ground water.


When evidence accumulates that the ground-water resource in a given locality is becoming
contaminated and that remedial measures must be taken, the first step, generally speaking, is to define
the extent, thickness, and direction and rate of movement of the contaminated ground water. This
usually calls for test-well drilling to explore the geologic and hydrologic characteristics of the materials
below the spill site. Geophysical surveys may be helpful in this connection by defining the extent of
contaminated water bodies. Monitoring of ground-water levels in the aquifers is helpful in determining
directions of movement. An intensive survey of ground-water quality at various depths below the
surface also must be made. In some instances, chemical tracers may be introduced into the ground to
study the movement and rate of flow of the ground water. Owing to the complex nature of many
wastes and the equally complex chemical reactions that may be taking place in the subsurface,
extensive laboratory work involving research, simulation, and field experiments is sometimes needed.

Once sufficient information is available to map the three-dimensional extent of the contaminated
ground water and to assess its environmental impact, measures can be devised to control and hopefully
to remove the contaminants. An obvious first step is to stop the polluting process at its source, but
this will not be an immediate benefit because the wastes already in the ground-water environment will
persist for a very long period of time. A second step is to attempt to remove the contaminated fluid
through ditching or through pumping from wells installed for the purpose. Another alternative is to
reverse the natural gradient of the water table, again by pumping from wells, so that the wastes can at
least be prevented from escaping beyond the affected area. In all of these procedures, it is essential
that careful attention be given to such elements as aquifer permeabilities, vertical hydraulic
interconnection of water-bearing beds, chemical reactions in the earth materials, and possibilities of
further spreading of the wastes through improperly designed test wells.


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