CHAPTER 2
Sources of Groundwater Contamination
The first step in groundwater contamination risk assessment is to identify potential
contaminant sources. As described in the introductory chapter, source identification
and characterization can be more difficult for groundwater than for other
environmental pathways. This is due to a number of factors. First, the presence of
groundwater contamination sources are generally hidden from sight. Even when their
existence is known, the characteristics of the sources are difficult to measure.
Second, sources that are only present in very small quantities may still pose a
potentially great health risk, depending on the toxicity of the substances. Third,
groundwater contamination sources are often very long-lived. Disposal of hazardous
material in the ground may pose a threat to groundwater for hundreds or even
thousands of years. Current groundwater contamination sources may be the
consequence of activities carried out years ago. Analogously, current waste disposal
activities may affect groundwater quality, and possibly human health, far into the
future. Finally, the health risk posed by a potential groundwater contamination
source will depend on changing hydrologie conditions. For example, a rise in the
water table, due to an increase in recharge or a decrease in pumpage, may transform
a previously isolated waste disposal site into an active contaminant source.
Different types of groundwater contamination sources can pose different threats
to human health and different problems in health risk assessment. Table 2.1 is a
tabulation of groundwater contamination sources compiled by the U.S. Congress
Office of Technology Assessment (1984). The man-made sources in Table 2.1 are
tabulated according to the activity for which they were originally designed. There are
many possible alternative categorization schemes. For example, sources can be
categorized based on the toxicity of the materials present or by whether they act as
point- or non-point sources. This chapter provides a brief discussion of some of the
important contaminant sources listed in Table 2.1. For further information, the
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Table 2.1
Groundwater Contamination Sources
(from U.S. Congress Office of Technology Assessment, 1984)
Open burning and detonation sites
Radioactive disposal sites
Category I - Sources designed to discharge
substances
Subsurface percolation (e.g., septic tanks and
cesspools)
Injection wells
Hazardous waste
Non-hazardous waste (e.g., brine disposal and
drainage)
Non-waste (e.g., enhanced recovery, artificial
recharge, solution mining, and in-situ mining)
Land application
Wastewater (e.g., spray irrigation)
Wastewater byproducts (e.g., sludge)
Hazardous waste
Non-hazardous waste
Category HI - Sources designed to retain
substances during transport or transmission
Pipelines
Hazardous waste
Non-hazardous waste
Non-waste
Materials transport and transfer operations
Hazardous waste
Non-hazardous waste
Non-waste
Category IV - Sources discharging substances as
consequence of other planned activities
Irrigation practices (e.g., return flow)
Pesticide applications
Fertilizer applications
Animal feeding operations
De-icing salts applications
Urban runoff
Percolation of atmospheric pollutants
Mining and mine drainage
Surface mine-related
Underground mine-related
Category II - Sources designed to store, treat,
and/or dispose of substances; discharge through
unplanned release
Landfills
Industrial hazardous waste
Industrial non-hazardous waste
Municipal sanitary
Open dumps, including illegal dumping (waste)
Residential (or local) disposal (waste)
Surface impoundments
Hazardous waste
Non-hazardous waste
Waste tailings
Waste piles
Hazardous waste
Non-hazardous waste
Materials stockpiles (non-waste)
Graveyards
Animal Burial
Above ground storage tanks
Hazardous waste
Non-hazardous waste
Non-waste
Underground storage tanks
Hazardous waste
Non-hazardous waste
Non-waste
Containers
Hazardous waste
Non-hazardous waste
Non-waste
Category V - Sources providing conduit or
inducing discharge through altered flow patterns
Production wells
Oil (and gas) wells
Geothermal and heat recovery wells
Water supply wells
Other wells (non-waste)
Monitoring wells
Exploration wells
Construction excavation
Category VI - Naturally occurring sources whose
discharge is created and/or exacerbated by
human activity
Groundwater-surface water interactions
Natural leaching
Salt-water intrusion/brackish water upconing (or
intrusion of other poor-quality natural water)
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reader is referred to useful compilations and discussions by Freeze and Cherry
(1979), Pye et al. (1983), and Gordon (1984).
2.1
Land Disposal of Municipal and Industrial Waste
Land disposal of solid waste is the groundwater contamination source of most current
concern to the general public in many developed countries and of most current
regulatory interest (see Chapter 9 and Appendix III). Whether it actually poses the
greatest risk to human health is not clear. Solid waste can be disposed in landfills,
facilities engineered to safely contain the waste. While landfills may often prevent
exposure of solid waste at the land surface, many landfills provide a direct connection
with groundwater. In the past, landfill siting was based on the availability of
inexpensive, undeveloped land requiring little modification for waste disposal, rather
than on hydrogeologic suitability. Disposed materials often are very susceptible to
leaching into groundwater.
Landfills may be grouped according to the type of materials they contain.
Municipal landfills accept only non-hazardous materials, but are still likely to contain
materials which pose potential health risks. Industrial landfills may contain either
"hazardous" or "non-hazardous" materials. Until recently, little was known about how
they were operated or what they contained. Open dumps and abandoned disposal
sites generally have no engineering design. Their connection with the groundwater
system and the type of materials present is often unknown. It is often in abandoned
disposal sites that large volumes of highly toxic materials are found. The most
hazardous solid waste disposal generally results from industrial and manufacturing
activities as well as some governmental energy and defense activities. Populations
of both developed and developing countries, where there is current or historical
industrial activity, face potential health risks from solid waste disposal.
2.2
Sewage Treatment and Disposal
The treatment and disposal of sewage present health risks in both developed and
undeveloped countries. In undeveloped countries, sewage may be directly applied
to the land surface. In more developed areas, sewage is generally transported to
municipal treatment plants or disposed of in septic tanks and cesspools. Groundwater contamination can result in all these cases. Sewage provides a source of
pathogens, nitrates, and a variety of organic chemicals to groundwater. Land
application of sewage can provide a direct contaminant source via infiltration.
Treatment plants can act as contaminant sources in several ways. Leaks may occur
in sewer lines and infiltration may occur from the ponds and lagoons within the
treatment plants. In addition, the sewage sludge that is a product of sewage
treatment processes is often disposed on land in conjunction with agricultural activity.
Depending on the characteristics of the sludge, the soil characteristics, and the
application process, such land application can act as a large non-point source of
groundwater contamination. Land disposal of treated waste water can pose
comparable risks.
The operation and cleaning of septic tanks and cesspools are often thought
to result in localized contamination sources, affecting only nearby wells. However,
the combination of a large number of septic tanks in an area may act as a source of
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regional groundwater contamination. Depending on hydrogeologic conditions, septic
tanks and cesspools may allow untreated sewage to enter the groundwater flow
system. In addition, use of solvents to clean out the systems can cause groundwater
contamination by synthetic organic compounds. The material cleaned out from
septic tanks must eventually be disposed of, often by land application.
2.3
Land Application of Liquid Wastes
Liquid wastes are applied to land in ponds or lagoons that are either designed to
percolate the liquid into the soil or to store and/or evaporate the liquid above
ground. In either case, such facilities act as potential groundwater contamination
sources. Facilities designed to intentionally infiltrate into the ground include cooling
ponds for power generation and for other industrial processes. The liquids in such
facilities may contain potentially hazardous materials. Storage and evaporation
ponds are often lined to prevent infiltration, but are likely to act as groundwater
contamination sources under some circumstances, depending on surface runoff
characteristics, the integrity and permeable of the liner(s), and the groundwater flow
system. Poorly designed evaporation ponds may, in many cases, function as
infiltration ponds.
2.4
Spills and Leaks from Storage and Transport of Liquids
A related category of groundwater contamination sources includes spills and leaks
resulting from transport and storage of liquids. Leaks and spills can occur as
hazardous liquids are transported via pipelines, trucks, or trains or stored in tanks
and other containers. This transportation and storage may either be above or below
ground. Depending on their magnitude, their location, and the local geohydrologic
conditions, these leaks and spills can be significant sources of groundwater
contamination. Spills due to transportation of hazardous materials via trucks or
trains may be due to traffic accidents, operating error, or equipment failure. Pipeline
leaks generally result from corrosion, failure of connections, or accidental damage
by excavation. They often involve petroleum products but other chemicals such as
fertilizers also pose threats.
In recent years, there has been increasing awareness of the large number of
potentially leaking underground storage gasoline tanks. For much of the twentieth
century, underground storage tanks were constructed of unprotected carbon steel.
Corrosion causes leaks in such tanks over some period of time, ranging from a few
years to tens of years. Although the leakage from individual tanks is often small, it
is often enough to contaminate a large volume of groundwater. In addition, the large
number of buried tanks-several million in the United States-makes them a
potentially significant groundwater contamination source. Above ground storage
tanks pose less of a threat than underground tanks. Leak detection and maintenance
is easier and the connection with the groundwater system is less direct. However
leaks from such tanks may still act as groundwater contamination sources.
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2.5
Well Injection of Liquid Wastes
Liquid wastes resulting from numerous industrial processes and from petroleum
production are sometimes injected directly into the groundwater system through
wells. The goal of deep-well injection is to put the waste below the usable
groundwater. The waste, in fact, may not be isolated from the active groundwater
system used for water supply and, thereby, poses a potential health risk. The wastes
can reach the active groundwater system either via natural geologic connection or via
abandoned wells. Clearly, shallow injection wells will pose a much more immediate
threat to groundwater quality. Injection wells are used as part of industrial,
agricultural, mining, and petroleum activities. Note that deep injection of potentially
hazardous liquids is also used for secondary recovery in the petroleum production.
Also, contamination can result from petroleum production when leakage occurs from
production wells-allowing cross-contamination of petroleum into shallower freshwater aquifers.
2.6
Agricultural Activities
Numerous agricultural activities can result in non-point sources of groundwater
contamination. Fertilizers, pesticides, and herbicides are applied as part of common
agricultural practice throughout the world. These applications can act as sources of
contamination to groundwater supplies serving large populations. Whether or not
fertilizers, pesticides, and herbicides become sources of groundwater contamination
depends on changing hydrogeologic conditions, application methods, and biochemical
processes in the soil. In developing countries, animal and/or human waste is used
for fertilizer. This is an example of the land application of sewage discussed earlier.
There are the same concerns with pathogens and nitrates. The manufactured
inorganic fertilizers widely used in developed countries, and finding increasing usage
in all countries, also pose the threat of nitrate contamination of groundwater systems.
Pesticide and herbicide application provides a source of numerous toxic organic
chemicals to groundwater supplies.
Even without the introduction of fertilizers, pesticides, and herbicides,
irrigation activities can lead to groundwater contamination. Naturally occurring
minerals in the soil can be leached at higher rates leading to hazardous concentration
levels in the groundwater. Evaporation of irrigation water can cause evaporative
concentration of certain chemicals in the root zone. Flushing of these chemicals can
then lead to hazardous concentration levels in groundwater.
Agricultural activities related to animals also can be groundwater contamination sources. These include the feeding of animals and the storage and disposal of
their waste. Animal wastes and feedlot runoff are commonly collected in some sort
of pit or tank creating the contamination threat described earlier for sewage disposal.
2.7
Mining Activities
Mines, both active and inactive, are potential groundwater contamination sources.
First of all, the mining excavation can create a direct connection between
groundwater and the land surface. Oxidation of exposed minerals can lead to acidic
leachate waters, particularly in coal mining. Leaching of heavy metals is also a
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threat. Drainage of materials from abandoned mines can act as a groundwater
contamination source for years after mining operation ceases. In addition,
dewatering for mining can alter the groundwater flow field, possibly inducing new
contaminant sources.
2.8
Radioactive Waste
Potentially very hazardous and lethal effects of exposure make radioactive wastes of
special concern. The mining of radioactive minerals, the processing of radioactive
materials, and, particularly, the disposing of radioactive wastes can create very longterm sources of groundwater contamination. As for land disposal of other wasteseither liquid or solid-the important factor for risk assessment is the degree of
isolation from the groundwater system. Although much work has gone into the
engineering design of nuclear waste depositories, much uncertainty remains. Special
uncertainties arise because radioactive chemicals with significant half-lives can
remain health threats for thousands of years, during which changes in geologic as
well as hydrologie conditions become important. Leaching of naturally occurring
radioactive materials is also a source of groundwater contamination. Radon gas, for
example, may be found in groundwater in or near some areas of crystalline rock or
shales.
2.9
Naturally Occurring Poor-Quality Water
Naturally formed waters such as ocean water and connate brines can be sources of
groundwater contamination under certain circumstances. Changes in pumping rates
can cause fresh-water aquifers to be contaminated by intrusion of seawater.
Similarly, changes in the groundwater flow field or leakage through imperfectly
sealed wells can cause contamination of groundwater supply by naturally occurring
brines or other poor-quality waters.
2.10 Surface Water and Atmospheric Contaminants
Groundwater is but one component of the hydrologie cycle. Groundwater quality is
very much influenced by surface-water conditions and vice versa. Contamination of
any surface water bodies that recharge the groundwater system is a source of
groundwater contamination. This includes "natural" recharge sources such as lakes
and rivers as well as "man-made" recharge sources such as artificial recharge
ponds/injection wells and infiltration of urban runoff. More generally, it is important
to consider the interaction of all environmental sources and pathways of pollution.
Environmental contaminant sources cannot be divided into separate, isolated
compartments. For example, atmospheric pollution can lead to deposition of
hazardous fallout to surface waters and to soils, and eventually lead to groundwater
contamination. Discussion of all the possible sources of surface-water and
atmospheric pollution is beyond the scope of this report.
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2.11
Conclusions
There are a number of possible sources of groundwater contamination that are not
listed in Table 2.1 or discussed above. For example, there are many possible
accidents associated with industrial activities that could lead to groundwater
contamination. Hopefully, however, this chapter at least provides a general
framework for grouping potential groundwater contamination sources. Given the
unique uncertainties inherent in identifying groundwater contaminant sources, source
characterization requires special attention. It is the first step in the groundwater risk
assessment process; if the source is poorly characterized, then risks are likely to be
poorly characterized as well.
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REFERENCES
Freeze, R.A., and Cherry, J.A. (1979) Groundwater. Englewood Cliffs, NJ: Prentice
Hall, Inc.
Gordon, W. (1984) A Citizens Handbook on Groundwater Protection. New York:
Natural Resources Defense Council, Inc.
Pye, V.I., Patrick, R., and Quarles, J. (1983) Groundwater Contamination in the
United States. Philadelphia: University of Pennsylvania Press.
U.S. Congress, Office of Technology Assessment (1984) Protecting the Nation's
Groundwater from Contamination, OTA-O-233. Washington, D.C.
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