Groundwater: What it is
CORNELL COOPERATIVE EXTENSION
Groundwater: What It IS and How to Protect It
Nancy M. Trautmann and Keith S. Porter
Center for Environmental Research
Robert J. Wagenet
Dept of Agronomy
Groundwater is the source of water for wells and springs. It is found
underground, within cracks in bedrock or filling the spaces between particles of
soil and rocks. The groundwater layer in which all available spaces are filled
with water is called the saturated zone. The dividing line between the saturated
zone and overlying unsaturated rock or sediments is called the water table.
Why Groundwater Is Important
Groundwater can be found under most of the land in the Northeast and is widely
used for household and other water supplies. Approximately half the population
of the United States relies on groundwater for drinking water, and more than 90
percent of rural residents obtain their water from groundwater through wells or
There are good economic reasons for this widespread dependence on groundwater.
In its natural state groundwater is usually of excellent quality and can be used
with no costly treatment or purification. It can be inexpensively tapped
adjacent to the point of use, thereby saving the costs of transporting water
long distances. In addition, costly storage facilities such as water tanks or
towers are not needed. Surface water, on the other hand, usually requires
storage and treatment, which are relatively expensive and difficult to manage
without technical resources. For rural residents relying on individual wells,
groundwater often is the only available water supply, and for many communities
it is by far the least expensive option for public water supply systems.
Until recently, groundwater quality has been taken for granted. Although some
wells and springs were known to contain naturally high levels of sulfur or
salts, and others were known to be contaminated by sewage bacteria, groundwater
was generally thought to be immune from contamination by the many chemicals we
use for industrial, agricultural, and household purposes. Only within the past
decade have we made the link between what we do at the land surface and what we
find in our groundwater supplies.
In dealing with threats to groundwater resources, communities and individuals
need knowledge and understanding of groundwater:where it comes from, how it
flows, and how it can be protected. This bulletin explains these principles in
nontechnical language, with emphasis on the information necessary to protect
Dependence of northeastern states on groundwater as a source of
State Total % of total Population Population % of % of
Population population served by served by public rural
(thousands) relying on groundwater groundwater supply popu-
groundwater from public from rural popu- lation
supplies supplies lation relying
(thousands) (thousands) relying on
CT 3032 37 590 531 24 98
DE 548 65 217 138 53 100
ME 994 37 153 219 20 91
MA 5689 31 1464 286 27 100
NH 738 61 261 191 48 98
NJ 7168 53 3032 746 47 100
NY 18191 32 4152 1734 25 100
PA 11794 30 1351 2144 14 100
RI 950 33 213 103 25 100
VT 445 56 100 150 35 96
TOTAL 205897 48 60600 38568 37 94
Source: V.I. Pye, R. Patrick, and J. Quarles, 1983, Groundwater Contamination
in the U.S., Univ. of Pennsylvania Press, Philadelphia.
Where Groundwater Comes From
Water entering the soil gradually percolates downward to become groundwater if
it is not first taken up by plants, evaporated into the atmosphere, or held
within soil pores. This percolating water, called recharge, passes downward
through the root zone and unsaturated zone until it reaches the water table.
Effective programs for protection of groundwater focus primarily on the recharge
process because this controls both the quantity and the quality of water
reaching the saturated zone. Water is far easier and less expensive to manage at
the land surface than after it becomes less accessible and more dispersed
The quantity of recharge in any particular location depends on the amount of
precipitation or irrigation, the type of soil, and the topography and geology of
the site. Seasonal fluctuations occur in the quantity of recharge, leading also
to fluctuations in depth of the water table. In winter and early spring when
snow is melting and plants are not yet taking up much water, the water table may
be close to or at the ground surface. Evidence of this includes wet basements
and agricultural fields that are too wet for cultivation or planting. As the
summer progresses, the water table commonly drops because evaporation and plant
uptake exceed recharge. During dry periods this drop may cause water shortages
in shallow wells, as well as drying up of some springs, wetlands, and small
Recharge is the only natural means of replenishing groundwater supplies, and the
water table will drop if the amount of water withdrawn exceeds the amount
recharged. This is not a common problem in the Northeast except during drought
periods. In more arid parts of the country, though, where groundwater is the
sole source of water supply for large cities or agricultural operations,
groundwater depletion can be a serious and continuing problem.
How Groundwater Moves
Groundwater does not generally consist of large underground lakes or streams
Rather, it is water that fills irregular spaces within rock fractures or between
particles of sand, gravel, or clay. Whereas water in a stream may move several
feet per second, groundwater may move only a few feet per month or even per
year. The major exception to this general rule is in limestone areas, where
groundwater may flow rapidly through large underground channels and caverns.
The geologic formation through which groundwater moves is called an aquifer.
This can be a layer of sand, gravel, or other soil materials, or a section of
bedrock with fractures through which water can flow. Drilling a hole into the
ground in most areas of the Northeast will yield some water. Only the major
aquifers, though, will have sufficient flow to maintain community water systems.
In New York State, the most productive aquifers consist of sand and gravel
deposits, which generally are found in river valleys. The quantity and quality
of recharge they receive depend on their depth from the ground surface, on the
geology of the overlying materials, and on the climate, land uses, and water
management practices in their recharge areas.
Recharge water generally moves downward through the soil until it reaches the
water table, then travels in a more horizontal direction, following the contours
of the aquifer. Eventually groundwater resurfaces, producing springs or feeding
water into streams, wetlands, or other surface water bodies.
One of the most familiar reasons for problems with well water is contamination
by bacteria, viruses, or parasites, which can seep into the groundwater from
sources such as septic systems, leaky sewer lines, barnyards, or fields spread
with manure. Water contaminated with sewage-related microorganisms can transmit
diseases such as cholera, typhoid, and hepatitis. These diseases kill many
people worldwide, but have been almost eliminated in the United States by
chlorination of public water supplies and regulation of septic system placement.
Of greater concern for groundwater contamination in this country are two broad
classes of chemicals: (l ) inorganic minerals, salts, and metals and (2)
synthetic organic compounds.
Inorganic Chemicals. All groundwater contains natural salts, minerals, and other
inorganic compounds. These usually are found at concentrations higher than those
in surface water because groundwater dissolves inorganic substances as it seeps
through the ground. Most inorganic compounds are harmless at the concentrations
commonly found in unpolluted groundwater, and some, such as potassium,
magnesium, and calcium, are even beneficial to human health. Others, such as
arsenic, barium, or mercury, can occur naturally in concentrations that are
considered harmful. Naturally high concentrations of hydrogen sulfide or salt
make some groundwater unpalatable, although not a health hazard.
Human activities are another source of inorganic substances in groundwater.
Chloride levels, for example, can become elevated by leaching of highway deicing
salts or effluent from septic systems. In coastal areas groundwater supplies
become high in chloride if pumping exceeds recharge, causing salt water to
intrude into what had been a fresh water aquifer. Salt water intrusion is a
problem in some Cape Cod and Long Island communities, as well as in other
coastal areas with heavy pumpage of groundwater. Chloride levels can also be
elevated naturally by flow of groundwater through areas with natural salt
Nitrate is another groundwater contaminant of concern in drinking water because
at high enough concentrations it can cause methoglominemia, or "blue baby
syndrome." (Fact Sheet 400.02, Nitrate: Health Effects in Drinking Water, in
this series gives more information about the health effects of nitrates in
drinking water.) Typical sources of high nitrate concentrations include septic
systems that are spaced too close together, fertilizers that leach from lawns or
agricultural fields, and liquids that percolate into the ground in areas where
animal manures are concentrated.
Organic Chemicals. Organic compounds are chemicals containing carbon and other
elements such as hydrogen, nitrogen, or chlorine. Many occur in nature, and many
others are manufactured for a wide range of purposes, including cleaning fluids,
wood preservatives, and pesticides. The production of synthetic organic
compounds has increased more than 10-fold in the past 40 years, and some of
these chemicals have become significant groundwater contaminants. In many cases
these can be traced to improper disposal of chemicals in lagoons, landfills, or
leaking storage tanks. Presence of synthetic organic compounds in groundwater is
a matter of serious concern because of their potential health effects in
drinking water. A few are known to be carcinogenic, and others can cause
problems such as liver damage, neurological disorders, or birth defects. (Fact
Sheet 400.03, Pesticides: Health Effects in Drinking Water, in this series gives
more information about the health effects of pesticides and other synthetic
organic compounds in drinking water.)
Not all chemicals will leach to groundwater. Which ones will do so and in what
quantities depend on the original quantity of the chemical, how soluble it is,
how strongly it is held by the soil, and how quickly it breaks down in the root
zone. Soil type and climatic conditions also affect leaching potential. One
example is aldicarb (brand name Temik), a pesticide widely used throughout the
country for a variety of crops including cotton, peanuts, potatoes, and citrus
fruits. In most areas aldicarb has been used without significant leaching to
groundwater. This has not been the case, though, on Long Island and in several
other locations with sandy, acidic soils and a shallow water table. As a result,
over 2,000 Long Island wells exceed the New York State drinking water guideline
for aldicarb. On Long Island carbon filtration units have had to be installed in
each affected household, and plans are being made to replace individual wells
with expensive community water supply systems.
Movement of Contaminants
Groundwater becomes contaminated when water percolating through the soil carries
pollutants downward to the water table. Gasoline or other liquids that seep into
the ground or leak from underground storage tanks can also become groundwater
contaminants. Once in the saturated zone, these chemicals move with the
groundwater, forming a region of contaminated water called a plume. Because
contaminants flow with groundwater through the saturated zone, the quality of
water in a particular well depends on the land areas, perhaps miles away, from
which this part of the aquifer was recharged.
It may be some years before a contaminant plume originating at the land surface
appears in a well some distance away. By the time this is detected, the
contamination is likely to be widespread, and it may take decades or even
centuries for the aquifer to purify itself naturally. In rivers and streams
dissolved contaminants are rapidly swept downstream, diluted, and subjected to
biological and chemical decay. Groundwater conditions, in contrast, act to
preserve contaminants. The slow water movement means that flushing of
contamination from an aquifer may take many years. Dilution does occur, but to a
limited extent, because mixing takes place only gradually as water moves through
small cracks or pores. Biological and chemical decay of chemicals are slow in
groundwater because of the cold temperatures, low oxygen levels, and limited
microbial activity. Because contaminants in groundwater tend to be preserved
rather than degraded, diluted, or rapidly swept away, the best way to protect
groundwater quality is to keep contaminants out of recharge waters.
Protection of Groundwater Quality
Protection of groundwater quantity and quality can best be accomplished by
controlling potential contaminant sources and by managing land uses in prime
recharge areas. Using knowledge of local geology and groundwater flow
directions, estimates can be made of the land areas contributing recharge to a
particular well or to an aquifer as a whole. Controls can then be established to
ensure appropriate land uses and chemical practices within the recharge areas.
The best protection is provided through land acquisition. Massachusetts recently
has initiated an Aquifer Land Acquisition Program, which provides state funding
for purchase of important recharge areas. Where acquisition is not feasible,
conservation easements and transfer of development rights are alternative means
of protecting recharge quality.
In many cases recharge areas cannot be set aside in their natural states.
Groundwater protection efforts then must focus instead on management of the
diverse potential contaminant sources. Possible techniques include public
education, inventory and monitoring of potential contaminant sources, and
tailoring of zoning ordinances and other local land use regulations for
protection of community groundwater supplies. Connecticut and Vermont have
established programs to assist communities in protecting recharge quality within
specially designated aquifer protection areas (see Mullikin 1984; Harrison and
Residential areas, commercial operations, and industries all require careful
design and management for protection of groundwater supplies. Potential
contaminants from residential areas include nitrates from lawn fertilizers or
septic systems, pesticides from lawn or household treatments, and synthetic
organic chemicals from septic tank cleaners and other household products ranging
from paint thinner to drain cleaners. Commercial operations can contaminate
groundwater through chemical spills or inadequate disposal practices. Substances
such as waste motor oil or used dry cleaning fluid can cause well closures if
they are dumped down a drain or onto the ground rather than handled
appropriately as toxic wastes. Similarly, industrial discharges to ground or
surface waters can make the water undrinkable unless toxic components receive
proper treatment or alternative disposal.
One means of reducing the chances of groundwater contamination from residential
areas is to limit the allowable housing density. Most houses in rural areas have
septic systems rather than public sewers, so wastewater is disposed of in the
ground. This wastewater is high in nitrogen and may also contain other
contaminants such as the synthetic organic chemicals found in cleaning agents,
petroleum products, or household pesticides. The farther apart the houses, the
wider the separation between possible contaminant sources, and therefore the
more the dilution they receive in groundwater.
Other possible groundwater protection techniques include sewering of sensitive
areas, mandatory inspection of septic systems, reduction of the amount of salt
used on roads, and regulation of chemical waste disposal practices of local
industries and commercial establishments.
Prohibitions or limits imposed on specific chemicals have been adopted by some
communities. Suffolk County on Long Island, for example, prohibits use of septic
tank cleaners because these products contain synthetic organic compounds that
have been detected in local wells at concentrations exceeding the N.Y.S.
drinking water guidelines. If nitrate levels are a problem in water recharging
from residential areas, then community ordinances can be established to limit
either the area used for lawns or the amount of fertilizer that can be applied
to them. The towns of Southampton and Easthampton on Long Island have recently
developed ordinances limiting the allowable turf area per lot, and the town of
Oyster Bay requires that any new lawns be made up of low maintenance turf
varieties so that fertilizer requirements will be reduced.
In some parts of the country, agricultural fertilizers and pesticides have
become significant groundwater contaminants. Farmers have a wide range of
choices in how to manage their farms, and these choices determine the efficiency
of chemical use as well as the success of the harvest. Pesticides and fertilizer
applications can be designed to efficiently meet crop needs while minimizing
losses to groundwater. The types of chemicals used, the amounts and timing of
applications, and a variety of other farm management practices all interact to
affect crop yields and leaching losses. Further bulletins in this series will
address farm chemical management in greater detail.
Some potential groundwater contaminant sources and methods for control
Source Contaminants Control methods
Septic systems Bacteria, viruses, Regulation of siting and
synthetic organic installation. Requirement of
compounds, nitrate periodic inspection or
pumping. Education about
hazardous household chemicals.
Underground fuel Gasoline and other Periodic inspection of tanks,
storage petroleum products including pressure testing.
Monitoring of fluid levels and
Sanitary landfills Heavy metals, Careful siting to protect
synthetic organic groundwater. Regulation of
compounds, nitrate construction and operation,
Road salt storage Sodium chloride or Protection of stockpiles from
and use other salts precipitation and runoff.
Minimization of salt use.
Fertilizers and Nitrate, synthetic Efficient nitrogen application
pesticides organic compounds, to meet crop needs. Use of
heavy metals integrated pest management or
other means of minimizing
pesticide needs. Regulation of
disposal of used containers.
Hazardous waste Industrial chemicals "Cradle to grave" accounting of
spills, leaks, or hazardous materials. Mandatory
improper disposal inspection of transportation
equipment. Heavy fines for
spills. Storage regulations for
Community action for groundwater protection
Action References for further information
Identify groundwater resources O'Donnell 1984
and recharge areas. Pacenka et al. 1984
Inventory existing local programs. Harrison and Dickinson 1984
Inventory present and future Harrison and Dickinson 1984
groundwater resources, land uses,
and water supply demands.
Identify inadequacies of existing Harrison and Dickinson 1984
programs. STCRPDB 1985
Choose and draft protection
Zoning techniques Dawson 1984
Aquifer protection districts MAPC 1982
Watershed protection districts STCRPDB 1985
Nonzoning regulatory techniques
Development review Dawson 1984
Underground fuel storage O'Donnell and White 1984
Hazardous material handling MAPC 1982
Storage and use of road salt STCRPDB 1985
Maintenance of septic systems
Education Dawson 1984
Conservation restrictions Harrison and Dickinson 1984
Transfer of development rights MAPC 1982
Land acquisition Mullikin 1984
Groundwater, historically, has been taken for granted, especially in the
Northeastern states where it tends to be plentiful and easily obtained. In
recent years, however, many communities have discovered how valuable their
groundwater supplies can be. Once groundwater becomes contaminated, the options
for treating it or for finding alternative supplies tend to be very expensive
and the prospects for cleaning up the aquifer may be many years in the future.
Both the quantity and the quality of groundwater supplies depend on the recharge
water that continually filters down through the soil to the saturated zone. Any
chemicals at the ground surface or within the soil profile can become
groundwater contaminants if they are carried downward by this recharge water.
The first step in protecting groundwater quality is to determine the locations
of prime recharge areas. The second step is to identify management options which
would help to protect the quality of recharge in these areas. The level of
management appropriate to a particular area depends on the vulnerability of the
aquifer, the extent to which it is relied on for high quality water supplies,
and the number and type of potential contaminant sources. Action for protection
of recharge quality can be as simple as not dumping used motor oil down the
drain or as comprehensive as a community wide aquifer protection plan
incorporating land-use and contaminant source control regulations. Recharge
protection is the key to any effective groundwater protection program.
For Further Reading
Dawson, A. 1984. Local authority for groundwater protection. Massachusetts
Audubon Society, Groundwater Information Flyer #4. Available from Massachusetts
Audubon Society, Public Information Office, Lincoln, MA 01773.
Gordon, W. 1984. A citizen's handbook on groundwater protection. Natural
Resources Defense Council, Inc. Available from NRDC, Inc., 122 East 42nd Street,
New York, NY 10168.
Harrison, E.Z., and M.A. Dickinson. 1984. Protecting Connecticut's groundwater:
A guide to groundwater protection for local officials. Connecticut Department of
Environmental Protection, Hartford, Conn.
Metropolitan Area Planning Council. 1982. Groundwater protection: A guide for
communities. Available from MAPC Resource Library, 110 Tremont Street, 5th
Floor, Boston, MA 02108.
Mullikin, E.B. 1984. An ounce of prevention: A groundwater protection handbook
for local officials. Available from Vermont Department of Water Resources, State
Office Building, Montpelier, VT 05602.
O'Donnell, A. 1984. Mapping aquifers and recharge areas. Massachusetts Audubon
Society, Groundwater Information Flyer #4. Available from previously cited
O'Donnell, A., and L. White. 1984. Underground storage tanks and groundwater
protection. Massachusetts Audubon Society, Groundwater Information Flyer #4.
Available from previously cited address.
Pacenka, S.P., et al. 1984. Protecting ground-water supplies in river valley
communities. Cornell Cooperative Extension Miscellaneous Bulletin 131. Available
The Resource Center
PO Box 3884
Ithaca, NY 14852-3884
Email orders: email@example.com
Southern Tier Central Regional Planning and Development Board. 1985. Final
report of the Central Southern Tier Groundwater Critical Recharge Area Project.
White, L. 1983. An introduction to groundwater and aquifers. Massachusetts
Audubon Society, Groundwater Information Flyer #1. Available from previously
--- 1984. Groundwater and contamination: From the watershed into the well.
Massachusetts Audubon Society, Groundwater Information Flyer #2. Available from
previously cited address.
Acknowledgments: Illustrations were drawn by Christine Cleveland and Donna
Curtin, and Mary Jane Porter served as production assistant. Funding was
provided by the New York Farmers' Fund. Many individuals reviewed the initial
drafts, including Cornell University faculty members, northeastern Cooperative
Extension agents, and employees of the U.S. Department of Agriculture and U.S.