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Groundwater and Contamination
Groundwater Information Flyer #2
Groundwater and Contamination: From the Watershed into the Well
Reprinted May 1985
Published by the Community Groundwater Protection Project
Massachusetts Audubon Society
This is the second in a series of Groundwater Information Flyers published by
the Massachusetts Audubon Society's Community Groundwater Protection Project.
The flyers provide information to help citizens and local officials protect
groundwater resources in their communities. Groundwater Information Flyer #1,
"An Introduction to Groundwater and Aquifers", presents basic hydrogeologic
principles pertaining to groundwater and the geology of aquifers. Groundwater
Information Flyer #2 applies those principles to groundwater supplies and
sources of contamination. This publication may be copied. Please give credit
to the Massachusetts Audubon Society.
WATER IN THE WATERSHED
Three quarters of Massachusetts' municipalities use groundwater for all or part
of their drinking water supply. To date, 31 communities have lost
approximately 30 million gallons of drinking water per day due to
contamination. Although the state provides some funding, technical assistance,
and regulatory programs, the authority to ensure a safe groundwater supply
rests primarily with municipalities. Thus, development and implementation of
groundwater protrection programs are the responsibility of local officials and
citizens. Aggressive groundwater protection programs are needed to avoid a
water supply crisis across the Commonwealth.
What is a Watershed?
All land is part of one watershed or another. When rain falls, much of the
water runs across the surface of the land toward a stream, river, pond, or lake
as surface runoff. The land area that drains runoff to the stream, or other
surface water body, is called a watershed, or drainage basin. Watershed
boundaries, also called drainage divides, are the high lands that divide one
watershed from another. Theoretically, a drop of rain that falls squarely on a
drainage divide is split into halves; one half flows into one watershed and the
other into the adjacent watershed. A watershed, or drainage basin, is a land
area that drains surface runoff to a stream, river, pond, or lake. Coastal
watersheds drain to the ocean.
Groundwater is found beneath the surface of the ground within drainage basins.
It does not move in underground rivers from distant watersheds. The source of
all groundwater in each watershed is the precipititation that falls there.
Groundwater divides usually occur approximately beneath surface water divides.
Occasionally, however, the groundwater divide may not coincide with the
watershed boundary. In that case, a recharge area for an aquifer in one
watershed may extend partially into the adjacent watershed, but these
conditions are relatively rare, and, in any case, quite local. Sophisticated
groundwater studies are required to determine exactly where groundwater divides
occur; such studies are needed, however, only in particular circumstances. For
example, it may be necessary to know whether a proposed development near a
divide could contaminate groundwater flowing toward a well in the adjacent
watershed. Groundwater divides usually occur approximately beneath surface
Since groundwater occurs within watersheds, and groundwater divides are usually
approximately beneath surface water divides, watersheds are often used as the
basic hydrologic unit for both surface water and groundwater planning
purposes. Massachusetts has been divided into 27 major drainage basins for
water resources planning. Each one includes a number of tributary watersheds,
or sub-basins, that are drained by smaller streams or rivers.
How Groundwater Moves Through the Watershed
Groundwater moves slowly from recharge areas, where precipitation is absorbed,
down to discharge areas, where it flows or seeps out of the ground and becomes
part of the surface water. When groundwater discharges into surface water,
they flow together. Streams and rivers flow down the valley of the watershed
until they join larger rivers and, eventually, reach the ocean. Thus,
groundwater typically flows toward a stream, while the stream flows toward the
Massachusetts is divided into 27 drainage basins for water-resource planning.
The fact that groundwater becomes surface water when it reaches discharge areas
can't be over-emphasized. Groundwater and surface water are interconnected and
can only be fully understood and intelligently managed when that fact is
acknowledged. For example, if pumping wells remove too much groundwater, there
will not be enough groundwater discharge to maintain stream flow and aquatic
habitats such as wetlands and ponds. (Contrary to popular belief, wetlands in
Massachusetts are not usually important groundwater recharge areas. Recharge
through wetland soild occurs very slowly and introduces only minor amounts of
surface water into the groundwater system.)
WATER IN THE WELL
Cone of Depression/Area of Influence
When a public supply well is pumping, groundwater flow changes direction in a
portion of the watershed. Instead of moving toward the natural discharge area,
the groundwater within the influence of the pump flows toward the well from
every direction. The pumping well creates an artificial discharge area by
drawing down (lowering) the water table around the well. This area of drawdown
is called the cone of depression.
Every pumping well is surrounded by a cone of depression, but each cone has a
differnt size and shape, depending on the duration and rate of pumping, the
aquifer's characteristics, the natural slope of the water table, and the
availability of recharge. The shape of the cone of depression is strongly
influenced by geologic materials, such as bedrock or clay, which serves as
groundwater barriers. Except for wells in bedrock, the cone of depression of a
private residential well is usually very small; the cone of depression of a
public supply well, however, can extend thousands of feet from the well.
The cone of depression is most easily illustrated by a diagram that shows a
cross-section of the well and the cone. However, to make this concept clear in
relation to the land surface, it is useful to visualize the area from above.
From this perspective, the cone of depression is termed the area of influence.
Although, technically, the terms refer to different views of the same
phenomenon, they are often used interchangeably.
The Cone of Depression Changes Size
When the amount of groundwater that is withdrawn by the pumping well is equal
to the amount of groundwater recharge within the area of influence, the cone
stops expanding. However, the cone of depression does not always remain the
same size. If there is no precipitation to recharge the aquifer, and the well
keeps pumping, the pump will pull water from a greater distance, and the cone
of depression will get deeper and wider. After heavy precipitation, with good
recharge, it will get smaller.
Land use can change the size and shape of the cone of depression and the
ability of the aquifer to supply water. If impermeable surfaces (such as
parking lots) cover a portion of the area of influence or its upland recharge
area, and the runoff from those surfaces flows overland to streams instead of
recharging the groundwater, the cone of depression for the pumping well will
have to expand to compensate for the lost groundwater recharge. As a result,
land areas formerly outside this area of influence will become primary recharge
areas for the well, if they can absorb precipitation. If there is no porous,
permeable land within reach of the pumping well that can provide the recharge
needed, the yield of the well may decrease. If enough of the potential
recharge area is covered with impermeable surfaces, or if nearby surface waters
are diverted for other purposes, the yield can be reduced so drastically that
the well must be abandoned.
Limit of the Well's Influence
A well draws water from only a portion of the watershed, specifically, the cone
of depression and upland recharge areas. Outside these areas, collectively
termed the areas of contribution, groundwater does not move toward the well.
Instead, it moves in its normal pattern from the recharge area down to the
discharge area. In some locations, such as the outwash plains of Cape Cod, the
cone of depression may extend farther than the area that actually contributes
water to the well. This phenomenon occurs on the down-
gradient side of the well where the water table slope is so great that it
counteracts the well's influence. In any case, the boundaries of the area of
influence can be determined by groundwater studies.
Most public supply wells in Massachusetts are located in buried valley aquifers
that are associated with a nearby stream or river (see Flyer #1). Most of these
wells draw surface water from the stream in a process called induced recharge.
Induced recharge occurs when the cone of depression reaches as far as the
stream, thereby lowering the water table beneath it. If there are no
impermeable barriers such as clay or thick deposits of organic muck in the
streambed, the pump will pull water from the stream down through the aquifer
and into the well. Under these conditions, polluted surface water can enter
the well and degrade the quality of the water supply. In Massachusetts,
induced recharge probably occurs in all but a few public supply wells located
in valley aquifers.
Four Areas That Should be Protected
There are four areas significant to groundwater supplies that must be
identified an protected to prevent contamination. They are all a part of a
Aquifers - Aquifers are geologic formations that are capable of yielding a
significant amount of water to a well or spring. In Massachusetts buried
valley aquifers are the sites of most public supply wells. Coastal outwash
plains in southern Plymouth County, Cape Cod, Martha's Vineyard, and Nantucket
are also excellent aquifers.
Cone of Depression - This is the area around the well where the water table is
lowered when the well is pumped. Since water is withdrawn from this area to
supply the well, it should receive utmost protection. Contamination that
enters groundwater within the cone of depression will eventually reach the
Recharge Areas - Recharge areas are porous, permeable geologic deposits
(usually sand and gravel) that can absorb precipitation and allow it to
percolate down to the water table and flow into the aquifer. These areas
usually include the land surface directly above the aquifer and the porous,
permeable areas adjacent to the aquifer. Although the most important recharge
areas are those that replenish the portions of an aquifer that supply a well,
all aquifer recharge areas should be protected, especially if there is a
potential for developing new wells in the aquifer in the future.
Surface Water - When the cone of depression intersects a lake, river, or
stream, surface water may be drawn into the well via induced recharge. This
occurs commonly in Massachusetts because most public supply wells are located
in valley aquifers near rivers and streams. In these cases, both the quality
and quantity of the surface water can affect the well. Therefore, it is
important to protect the surface water that contributes to recharge. To do
that, it is necessary to control land use in the watershed so that contaminants
will not reach the river or stream, and to ensure that upstream use of the
water does not decrease the quantity required to supply the well.
THE WATERSHED AND THE WELL
There are many sources and types of groundwater contamination, both natural and
man-made. Human waste disposal practices, such as those for sanitary, solid,
and industrial waste, are the most serious sources of groundwater pollution.
Contamination also results from hazardous material spills and leaks and from
the application of fertilizers, pesticides, and road salt. Because
contamination is a complex problem, only general information about this topic
can be presented here.
How is Groundwater Contaminated?
Contaminants often enter groundwater from the land surface where they are
dumped or spilled and percolate down through the zone of aeration, until they
reach groundwater. Some contaminants, such as fuel from leaking tanks and
effluent from septic systems, occurs underground. In either case, once
pollutants enter groundwater, they flow according to the same hydrologic
principle: from recharge areas toward discharge areas. Pollutants move along
the groundwater flow path at a rate varying between a fraction of an inch and a
few feet per day.
How Does Pollution Move Through an Aquifer?
In surface water such as a stream or lake, pollutants can disperse throughout
the water body, mix thoroughly with the water, and become diluted. In
contrast, contamination in groundwater remains concentrated, with only a little
dispersion, because little mixing takes place. As a result, contamiants in
groundwater usually form a concentrated slug of pollution that moves with the
groundwater as a contamination plume. Moreover, different contaminants settle
at different levels when they reach the zone of saturation. Some, like most
hydrocarbon components of oil or gasoline, are insoluble and lighter than water, and therefore, float on top of the water table. These chemicals can spread easily underground if the contamination occurs in permeable surficial deposits. Other contaminants, such as salt, dissolve in water and still others, depending on their density and solubility, sink to the bottom of the zone of saturation. The porosity, permeability, and chemical composition of the geologic formation and the slope of the water table all affect the way contamination moves once it reaches groundwater.
Natural Processes May Decrease Pollution
In general, it is very difficult if not impossible to correct groundwater
pollution. Depending on the level and type of groundwater contamination,
however, some pollutants can be removed or reduced by natural processes. As
they flow through the zone of aeration, some contaminants are removed or
reduced by natural means. For example, certain heavy metals may be taken up by
plants, and bacteria can degrade or change a variety of organic compounds.
Other pollution-reduction mechanisms include absorption (chemical and physical
bonding to soil particles), filtration, and chemical reactions.
While some contaminants, such as phosphates, may be removed before they reach
groundwater, others, such as synthetic organic solvents, are resistant to
breakdown. The soil's ability to attenuate (reduce) contamination depends
partly on its characteristics and partly on the volume and type of contaminants
involved. Very small quantities may never reach goundwater; very large
quantities may reach groundwter without any attenuation, and some contaminants,
such as chlorides, resist degradation in any case.
Contamination in the Well
Contamination can enter the well from natural recharge areas and, by induced
recharge, from polluted surface water. These two pathways will be discussed
Natural Recharge Areas - Contaminants can enter a well from the same land area
that supplies the well with water. If contaminated groundwater enters the area
of influence, or its upland recharge areas, the contamination will eventually
reach the pumping well. Likewise, if the area of influence expands,
contaminated groundwater within the expanded area of influence will also be
pulled toward the well. The contamination will reach the well if the pump
continues to pull water from the expanded area of influence. Often, the area
of influence contracts again, for example after heavy rains. If this occurs,
the contaminated groundwater may never reach the well. Instead, the natural
groundwater flow pattern will prevail once more and contaminants may flow
toward the natural discharge area instead of toward the well. To prevent
contamination of the well, the area of influence and upland recharge areas
should be identified and protected.
Induced Recharge - If surface water is being pulled into the well via induced
recharge, a pollutant that enters the body of water upstream of the well is a
potential source of contamination. Runoff from farms, golf courses, and
parking lots, waste-water discharge from industries, and leachate from septic
systems are examples of potential pollution sources.
Common Groundwater Contaminants and Contamination Sources
There are many types and sources of groundwater contamination in Massachusetts.
Only the most common ones will be discussed here.
Iron and Manganese - Iron, one of the earth's most abundant elements, occurs
naturally throughout Massachusetts; manganese is less common but it is often
found in association with iron in groundwater. Iron and manganese in drinking
water do not pose a health hazard; in fact, iron is needed for oxygen transport
in the blood, so it is essential to good health. However, both iron and
manganese can impart an unpleasant taste to water and can stain plumbing
fixtures and laundry. Water departments and boards of health report that these
two problems provoke the most water-quality complaints. Many potential
groundwater supplies have not been used because of high iron and manganese
concentrations. However, it is likely that water shortages will force
communities to treat and use most of these potential water supply sources in
Road Salt - Every winter, approximately one-half million tons of road salt are
spread on Massachusetts' public ways to melt ice. Most of the salt used is
sodium chloride; calcium chloride is used relatively little because it is much
more expensive. Both types of salt are very soluble in water and move easily
into groundwater. Aquifers and recharge areas crossed by highways or located
near uncovered salt storage piles are liable to be contaminated by sodium.
Excessive sodium in the diet can induce hypertension in people who are
susceptible to it; federal and state limits for sodium in drinking water are 20
parts per million (ppm). Almost one third of all public drinking water
supplies have exceeded that limit. Weston, Weymouth, and Auburn closed public
supply wells after extremeley high sodium levels were discovered. Road salt
has also contaminated private wells in western Massachusetts.
Organic Solvents - A number of industrial, domestic, and public supply wells in
Massachusetts have been contaminated by organic solvents. The most common
chemicals of this type found in groundwater are trichloroethylene (TCE), 1,1,1,
trichloroethane, tetrachloroethylene, and carbon tetrachloride, In some cases,
the contamination could not be traced to its source but known or suspected
sources include waste chemical storage sites, areas with illegally stored and
dumped barrels of hazardous wastes, industrial sites with complex occupancy
histories, leaking sewer lines, and smaller generators of hazardous wastes such
as machine shops and barrel-and-truck-washing facilities.
The use of organic solvents is not limited to industry. These chemicals are
found in many household products, including stain and spot removers,
degreasers, paint and varnish removers, drain cleaners, and septic system
Commercial Fertilizers - Since World War II, the use of commercial fertilizers
on crops, lawns, and golf courses has increased steadily. The major
constituents of commercial fertilizers are nitrogen, phosphorous, and
potassium, all nutrients required by plants. Potassium and phosphate do not
move into groundwater as readily as nitrogen compounds do. Nitrogen in
fertilizer is oxidized to form nitrates; nitrates percolate into the soil with
rain and irrigation water and can contaminate groundwater. Where fertilizers
are applied year after year, nitrate levels in groundwater may gradually
increase and eventually exceed the 10 ppm limit allowed for drinking water by
state and federal standards.
Pesticides - Pesticide use has also increased steadily since World War II. The
term "pesticide" refers to a wide range of chemicals, including herbicides,
insecticides, rodenticides, and fungicides that are used to kill or control
organisms, both plant and animal. While some pesticides are known sources of
groundwater pollution, not enough research has been done on the movement of
pesticides underground. In some cases pesticides may degrade or combine with
other substances to form more toxic products. Although there may be
exceptions, pesticides that are soluble in water are generally a greater threat
to groundwater than those that are not. While much information is available
about health effects from a single large dose (acute exposure) of these toxic
compounds, far less is known about the health effects that result from exposure
to small amounts over a long period of time (chronic exposure). State and
federal laws regulate the amounts of six pesticides (2,4-D, 2,4,5-TP (Silvex),
endrin, lindane, methoxychlor, and toxaphene) that may be present in public
drinking water supplies. Although the state annually requires drinking water
quality tests for a variety of chemical and biological substances, pesticides
are not tested for as often.
Septic Systems and Cesspools - Septic systems that are carefully sited,
constructed, and maintained can be effective and inexpensive wastewater
treatment systems. One benefit of these onsite systems is that they recharge
groundwater while municipal sewer pipes transport the water away for
treatment. Even the best septic systems, however, release some bacteria and
nitrates into the ground. Septic systems that are poorly designed, sited, or
constructed, and most cesspools, can be sources of severe pollution. Effluent
from septic systems and cesspools can contain bacteria and viruses, nitrates,
heavy metals, detergents, and elevated levels of choride, sulfate, calcium,
magnesium, potassium, and phosphate. Pollution from this effluent poses a
threat to groundwater quality. Groundwater contamination can also be caused by
hazardous chemicals that homeowners pour down their drains and by septic
systems cleaners that contain strong acids or organic solvents.
Nitrates, bacteria, and viruses from septic systems are major health hazards.
Greater than 50 ppm of nitrates can cause a blood disease called
methemoglobinemia, or "blue baby" in infants, and the possible formation of
carcinogenic nitrosamines. A decrease in the concentration of bacteria and
viruses in groundwater depends on soil type, amount of effluent, and distance
that the effluent travels. In Massachusetts, a distance of 400 feet is
considered adequate for the removal of most pathogens. However, when many
septic systems are concentrated in a small area with highly permeable soil, it
cannot be assumed that all bacteria and viruses are being removed from the
effluent. In fact, some micro-organisms persist in groundwater and can be
transported through an aquifer for hundreds of yards. When many septic systems
are located within the area of influence of a public supply well, the water
supply is likely to be degraded.
Underground Fuel Storage Tanks - Underground tanks are a potential source of
groundwater contamination. Fuel oil, diesel fuel, and many other chemicals are
stored in underground tanks in Massachusetts. Gasoline leaks from underground
tanks have contaminated groundwater in numerous Massachusetts towns including
Truro, Dover, Walpole, Auburn, and Westport. Many gasoline tanks were
installed in the 1950s and 1960s and are now reaching the end of their useful
lives; corrosion is the cause of most steel tank failures. Even small leaks
can have serious consequences - just one part per million of gasoline
discharged into groundwater can render it unsafe for drinking.
Dumps and Landfills - Leachate is the liquid that is created beneath dumps and
landfills when precipitation percolates through the decomposing solid waste.
The precipitation dissolves soluble materials and carries them along with the
water flow. Leachate can contain large quantities of both organic and
inorganic contaminants. The volume and characteristics of landfill leachate
depend on the amount of water that passes through the refuse and the materials
that are buried at the site. Unless landfills are covered with impermeable
material (such as clay) to prevent precipitation from percolating through them,
leachate continues to be produced for many years after dumps and landfills are
Leachate can seep out of dumps and landfills into surface water or it can
percolate downward into groundwater and move in a contamination plume toward a
discharge area. Leachate moves most easily through sand, gravel, or other
porous, permeable material, therefore there is a greater potential from
contamination of aquifers near landfills located in these deposits.
Unfortunately, sand and gravel pits were often used as dump and landfill sites
Since groundwater supplies are found quite locally within drainage basins, and
the authority and responsibility for groundwater protection rest with
municipalities, what is the best way for local officials and citizens to
protect their groundwater sources? First, aquifers, recharge areas, and
potential contamination sources must be identified and mapped; then a
groundwater protection program can be developed and put into place. If
adjacent towns share the aquifer, cooperative efforts for groundwater
protection are very important. Groundwater Information Flyer #3 will explain
how to identify and map the crucial areas. Flyer #4 will describe the powers
and duties local boards and departments have to protect groundwater.
Baldwin, Helene and C.L. McGuinness. A Primer on Groundwater. United States
Geologic Survey, 6th printing, 1980. Library of Congress Card No.
Dunner, Thomas and Luna Leopole. "Groundwater". Chapter 7 in Water in
Environmental Planning. San Francisco: W.H. Freeman & Co.*
Freeze, R. Allan and John A. Cherry. Groundwater. Englewood Cliffs: Prentice
Hall, Inc., 1979.
Frimpter, Michael. Probable High Ground-Water Levels in Massachusetts. U.S.
Geological Survey Water Resources Investigations Open-File Report
Frost, L.R., Jr., S.J. Pollack and R.F.Wakalee. Hydrologic Effects of Highway
De-icing Chemicals in Massachusetts. U.S. Geological Survey Open-File
Report 81-209, 1981.
Metropolitan Area Planning Council. A guide to the safe use and disposal of
hazardous household products. Boston, 1982.*
----Septic Systems: A Manual for Owners. Boston, 1981.*
----Underground Fuel Storage Manual. Boston, 1982.*
Roy, Steven P. A Process to Identify Potential Salt Contaminated Groundwater
Areas. Massachusetts Department of Environmental Quality Engineering,
----Road Salts and Water Supplies: Best Management Practices. Mass. Department
of Environmental Quality Engineering, 1981.*
Strahler, Arthur N. The Environmental Impact of Groundwater Use on Cape Cod.
Orleans: Association for the Preservation of Cape Cod, 1972.
U.S. Environmental Protection Agency Report No. EPA-570/9-017. A Guidance
for Protection of Groundwater Resources from the Effects of Accidental
Spills of Hydrocarbons and Other Hazardous Substances. 1979.*
UOP, Inc. Johnson Division. Groundwater and Wells. 7th Printing. Saint
*These publications and others are available in groundwater resource libraries
at the following Massachusetts Audubon Society sanctuaries:
Arcadia Wildlife SAnctuary, 127 Combs Rd., Easthampton, MA 01027
Berkshire Sanctuaries, 472 West Mountain Rd., Lenox, MA 01240 (413)637-0320
Endicott Regional Center, 346 Grapevine Rd., Wenham, MA 01984 (617)927-1122
South Shore Sanctuaries, 2000 Main St., Marshfield, MA 02050 (617)837-9400
Stony Brook Nature Center, North St., Norfolk, MA 02056 (617)528-3140
If you are concerned about the environment, join us. For membership
information, please write to: Massachusetts Audubon Society, Membership,
Lincoln, MA 01773, or call (617)259-9500.
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