Open-File Report 95-376
Nashville, Tennessee--1995
Ground-Water Quality Protection
Why It's Important To You
By ANK WEBBERS
(Printable PDF available)
Prepared by the
U.S. GEOLOGICAL SURVEY
in cooperation with the
U.S. ARMY, FORT CAMPBELL,
KENTUCKY
Introduction
Ground water is a valuable resource often used for industry, commerce,
agriculture, and most importantly drinking water. In the 1980's, ground
water provided 35 percent of the municipal water supplies in the United
States and 95 percent of the rural, domestic drinking water. Except for
turning on the faucet, most of us who use water often take for granted
its availability and don't really know where our water comes from or recognize
why it may be so vulnerable to contamination.
In the 1970's and 1980's, the United States Environmental Protection
Agency (USEPA) established national guidelines for safe drinking-water
standards in response to a large number of reports of diseases caused by
contaminated ground water. The USEPA developed the concept of Wellhead
Protection which focuses on protecting and preserving the quality
of ground-water supplies. Following USEPA guidelines, many states, including
Tennessee, established local Wellhead Protection programs and rules for
ground-water supplies that were used by the public. The Wellhead Protection
programs are based on a thorough knowledge of the geologic and hydrologic
characteristics of ground-water occurrence and flow. Scientists of the
U.S. Geological Survey (USGS) and other organizations use their skills
and expertise to provide such information to communities that depend primarily
on ground water for their drinking-water supply.
Purpose
This brochure acquaints the reader with some of the common characteristics
of ground water in carbonate (limestone and dolomite) rocks
in Tennessee. It also emphasizes the importance of protecting the quality
of these water supplies for present and future use.
Some Basics About Ground Water in Carbonate Aquifers
Ground water is a term that refers to the water occurring
below ground in zones where open space is saturated or filled with water.
Any zone that allows the movement of water and can supply a usable quantity
of water to a well or spring is called an aquifer . An aquifer
can be composed of loose sediment or cemented rock. In aquifers composed
of sands and gravel, water flows through the openings between the grains,
whereas in cemented aquifers, water can only flow through an available
network of connected fractures or cracks in the rock. Aquifers are replenished
by infiltrating rainfall or surface streams that can penetrate overlying
soils. The water table (ground-water surface ) in an aquifer
rises in response to recharge from rainfall and streams, and falls
in response to drought, pumping conditions, or other withdrawals. Any changes
that occur in the elevation of the water table reflect a net recharge to
or discharge from the aquifer.
(Printable PDF available)
The aquifers in Tennessee are composed of several different rock types.
Approximately two thirds of the State is underlain by cemented aquifers
of limestone and/or dolomite rock.
Typical Features of Carbonate Rock
Land forms that develop in carbonate terrain are referred to as karst,
and are characterized by gently rolling hills, with depressions and sinkholes.
Some sinkholes are created by collapse of overlying soils into underground
cavities in the carbonate rock. Sinkholes also form by the weathering and
dissolving of surface carbonate rocks. Drainage from sinkholes occurs through
the underground network of conduits (tunnels) formed by the dissolving
reaction of mildly acidic ground water and carbonate rock. As the rock
dissolves, small fractures and cracks become larger and may connect to
form an extensive subterranean drainage system. Under certain conditions
caves may form, and if the water table falls, surface streams may disappear
into the underground tunnels.
Studies of water moving through underground tunnels have shown that
ground water can travel up to several miles in a single day! In karst terrain
it is not unusual for a disappearing surface stream to reemerge elsewhere
as a natural spring. In the event that a potential migrating contaminant
can enter a stream, it is highly likely that the same contaminant may resurface
at a spring.
Carbonate rock exhibiting fractures and caves. Note water table has
dropped to stream level.
Example of a Vulnerable Water Supply
In northwestern Middle Tennessee, approximately 40,000 residents of the
Fort Campbell Military Reservation rely primarily on Boiling Spring, a
spring that issues from carbonate rock, for their water supply. Boiling
Spring has an average discharge of about 5 million gallons per day and
receives its water from the surface runoff, streams, and ground water in
adjacent areas up gradient (at higher elevations) of the spring.
The area where surface runoff, precipitation, or stream flow seeps into
the ground water and moves to a water supply well or spring, like Boiling
Spring is referred to as the recharge zone. Any contaminants present
in the recharge zone could easily be transported in waters contributing
to spring discharges or supply wells. Contamination in the recharge zone
of karst aquifers could potentially harm the quality of ground-water supplies
like Boiling Spring. The ability to define the recharge zone as well as
protect it from potential contamination is very important.
Techniques Used To Define Recharge Areas in Karst Terrain
Scientists use several methods to define the recharge zone of springs or
well fields in karst terrain. Field techniques include (1) introducing
harmless dye tracers into the aquifer at points upgradient of the water
supply, (2) measuring water levels in wells in the surrounding area, and
(3) measuring the gains and losses of water in local streams near the water
supply.
Dye tracing is considered the most successful method for confirming
ground-water flow directions in karst aquifers. Harmless dyes which are
injected in sinkholes or wells and detected later at the water supply help
demonstrate that an underground connection exists between the two points.
Maps of the water table are useful for interpreting the general direction
of ground-water flow in most aquifers. The maps are made by plotting measured
water levels in wells to determine elevations of the water table at several
points in an area. In karst terrain, the direction of ground-water flow
may temporarily change if the amount of recharge received in a particular
region of an aquifer exceeds that in another region. This can result in
a local reversal of ground-water flow.
Measuring depth to water in an observation well.
The measurement of flow rates in streams also provides useful information
on the source of recharge water to springs and well fields. By determining
the changes in flow per square mile of drainage areas, scientists can determine
where streams contribute water to or receive water from an aquifer.
Collecting a streamflow rate measurement.
Public and Private Roles in Ground-Water Protection
Individual home and business owners can play a major part in ground-water
protection programs by becoming knowledgeable about proper storage and
disposal of materials that could contaminate the ground water. The most
recognized sources of contaminants that cause problems for ground-water
users include but are not limited to neighborhood dumps, animal wastes,
leaking gasoline tanks, and pesticides. A major source of contaminants
in the ground water of some rural karst areas is associated with the use
of sinkholes as dumping stations. Contaminants associated with garbage
dumped in sinkholes may drain through the sinkhole and recharge an aquifer
that supplies water to a nearby community. Improper handling, storage,
maintenance and disposal of waste materials not only threaten drinking-water
quality, but constitute violations of State and Federal laws.
Questions and Answers Related to Wellhead Protection
Listed below are answers to some commonly asked questions that may be of
concern to anyone living in or near a wellhead protection zone:
Q: How do I know if my home or business is located within a wellhead
protection area?
A: Call your local water supplier. Determine if your water comes from
a spring or community wellfield with several wells. If you have your own
waterwell, recognize what materials are potential contaminants and keep
them away from the well.
Q: What are the most common contaminant sources that threaten ground
water?
A: The most common sources include pesticides, herbicides, fertilizers,
household cleaners, motor oils, sewage overflow, dumps with leaking containers,
street runoff, leaking septic systems, leaking underground gasoline tanks,
and livestock waste. Proper storage and disposal of potentially contaminating
materials should always be considered.
Q: I have an old water well on my property but I don't use it any
more. Can I dump wastes down this well?
A: Never throw any type of waste down an old well. The well may be directly
connected to an aquifer that supplies water to your neighbor, and you could
be contaminating his or her drinking water. Never dump waste or trash in
a sinkhole on your property. In karst areas a sinkhole is probably connected
to an aquifer that could supply drinking water for residents throughout
the area.
Q: What can I do to help protect ground-water supplies?
A: Make sure that all potential polluting materials on your property
are inventoried, and stored, used, and disposed of in an environmentally
safe manner. For example, do not store your cleaning fluids, gas cans,
or antifreeze near an open well. Understand why and how your water supply
and/or your neighbor's could be contaminated if potential contaminating
materials enter the ground water.
A Final Note
In karst terrain, a contaminant can enter a network of conduits in an aquifer
and become a widespread health problem. Federal and local funding for ground-water
cleanups and treatment may be available, but costs can exceed many millions
of dollars. Costly remedial actions could be avoided or minimized when
individuals are aware that ground water is vulnerable to contamination,
particularly in karst. The practice of good "out-of-doors" housekeeping
is necessary. From the standpoint of economic and environmental responsibility,
it is critical that communities work together to protect the quality of
ground-water resources so that current and future generations will continue
to have clean water.
For more information, contact:
District Chief
U.S. Geological Survey
640 Grassmere Park, Suite 100
Nashville, Tennessee 37211
Copies of this report may be purchased from:
U.S. Geological Survey
Branch of Information Services
Box 25286
Denver, Colorado 80225-0286
More information on the activities of the U.S. Geological Survey in
Tennessee can be obtained by accessing the Tennessee
home page, and more information on the Water Resources Division of
the U.S. Geological Survey can be found by accessing the Water
Resources Division home page.
