Ground Water in the Coastal Plain of North Carolina
Prepared by
Rodney L. Huffman, Extension Agricultural Engineering Specialist
Published by: North Carolina Cooperative Extension
Service
Publication Number AG-450
Last Electronic Revision: March 1996 (JWM)
Ground water is one of our most important natural resources. Fifty-five
percent of the population depends on ground waterfor drinking. More and more, we
are learning that this resource is in danger. The expansion of population, industry,
and agriculture places greater demands on the natural system. We must plan carefully if we are to
enjoy growth and progress and still protect our ground-water
resources for ourselves and future generations.
This publication will provide an understanding of ground water and its
sources. Understanding will make our task of preserving this valuable resource
much easier.
Ground Water - What Is it?
The ground beneath our feet is
not completely solid. It is more
like a sponge with pores of many
shapes and sizes. When rain falls,
it soaks into the ground and
moves throughout this pore space.
Pore space may account for up to
50 percent of the total volume of
some soils.
Near the soil surface, in the
unsaturated zone, the pores
contain a combination of air and
water. Further down is the
saturated zone where all of the
pore space is filled with water.
This water is called ground water.
The water table is the boundary
between the saturated zone and
the unsaturated zone. A well
must reach down below the water
table, into the saturated zone, to
obtain ground water.
Aquifers and Confining
Beds
The word aquifer comes from the
Latin for nwater bearingn and is
used for any geologic formation
that contains water in sufficient
quantity and with sufficient
mobility to be useful as a water
source (for example, a layer of sand
or gravel).
When water mobility is very
limited (such as in a layer of clay or
silt), the formation is called a confining bed or an aquitard.
Recharge and Discharge
Aquifer recharge is the movement of water from the surface
down into an aquifer. In a recharge area, the net movement of
water is downward. Recharge
usually occurs in the upland areas
between streams.
On the other hand, a discharge
area is an area where the net
movement of water is toward the
surface. Ground water discharge
usually occurs in low areas close to
streams and through the banks and
beds of streams.
Aquifer Systems of the Coastal Plain
The four major aquifers in North Carolina's coastal plain region are, in
order of increasing age and depth:
- the surficial aquifer
- the Yorktown aquifer
- the Castle Hayne aquifer
- the Cretaceous aquifer
The single-hatched areas of Figures 1 through 4 show the range of each
aquifer. The double-hatched areas in the figures indicate where each aquifer
is an important source of water.
Surficial Aquifer
The surficial aquifer is the
saturated portion of the upper layer
of sediments. The thickness of this
layer, from the surface down to the
first major confining bed, is typically from 20 to 50 feet. The surficial aquifer is unconfined, meaning
that its upper surface is the water
table rather than a confining bed.
(The surficial aquifer is sometimes
called the water-table aquifer.)
The composition of the surficial
aquifer varies across the region, but
it is generally 50 percent to 70
percent sand, allowing high infiltration rates.
Many shallow wells tap the
surficial aquifer, especially near the
coast and on the Outer Banks,
where deeper waters are too salty.
The surficial aquifer is also important in the Sandhills area. Because
the water in the surficial aquifer
tends to be acidic, it is somewhat
corrosive. The surficial aquifer is
particularly vulnerable to contarnination.
Yorktown Aquifer
The Yorktown aquifer lies
below the surficial aquifer in the
northern half of the coastal plain.
The Yorktown is thin toward the
west, sometimes less than 20 feet.
It thickens eastward, to as much as
300 feet in Dare County. The
Yorktown is mostly fine sand, silty
and clayey sand, and clay with
shells and beds of shells throughout.
The Yorktown aquifer is an
important source of water in the
northeastern part of the region,
where deeper aquifers are too
salty. It is not used as much in the
western part, since more productive sources are available.
Castle Hayne Aquifer
The Castle Hayne aquifer,
underlying the eastern half of the
coastal plain, is the most productive aquifer in the state. It is
primarily limestone and sand. The
Castle Hayne is noted for its thickness (more than 300 feet in places)
and the ease of water movement
within it, both of which contribute
to high well yields. It lies fairly
close to the surface toward the
south and west, deepening rapidly
toward the east.
Chloride content exceeds 250
parts per million east of a line
between Gates and Carteret
counties. Water in the Castle
Hayne aquifer ranges from hard to
very hard because of its limestone
composition. Ironconcentrations
tend to be high near recharge areas
but decrease as the water moves
further through the limestone.
Cretaceous Aquifer
The Cretaceous aquifer is a grouping of several of the oldest and deepest sedimentary deposits that lie directly over the basement rock. The Cretaceous is the primary source of water for the western half of the coastal plain with the exception of the Sandhills. To the east, the Cretaceous dips underneath the Castle Hayne. Toward the west, it rises near the surface, covered only by the surficial deposits, and in the northern part it rises to just below the Yorktown.Water cannot move as easily in the Getaceous as it does in the Castle Hayne, but the Cretaceous aquifer is very thick, allowing deep and productive wells. Water from the Cretaceous is generally soft and slightly alkaline, requiring no treatment for most uses. In a few areas, however, natural fluoride concentrations exceed the national drinking-water standard.
Table 1 indicates typical well
depths and yields in the four
aquifers described above. Yields
obtained from private wells usually
fall near the lower values. Larger
well diameters and special construction methods may be required
to obtain the higher yields.
Looking Forward
Not long ago, it was assumed that
ground water was beyond the influence of human activity. Dumping
and burial of wastes were widespread. Many of those wastes have
moved down through the soil,
appearing in the ground water
years or decades later. Such contamination is practically impossible
to clean up. We are only now
learning the staggering price of yesterday's ignorance.
The coastal plain area of North
Carolina has one of the most
abundant supplies of ground water
in the world. Yet, the U.S. Geological Survey reports slowly declining
water levels in the coastal plain
aquifers caused by increased
pumping rates. It is not clear what
effect this may have on the future
quality of the ground water.
We can all help to maintain the
highest possible quality through
careful management of fertilizers,
pesticides, and wastes. Proper
construction, maintenance, and
management of wells and wellhead
areas are vital to prevent direct
contarnination of our ground water.
As individuals and as a society, we
must act wisely today so that future
generations can share this precious
resource.
These publications are available at your county Cooperative
Extension Service office. Or, you may order them from Agricultural
Publications, Campus Box 7603, North Carolina State University, Raleigh,
NC 27695-7603 (prepayment is required).
Distributed in furtherance of the Acts of Congress of May 8 and June 30,
1914. Employment and program opportunities are offered to all people
regardless of race, color, national origin, sex, age, or disability. North
Carolina State University, North Carolina A&T State University, U.S.
Department of Agriculture, and local governments cooperating.
AG-450
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