Ground Water in the Piedmont and
Blue Ridge Provinces of North Carolina
Prepared by:
R.L. Huffman
Extension Agricultural Engineering Specialist
Published by: North Carolina Cooperative Extension
Service
Publication Number: AG 473-6
Last Electronic Revision: March 1996 (JWM)
Ground water is one of our most important natural resources, as over half of
the population of North Carolina depends on it for drinking water. But the
increasing pressures of population, industry, and agriculture have placed
greater demands on this natural system. If we are to enjoy growth and progress
and still protect the ground water in this state, we need to plan carefully to save
this precious resource.
What is Ground Water?
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 through this pore space. In
typical soils, 30 to 50 percent of the total
volume is pore space. Bedrock has
considerably less pore space, often less
than 1 percent of the total volume.
The subsurface can be divided into
two zones. Close to the surface, in the
unsaturated zone, the pores hold both air
and water. Further down, in the saturated
zone, all of the pore space is filled with
water. Water in the saturated zone is
called ground water. The boundary
between the saturated zone and the unsaturated zone is the water table. A well
must reach down below the water table,
into the saturated zone, to obtain ground
water.
Aquifers and Confining Beds
The word aquikr means "water bearing"
in Latin, and it applies to any geologic
formation that contains water in sufficient quantity
and with sufficient mobility to be useful as a water source. Beds of
sand or gravel usually make excellent
aquifers. Although nearly every geologic
formation contains water, not all formations
allow water to move easily through
them. Such compact formations are
known as confining beds or aquitards.
Recharge and Discharge
Recharge is the result of the net
movement of water down from the ground
surface into an aquifer. Recharge areas are
usually found in upland regions between
streams. When water moves from the
aquifer to the surface, discharge occurs.
Ground water discharge areas are usually
located in low areas close to stream beds.
In hilly or mountainous areas, discharge
may occur as seeps and springs higher
on the slopes.
Geology of the Piedmont
and Blue Ridge Provinces
The various landscapes of central and
western North Carolina are grouped into
two provinces based upon similar landforms (Figure 1). Geologists use the term province to
denote broad areas having similar origins and
structures.
Figure 1. Physiographic provinces of North Carolina
In many respects, the piedmont and Blue
Ridge provinces are very similar. Both are in
a relatively advanced state of weathering. Almost all of the soils have
formed "in place" by the weathering of underlying rock. (This
pattern contrasts with the state
of the water-worked sediments
of the coastal plain, glaciated
regions of the upper Midwest,
or wind-deposited soils in the
central United States.) The
chemical and physical weathering produces a clay-rich layer
overlying the bedrock that is typically 30 to 70 feet
thick but may be as much as 300 feet deep.
There are several geologically distinct areas in the
piedmont province (Figure 2). These areas, known as
Triassic basins, formed when deep rifts filled with
sediment. The largest of these basins stretches 90
miles from central Moore County to southern Granville County. The three smaller basins are located in
Anson, Montgomery, and Richmond counties; Stokes
and Rockingham counties; and Davie and Yadkin
counties. Many of the sedimentary rocks in these
basins are fine-textured and sparsely fractured.
As North Carolina's mountains and ridges were
formed, the rock formations underwent extensive up-
lifting, folding, and faulting. The stresses of these
movements caused the rock to crack, creating a
system of fractures. Although individual fractures
may extend hundreds of feet. they are typically less
than 1/16 of an inch wide.
These bedrock fractures provide a network of
channels for water movement. However, they provide
very little storage volume. The porosity is often less
than 1 percent. Most of the water is stored in the
unconsolidated materials overlying the bedrock,
where the porositsy may be 20 to 40 percent. The
whole system can be compared to a large tank with a
network of small pipes connected below it.
Figure not available.
Figure 2. Geologic units with lower well yields.
Wells
Wells provide access to ground water. We consume
ground water in preference to surface water because
it undergoes a natural filtration process as it moves
down through the soil. Most harmful organisms are
filtered out. The soil can react chemically with contaminants
in the water, often breaking them down or
immobilizing them before they reach the ground
water. These natural processes make ground water
the safest choice for drinking.
To be effective, the well must provide access to
high-quality water, and it must protect that water
from contamination. In an improperly constructed
well, contaminants can bypass the natural filtration
system and move directly into the ground water,
fouling the drinking water supply.
Figure 3. Typical drilled well in the piedmont and Blue
Ridge provinces.
Construction
Proper construction of wells is necessary to obtain
and protect water quality. Figure 3 illustrates basic
construction of a well drilled into fractured bedrock.
North Carolina regulations set minimum separations
between wells and septic or sewer systems. Contact
your local health department for specific requirements in your area or see Cooperative Extension
Service publication, AG-469, Your Water Supply: Well
Construction and Protection, for more information.*
The water in the soil above the bedrock is not
considered safe and must be excluded from the well.
For this reason, state well construction standards
require that the well have a solid casing that extends
down to the bedrock. The well casing must also be
grouted to prevent the channeling of water along its
outside.
Choosing a Location
In the piedmont and Blue Ridge provinces, ground
water enters a well by flowing through the fractured
rock. To obtain the greatest flow, the well must be
drilled where the fracturing is most intense so that the
well bore will intersect as many fractures as possible.
Also, the "tank" (the more porous unconsolidated
materials) should be large enough so that the supply
to the "pipes" (the fractures in the bedrock) is plentiful.
Both of these conditions are usually met on the
lower reaches of the landscape - that is, in draws and
on the sides of valleys of perennial streams. Upland
areas, in general, have fewer fractures and thinner
blankets of unconsolidated materials. The best well
locations are marked in Figure 4.
The floodplains along rivers hold deposits of
unconsolidated sediments, usually containing sand
and gravel beds. These beds are highly porous and
allow water to move freely through them. Wells
located in these materials may obtain high yields, but
they also may draw water from the stream if pumping
is sustained at high rates. Surface waters from the
stream will contain contaminants that might not be
adequately filtered by the short travel through coarse
materials.
A driller with experience in the local geology
should be able to select the most promising site on a
piece of property. For particularly difficult areas,
consult a geologist.**
Figure 4. Topographic and geologic features of the
piedmont and Blue Ridge provinces.
(From Heath, Ralph
C., Ground Water Regions of the United States, USGS
Water Supply Paper 2242,1984)
Typical Well Yields
Data from more than 6,200 drilled wells in the piedmont and Blue Ridge provinces of North Carolina
show that, on average, wells in draws and valleys
yield about three times as much water as wells on
hills and ridges in the same area. Wells on slopes and
flats yield about one and one half to two times as
much as wells on hills and ridges. Well yields generally increase with larger diameters and greater
depths.
The many rock types underlying these areas
influence water availability. Average yields from the
mountains are around 23 gallons per minute (gpm).
Throughout most of the piedmont, the average yield
is 18 to 21 gpm. In the Carolina Slate Belt, yields
average 15 to 16 gpm. The lowest yielding areas of the
state are the Milton Belt (12 to 13 gpm), and the Smith
River Allocthon and the Triassic Basins (11 to 12 gpm)
(Figure 2).
Looking Forward
Until recently, it was assumed that ground water was
virtually unaffected by human activity. But "out-of-sight,
out-of-mind" thinking contributed to widespread
dumping and waste burial. Many of those
wastes have migrated down through the soil to
contaminate the ground water years or decades later.
Such contamination is practically impossible to clean
up. Only now are we learning the staggering price of
yesterday's ignorance.
We can all help to maintain the highest possible
quality of our ground water by carefully managing
our fertilizers, pesticides, petroleum products, and
wastes. Also, proper construction, maintenance, and
management of the well and wellhead area are vital in
preventing direct contamination of the water we
drink. As individuals and as a society, we must act
wisely today so that future generations may share this
precious resource.
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 473-6