North Carolina
Aquifers
The aquifers in North Carolina are
highly varied in their character and water producing capabilities. Several
of these aquifers can be traced over large geographic areas and hence form
principal aquifers; significant sources of ground water for potable water
supplies and other agricultural or industrial interests in large portions
of North Carolina. Other aquifers in the State provide less significant
amounts of water and cover smaller areas of the State. The following
material describes briefly some attributes of the principal aquifers,
which include the Lower Cape Fear, Upper Cape Fear, Black Creek, Peedee,
Castle Hayne, Yorktown, Surficial, and Bedrock aquifers. Minor aquifers in
the State include the Lower Cretaceous, Beaufort, and Pungo
River.
Aquifers, or more accurately, aquifer systems, are
hydraulically connected materials (sands, limestone, and fractured rock)
that provide water through a properly constructed well open to those
materials. In the coastal plain, an aquifer is typically composed of one
to several layers of eastward thickening, permeable sands or limestone
split by discontinuous, clay-rich materials. Confining units, consisting
of clay-rich sediments, exist above and below an aquifer. These confining
units are more continuous clay layers and separate the aquifers. The
surficial or unconfined aquifer overlies all the confined aquifers in the
coastal plain.
In the Piedmont and Blue Ridge Provinces of North
Carolina, two major aquifer systems exist, and usually interact with one
another. The surficial materials or regolith of these provinces forms the
unconfined aquifer and the fractured rock beneath, is the unconfined to
semi-confined bedrock aquifer. Usually the surficial aquifer feeds the
fractures in the bedrock aquifer.
Several of the principal aquifers
deserve further classification. The Upper and Lower Cape Fear, Black
Creek, Peedee, and Castle Hayne aquifers form regional aquifers. In large
portions of these aquifers, sands and limestone materials are so well
connected that withdrawals cause pressure reductions many miles from the
pumping center. This is good news in that wells are very high yielding.
The negative outcome is that pumping at one well affects water levels in
wells for miles around. This can be seen in water level data from the
central coastal plain in the Upper Cape Fear, Black Creek, and Peedee
aquifers; Robeson, Bladen and Columbus counties in the Upper Cape Fear
aquifer; northeastern North Carolina in the Lower Cape Fear aquifer; and
Beaufort and surrounding counties in the Castle Hayne
aquifer.
Recent Carbon 14 age dating of the ground water in the
Black Creek and Upper Cape Fear aquifers reinforces our understanding of
the ground water flow patterns in these aquifers. The ages of the water
increased with depth and with distance from the fall line eastward and
ranged from 400 years to over 26,000 years [1]. The major source of
recharge to these aquifers comes from where they are in communication with
the surficial aquifer near their westward limit. The water flows down
gradient for tens of thousands of years before being withdrawn by water
users in the coastal plain.
The following correlation chart
illustrates the relation between North Carolina coastal plain aquifers,
geologic time, geologic formations, and similar information from South
Carolina and Virginia.
Principal Aquifers (links to the latest potentiometric surface maps
available)Historical
potentiometric surface maps for the regional aquifers can be seen
here.
- Lower
Cape Fear aquifer: This aquifer is present in the northwestern
portion of the coastal plain at elevations of -9 to -3260 feet,
averaging -798 feet (referenced
to mean sea level). The Lower Cape Fear aquifer ranges from 1 to 3081
feet thick and averages 394 feet thick. The
aquifer is composed of fine to coarse sands. Wells typically yield
200-400 gallons per minute.
- Upper
Cape Fear aquifer: This aquifer is present in the western
portions of the coastal plain at elevations of 295 to -2394 feet,
averaging -387 feet. The Upper
Cape Fear aquifer ranges from 3 to 3892 feet thick and averages 185 feet thick. The aquifer
is composed of very fine to coarse sands and occasional gravels. Wells
typically yield 200-400 gallons per minute.
- Black
Creek aquifer: This aquifer is present in the central and
southwestern portions of the coastal plain at elevations of 318 to -1477
feet, averaging -172 feet. The
Black Creek aquifer ranges from 14 to 448 feet thick and averages 160 feet thick. The
aquifer is composed of very fine to fine "salt and pepper" sands. Wells
typically yield 200-400 gallons per minute.
- Peedee
aquifer: This aquifer is present in the central to southeastern
portion of the coastal plain at elevations of 114 to -1842 feet,
averaging -164 feet. The Peedee
aquifer ranges from 2 to 1001 feet thick and averages 142 feet thick. The aquifer
is composed of fine to medium sand. Wells typically yield up to 200
gallons per minute.
- Castle
Hayne aquifer: This aquifer is widely used in the eastern
portions of the coastal plain at elevations of 65 to -1091 feet,
averaging -143 feet. The Castle
Hayne aquifer ranges from 6 to 1105 feet thick and averages 165 feet thick. The
aquifer is composed of limestone, sandy limestone, and sand. It is the
most productive aquifer in North Carolina. Wells typically yield 200-500
gallons per minute, but can exceed 2000 gallons per minute.
- Yorktown aquifer: This aquifer is present throughout most of
the northern coastal plain at elevations ranging from 97 to -222 feet,
averaging -17 feet. The Yorktown
aquifer ranges from 4 to 992 feet thick and averages 133 feet thick. Several
localities tap this aquifer and produce high yielding wells including
Roanoke Island, Kill Devil Hills, and Elizabeth City. Yorktown aquifer
is composed of fine sand, silty and clayey sand, shell beds, and coarser
sand beds. Wells typically yield 15-90 gallons per minute.
- surficial aquifer: This aquifer is widely used throughout the
State for individual home wells. The surficial aquifer is the shallowest
and most susceptible to contamination from septic tank systems and other
pollution sources. Commonly, large diameter wells (up to 3 feet in
diameter) are drilled up to 60 feet deep to store large quantities of
water in the well casing. The surficial aquifer is also very sensitive
to variations in rainfall amounts -- they are the first to dry-up in a
drought. On the Outer Banks shallow wells are subject to rainfall
amounts, saltwater intrusion, poor quality ground water, and ocean
overwash. Wells typically yield 25-200 gallons per minute.
- Fractured Bedrock aquifer: This aquifer is widely used for
home water supply in the western coastal plain, Piedmont, and Blue Ridge
Provinces. Usually six inch wells are drilled to intercept water bearing
fractures which are more common in valleys or draws. Thick sequences of
regolith (surficial aquifer) above fractured bedrock can improve yields
to 200 gallons per minute or more. industries and county or municipal
well fields look for these higher yielding bedrock wells. Wells
typically yield 5-35 gallons per minute.
Minor Aquifers
- lower Cretaceous aquifer: This aquifer is rarely used. It is
only fresh in the northern part of the coastal plain below Hertford and
Northampton Counties. The aquifer consists of fine to medium sand with
occasional coarse sand and limestone beds.
- Beaufort
aquifer: This aquifer is present in the eastern-central portion
of the coastal plain at elevations of 27 to -1737 feet, averaging -333 feet. The Beaufort aquifer
ranges from 10 to 253 feet thick and averages 84 feet thick. This
aquifer is composed of fine to medium glauconitic sand, clayey sand,
with occasional shell and limestone beds.
- Pungo River aquifer: This aquifer is present in the
north-central coastal plain and consists of fine to medium
sand.
Aquifer MapsMaps of the North Carolina regional aquifers
follow. Please consider this information as "work in progress." Maps are
based on data collected from USGS and NCDENR sources and will change as
mapping and interpretation in DWR project areas progress. Sources include
the following: USGS Open-File 87-690, USGS WRIR 93-4049, 89-4128, &
87-4178; and DWR's Hydrogeologic Assessments of Wilmington Harbor, North
Albemarle, CUA #1, Southern Coastal Plain, and the Central Coastal Plain.
Arcview shape files include a polygonal dataset of the extent of the
fresh, transitional, and salty parts of the aquifer; the distribution of
boreholes as point data; and line data representing the contours of the
aquifer top.
Aquifer
| Image
| Shape Files (zip)
| Date
| Legend |
Yorktown |
|
|
11/2009 |
|
Castle Hayne |
|
|
11/2009 |
Beaufort |
|
|
11/2009 |
Peedee |
|
|
11/2009 |
Black Creek |
|
|
11/2009 |
Upper Cape Fear |
|
|
11/2009 |
Lower Cape Fear |
|
|
11/2009 |
The block diagram to the right illustrates how the
linked maps show the distribution of salty and fresh water in an
aquifer. The width of the transition zone corresponds to the dip of
the fresh-salt water interface. A well in the fresh extent shown on
the map will be fresh in that aquifer (<250 ppm chloride
concentration). A well in the transition extent shown on the map
will encounter salty water in that aquifer at some depth. A
well in the salty extent shown on the map will be salty in that
aquifer. | Approximate recharge areas
for some of these aquifers are to be shown on a recharge map that is
under construction.
Aquifer recharge areas indicate where
the respective aquifer may be recharged by the surficial aquifer. Local
conditions may not allow recharge to confined aquifers. Please refer to
the typical water
budget for the NC coastal plain. As is clear in this budget diagram, a
very small percentage of rainfall infiltrates into the confined
aquifer system. Most rainfall is lost to evapotranspiration, runoff, or
infiltrates into the shallow ground water system then discharges to local
rivers and streams. This discharge to surface drainage is commonly
referred to as "base flow."
Those aquifers not shown are recharged
laterally or by vertical movement through leaky confining units.
[1] 14C
GROUNDWATER AGE AND THE IMPORTANCE OF CHEMICAL FLUXES ACROSS AQUIFER
BOUNDARIES IN CONFINED CRETACEOUS AQUIFERS OF NORTH CAROLINA, USA,
Casey Kennedy, David P. Genereux, Department of Marine, Earth &
Atmospheric Sciences, NCSU, Raleigh, NC, in Radiocarbon, Vol 49, Nr 3,
2007, p. 1181-1203 |