May 29, 2024

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WHAT IS AN AQUIFER

By Tom Watson

Note. We hear and read a lot about the Edwards Aquafer. I hope to write a short article to give the basics about the Edwards, but I find it very complicated. Until I get enough information I thought that the article below would give Writer’s Corner readers a basic understanding of aquifers and aquicludes. It was written for a community paper, The Taberna Tribune, in New Bern, NC. The geography is different, but the principles are the same.

WHAT IS AN AQUIFER?

By Tom Watson

Recent articles by Sue Book in the New Bern Sun Journal and a television presentation on Around the Town with Linda Staunch have emphasized the need to conserve water as the eastern North Carolina population grows. The articles and the television show discuss the plans to drill wells into the Castle Hayne, and Peedee aquifers to reduce the current draw from the Black Creek aquifer. This leads to the questions of “What is an aquifer?” and “Why do we need to change?”

Last month Max Freeze wrote an article for the Taberna Tribune and gave the following explanation or description of aquifers.

“First of all an aquifer is not a man-made thing, as a recent letter to the editor inferred.   We do not “build” multi-million dollar aquifers.  Nature has been doing that for us for the past 15 millions years or so.  The earth beneath our feet for many hundreds of feet consists of stratum, or layers of porous rock separated by nearly impervious layers of other types of rock.  Water seeps down into aquifers from the surface and slowly makes its way downhill, sandwiched between other layers which keep the water in the aquifers from becoming blended.”

Max’s description of aquifers is quite good, though very brief. I have had some experience with wells and aquifers and thought some of you might be interested in a bit more detail about aquifers and New Bern’s water plans. To learn more I visited New Bern’s City Engineer, David Muse, and talked to Dr. Richard Spruill, Professor of Hydrology, at East Carolina University. I came away satisfied that in the area of future water supply excellent plans are being made and implemented. In this article I plan to give an idea of what aquifers are and where our water comes from. In the next issue I will write about New Bern’s plans.

Some terms that will be used are:

Aquifer. An underground layer of water-bearing rock or sand.

Aquiclude. A layer of rock or clay through which little or no water flows.

Confined aquifer. An aquifer confined between two aquicludes. (Also called an artesian aquifer.)

Groundwater. Water that flows underground. Water in rivers, streams, and lakes is “surface water.”

Recharge area or zone. The geographic region where the water that feeds a particular aquifer falls as rain.

Water table. The distance down from the surface that is saturated with water. If you dig down until you hit water, you have reached the water table. Its level fluctuates as rainfall varies. The water table roughly follows the topography.

Artesian well. A well that goes into a confined aquifer. Where there is sufficient pressure, water will flow to the land surface without a pump. This pressure exists because the altitude of the recharge area is sufficiently higher than the altitude of the well.

Sectional illustration of underground aquifers.

See the sectional sketch above. Only two aquifers are drawn: the water table or surficial aquifer, and the first confined, or artesian, aquifer. In the sketch are three wells, ‘A’, ‘B’, and ‘C’. All are cased (pipes driven through the sand, clay, and rock, with slots (screens) in the aquifer zone to let water come into the pipe). None has a pump. The water level in well ‘A’ stands at the water table level of the confined aquifer. The water level of well ‘B’ stands at the local water-table level. In well ‘C’ water is flowing because the elevation of its aquifer’s recharge zone is higher than the ground level at the well’s location, and also because the aquifer is confined, and is fully charged. In the 1920’s free flowing artesian wells were common in East Carolina. Today, because wells serving individuals, cities, factories, and agriculture have pumped more water than has been recharged, aquifers are not full, and the once artesian wells are no longer free flowing.

If you drill a well anywhere and go far enough down, you will drill through several aquifers. New Bern pumps its water from wells near Cove City. A test well driven 1,090 ft down near Cove City ran into the following aquifers.

Aquifer                   Feet down from surface

Surficial (Water Table)        13

Castle Hayne                       101

Beaufort                              213

Peedee                                307

Black Creek                        507

Upper Cape Fear                  869

Lower Cape Fear                 1005

Aquifers get their water (recharge) from rainfall over large areas. Powered by gravity, groundwater flows very slowly from higher elevation to lower, eventually reaching the sea. The water in the deepest aquifers fell to earth farther away than the shallower ones. Water in coastal Carolina’s deeper aquifers may have fallen in the Appalachian Mountains. Of the water that falls as rain or snow, surprisingly little gets into the confined aquifers. More than half evaporates or is transpired by trees and other vegetation; most of what’s left runs off to streams, rivers, and lakes. The small remainder goes mainly to the water table aquifer. Only around 3% enters the confined aquifers as recharge.

The water we pump today did not fall yesterday or last week. Most of the water in our aquifers fell to earth more than a hundred miles from here. Assuming a movement of 5 ft/day, it takes 290 years for water to move 100 miles. The point is – if we use too much from the aquifers today, they cannot be recharged in a short time. A wet season can raise the local water table but has little effect on the confined aquifers. We don’t need to panic, but we should not waste water.

When the water level in an aquifer drops several things happen: pumps don’t pump as much, so more pumps have to be added, a vacuum forms between the water level and the upper aquiclude. This vacuum in the spaces between the grains of sand or gravel may be filled by the land subsiding, by possible air infiltration backing up through unused wells, or in our region, by salt water. The level in the Black Creek aquifer has been dropping dramatically as coastal Carolina population has increased. This is why the state of North Carolina “mandated” that alternate sources had to be found to protect the aquifer from salt water intrusion.

Until recently New Bern pumped 100%of its water from the Black Creek Aquifer. Its level was dropping, and the recharge time was getting longer. New wells have been drilled and now 25% of our water is pumped from the Peedee Aquifer, thus relieving some of the strain on the Black Creek Aquifer. The Peedee water is hard and so New Bern no longer has the soft water of the Black Creek days.

Anyone interested in learning more about aquifers can go to North Carolina Department of Environment and Natural Resources web site at www.ncwater.org . Under Educational and Technical Assistance, click “groundwater” and under Data and Modeling click “groundwater data.”

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