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Aug 2, 2003
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Aquifer Is More Sponge Than Lake

BY CLARK COX: Senior Writer

Middendorf Aquifer: It’s an esoteric name, but that hasn’t kept people from talking about it.

The term, once used primarily by geologists and, to a lesser extent, by other scientists, is now bandied about regularly at public meetings. After last year’s drought, public concern arose about the state of the aquifer, which provides much of Moore County with its water.

Lack of knowledge about the aquifer hasn’t prevented some laymen from making statements about it. Some of these statements are incorrect — but, having been made in public meetings, they have made their way into print.

Few people understand what the aquifer is, how it got its name and what it does as well as Dennis Brobst, director of Moore County Public Utilities. He operates the systems that provide water to much of Moore County, including the Pinehurst system, where the matter has come in for much attention in recent years.

“We have a good idea of what the aquifer is, but it is not completely mapped, by any means,” Brobst says. “What we need at this point is a bunch of guys with ‘Ph.D.’ after their names — U.S. Geological Survey types, real hydrogeologists — to come in here and work about five years, compiling data and giving us a complete map.

“We’d know better then how much water was available to us in the aquifer, and whether we’re adding to it through rainfall and surface-water flow, or whether we’re taking away from it by pumping water out of our wells.”

Pinehurst Mayor Steve Smith agrees with Brobst, but he suggests that mapping the aquifer may take as long as 10 years.

“What we need is an in-depth study,” Smith says. “There’s no question about that. But such a study would be very expensive unless some university or the U.S. Geological Survey would be willing to pay for it. Usually the communities that would benefit from the study do the paying.”

Public discussion of the aquifer has led some to believe that it is a large underground lake underlying Pinehurst. Not so. Such subterranean pools are rare. Properly speaking, the Middendorf Aquifer is not the water, but the formation of rock and sand that holds it.

It’s more like a sponge than a lake bed, experts say. Water flows through the sponge, going from higher to lower pockets.

“The water flows downhill,” Brobst says. “In most places, there’s an upper aquifer and a lower aquifer. Water that is not disturbed will eventually find its way to the lower aquifer. We’re not breaking any laws of physics here.”

What Is the Aquifer?

Geologists describe the Middendorf Aquifer as sedimentary sandstone, sand and mudstone that has washed down to the North Carolina Coastal Plain from higher elevations to the west. It is mottled gray with an orange cast.

Pinehurst is at the “peak,” or highest point, of the aquifer.

“That’s a disadvantage in a way, in that the water naturally flows downhill, away from Pinehurst,” Brobst says. “But it’s an advantage in that the community at the peak of the aquifer determines its own fate. There are no communities ‘upstream’ to take usable water out of the aquifer.”

But being situated at the peak of the aquifer limits the amount of water available to Pinehurst. Much concern has been expressed in recent years because many people digging irrigation wells in Pinehurst — and there have been hundreds — have drilled through the upper aquifer and taken water from the village’s drinking-water supply in the lower aquifer.

“I’m convinced that there is an upper aquifer and a lower aquifer,” says George Hillier, mayor pro tem of Pinehurst. “I’m convinced that they exist, even though we don’t have good maps of them yet.”

How deep the upper aquifer and lower aquifer are depends on “where you drill a well,” Brobst says.

Most wells dug in the Pinehurst area strike water first at about 40 or 50 feet. There is almost always a layer of impermeable clay, below which is the “lower aquifer,” containing water that is purer and potable.

“We usually strike the lower aquifer at less than 200 feet deep,” Brobst says. “And 200 feet is shallow for a well. In Maryland, I was accustomed to wells that are 900 feet deep or deeper.”

Brobst says there always seems to be a layer of clay separating the two layers.

“But that clay is located at different depths,” he says. “That allows the water to flow from upper aquifer to lower. What I’m saying is that the upper and lower Middendorf aquifers are discontinuous.”

A peculiar characteristic of much of the aquifer also causes the water to flow unusually quickly away from Pinehurst: Studies have demonstrated that the sandstone of the Middendorf Aquifer is more porous than most. Scientists place the average porosity of sandstone at about 25 percent. Nearer the coast, the Middendorf’s porosity is over 40 percent.

The Aquifer’s Extent

By any measure, the Middendorf Aquifer is huge.

It “runs out” somewhere around N.C. 211. Wells drilled north of that road often come up dry.

But from Pinehurst, the aquifer flows slightly downhill to Aberdeen — which gets water from its own system of wells — and Hoke, Scotland and Robeson counties and to the Fuquay-Varina area. It is still relatively near the surface in those areas.

Even Southern Pines and other Moore County communities that draw their water from Drowning Creek make use of the Middendorf Aquifer, Brobst says. That’s because the aquifer — along with surface water — feeds the creek.

“Surface water is just groundwater with no ground on top of it,” he says.

The aquifer continues to go downward throughout much of the Coastal Plain to Cape Hatteras on the Outer Banks, where several other aquifers overlay it. Through part of this area, the aquifer is known as the “Black Creek Aquifer,” after a prominent stream in the section where the aquifer was first described in North Carolina. But it is the same aquifer.

To the southwest, still going downward for the most part, the Middendorf Aquifer underlies much of South Carolina, Georgia and Alabama, where it is known as the Tuscaloosa Aquifer. There is then some discontinuity, but there are “remnants” of the sedimentary formation making up the Middendorf Aquifer as far west as Texas, where it is commonly known as the Woodbine Aquifer.

The Aquifer’s Name

There is some disagreement about how the Middendorf Aquifer got its name.

Because Pinehurst sits on a high “outcrop” of the aquifer, Donald Van Roosen has suggested that the aquifer may have been named for a Middendorf family who came to the area in 1892, before the village was founded.

(Van Roosen is chairman of the Water Resources Committee of the Village of Pinehurst Planning and Zoning Board, which has studied the Middendorf Aquifer in some detail.)

Brobst says the aquifer must have been named for the geologist who discovered it. There have been prominent geologists named M.A. Middendorf and Emmo Middendorf, but they have both lived and worked in recent decades, after the term “Middendorf Aquifer” began to be used. And Emmo Middendorf was a European and probably never worked in this country.

The most prevalent theory is that the aquifer is named for the town of Middendorf, S.C., near which the formation that makes up the aquifer was first described. Most aquifers have such geographical designations.

Sure enough, a geologist named Berry is said to have described “a formation” near Middendorf, S.C., in 1913. The name “Middendorf Aquifer” has been in common scientific parlance since about that time.

Managing the Aquifer

Most geologists are of the opinion that the Middendorf Aquifer is being “recharged” by surface water at a rate sufficient to provide Pinehurst and surrounding communities with water for a long time to come.

But that’s only an opinion. The recharge rate is anybody’s guess. There are different points of view.

Brobst has one pretty good way of measuring the recharge rate. He does “static water-level tests,” measuring the level of water in wells after the pumps have been turned off.

Pinehurst has 21 public production wells, one of which has been closed because of contamination, and two test wells.

“Soon, we hope to have two more production wells,” Brobst says.

Each well, by state regulation, runs a maximum of 12 hours each day. Pumping at this rate, the wells can produce up to 1.6 million gallons a day (MGD) of water.

Pinehurst uses about 1.4 MGD in the winter months and as much as 3.5 MGD in the summer. Water purchased and piped in from other community systems makes up much of the difference.

Most of Pinehurst’s water usage is residential. (FirstHealth Moore Regional Hospital, the village’s largest water user, accounts for 100,000 to 110,000 gallons a day. Pinehurst Inc. irrigates its golf courses with impounded water from Lake Pinehurst.) But for a primarily residential community, that’s a lot of water usage. Much of it is for irrigation. If the irrigation is done right, most of the water evaporates or is taken up by plants. Very little goes back to the aquifer.

“We know the aquifer reasonably well, short of completely mapping it,” Brobst says. “If we space our public wells out and manage the aquifer carefully, it will be a viable source of water for years to come.

“But we need to manage that resource.”

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