Thirty-five million years ago, an asteroid blasted a hole in the ocean floor near what is now the mouth of the Chesapeake Bay, and thirsty Virginians are still living with the consequences.

A federal geologist based in Baltimore says the asteroid's 55-mile-wide crater is to blame for bad-tasting ground water in parts of southeastern Virginia.

David S. Powars, of the U.S. Geological Survey, told a Toronto meeting of the Geological Society of America recently that he believes the crater disrupted the layer cake of underground aquifers in the area. Fresh ground water can't reach the salty water inside the crater to flush it out.

The result is undrinkable ground water inside an arc from Virginia's lower Eastern Shore to the fast-growing Hampton and Newport News areas, which now rely primarily on surface-water reservoirs.

Ground water inside the crater is "really bad stuff," Powars says.

"It's terrible water. We'll be desalinating bay water before we mess with that stuff," he says.

One well drilled on the Eastern Shore near the crater's center produced water 32 percent saltier than seawater.

Well drillers in the area have known about the bad ground water for years. A federal geologist, D. J. Cederstrom, first described it in 1943 as "Virginia's inland salt water wedge." But it remained an anomaly no one could explain.

"Outside, you have beautiful fresh water, some of the best you'd want," Powars says. "Inside, the quality decreases. It gets worse and worse in what we used to think would be the same aquifer."

Policy-makers need to understand the crater's hydrogeology, Powars says, because "with the population expected to grow, they are in trouble for water."

After encountering opposition to a 100-mile water pipeline built from Lake Gaston, N.C., and a proposed reservoir in King William County, Va., local governments are looking increasingly to the brackish ground water beneath their feet.

The city of Chesapeake spent $75 million on a new well field and desalinization plant that will soon produce 3 million gallons of fresh water a day, to replace unreliable surface-water supplies.

Newport News has nearly completed a $17 million well system and desalinization plant. It will produce 5 million gallons of potable water daily, a 10 percent boost to the supply.

"If the impact crater wasn't there, Newport News probably wouldn't have to have this [desalinization] plant," says Scott Bruce. He is environmental program manager for the state's Department of Environmental Quality, and Powars' co-author.

But the new plant may be in a precarious location. Its wells are just outside the rim of the buried crater. It's where fresh ground water flowing from the west grows saltier abruptly as it nears the crater.

Powars worries about what will happen when demand for water rises. Will more pumping draw in fresh water from the west or salt water out of the crater to the east?

"If they do this, they might start pulling salt water out of the crater. It would then get a lot saltier," he says. And desalinization, an energy-intensive process, would get more costly. The plant might have to be redesigned.

The presence of an asteroid crater beneath the bay's mouth was first reported by Powars and geologist C. Wylie Poag in 1993, based on seismic mapping and drill cores.

About 35 million years ago, they said -- about midway between the extinction of the dinosaurs and the appearance of the first humans -- a chunk of rock or ice a mile wide shot through the atmosphere and into the Atlantic off the Virginia coast. (The Eastern Shore did not exist yet, and the ocean reached to the Fall Line, where Richmond, Washington and Baltimore are today.)

"It cut through the water and unconsolidated sediment a couple thousand feet thick just like butter," Powars says. Then it rammed more than 1.2 miles into the underlying bedrock.

The impact blasted open the bedrock and blew a plume of vapor, melted rock and debris high into the atmosphere. Seawater, and two-thirds of what was thrown out of the hole, fell or washed back in. The rest was blown away.

No known extinction is linked to the impact, but it touched off a series of towering tidal waves, or tsunami, that crashed ashore ,, from New England to Georgia.

"Anything living on the East Coast would have been washed away," Powars says, perhaps including the early horses, camels and rhinoceroses living at the time.

As the waves drained off the land, they dragged along a slurry of debris, depositing a jumbled layer of crushed rock, boulders, shells and organic matter across the crater. Powars calls it "tsunami-breccia."

The crater is still there, since buried by 400 feet to 1,200 feet of fine silt, sand and clay.

"A good way to think of it is as an inverted sombrero," Powars says. There is a central crater more than a mile deep and 15 miles wide. It has a central peak, and concentric ridges and valleys surrounded by a shallower, wider basin 55 miles across. The later marine sediments have not yet completely filled the hole, leaving a broad, shallow depression in the ocean bottom.

In most of Tidewater Virginia, Maryland and Delaware, ground water flowing east in the sediments beneath the coastal plain generally remains potable even quite near the ocean.

But in towns on the lower James, York and Rappahannock rivers, measurements of dissolved solids (which include salt) jump in just a few miles from a drinkable 500 parts per million to a "moderately saline" 5,000 ppm. At Cape Charles, on Virginia's Eastern Shore, the ground water is classified as "brine." It's saltier than sea water, with 39,000 ppm of dissolved solids.

Some early hydrologists thought the salt "wedge" was an intrusion from the ocean, to the east. But that didn't explain why the water was so much better north and south of the Norfolk

area. Others blamed decades of ground-water pumping for local industrial processes. But then why was the water worse at rural Cape Charles?

Only when Powars began to compare his maps of the crater with ground-water-quality maps did the link began to dawn on him. "All of a sudden, everything came together," he says.

zTC Scientists still aren't sure why the impact left ground water inside the crater so briny. Its high salt content makes it heavy and dense compared with fresh water. That -- plus the disruption of the older aquifers -- may explain why the fresh ground water outside the crater has never flushed it out.

The discovery "has totally changed the conceptual framework of the aquifer system" in the area, says Bruce.

"There's a big bull's-eye out there that has been disrupted and changed," Bruce says. If the region is to make wise water resource decisions from now on, "we've got to get in there and start learning about those sediments."

Pub Date: 11/13/98