[meteorite-list] Chesapeake Bay Impact Crater (Part 4 of 7)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:14 2004
Message-ID: <200106271606.JAA03089_at_zagami.jpl.nasa.gov>

http://www.pilotonline.com/special/meteor/part4.html

(Salty) water, water everywhere
By DIANE TENNANT
The Virginian-Pilot
June 27, 2001

Part 4 of 7

The water we drink, wash cars with, sprinkle lawns with, play in; the water
that drives new industry and subdivisions and jobs; the water that grows
cotton and soybeans and peanuts and cattle; the water that drizzles out in
long, hot, drought-filled summers; this water most often comes from under
the ground.

It slips through layer cakes of rock and sand and clay. It flows, if the
creeping movement of groundwater can be called a flow, through pores in the
earth. Clay has the most, but those pores are the tiniest and some carry a
static charge that repels the water molecules. So the water moves, instead,
through sand.

Aquifers are not, as some people believe, underground pools of water. They
are layers of sediment that allow water to collect between the grains.
Imagine a water balloon filled first with sand, then with water.

The sediment layers collected over millions of years. As ice ages came and
went, drawing the sea level down, then melting it back up, the shoreline
traveled from Richmond to Virginia Beach, then 50 miles farther east than it
is today, then back to Virginia Beach, then back to Richmond.

It did this numerous times, until there were nine different sandy layers
where water could pool into aquifers, separated by nine different clay
layers that restricted water from moving up or down. The only way to remove
it was through wells, deep vertical pipes that punctured the layers of clay
and sipped from the layers of sand.

The impact wrecked all that. The aquifers, where they cross the outer rim of
the crater, are broken. Geologists call this ``truncated.'' It means they
are, for practical purposes, gone. The sandy aquifer beds were blasted into
the atmosphere. Did the groundwater from undisturbed parts of the aquifers
flow on into the crater? Or did it find the jumbled breccia too tough to
penetrate? No one knows, but it appears that the groundwater may be taking
the path of least resistance around the north and south rims, leaving little
pressure to flush out the salty water trapped in the crater.

The question, Scott Bruce says, is how much water can you get out of a well
that's sunk into the crater. The answer, so far, is not much. And what has
come out is a brine.

David Powars pulls out a color-coded map showing salt levels in various
wells along the East Coast. ``Once you're in the blue, that's as salty as
seawater,'' he explains. ``You can see this big freshwater plume up here in
the north, way deep. That's from the big glaciers; you had so much fresh
water melted it completely flushed out the system. You can go off the shore
in New Jersey, and there's fresh water below the ocean. That's pretty neat.

``Then you come down here,'' he says, pointing to southeastern Virginia,
``and get a couple different stories. See how much this blue line comes up?
To 18,000 milligrams per liter, which is pretty bad. You're not drinkin'
that water. And here it's getting shallower. I thought this was just too
cool, to show bull's-eye, bang, the saltwater keeps coming up. So at less
than 1,000 feet in here, where the crater is, you find the salinity's
high.''

Four aquifers formed across the top of the crater, laid down on the breccia
over millions of years. But five existed before the meteorite arrived. When
it hit, it shattered them, and two of those -- the Upper and Middle Potomacs
-- are the ones most used by public water systems in Hampton Roads.

More than 1.5 million people rely on public water supplies here, and the
population is growing. Of the 13 cities and counties that provide public
water locally, six depend solely on groundwater. Four blend groundwater with
surface water. Another 327 private water systems and 320 noncommunity wells
(such as schools and malls) also draw groundwater.

Public utilities have to figure out how the crater is affecting the water,
said Scott Emry, geologist with the Hampton Roads Planning District
Commission. ``We believe that everything within that crater will be salty.
All this is so brand new that we're still going through the analysis phase
and scratching our heads and saying, `What does that mean?' ''

To stop the itch, commission members chipped in money to study the
long-buried impact crater. This year's contribution is $88,250, matched
dollar for dollar by the U.S. Geological Survey.

Part of the work includes looking for an isotope in water squeezed from the
core samples, a type of chloride that might tell scientists the origin of
the salt and even how long the water has been in the crater. Both are
important in understanding where the salt may end up as a result of pumping
the aquifers.

Powars points to a different line of blue on the chart. ``That's super bad.
At 2,500 feet. Look at that. Off the chart. That's worse than the worst
water almost known. That's the one at Moores Bridges in Norfolk.

``That's been known as a real wicked, super nasty, nasty bad water. I mean,
whew, really bad stuff. Nobody's been able to explain that, but we're
starting to get there now.''

An aquifer is under pressure, both from the soil and rock around it, and
from the water pushing in from the west. When a pipe is sunk into one, the
pressure causes the water in it to actually rise higher than the level of
the aquifer.

The state has a rule: Pumping must never cause the water in the pipe to drop
below 80 percent of its original level. Reduced pressure in one aquifer may
encourage water from other aquifers to jump ship, or even suction saltwater
in sea beds back toward the land. If the pressure drops, the water balloon
will start to deflate, and the land on top of it will begin to sink.

Even before the crater was discovered, the state had identified two areas
believed to have ``significant stress'' on their water supplies. One is the
Eastern Groundwater Management Area, from Virginia Beach to Richmond, by way
of the Peninsula, and down I-95 to the North Carolina line. The other is the
Eastern Shore.

By requiring permits for large-volume groundwater pumping in those areas,
the state hopes to keep the aquifers from running dry. But Hampton Roads
uses more than 60 million gallons of groundwater a day, not counting
agricultural wells and private wells.

``From a study done over on the Eastern Shore, before this crater was ever
studied, they figured out, the USGS did, trying to figure out water
resources back in the '60s, you go down below 300 feet, you're probably not
going to find fresh water,'' Powars says. ``And now that we've done some
deeper tests over there, you can forget it. I've long said the Eastern Shore
of Virginia, you're in trouble. Well, the answer is desalinate.''

Several cities and counties already have begun that process. Gloucester
County is building a plant to take the salt out of drinking water.
Chesapeake, Suffolk and Newport News are already on line. James City County
is considering one. The question is what this pumping, from wells just
outside the crater rim, will do to salt levels in the groundwater. The rim
might act as a barrier. It might not. And other factors may be at play.

``What boggles everybody's mind is that all of the models say everything
should have quieted down, cooled down, there shouldn't be any remnant heat
(from the crater explosion),'' Powars says. ``But when you crack the earth
seven miles down . . . you might easily have awoken an old lava zone there.
Somehow, if you add heat into this thing, the temperature could maybe do
some weird things with salinity and stuff. We've played with this a little
bit, but it's pretty much been behind closed doors. We haven't told anybody
in public, because this is arm-waving stuff.''

And then there are the earthquakes.

Reach Diane Tennant at 446-2478 or dianet_at_pilotonline.com
Received on Wed 27 Jun 2001 12:06:29 PM PDT