As the bay rises, some land sinks


The water in the Chesapeake Bay has risen about a foot in the last century, drowning low-lying bay islands, invading delicate marshlands and gobbling up valuable waterfront real estate.

But scientists say rising sea levels aren't the whole story. Something is also causing the land around the Chesapeake to sink, and they want to find out whether it is because a growing bay-side population is pumping too much ground water out from under it.

"We're hoping, if we find it's related to ground-water usage, it can be turned around . . . or at least slowed down," said Dr. Tonie vanDam, a geophysicist with the National Oceanographic and Atmospheric Administration (NOAA).

Restrictions on the use of ground water and a greater reliance on surface reservoirs could help to slow the bay's encroachment on the land.

Dr. vanDam is a co-investigator with Dr. Steve Nerem of NASA and Dr. Mark Schenewerk of NOAA in a $250,000 project called BAYONET, described last week at the annual spring meeting of the American Geophysical Union in Baltimore.

BAYONET will use a network of navigational satellites and ground stations to measure any subsidence of the land around the bay.

"Whole islands have disappeared," said Dr. vanDam. Formerly assigned to the Goddard Space Flight Center in Greenbelt, she now works in NOAA's office at the University of Colorado in Boulder.

Cows Island, Hambleton Island and Herring, Powell, Punch, Sharps, Swan and Turtle Egg islands are just some of those that have vanished.

St. Clements Island, where English colonists first settled in Maryland, has lost 90 percent of the 400 forested acres that greeted them in 1634, according to Dr. Stephen P. Leatherman, director of the University of Maryland's Laboratory for Coastal Research.

Bay cliffs in parts of Calvert County retreat four to five feet each year. Between 1938 and 1988, the Blackwater National Wildlife Refuge lost 20 square miles of wetlands -- a quarter of the total.

"These wetlands are home to numerous ecosystems," said Dr. vanDam. "The Chesapeake Bay is one of the biggest fish-producing areas in the country, and if you lose these marshes, the health of the whole bay may be . . . in jeopardy."

Sea levels have been rising globally since the planet emerged from the last ice age 10,000 years ago, and ice sheets have melted into the sea. The Chesapeake itself formed as the rising Atlantic Ocean flooded the ancient Susquehanna River valley.

The rise has accelerated in some parts of the world in the last century, and some scientists argue that human industrial activities -- principally the burning of fossil fuels -- have hastened the global warming.

Globally, sea levels are rising at a rate of about 1.7 mm per year (almost 0.07 inches). In the Chesapeake, however, sea level is rising at 3.4 mm per year -- twice as fast as the global average.

That suggests something peculiar to the bay is also at work. Dr. vanDam believes as much as 1 mm of the 1.7 mm difference is because of something called "post-glacial rebound."

She explained that during the last ice age, land to our north was covered and weighed down by a mile-thick glacier. When it melted away, the weight was removed, and the Earth's crust is still rebounding -- rising -- where the glacier once stood. To the south, meanwhile, the Chesapeake region has been sinking, a bit like the child on the south end of a seesaw after the one on the north end jumps off.

That leaves 0.7 mm of annual sea level rise in the Chesapeake still unexplained.

Some scientists have suggested that the bay floor may be sinking under the increasing weight of sediment as farmers have cleared forests and accelerated erosion into the bay.

But Dr. vanDam dismisses that idea. The weight of all that sediment is "way too small to account for the discrepancy we see," she said.

Among the remaining explanations is the extraction of ground water. Just as a sponge shrinks as it dries, so porous soil compresses as the water in it is removed.

In Galveston, Texas, Dr. vanDam said, pumping of ground water was found responsible for subsidence of the land by nearly 10 feet. New ground-water controls and a greater reliance on surface water stopped it.

Around the bay, seven instruments are being installed to measure changes in ground level.

Called Global Positioning System (GPS) receivers, the devices were developed by the military to aid troops and ships in determining their precise locations on the planet relative to a constellation of Earth-orbiting satellites.

BAYONET's GPS receivers convert the navigational signals into precise calculations of the distances between the receiver, the satellite and the center of the Earth.

If the ground on which a receiver sits is sinking -- getting closer to the center of the Earth -- the GPS data show it.

The first receiver was installed in October 1993 at Solomons, in Calvert County, where tide gauges have recorded the fastest relative rise in sea level in the bay. Others have been installed at Annapolis, Greenbelt and Wallops Island, Va.

Additional GPS receivers likely will be set up this summer in the Blackwater refuge, the Hampton Roads area in Virginia and Baltimore.

Surprisingly, Dr. vanDam said, the Solomons receiver so far has signaled that the land there is rising, suggesting that relative sea level is falling. "This is totally inconsistent with the [long-term] tide gauge records," she said.

She believes the numbers reflect a short-term fluctuation in what is otherwise a long-term trend in the opposite direction. "We're hoping that with longer-term records, and more records, we'll be able to resolve it," she said.

After all the instruments are operating, she said, scientists should be able to determine whether changes in ground level are uniform across the region -- suggesting broad geological forces at work -- or variable. If each station shows a different rate of change, it would indicate that the causes are local, and ground-water extraction could be to blame.

To test the theory, the GPS ground subsidence data will be compared with records kept by the Maryland Department of Natural Resources on ground-water levels around the region.

The water in wells monitored by the state near Solomons, for example, has dropped nearly 33 feet in the last decade. "That's an incredible amount," Dr. vanDam said, and it reflects increased pumping by the nearby Patuxent Naval Air Station and the growing population around Lexington Park.

If local ground-water declines can be correlated with subsidence in the ground level, BAYONET scientists would have good reason to argue that the extraction of ground water is lowering ground levels, and accelerating the apparent rise in bay waters in the area.

"We're pretty sure that's what we're going to see," Dr. vanDam said.

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