Calvert, Charles and St. Mary's counties have informally agreed to collaborate on a search for a new public water source for Southern Maryland, a $1.4 million study whose results may well determine how the region grows in the coming decades, officials said.

"We need to know what's below the Aquia Aquifer because that's the future. That's where St. Mary's future water supply will come from," said Steven L. King, director of the St. Mary's Metropolitan Commission, the county's water and sewer governing body, during a presentation Tuesday before the St. Mary's commissioners.

"You talk about adequate public facilities, water is probably number one," said County Commissioners President Julie B. Randall (D-At Large).

Most of Southern Maryland draws water from two major aquifers, the Aquia and the Piney Point/Nanjemoy, deep reserves of water held in layers of fine sand, sandwiched between thick, impermeable layers of clay. They are old, vast reserves. Carbon testing revealed that water in the Aquia, for instance, is 8,500 years old. That age indicates they are slow to replenish.

But explosive growth in recent decades has accelerated the decline of water in the aquifers as thousands of new wells have tapped into the same source, according to Harry Hansen, chief of hydrology for the Maryland Geological Survey, a division of the Department of Natural Resources.

Projections based on mathematical models suggest that continued population growth and demand for more water could put critical stress on current sources, Hansen, who did not attend Tuesday's commissioner meeting, said in an interview. The state oversees and must issue permits for all well drilling in Maryland.

Hansen, a hydrologist, will lead the search for the Patapsco, an aquifer below the Aquia and Piney Point/Nanjemoy aquifers that is perhaps larger and deeper than either of them.

He said an observation well built on Solomons Island by the state and the U.S. Geological Survey in 1947 shows the decline over the decades in the region's current water sources.

He pointed to decreasing natural pressure in the aquifers. For example, in the 1950s the natural pressure from the Aquia brought water up to within 25 feet from the land surface. In 1977, this so-called drawdown level, had increased to 50 feet below ground. In June 1998, that level was 133 feet below land surface, and in June this year, it was 149 feet, Hansen said.

"It's a significant decline," he said.

Those changes already have had practical results in parts of St. Mary's County: Homeowners near Breton Bay in Leonardtown have been forced to lower pumps as their wells have gone dry, while others were forced to drill new wells because old wells were not designed to accommodate the pumps.

New wells are being dug deeper, but this too has a limit, said King, of the Metropolitan Commission.

"There's a danger of drawing down too close to the top of the aquifer: Land subsides," King said. In New Orleans, for example, over-pumping aquifers led to a collapse of the soil above them, he said.

With an alternate water source, the decline in the Aquia could be stabilized, Hansen said.

The tri-county study will involve drilling at least six wells to determine the extent and depth of the Patapsco aquifer. The final report on the study will not be ready until 2003, King said.

"While it doesn't seem like a quick turn-around time," the work is laborious and slow, said Thomas Russell, assistant director of the Metropolitan Commission.

King said officials from the three counties will discuss details of the study in the coming weeks.