There was a time once, in those quieter days before strip malls and adequate public facilities ordinances, when the groundwater in Southern Maryland was so plentiful that just poking a well down into the nearest aquifer could send water rushing up the pipes and out onto the ground.

Then came crowds of people -- drinking, washing, bathing -- and the impact was like too many mosquitoes sucking the same forearm or too many straws in the same milkshake. The groundwater began its retreat.

Since 1975, the level of the Magothy aquifer, one of five major aquifers in the area, has declined by 90 feet under Waldorf in Charles County; in parts of St. Mary's County under Lexington Park, the Aquia aquifer has dropped to 200 feet below sea level.

It was this decline, noticed by local well drillers since the 1970s, that led state officials in 2000 to begin drilling six exploratory wells into aquifers in Calvert, Charles and St. Mary's counties as part of a multiyear study of the region's water supply.

The preliminary report on the $1.4 million study, prepared by the Maryland Geological Survey and circulated among county government planning offices this month, indicates that current rates of withdrawing water, particularly from the Waldorf area, are not sustainable for the next 25 years. By 2030, the population in Southern Maryland is projected to increase by more than 35 percent -- up to about 440,000 people -- the fastest growth of any region in the state.

"It's definitely an issue of big concern in Southern Maryland," said David Bolton, chief of hydrogeology at the Maryland Geological Survey. "The water is being withdrawn at a greater rate than it's being recharged. When you have that going on, you wind up losing your water."

Homeowners are not yet feeling widespread impacts from the decline, but experts say that eventually some areas of Southern Maryland might need alternative water sources. And water officials agree that it is important to closely monitor the supply.

In the last half-century, the amount of water pumped from Southern Maryland earth roughly tripled, to 43.4 million gallons per day. The study found that the rate of water removal from the Magothy aquifer could not be increased significantly above 2002 levels without dipping below what is known as the "80 percent management level" by 2030. If it reaches that level, it would mean that 80 percent of the pressure in the aquifer has been lost, at which point the state would recommend tapping into a different aquifer, said Robert S. Summers, director of the Water Management Administration in the Maryland Department of the Environment.

"It does not mean we're out of water. . . . It's a warning level," Summers said. "We need to begin planning . . . because 25 years is not much time."

The report also said that future drawdowns could allow the brackish water of the Potomac River to seep into fresh water supplies in the Indian Head area of Charles and even drain down local wetlands.

Southern Maryland relies on underground aquifers because the Potomac is mixed with ocean water that far south; the water would need an expensive desalinization treatment to be used as drinking water.

Charles County already has a backup supply with a capacity of 1.4 million gallons pumped from Washington Suburban Sanitary Commission resources, although only a trickle is used to keep the equipment ready, said Jerome L. Michael, Charles's director of public utilities.

"It's much more expensive than what we're able to produce on our own," Michael said. But a more large-scale treatment of river water "might be something we have to look at." Other options include building reservoirs and relying more heavily on deeper aquifers.